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..................... + or - 10% or 2dB whichever is the greater. 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 ~Ever Ready~ RED body 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. ---------------------------------------------------------------------------- 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..... ----------------------------------------------------------------------------