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-
- An Audio Wattmeter.
- -------------------
-
- Technical specifications:-
- --------------------------
-
- Range........................ 1 Watt to 100 Watts RMS.
- Load Impedance............... 7.5 Ohms resistive.
- Isolated..................... Yes, using a transformer, (1:1 ratio).
- Calibrated Test Frequency.... 400Hz Sinewave.
- Useful Frequency Range....... 40Hz to 1000Hz.
- Accuracy..................... Approximately + or - 2dB.
- Display Type................. Large Digital Readout.
- Duty Cycle................... Continuous up to 25 Watts RMS.
- 1 Hour between 25 Watts and 50 Watts RMS.
- 5 Minutes from 50 Watts to maximun reading.
- Settling Time Per Reading.... Approximately 5 Seconds.
-
- Note:-
- ------
-
- These specifications apply to an A1200 only, and may not be exactly the
- same for an A500, A500+, A600 etc, etc..
-
- ----------------------------------------------------------------------------
-
- Overview:-
- ----------
-
- Many modern Linear Audio Amplifiers have ~Bridge~ type outputs and this
- makes measurement of power output tricky. This Audio Wattmeter overcomes
- this problem using a Mains Transformer as an isolating transformer, using
- one of the 120 Volt windings as an input from the amplifier and using the
- other 120 Volt winding as the coupling for the detector circuit, the
- load for the amplifier being the two 15 Ohm, 50 Watt resistors wired in
- parallel. The accuracy of the prototype was far better than the above
- specifications suggest. This can be used to test all types of Linear
- Audio Amplifiers with various means of Output coupling, up to about
- 100 Watts (RMS). The unit was checked at 400Hz Sinewave using ~Project5~,
- the Stereo LF Audio Function Generator as the signal source, driving a
- Public Address amplifier capable of giving 120 Watts output. This was
- directly connected to the two wires of the Wattmeter input, presenting a
- 7.5 Ohm load to the amplifier.
-
- ----------------------------------------------------------------------------
-
- Construction:-
- --------------
-
- 1) Using the two resistors R1,R2 as templates, drill four holes through
- the HEATSINK as shown in the HardWare drawing. Countersink the two
- innermost holes on the underside of the HEATSINK. Deburr as neccessary.
- 2) Fit the resistors to the HEATSINK using the two innermost holes and
- two countersunk screws, nuts and washers.
- 3) Using the BLUE and YELLOW connecting wire, connect the two power
- resistors up so that they are wired as per the HardWare drawing. Leave
- a long BLUE wire and a long YELLOW wire free, (these will be the INPUT
- wires). Also leave a short BLUE wire and a short YELLOW wire free,
- (these will be connected to the transformer later).
- 4) Centralise the HEATSINK inside the BOX and drill two holes A and B
- through the BOX using the resistors as templates. Countersink these two
- holes on the underside of the BOX. Fit the HEATSINK inside the BOX
- using two countersunk screws, nuts and washers. See HardWare drawing.
- 5) Drill two holes for the transformer T1 using the transformer holes as
- a template. Countersink these two holes on the underside of the BOX.
- Fit the transformer T1 using two countersunk screws, nuts and washers.
- 6) Connect up the short BLUE and YELLOW wires to the transformer T1 as
- per the HardWare drawing, (120 Volt winding).
- 7) Using a TAGSTRIP of your choice as a template drill hole(s) through the
- BOX. Countersink the hole(s) on the underside of the BOX. If a TAGSTRIP
- is unavailable then use TERMINAL BLOCK instead.
- 8) Fit the components D1,D2,D3,D4,C1,C2,R3,RV1 to the TAGSTRIP as per the
- HardWare drawing. Note the diode and capacitor polarities.
- 9) Fit this assembled TAGSTRIP inside the BOX using countersunk screws,
- nuts and washers as required.
- 10) Using the BLACK and RED connecting wire, connect the other two tags of
- the transformer T1 to tags 2 and 3 of the TAGSTRIP. Tag 7 is NOT used.
- 11) Remove the outer insulation of the RG58/UR76 coaxial cable approximately
- 2" (50mm) long. Part the braiding from the inner conductor. Connect
- the braiding to tag 1 of the TAGSTRIP. Connect the inner conductor
- to tag 6 of the TAGSTRIP. Insulate any bare wires as required.
- 12) Drill one 1/4" (6.3mm) hole at either end of the BOX to allow the
- cables to go through. Feed the long BLUE and YELLOW wires through one
- end and the coaxial cable through the other.
- 13) Fit an RCA/PHONO plug to the opposite end of the coaxial cable, inner
- conductor to the pin, braid to the body of the plug.
- 14) You are now ready to calibrate the Audio Wattmeter.
-
- Note:-
- ------
-
- The two 20 Volt windings on the transformer T1 are NOT used.
-
- ----------------------------------------------------------------------------
-
- Calibrating the Audio Wattmeter:-
- ---------------------------------
-
- You WILL require a 9 Volt nominal battery type ~6F22~.
- I used an ~EVEREADY~ RED body ~ULTRA PLUS~ type, although any
- ZINC/CHLORIDE type will probably be alright.
-
- 1) Switch OFF the AMIGA.
- 2) Connect up the Analogue to Digital Converter (ADC) to the parallel port.
- 3) Connect the RCA/PHONO plug to the input of the ADC.
- 4) Connect your 9 Volt nominal battery type ~6F22~, +ve terminal to C1 +ve,
- (tag 4 on the TAGSTRIP) and -ve terminal to C1 -ve, (tag 1 on the
- TAGSTRIP). These are shown as TP1 (+ve) and TP2 (-ve) on the Circuit
- drawing. Do NOT reverse these connections.
- 5) Switch ON the AMIGA.
- 6) Go to the ~Project6~ drawer and start the ~Parallel_Test~ software.
- 7) Adjust RV1 to give 72 (flashing 73) in the window.
- 8) Switch OFF the AMIGA.
- 9) If this reading cannot be achieved then change the value of the SELECT
- ON TEST resistor R3. My value was 82 KilOhms.
- 10) Do (1) to (8) again until the correct results are obtained.
-
- ----------------------------------------------------------------------------
-
- Using the Audio Wattmeter:-
- ---------------------------
-
- 1) Read the ~Warning~ document.
- 2) Connect up the system under test as follows:-
- Disconnect ALL Loudspeakers on the Channel under test ONLY.
-
- Audio test lead from LH or RH Audio Channel of computer.
- +-------------------------------------------------------
- |
- | +-----------+ Yellow wire. +-----------+
- | | Amplifier +--------------+ Audio | RG58 +-----+ 25 Pin ~D~
- +-----+ under | O/P I/P | +----------+ ADC +------+
- I/P | test. +--------------+ WattMeter.| O/P I/P +-----+ Plug.| O/P
- +-----------+ Blue wire. +-----------+ |
- |
- Parallel Port of the computer.| I/P
- ------------------------------+
-
- (I/P=Input, O/P=Output, ADC=Analogue to Digital Converter.)
-
- 3) Switch ON all of the equipment
- 4) From ~Project5~ start up the ~Function_Gen~ software.
- 5) From ~Project7~ start up the ~Audio_Wattmeter~ software.
- 6) Set the ~Function_Gen~ software to give 400Hz at ?mV Sinewave on either
- channel. The value ~?~ will be on the specification sheet of the
- Amplifier under test for any given input.
- 7) Adjust the Amplifier controls and start testing.
- 8) When all tests are complete switch OFF all of the equipment and
- disconnect all of the hardware.
-
- ----------------------------------------------------------------------------
-
- Acknowledgements:-
- ------------------
-
- First of all many thanks to my wife Tricia who allows me to spend many hours
- on my computers.
-
- Also to David Benn and Herbert Breuer for the ACE/AIDE combination supplied
- on an AMIGA FORMAT floppy disk which made it easy to compile this program.
-
- Also to Laurence J Greatorex for giving these a shakedown on his A1200 with
- a Turbo LC '030 accelerator board and 32MB of fast ram. To all the other
- people that have given any of these projects a rigorous testing.
- And finally to anyone else who I may have neglected or forgotten.
-
- ----------------------------------------------------------------------------
-
- Mr Barry Walker,
- 70 King George Road,
- Loughborough,
- Leicestershire,
- LE11 2PA,
- England.
-
- Email to:-
-
- 106161.3245@compuserve.com
- or
- wisecracker@tesco.net
-
- BYE.....
-
- ----------------------------------------------------------------------------
-