home *** CD-ROM | disk | FTP | other *** search
- Path: sparky!uunet!usc!zaphod.mps.ohio-state.edu!sdd.hp.com!ncr-sd!crash!cmkrnl!jeh
- From: jeh@cmkrnl.com
- Newsgroups: rec.audio.car
- Subject: Re: ohms and watts and such
- Message-ID: <1992Nov17.213127.884@cmkrnl.com>
- Date: 17 Nov 92 21:31:27 PST
- References: <1e9njgINN1fh@bigboote.WPI.EDU>
- Organization: Kernel Mode Consulting, San Diego, CA
- Lines: 64
-
- In article <1e9njgINN1fh@bigboote.WPI.EDU>, walkerr@bigwpi.WPI.EDU
- (Robert A Walker) writes:
- > OK if i have an amp that puts out 60 watts a channel
- > and i run one channel to a dual voice coil sub with 8 ohms resiatance per
- > coil, and i run these in parelell. Is the total output 60 watts, oh yeah
- > if the 60 watts is at 8 ohms. What does my total resistance become.
-
- Paralleling two 8-ohm voice coils yields a net load of 4 ohms.
-
- Whether or not the power stays at 60 watts depends on the design of the amp.
- Lowering the resistance (impedance, actually) allows more current to flow for a
- given voltage. Since power (watts) = voltage (volts) * current (amps), and
- current = voltage / resistance (impedance in this case), lowering the impedance
- => more amps => more watts -- IF the amp has additional current capability.
-
- But if, when the amp is running 60 W into an 8-ohm load, the amp is already
- supplying as much current as it can -- ie if its power supply is "maxed out" at
- that point -- lowering the impedance (by paralleling two voice coils, or simply
- using a single speaker with 4-ohm impedance) won't pull any more power out of
- the amp.
-
- For a moment, think of the amp as a battery. (this analogy has serious flaws,
- but it's good enough for now.) Connect a light bulb to the battery and the
- battery puts out a certain amount of current. Connect another light bulb in
- parallel and (assuming that the battery has the current available) the same
- amount of current will flow through the second light bulb as well, so you get
- twice the power usage (watts) and twice the light. You can keep adding light
- bulbs until you're drawing as much current as the battery can supply. If you
- keep adding light bulbs after that, each add'l bulb will make the other ones
- get dimmer.
-
- > I've noticed that onthe specs of some amps that they put out 100 watts at 8 ohms
- > 200 at 4 400 at 2 and so on, do all amps follow this pattern?
-
- Again -- it depends on the design of the amp.
-
- > If my amp
- > can handle only a four ohm load, how should i connect a dual voice coil
- > sub with 8 ohms per coil?
-
- When an amp is rated to handle a four ohm load, it will handle larger (higher
- impedance) loads just fine.
-
- > I presently have them in parellel and they sound good,
-
- given that the amp is rated to drive four ohm loads, this is fine.
-
- > putting the two coils on each channel in series would give 16
- > ohms and would sound lousy, or at least quieter.
-
- yep. Look at the math. What you've done is double the impedance. This cuts
- the current in half, which cuts the power (watts) in half. Of course that's
- only a difference of 3 dB, which is well past the threshold of "noticeability",
- but it's hardly a major change.
-
- > Am I hopelessly lost or am i on the right track?
-
- You're on the right track. You will get at least 60 W into the four-ohm load,
- maybe more. Probably not 120 W, though -- most car audio amps' power supplies
- aren't designed with much margin.
-
- --- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
- Internet: jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
- Uucp: ...{crash,eisner,uunet}!cmkrnl!jeh
-