How to set the DPA 220
- Set the R13 to the middle position and the R23 to minimum resistance.
- Connect a signal generator to the input and an oscilloscope to the output.
- Take out a fuss in one branche of power supply and connect a current meter instead.
- Slowly raise the supply voltage above zero and watch the current. Since about +/- 3 V the amp should start to work and there should be symmetrically limited signal at the output. If the current does not grow too high (30 mA), raise the voltage to the full +/- 40 V and raise the input signal to check the symmetry of limitation.
- If everything's all right, connect 4/8ohms/100W load and raise the signal level to 3 dB under limitation. When the power transistors grow hot (5 minutes?) set the signal generator to 20 kHz at the output voltage of the amp 1V. A distortion should be visible on the sinusoid - eliminate it by raising the BIAS current by R28. A correctly set amp should have a BIAS current about 50 mA in each branch of power supply.
- Then connect a DC voltage meter to the output and by R13 set as small DC output voltage as possible. 10 mV is acceptable, 1 mV is possible with a bit of patience.
- You can check frequency response if you want.
- In the end you can check the current securing circuit :
- Shorten the output with a 0.1 ohm resistor
- Set the input signal to 1 kHz/500mV.
- Slowly raise the supply voltage - there should appear a sharply limited signal on the output with small glitches on the front edges.
- If the circuit works as described, you can try complete short circuit at the nominal voltage - the current consumption should be about 3A in each branch of the supply.
My own advice: when you start and set the amp for the first time - or any other circuit - be absolutely careful. When changing configuration of signal generator, oscilloscope, meters and load, ALWAYS switch off the power supply. I even unplug it ever since i burned a TDA2040.
Another practice has proven useful over the years: for the first power-up, replace fuses in both power supply rails with lightbulbs - I'm using regular 100W / 240V bulbs. Naturally you mustn't apply load to the amp's output in this setup. If something goes wrong, the lightbulbs start to glow and basically limit the current flowing through the circuit. If everything's right, the lightbulbs remain cool and the amp behaves as expected. Then you can try to make them glow by turning the bias-current trimpot. If you succed, you know you have another clue everything is working fine. Then you turn the bias current back to zero, put the fuses back and you can proceed with the aforementioned procedure.
In general, it cost me about a handful of transistors to gain this knowledge.
Note that the frequency response is a full graph and measured 1 dB under limitation (=maximum sinus power) - when measuring voltage, value in decibels = 20 log (amplification) - 6 dB means amplification 2x.