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- Subject: FAQ: rec.audio.* Amplifiers 2/99 (part 4 of 13)
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- Summary: Answers to common questions about audio equipment, selecting,
- buying, set-up, tuning, use, repair, developments, and philosophy.
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- Archive-name: AudioFAQ/part4
- Last-modified: 2004/01/12
- Version: 2.16
-
- 11.0 Amplifiers
- Note: A receiver contains an amplifier, so the following
- questions apply to both receivers and amplifiers. In the
- following text, "amp" and "amplifier" are used synonymously.
-
- 11.1 What is Biamping? Biwiring?
- Most speakers are connected to an amplifier by one pair
- of terminals on each speaker. Within these speakers, a
- crossover distributes the signal (modified appropriately)
- to each of the drivers in the speaker.
-
- Some speakers are set up to be either biwired or biamped. A
- much smaller number allows triwiring and triamping. The same
- principles apply but use three sets of wires or three amplifiers
- instead of two. Most speakers that support biamping/biwiring
- have two pairs of terminals and some mechanism for shorting
- the two pairs together when used in the normal way. This
- mechanism is most likely a switch or a bus bar. To help
- the descriptions below, I will refer to these two pairs as
- LO and HI (because normally one pair connects to the woofer
- and the other pair connects to the tweeter/midrange).
-
- Biwiring means that a speaker is driven by two pairs of wires
- from the same amplifier output. One cable pair connects HI to
- the amp, and the other cable pair connects LO to the same amp
- output that you connected the HI cable to. Biwiring is
- controversial; some folks hear a difference, some do not. One
- plausible explanation for this involves magnetic induction of
- noise in the relatively low current HI cable from the high
- current signal in the LO cable. Accordingly, Vandersteen
- recommends the two cable pairs for a channel be separated by at
- least a few inches. In any case, the effect appears to be small.
-
- Biamping means that the two pairs of terminals on a speaker are
- connected to distinct amplifier outputs. Assuming you have two
- stereo amplifiers, you have two choices: either an amp per
- channel, or an amp per driver. For the amp per channel, you
- connect each terminal pair to a different channel on the amp
- (for example, the left output connects to HI and the right side
- to LO). In the other configuration, one amp connects to the LO
- terminals, and the other amp is connected to the HI terminals.
-
- The point of biamping is that most of the power required to
- drive the speakers is used for low frequencies. Biamping allows
- you to use amps specialized for each of these uses, such
- as a big solid-state amplifier for the LO drivers and higher
- quality (but lower power) amp for the higher frequencies.
- When you have two identical stereo amps, some folks
- recommend distributing the low-frequency load by using an amp
- per channel. In any case, whenever you use two different
- amplifiers, be careful to match levels between them.
-
- Biamping also allows you to use high-quality electronic
- crossovers and drive the speaker's drivers (the voice coils)
- directly, without the series resistance and non-linear
- inductance of a passive crossover. Biamping which uses the
- speaker's crossover is therefore much less desirable. Replacing
- a good speaker's crossover with an electronic crossover has
- advantages, but involves some very critical tradeoffs and tuning
- which is best left to those well-equipped or experienced.
-
- See also section 16.0 below, on wire and connectors in general.
-
- 11.2 Can amplifier X drive 2 ohm or 4 ohm speakers? How do I raise the
- impedance of a speaker from (say) 4 ohms to 8 ohms?
- Almost any amplifier can drive almost any load if you don't turn
- the volume up too high. Tube amplifiers are one exception.
- Some amps clip if you play them too loud. This is bad and
- damages speakers. Other amplifiers shutdown if they are asked
- to play too loud. Many will overheat, with bad consequences.
- However, in almost all cases, it takes seriously loud sound or
- low speaker resistance (less than 4 ohms) to do damage. Running
- two sets of 8 ohm speakers at once with common amplifiers
- represents a 4 ohm load. Four sets of 8 ohm speakers makes a 2
- ohm load. Two sets of 4 ohm speakers also makes a 2 ohm load.
- If you stay sober and don't turn it up past the point where it
- distorts, you are PROBABLY safe with most amplifiers and most
- loads. See 11.3 for more information.
-
- You can raise the impedance of a speaker by a few different
- methods. However, each has drawbacks. If your amplifier won't
- drive your speakers, AND you are sure that the problem is that
- the speakers are too low impedance, you might try one of these
- techniques.
-
- A) Add a 4 ohm resistor in series with the speaker.
- This requires a high power resistor, because the
- resistor will dissipate as much power as the speaker.
- Doing this will almost always hurt sound quality, too.
- This is caused, in part, by the fact that speakers do
- not have constant resistance with frequency. See 11.3
- for more information on this.
-
- B) Use a matching transformer. There are speaker matching
- transformers which can change from 4 ohm to 8 ohm, but
- a high quality transformer like this can cost as much
- as a common receiver. Also, even the best transformer
- will add some slight frequency response and dynamic
- range errors.
-
- C) Use two identical speakers in series. If you have two
- 4 ohm speakers which are the same make and model, you
- can wire them in series and make an equivalent speaker
- with 8 ohm impedance. The sound from that "new speaker"
- will not be as precisely localized as it would from one
- speaker, so your stereo image may be hurt. Also, it
- requires that you buy twice as many speakers as you
- might have bought otherwise. However, this technique
- has one side benefit. Two speakers can handle twice the
- power of one.
-
- 11.3 How do I drive more than two speakers with one stereo amplifier?
- One amp can drive many speakers. However, there are two limits
- to this practice. The first is that you can overheat or damage
- an amplifier if you drive too low of an impedance to loud
- listening levels. Avoid loading any amplifier with a lower
- impedance than recommended. Adding two speakers to one amp
- output loads that output with half the impedance of one speaker.
- (See also 11.2 above)
-
- The second is that with tube amplifiers, which are uncommon
- in today's common system, it is important that the speaker
- impedance and the amplifier output impedance be well matched.
-
- When driving two or more speakers from one amp output, always
- wire them in parallel, rather than series. Series connection,
- while safe in terms of impedance levels, can hurt sound quality
- by raising the impedance that the speakers themselves see.
- Also, when different speakers are wired in series, amplifier
- voltage will divide between the speakers unevenly, because
- different speakers have different impedance-versus-frequency
- characteristics.
-
- Many amplifiers have connectors for two pairs of speakers. In
- general, these amplifiers also have a speaker selector switch.
- Most amplifiers connect speakers in parallel when both are
- selected, although some less expensive ones will wire the
- speakers in series. It is common for these amplifiers to require
- 8 ohm speakers only, because the amplifier is built to drive
- either 4 or 8 ohms, and two sets of 8 ohm speakers in parallel
- loads the amplifier like one set of 4 ohm speakers. It is
- almost always safe to connect one set of 4 ohm speakers to
- an amplifier with two sets of outputs, provided that you
- NEVER use the second terminals for any other speakers.
-
- 11.4 How big an amplifier do I need?
- Unfortunately, amplifier power ratings and speaker power ratings
- are almost always misleading. Sometimes, they are factually
- wrong. Speaker ratings are almost useless in evaluating needs.
-
- To start with, sound pressure, measured in dB, often stated as
- dB SPL, is a function of the log of the acoustic "sound" power.
- Further, human hearing is less sensitive to differences in power
- than the log transfer function would imply. This means that the
- perceived difference between a 50 watt amplifier and a 100 watt
- amplifier, all else equal, is very small! One columnist said
- that a 250 watt amplifier puts out twice the perceived
- loudness of a 25 watt amplifier, but quantitative statements
- about perception should always be treated with caution.
- That statement came from Electronics Now Magazine, Jan 1994,
- Page 87, Larry Klein's "Audio Update" Column, which is also
- good reading on the subject of required amplifier power.
-
- There is a wide variation in the "efficiency" and "sensitivity"
- of the various speakers available. I have seen good speakers
- with under 80 dB per watt efficiency and have also seen good
- speakers with over 96 dB per watt efficiency, measured one meter
- from the speaker. This difference of 16 dB represents a factor
- of 40 difference in power requirement!
-
- So the first step in determining amplifier requirements is to
- estimate relative speaker efficiency. Other factors include how
- loud you will want to listen, how large your room is, and how
- many speakers you will drive with one amplifier. This
- information will give you a rough starting point. For an
- example, a typical home speaker will produce 88 dB at 1 watt.
- In an average room, a person with average tastes will be happy
- with this speaker and a good 20 watt per channel amplifier.
- Someone who listens to loud music or wants very clean
- reproduction of the dynamics of music will want more power.
- Someone with less efficient speakers or a large room will also
- want more power.
-
- Past that point, you will have to use your ears. As with all
- other decisions, your best bet is to get some candidates, borrow
- them from a friendly dealer, take them home, and listen to them
- at your normal and loudest listening level. See if they play
- cleanly when cranked up as loud as you will ever go, into your
- speakers in your room. Of course, it is also important to be
- sure that the amp sounds clean at lower listening levels.
-
- 11.5 Do all amplifiers with the same specifications sound alike?
- Some say that they do. Some say that they don't. Some
- demonstrated that many amplifier differences can be traced to
- very slight frequency response difference. Let your own ears
- guide you. If you want to compare amplifiers, you can do it
- best in a controlled environment, such as your home, with your
- music and your speakers. Also be very careful to match levels
- precisely. All you need to match levels of amplifiers is a high
- input-impedance digital voltmeter set to AC volts and a test
- recording or signal generator. For best accuracy, set levels
- with the speakers wired to the amplifier.
-
- 11.6 Is this amplifier too big for that set of speakers?
- There is no such thing as an amplifier that is too big. Small
- amplifiers are more likely to damage speakers than large ones,
- because small amplifiers are more likely to clip than larger
- ones, at the same listening level. I have never heard of
- speakers being damaged by an overly large amplifier. I have
- heard of 100 watt speakers being damaged by a 20 watt
- amplifier, however, in really abusive hands. This will happen
- because when an amplifier clips, it will generate much more
- energy at high frequencies than normal music would contain.
- This high energy at high frequencies may be less than the
- continuous power rating of the speaker, but higher than the
- actual energy rating of the tweeter. Tweeters tend to be
- very fragile components
-
- 11.7 Where can I get a cheap low-power amplifier?
- There are very few available. One source is to buy a cheap boom
- box and only use the amplifier. Another source is Radio Shack.
- A third alternative is to buy a car stereo booster and get a
- 12V power supply for it. Finally, you can build an amp pretty
- easily if you are handy, but it probably won't be that cheap.
- Sound Values has a 60 watt amp kit complete for about $200, and
- Old Colony sells some amp kits for a bit more. These kits have
- been built by satisfied rec.audio.* posters. (See 11.15,
- 11.16, 11.17)
-
- 11.8 Is the stuff sold by Carver really awesome?
- There is a lot of repeated rumor and prejudice for and against
- Carver equipment based on anecdotes of older Carver equipment.
- Sometime in 1994, Bob Carver left the Carver Company, so it is
- reasonable to expect significant changes in the company and
- their product line. One of Carver's claims to fame is lots of
- watts per pound of weight. As with almost everything else, the
- best policy is to listen for yourself and see what you think.
-
- 11.9 What is a preamplifier?
- A preamplifier is an amplifying electronic circuit which can be
- connected to a low output level device such as a phono cartridge
- or a microphone, and produce a larger electrical voltage at a
- lower impedance, with the correct frequency response. Phono
- cartridges need both amplification and frequency response
- equalization. Microphones only need amplification.
-
- In most audio applications, the term 'preamplifier' is actually
- a misnomer and refers to a device more properly called a
- 'control amplifier'. Its purpose is to provide features such
- as input selection, level control, tape loops, and sometimes,
- a minimal amount of line-stage gain. These units are not
- preamplifiers in the most technical sense of the word, yet
- everyone calls them that.
-
- 11.10 What is a passive preamplifier?
- A passive preamplifier is a control unit without any
- amplification at all. It is a classic oxymoron, because it has
- no capability to increase the gain of the signal. It is only
- used with line level sources that need no gain beyond unity.
-
- 11.11 Do I need a preamp? Why?
- The tasks of a preamp are to:
- Switch between various input signals,
- Amplify any phono inputs to line level,
- Adjust the volume,
- Adjust the treble and bass if necessary,
- Present the right load impedance for the inputs, and
- Present a low source impedance for the outputs.
-
- If you have a turntable, you NEED a preamp with a phono input.
- This is because the turntable has an output which is too
- small for driving amplifiers and because the output of the
- turntable requires frequency response equalization. You
- can't connect any other source to a phono input other than a
- turntable (phono cartridge). Also, you can't connect a phono
- cartridge or turntable to any input other than a phono input.
-
- Microphones also require special preamplifiers. Some microphones
- also require "phantom power". Phantom power is operating power
- for the microphone which comes from the preamp. Microphone
- preamps are often built into tape decks and microphone mixers.
-
- If you only have high level inputs, such as the output of a CD
- player and the output of a tape deck, the main value of a preamp
- is selecting between inputs and providing a master volume
- control. If you only listen to CDs, it is plausible to skip
- the preamp entirely by getting a CD player with variable level
- outputs and connecting them directly to a power amplifier.
-
- Some caveats apply. One, the variable outputs on a CD player are
- often lower sound quality than fixed outputs. Two, some sources
- have high or nonlinear output impedances which are not ideal for
- driving an amplifier directly. Likewise, some amplifiers have
- an unusually low or nonlinear input impedance such that common
- sources can't drive the input cleanly. A good preamplifier
- allows use of such devices without sacrificing sound quality.
-
- Unfortunately, the only way to be sure that a preamplifier is
- of value with your sources and your amplifier is to try one.
-
- 11.12 Should I leave equipment on all of the time or turn it on and off?
- Some gear draws significant electricity, so you will waste money
- and fossil fuel if you leave it on all of the time. As an
- example, a common amplifier consumes 40 watts at idle. High-end
- gear uses far more electricity, but ignoring that, 40 watts x
- 168 hours x 52 weeks x US $0.0001 per watt hour (rough estimate)
- is $35/year. Now add a CD player, a preamp, and a tuner, and it
- really adds up.
-
- High-end enthusiasts claim that equipment needs to warm up to
- sound its best. If you care about the best sound, give your
- equipment at least 20 minutes to warm up before serious
- listening. Warm up will allow the inside temperature to
- stabilize, minimizing offsets, bring bias currents up to their
- proper values, and bringing gain up to operating level.
-
- Either way, good gear will last a very long time. Tubes are
- known to have a finite life, but good tube designs run tubes
- very conservatively, giving them life exceeding 10 years of
- continuous service. Some amplifiers run tubes harder to get
- more power out, and thereby may be more economical to turn off
- between use.
-
- | Electrolytic supply capacitors will fail after enough time at
- | temperature. They will last longer if turned off between use.
- | However, like tubes, capacitors can last tens of years of
- | continuous use, as can power transformers, semiconductors, and
- | the like. Better quality electrolytic capacitors are rated for
- | operation at 105 degrees C. If you're replacing the
- | electrolytic capacitor in a power supply, look for capacitors
- | with this higher temperature rating, rather than 85 degree C
- | capacitors.
-
- | Electrolytic capacitors have a funny problem that justified a
- simple break-in or reforming when they are restarted after many
- years of rest. It involves bringing up the power line voltage
- slowly with a variable transformer. For tips on reforming
- capacitors, consult "The Radio Amateur's Handbook", by the
- ARRL.
-
- Semiconductors seem to fail more often because of bad surges and
- abuse than age. Leaving gear off may be best for semiconductors
- and other surge-sensitive gear if you expect power line surges,
- as come from an electrical storm or operation of large motors.
-
- Fuses seem to age with temperature and get noisy, but they are
- so inexpensive that it should not bias your decision. However,
- some are inconvenient to change, and may require opening the
- case and even voiding the warranty.
-
- 11.13 Do tube amps sound better than transistor amps? FETs?
- Lets first list some commonly used active electronic
- | components and their good and bad attributes. What follows
- | are some generalizations. There may be exceptions to these
- | generalizations, but they are based on solid facts.
- |
- TUBE: (Valve, Vacuum Tube, Triode, Pentode, etc.)
- Tubes operate by thermionic emission of electrons from a
- hot filament or cathode, gating from a grid, and collection
- on a plate. Some tubes have more than one grid. Some tubes
- contain two separate amplifying elements in one glass
- envelope. These dual tubes tend to match poorly.
-
- The characteristics of tubes varies widely depending on the
- model selected. In general, tubes are large, fragile, pretty,
- run hot, and take many seconds to warm up before they operate
- at all. Tubes have relatively low gain, high input resistance,
- low input capacitance, and the ability to withstand momentary
- abuse. Tubes overload (clip) gently and recover from overload
- quickly and gracefully.
-
- Circuits that DO NOT use tubes are called solid state, because
- they do not use devices containing gas (or liquid).
-
- Tubes tend to change in characteristic with use (age). Tubes
- are more susceptible to vibration (called "microphonics") than
- solid state devices. Tubes also suffer from hum when used with
- AC filaments.
-
- Tubes are capable of higher voltage operation than any other
- device, but high-current tubes are rare and expensive. This
- means that most tube amp use an output transformer. Although
- not specifically a tube characteristic, output transformers
- add second harmonic distortion and give gradual high-frequency
- roll-off hard to duplicate with solid state circuits.
-
- TRANSISTOR: (BJT, Bipolar Transistor, PNP, NPN, Darlington, etc.)
- Transistors operate by minority carriers injected from emitter
- to the base that are swept across the base into the collector,
- under control of base current. Transistors are available as PNP
- and NPN devices, allowing one to "push" and the other to "pull".
- Transistors are also available packaged as matched pairs,
- emitter follower pairs, multiple transistor arrays, and even
- as complex "integrated circuits", where they are combined with
- resistors and capacitors to achieve complex circuit functions.
-
- Like tubes, many kinds of BJTs are available. Some have high
- current gain, while others have lower gain. Some are fast,
- while others are slow. Some handle high current while others
- have lower input capacitances. Some have lower noise than
- others. In general, transistors are stable, last nearly
- indefinitely, have high gain, require some input current, have
- low input resistance, have higher input capacitance, clip
- sharply, and are slow to recover from overdrive (saturation).
- Transistors also have wide swing before saturation.
-
- Transistors are subject to a failure mode called second
- breakdown, which occurs when the device is operated at both
- high voltage and high current. Second breakdown can be avoided
- by conservative design, but gave early transistor amps a bad
- reputation for reliability. Transistors are also uniquely
- susceptible to thermal runaway when used incorrectly. However,
- careful design avoids second breakdown and thermal runaway.
-
- MOSFET: (VMOS, TMOS, DMOS, NMOS, PMOS, IGFET, etc.)
- Metal-Oxide Semiconductor Field Effect Transistors use an
- insulated gate to modulate the flow of majority carrier current
- from drain to source with the electric field created by a gate.
- Like bipolar transistors, MOSFETs are available in both P and N
- devices. Also like transistors, MOSFETs are available as pairs
- and integrated circuits. MOSFET matched pairs do not match as
- well as bipolar transistor pairs, but match better than tubes.
-
- | MOSFETs are also available in many types. However, all have
- | virtually zero input current. MOSFETs have lower gain than
- | bipolar transistors, clip moderately, and are fast to recover
- | from clipping. Although power MOSFETs have no DC gate current,
- | finite input capacitance means that power MOSFETs have finite
- | AC gate current. MOSFETs are stable and rugged. They are not as
- | susceptible to thermal runaway or second breakdown when
- | compared to bipolar transistors, although a badly designed
- | MOSFET circuit can still self-destruct. MOSFETs can't
- | withstand abuse as well as tubes.
- |
- JFET:
- Junction Field Effect Transistors operate exactly the same
- way that MOSFETs do, but have a non-insulated gate. JFETs
- share most of the characteristics of MOSFETs, including
- available pairs, P and N types, and integrated circuits.
-
- JFETs are not commonly available as power devices. They make
- excellent low-noise preamps. The gate junction gives JFETs
- higher input capacitance than MOSFETs and also prevents them
- from being used in enhancement mode. JFETs are only available
- as depletion devices. JFETs are also available as matched
- pairs and match almost as well as bipolar transistors.
-
- IGBT: (or IGT)
- Insulated-Gate Bipolar Transistors are a combination of a MOSFET
- and a bipolar transistor. The MOSFET part of the device serves
- as the input device and the bipolar as the output. IGBTs are
- | now available as P and N-type devices. IGBTs are slower than
- other devices but offer the low cost, high current capacity of
- bipolar transistors with the low input current and low input
- capacitance of MOSFETs. IGBTs suffer from saturation as much
- as, if not more than bipolar transistors, and also suffer from
- second breakdown. IGBTs are rarely used in high-end audio, but
- are sometimes used for extremely high power amps.
-
- Now to the real question. You might assume that if these
- various devices are so different from each other, one must be
- best. In practice, each has strengths and weaknesses. Also,
- because each type of device is available in so many different
- forms, most types can be successfully used in most places.
-
- Tubes are prohibitively expensive for very high power amps.
- Most tube amps deliver less than 50 watts per channel.
-
- JFETs are sometimes an ideal input device because they have
- low noise, low input capacitance, and good matching. However,
- bipolar transistors have even better matching and higher gain,
- so for low-impedance sources, bipolar devices are even better.
- Yet tubes and MOSFETs have even lower input capacitance, so
- for very high source resistance, they can be better.
-
- Bipolar transistors have the lowest output resistance, so
- they make great output devices. However, second breakdown
- and high stored charge weigh against them when compared to
- MOSFETs. A good BJT design needs to take the weaknesses of
- BJTs into account while a good MOSFET design needs to
- address the weaknesses of MOSFETs.
-
- Bipolar output transistors require protection from second
- breakdown and thermal runaway and this protection requires
- additional circuitry and design effort. In some amps, the
- sound quality is hurt by the protection.
-
- All said, there is much more difference between individual
- designs, whether tube or transistor, than there is between tube
- and transistor designs generically. You can make a fine amp
- from either, and you can also make a lousy amp from either.
-
- Although tubes and transistors clip differently, clipping
- will be rare to nonexistant with a good amp, so this
- difference should be moot.
-
- Some people claim that tubes require less or no feedback
- while transistor amps require significant feedback. In
- practice, all amps require some feedback, be it overall,
- local, or just "degeneration". Feedback is essential in
- amps because it makes the amp stable with temperature
- variations and manufacturable despite component variations.
-
- Feedback has a bad reputation because a badly designed
- feedback system can dramatically overshoot or oscillate.
- Some older designs used excessive feedback to compensate
- for the nonlinearities of lousy circuits. Well designed
- feedback amps are stable and have minimal overshoot.
-
- When transistor amps were first produced, they were inferior to
- the better tube amps of the day. Designers made lots of mistakes
- with the new technologies as they learned. Today, designers
- are far more sophisticated and experienced than those of 1960.
-
- Because of low internal capacitances, tube amps have very
- linear input characteristics. This makes tube amps easy to
- drive and tolerant of higher output-impedance sources, such
- as other tube circuits and high-impedance volume controls.
- Transistor amps may have higher coupling from input to output
- and may have lower input impedance. However, some circuit
- techniques reduce these effects. Also, some transistor
- amps avoid these problems completely by using good JFET
- input circuits.
-
- There is lots of hype out on the subject as well as folklore
- and misconceptions. In fact, a good FET designer can make a
- great FET amp. A good tube designer can make a great tube amp,
- and a good transistor designer can make a great transistor amp.
- Many designers mix components to use them as they are best.
-
- As with any other engineering discipline, good amp design
- requires a deep understanding of the characteristics of
- components, the pitfalls of amp design, the characteristics
- of the signal source, the characteristics of the loads, and
- the characteristics of the signal itself.
-
- As a side issue, we lack a perfect set of measurements to
- grade the quality of an amp. Frequency response, distortion,
- and signal-to-noise ratio give hints, but by themselves are
- insufficient to rate sound.
-
- Many swear that tubes sound more "tube like" and transistors
- sound more "transistor like". Some people add a tube circuit
- to their transistor circuits to give some "tube" sound.
-
- Some claim that they have measured a distinct difference between
- the distortion characteristics of tube amps and transistor amps.
- This may be caused by the output transformer, the transfer
- function of the tubes, or the choice of amp topology. Tube amps
- rarely have frequency response as flat as the flattest
- transistor amps, due to the output transformer. However, the
- frequency response of good tube amps is amazingly good.
-
- For more information on tubes, get one of the following old
- reference books, or check out audioXpress Magazine (see the
- magazine section of the FAQ for more info on audioXpress).
-
- The Receiving Tube Manual (annual up to 1970)
- The Radiotron Designers Handbook
- Fundamentals of Vacuum Tubes" by Eastman 1937, McGraw-Hill
-
- 11.14 What about swapping op-amps?
- | In the late 1980s, it was common for mid-range audio to use
- | discrete transistors and a few carefully placed op amps. In
- | the 2000s, integrated circuits are much more sophisticated
- | and highly integrated. The idea of swapping out an inferior
- | op-amp for a better part as an easy way of improving sound is
- | far less meaningful today than it was in the 1980s.
- |
- | There are many good op amps available today. Some are
- | engineered for use in audio. If you want to build something
- | for yourself, such as a filter or buffer, select a quality
- | op-amp that is meant for audio use. Also, pay careful attention
- | to the power supplies and grounding. Remember that all op-amp
- | circuits process signals with respect to ground, whether they
- | have a ground terminal or not.
- |
- | But if you have a modern piece of equipment, don't waste your
- | time trying to replace the op amps in it with better parts.
- | You may make things worse, rather than better.
- |
- | As an alternative, you could consider replacing ceramic or
- | electrolytic capacitors in the audio paths with quality film
- | capacitors. This is a safer idea and more likely to improve
- | the sound. For supply bypassing, ceramic capacitors are OK,
- | but they are bad if used in between stages or as part of a
- | filter or equalization network. Electrolytic capacitors
- | are also poor if used in the signal path. You can improve
- | the sound by adding a large value film capacitor in parallel
- | with the existing electrolytic capacitor.
-
- 11.15 Where can I buy electronic parts to make an amplifier?
- There are many commercial parts distributors that sell only to
- Corporations. Their prices are often list, their supply is
- often good, and their service varies. Common ones are Arrow
- Electronics, Gerber Electronics, Hamilton Avnet, and Schweber
- Electronics. See your local phone book.
-
- There are also distributors that cater to smaller buyers. These
- typically have only one office. Some have lousy selections but
- great prices. In the following list, (+) means that the dealer
- has a good reputation, (?) means that the dealer has
- insufficient reputation, and (X) means that some have reported
- problems with this dealer. (C) means they have a catalog.
-
- All Electronics Corporation (Surplus, Tools, Parts) (?) (C)
- PO Box 567
- Van Nuys CA 90408 USA
- 800-826-5432
- 818-904-0524
- Allied Electronics (Full Line of Parts) (+) (C)
- 800-433-5700
- Antique Electronics Supply (Tubes, capacitors, etc) (?)
- 688 First St
- Tempe AZ 85281 USA
- 602-894-9503
- Billington Export Ltd. (Valves and CRTs)
- I E Gillmans Trading Estate
- Billinghurst, RH14 9E3 United Kingdom
- Tel (0403) 784961
- Chelmer Valves (Valves)
- 130 New London Rd
- Chelmsford, CM2 0RG United Kingdom
- DigiKey Corporation (Full Line of Parts) (+) (C)
- 701 Brooks Avenue South
- PO Box 677
- Thief River Falls MN 56701-0677 USA
- 800-344-4539
- Electromail (Wide range of parts, similar to Radio Shack)
- PO Box 33, Corby, Northants NN17 9EL United Kingdom
- Tel 0536 204555
- Langrex Supplies Ltd. (Obsolete Valves)
- 1 Mayo Rd.
- Croyden, Surrey, CR0 2QP United Kingdom
- Maplin (General parts supplier)
- PO Box 3
- Rayleigh, Essex, SS6 2BR United Kingdom
- Tel 01702 556751.
- Marchand Electronics (?) (Crossover kits)
- 1334 Robin Hood Lane
- Webster NY 14580 USA
- 716-872-5578
- MCM Electronics (Speakers, A/V Repair Parts, Etc) (+) (C)
- 650 Congress Park Dr
- Centerville Ohio 45459-4072 USA
- 513-434-0031 or 800-543-4330
- MesaBoogie (Tubes, instrument speakers) (?)
- 707-778-8823
- Michael Percy (Connectors, MIT, Wonder Caps, Buf-03) (+)
- PO Box 526
- Inverness CA 94936 USA
- 415-669-7181 Voice
- 415-669-7558 FAX
- Mouser Electronics (Full Line of Parts) (+) (C)
- PO Box 699
- Mansfield TX 76063-0699 USA
- 800-346-6873
- 817-483-4422
- Newark Electronics (Full Line of Parts) (+) (C)
- Old Colony Sound (Audio parts and audio kits) (+) (C)
- PO Box 243
- Peterborough NH 03458-0243 USA
- 603-924-9464
- Parts Express (Speakers, Cables, Connectors) (+) (C)
- 340 East First Street
- Dayton OH 45402-1257 USA
- 937-222-0173
- PM Components (High end audio parts and valves)
- Springhead road
- Gravesend
- Kent, DA11 3HD United Kingdom
- Tel (0474) 560521
- PV Tubes (Valves and Transformers)
- 104 Abbey St.
- Accrington, Lancs, BB5 1EE United Kingdom
- Tel (0254) 236521
- Radio Shack (Parts, Low-End Audio) (+) (C)
- RATA Ltd (Audio parts and cables: Kimber, Ansar, Vishay)
- Edge Bank House
- Skelsmergh
- Kendal, Cumbria, LA8 9AS United Kingdom
- Tel (0539) 823247
- SJS Acoustics (High-end parts, valves, transformers)
- Ben-Dor
- Lumb Carr Rd.
- Holcombe, Bury, BL8 4NN United Kingdom
- Sowter Transformers (Mains and output transformers)
- EA Sowter Ltd. PO box 36
- Ipswich, IP1 2EL United Kingdom
- Tel (0473) 219390
- Tanner Electronics (Surplus Parts) (+)
- 214-242-8702
- Toroid Corp of Maryland (Toroidal power transformers) (+)
- (also sells without secondary, ready to finish)
- Toroid Corporation of Maryland
- 2020 Northwood Drive
- Salisbury, MD 21801 USA
- 410-860-0300
- Fax 410-860-0302
- USA Toll Free 888-286-7643
- sales@toroid.com
- http://www.toroid.com
- Triode Electronics (Tubes, transformers, boxes) (?)
- 2010 Roscoe St
- Chicago IL 60618 USA
- 312-871-7459
- Welborne Labs (Connectors, Linear Tech ICs, Wima Caps) (?)
- P.O. Box 260198
- 971 E. Garden Drive
- Littleton, CO 80126 USA
- 303-470-6585 Voice
- 303-791-5783 FAX
- Wilson Valves (Valves)
- 28 Banks Ave.
- Golcar, Huddersfield, HD7 4LZ United Kingdom
-
- 11.16 Where can I buy audio amplifier kits?
- Alas, Heath is no longer making Heathkits. Alternatives:
- AP Electronics (High grade components and kits)
- 20 Derwent centre
- Clarke St.
- Derby DE1 2BU United Kingdom
- | Audio Kits, div. Classified Audio Video Inc. (kits from
- | Erno Borbely designs)
- | support@audiokits.com
- | http://www.audiokits.com
- Audio Note (Audio parts, kits, and high quality amps)
- Unit 1
- Block C, Hove Business Centre
- Fonthil Rd.
- Hove, East Sussex, BN3 6HA United Kingdom
- Tel (0273) 220511
- Audio Synthesis (Many kits from Ben Duncan designs) (?)
- 99 Lapwind Lane
- Manchester M20 0UT, UK
- 061-434-0126 Voice
- 060-225-8431 FAX
- BORBELY AUDIO, Erno Borbely (JFET & tube preamp kits, MOSFET &
- tube power amplifier kits. Also audiophile components)
- Angerstr. 9
- 86836 Obermeitingen, Germany
- Tel: +49/8232/903616
- Fax: +49/8232/903618
- E-mail: BorbelyAudio@t-online.de or EBorbely@aol.com
- http://www.borbelyaudio.com
- Crimson (UK) (?)
- Hafler (+) (may be out of the kit business)
- Hart Electronic Kits (Audiophile kits and components)
- Penylan Mill
- Oswestry
- Shropshire, SY10 9AF United Kingdom
- Tel (0691)652894
- Old Colony Sound (+) (See 11.15)
- PAiA Electronics (?) (Musician-related kits)
- 3200 Teakwood Lane
- Edmond OK 73013 USA
- 405-340-6378
- Sound Values (+) (See 11.7)
- 185 N Yale Avenue
- Columbus OH 43222-1146 USA
- 614-279-2383
-
- 11.17 Where can I read more about building amplifiers, preamps, etc.?
- Audio Amateur Magazine
- Audio Amateur Publications
- PO Box 494
- Peterborough NH 03458 USA
- 603-924-9464
- Analog Devices Audio/Video Reference Manual
- Electronic Music Circuits, by Barry Klein
- | Available only from author direct at
- | barry.l.klein@wdc.com or barryklein@coxnet.net
- Howard D Sams & Co ISBN 0-672-21833-X
- Electronics Australia (Magazine with audio projects)
- AUD47 per year 12 issues, often discounted
- PO Box 199
- Alexandria, Austrailia
- +612 353 9944 or +612 353 6666
- Elektor Electronics (How it works and you-build articles)
- (no longer published in US. Still available in Europe)
- PO Box 1414
- Dorchester DT2 8YH, UK
- Enhanced Sound: 22 Electronic Projects for the Audiophile
- (Some basic projects and some "how it works")
- by Richard Kaufman
- Tab Books #3071/McGraw Hill
- ISBN 0-8306-9317-3
- audioXpress Magazine
- Audio Amateur Publications
- PO Box 494
- Peterborough NH 03458 USA
- 603-924-9464
- IC Op-Amp Cookbook, Third Edition by Walter G. Jung
- ISBN 0672-23453-4, Howard W. Sams, Inc.
- Journal of the Audio Engineering Society (Theory & Experiment)
- Audio Engineering Society
- 60 East 42nd Street
- New York City NY 10165-0075 USA
- 212-661-2355
- Popular Electronics
- Radio-Electronics
- Radiotron Designer's Handbook, Fourth Edition (old, tube info)
- The Technique of Electronic Music, by Thomas H Wells
- Schirmer Books ISBN 0-02-872830-0
- Vacuum Tube Amplifiers, MIT Radiation Lab series
- Wireless World
- Some of the above titles, as well as a catalog of technical
- books, are available from:
- OpAmp Technical Books, Inc.
- 1033 N Sycamore Avenue
- Los Angeles CA 90038 USA
- 800-468-4322 or 213-464-4322
-
- 11.18 What is Amplifier Class A? What is Class B? What is Class AB?
- What is Class C? What is Class D?
-
- All of these terms refer to the operating characteristics
- of the output stages of amplifiers.
-
- Briefly, Class A amps sound the best, cost the most, and are the
- least practical. They waste power and return very clean signals.
- Class AB amps dominate the market and rival the best Class A
- amps in sound quality. They use less power than Class A, and
- can be cheaper, smaller, cooler, and lighter. Class D amps are
- even smaller than Class AB amps and more efficient, because
- | they use high-speed switching rather than linear control.
- | Starting in the late 1990s, Class D amps have become quite
- | good, and in some cases rivaling high quality amps in sound
- | quality. Class B & Class C amps aren't used in audio.
-
- In the following discussion, we will assume transistor output
- stages, with one transistor per function. In some amplifiers,
- the output devices are tubes. Most amps use more than one
- transistor or tube per function in the output stage to increase
- the power.
-
- Class A refers to an output stage with bias current greater
- than the maximum output current, so that all output transistors
- are always conducting current. The biggest advantage of Class A
- is that it is most linear, ie: has the lowest distortion.
-
- The biggest disadvantage of Class A is that it is inefficient,
- ie: it takes a very large Class A amplifier to deliver 50
- watts, and that amplifier uses lots of electricity and gets
- very hot.
-
- Some high-end amplifiers are Class A, but true Class A only
- accounts for perhaps 10% of the small high-end market and none
- of the middle or lower-end market.
-
- Class B amps have output stages which have zero idle bias
- current. Typically, a Class B audio amplifier has zero bias
- current in a very small part of the power cycle, to avoid
- nonlinearities. Class B amplifiers have a significant advantage
- over Class A in efficiency because they use almost no
- electricity with small signals.
-
- Class B amplifiers have a major disadvantage: very audible
- distortion with small signals. This distortion can be so bad
- that it is objectionable even with large signals. This
- distortion is called crossover distortion, because it occurs at
- the point when the output stage crosses between sourcing and
- sinking current. There are almost no Class B amplifiers on the
- market today.
-
- Class C amplifiers are similar to Class B in that the output
- stage has zero idle bias current. However, Class C amplifiers
- have a region of zero idle current which is more than 50% of
- the total supply voltage. The disadvantages of Class B
- amplifiers are even more evident in Class C amplifiers, so
- Class C is likewise not practical for audio amps.
-
- Class A amplifiers often consist of a driven transistor
- connected from output to positive power supply and a constant
- current transistor connected from output to negative power
- supply. The signal to the driven transistor modulates the
- output voltage and the output current. With no input signal,
- the constant bias current flows directly from the positive
- supply to the negative supply, resulting in no output current,
- yet lots of power consumed. More sophisticated Class A amps
- have both transistors driven (in a push-pull fashion).
-
- Class B amplifiers consist of a driven transistor connected
- from output to positive power supply and another driven
- transistor connected from output to negative power supply. The
- signal drives one transistor on while the other is off, so in a
- Class B amp, no power is wasted going from the positive supply
- straight to the negative supply.
-
- Class AB amplifiers are almost the same as Class B amplifiers
- in that they have two driven transistors. However, Class AB
- amplifiers differ from Class B amplifiers in that they have a
- small idle current flowing from positive supply to negative
- supply even when there is no input signal. This idle current
- slightly increases power consumption, but does not increase it
- anywhere near as much as Class A. This idle current also
- corrects almost all of the nonlinearity associated with
- crossover distortion. These amplifiers are called Class AB
- rather than Class A because with large signals, they behave
- like Class B amplifiers, but with small signals, they behave
- like Class A amplifiers. Most amplifiers on the market are
- Class AB.
-
- Some good amplifiers today use variations on the above themes.
- For example, some "Class A" amplifiers have both transistors
- driven, yet also have both transistors always on. A specific
- example of this kind of amplifier is the "Stasis" (TM)
- amplifier topology promoted by Threshold, and used in a few
- different high-end amplifiers. Stasis (TM) amplifiers are
- indeed Class A, but are not the same as a classic Class A
- amplifier.
-
- Class D amplifiers use switching techniques to achieve even
- higher efficiency than Class B amplifiers. As Class B
- amplifiers used linear regulating transistors to modulate
- output current and voltage, they could never be more efficient
- than 71%. Class D amplifiers use transistors that are either on
- or off, and almost never in-between, so they waste the least
- amount of power.
-
- Obviously, then, Class D amplifiers are more efficient than
- Class A, Class AB, or Class B. Some Class D amplifiers have
- >80% efficiency at full power. Class D amplifiers can also have
- low distortion, although theoretically not as good as Class AB
- or Class A.
-
- To make a very good full-range Class D amplifier, the switching
- frequency must be well above 40kHz. Also, the amplifier must be
- followed by a very good low-pass filter that will remove all of
- the switching noise without causing power loss, phase-shift, or
- distortion. Unfortunately, high switching frequency also means
- significant switching power dissipation. It also means that the
- chances of radiated noise (which might get into a tuner or
- | phono cartridge) is much higher. If the switching frequency is
- | high enough, then less filtering is required. As technology
- | improves, industry is be able to make higher switching
- | frequency amplifiers which require less low-pass filtering.
- | Eventually, Class D amplifier quality could catch up with Class
- | A amplifiers. Some believe that it already has.
-
- Some people refer to Class E, G, and H. These are not as well
- standardized as class A and B. However, Class E refers to an
- amplifier with pulsed inputs and a tuned circuit output. This
- is commonly used in radio transmitters where the output is at
- a single or narrow band of frequencies. Class E is not used
- for audio.
-
- Class G refers to "rail switched" amplifiers which have two
- different power supply voltages. The supply to the amplifier
- is connected to the lower voltage for soft signals and the
- higher voltage for loud signals. This gives more efficiency
- without requiring switching output stages, so can sound better
- than Class D amplifiers.
-
- Class H refers to using a Class D or switching power supply
- to drive the rails of a class AB or class A amplifier, so that
- the amplifier has excellent efficiency yet has the sound of a
- good class AB amplifier. Class H is very common in professional
- audio power amplifiers.
-
- 11.19 Why do I hear noise when I turn the volume control? Is it bad?
- Almost all volume controls are variable resistors. This goes
- for rotary controls and slide controls. Variable resistors
- consist of a resistive material like carbon in a strip and a
- conductive metal spring wiper which moves across the strip as
- the control is adjusted. The position of the wiper determines
- the amount of signal coming out of the volume control.
-
- Volume controls are quiet from the factory, but will get noisier
- as they get older. This is in part due to wear and in part due
- to dirt or fragments of resistive material on the resistive
- strip. Volume control noise comes as a scratch when the control
- is turned. This scratch is rarely serious, and most often just
- an annoyance. However, as the problem gets worse, the sound of
- your system will degrade. Also, as the problem gets worse, the
- scratching noise will get louder. The scratching noise has a
- large high-frequency component, so in the extreme, this noise
- could potentially damage tweeters, although I have never seen
- a documented case of tweeter damage due to control noise.
-
- Some controls are sealed at the factory, so there is no
- practical way to get inside and clean out the dirt. Others have
- access through slots or holes in the case. These open controls
- are more subject to dirt, but also are cleanable. You can clean
- an open volume control with a VERY QUICK squirt of lubricating
- contact cleaner, such as Radio Shack 64-2315. Even better is a
- non-lubricating cleaner, such as Radio Shack 64-2322. With any
- cleaner, less is better. Too much will wash the lubricant out
- of the bearings and gunk up the resistive element.
-
- You can also clean some controls by twisting them back and forth
- vigorously ten times. This technique pushes the dirt out of the
- way, but is often just a short term fix. This technique is also
- likely to cause more wear if it is done too often. Try to do it
- with the power applied, but the speaker disconnected, so that
- there is some signal on the control.
-
- Sealed and worn controls should be replaced rather than cleaned.
- Critical listeners claim that some controls, such as those made
- by "Alps" and by "Penny and Giles" sound better than common
- controls. Regardless of the brand, however, it is essential
- that whatever control you buy have the same charcteristics as
- the one you are replacing. For most volume controls, this
- means that they must have AUDIO TAPER, meaning that they are
- designed as an audio volume control, and will change the level
- by a constant number of dB for each degree of rotation.
-
- Badly designed circuits will wear out volume controls very
- quickly. Specifically, no volume control is able to work for
- a long time if there is significant DC current (or bias current)
- in the wiper. If the output of the control goes to the input of
- an amplifier, the amplifier should be AC coupled through a
- capacitor. If there is a capacitor there, it might be leaky,
- causing undesirable DC current through the volume control.
-
- If you have a circuit with no blocking capacitor or a bad
- blocking capacitor, you can add/replace the capacitor when
- you replace the control. However, get some expert advise
- before modifying. If you add a capacitor to a device which
- doesn't have one, you will have to make other modifications
- to insure that the amplifier has a source for its bias current.
-
- 11.20 What is amplifier "bridging" or "monoblocking"? How do I do it?
- When you're told a stereo power amplifier can be bridged,
- that means that it has a provision (by some internal
- or external switch or jumper) to use its two channels
- together to make one mono amplifier with 3 to 4 times the
- power of each channel. This is also called "Monoblocking"
- and "Mono Bridging".
-
- Tube amps with multiple-tap output transformers are simple to
- bridge. Just connect the secondaries in series and you get
- more power. The ability to select transformer taps means that
- you can always show the amplifier the impedance it expects, so
- tube amp bridging has no unusual stability concerns.
-
- The following discussion covers output transformer-less amps.
- Bridging these amps is not so simple. It involves connecting
- one side of the speaker to the output of one channel and the
- other side of the speaker to the output of the other channel.
- The channels are then configured to deliver the same output
- signal, but with one output the inverse of the other. The
- beauty of bridging is that it can apply twice the voltage to
- the speaker. Since power is equal to voltage squared divided
- by speaker impedance, combining two amplifiers into one can
- give four (not two) times the power.
-
- In practice, you don't always get 4 times as much power. This
- is because driving bridging makes one 8 ohm speaker appear like
- two 4 ohm speakers, one per channel. In other words, when you
- bridge, you get twice the voltage on the speaker, so the
- speakers draw twice the current from the amp.
-
- The quick and dirty way to know how much power a stereo amp can
- deliver bridged to mono, is to take the amp's 4 ohm (not 8 ohm)
- power rating per channel and double it. That number is the
- amount of watts into 8 ohms (not 4 ohms) you can expect in mono.
- If the manufacturer doesn't rate their stereo amp into 4 ohms,
- it may not be safe to bridge that amp and play at loud levels,
- because bridging might ask the amp to exceed its safe maximum
- output current.
-
- Another interesting consequence of bridging is that the amplifier
- damping factor is cut in half when you bridge. Generally, if you
- use an 8 ohm speaker, and the amplifier is a good amp for driving
- 4 ohm speakers, it will behave well bridging.
-
- Also consider amplifier output protection. Amps with simple
- power supply rail fusing are best for bridging. Amps that rely
- on output current limiting circuits to limit output current
- are likely to activate prematurely in bridge mode, and virtually
- every current limit circuit adds significant distortion when it
- kicks in. Remember bridging makes an 8 ohm load look like 4 ohms,
- a 4 ohm load look like 2 ohms, etc. Also, real speakers do not
- look like ideal resistors to amps. They have peaks and dips in
- impedance with frequency, and the dips can drop below 1/2 the
- nominal impedance. They also have wildly varying phase with
- frequency.
-
- Finally, some amplifiers give better sound when bridged than
- others. Better bridging amps have two identical differential
- channels with matched gain and phase through each input, left
- and right, inverting and non-inverting. Simpler bridging
- amplifiers have one or two inverting channels, and run the
- output of one into the input of the second. This causes the
- two outputs to be slightly out of phase, which adds distortion.
- There are also other topologies. One uses an additional stage to
- invert the signal for one channel but drives the other channel
- directly. Another topology uses one extra stage to buffer the
- signal and a second extra stage to invert the signal. These are
- better than the simple master/slave arrangement, and if well
- done, can be as good as the full differential power amp.
-
- COPYRIGHT NOTICE
- The information contained here is collectively copyrighted by the
- authors. The right to reproduce this is hereby given, provided it is
- copied intact, with the text of sections 1 through 8, inclusive.
- However, the authors explicitly prohibit selling this document, any
- of its parts, or any document which contains parts of this document.
-
- --
- Bob Neidorff; Texas Instruments | Internet: neidorff@ti.com
- 50 Phillippe Cote St. | Voice : (US) 603-222-8541
- Manchester, NH 03101 USA
-
- Note: Texas Instruments has openings for Analog and Mixed
- Signal Design Engineers in Manchester, New Hampshire. If
- interested, please send resume in confidence to address above.
-