Help needed - how to test an amp ?

[karul]

I've already build a LM1875 guitar amp. Now I want to build another one with TDA7293 and I have some questions regard testing. What kind of tests should I do, to be sure that everything is ok, before I safely connect a speaker to the amp?

Quote from Rod Elliot Project 127, TDA7293 power amp :
link -> Project 127

"Connect to a suitable power supply - remember that the supply earth (ground) must be connected! When powering up for the first time, use 100 ohm 5W "safety" resistors in series with each supply to limit the current if you have made a mistake in the wiring. If available, use a variable bench supply - you don't need much current to test operation, and around 500mA is more than enough. If using a current limited bench supply, the safety resistors can be omitted. Do not connect a speaker to the amplifier at this stage!

If using a normal power supply for the amp tests, apply power (±35V via the safety resistors) and verify that the current is no more than 60mA or so - about 6V across each 100 ohm resistor. No load current can vary, so don't panic if you measure a little more or less. Verify that the DC voltage at both outputs is less than 100mV. Using another 100 ohm resistor in series with a small speaker, or an oscilloscope, apply a sinewave signal at about 400Hz to the input and watch (or listen) for signal. The signal level needs to be adjusted to ensure the amp isn't clipping, and the waveform should be clean, with no evidence of parasitic oscillation or audible distortion.

If everything tests out as described, wire the amplifier directly to the power supply and finish off any internal wiring in the amp. Once complete, it's ready to use."

I don't have an oscilloscope to perform the test.

So, what would be the proper size (how many watts) of the "small speaker" ?
100K resistor should be 1/4W, or more is needed?

Is there another "harmless" way to test the amp, before connecting the bigger (real one) speaker (and avoid unwanted damage to the expensive speaker)?

[GloBug]

Make and learn to use a bulb tester, it's easy.

Once you verified the amp will power up OK, measure the DC offset on the speaker terminals before and after connecting and junk speaker. It should be in the low millivolt range.

[karul]

thanks GloBug,

I've read about the bulb tester, and it's on my TO-DO list. As I've understand it, the bulb test will show me that there is an error with powering - but it will not show weather the chip is oscillating or not. Or am I wrong?

I didn't quite understand the "DC offset" part.

If I apply a sinewave signal to the input, the voltage on speaker terminals (no speaker connected) should be in a millivolt range?

[littlerick]

I used a bulb tester to check mine was working... Then I connected a pair of old speakers to test for sound. That way if anything went wrong I wouldnt blow my decent speakers.

I have no testing equipment either, but this worked out ok for me.

[GloBug]

The bulb tester is not for detecting oscillations. Someone else will have to help you there.

The DC offset basically detects if your amplifier is passing to much DC current to your speakers. Too much DC would/could ruin your speakers.

Someone else will have to help you with that last part.

[pacificblue]

You can deduce from certain symptoms that an amplifier is oscillating, but the absence of those symptoms does not means the amp is not oscillating, and those same symptoms can also occur without oscillations. You will need an oscilloscope to reliable detect oscillations.

Concerning DC offset, an ideal amplifier would have an output voltage of 0 V with no signal at the input. Real amplifiers will not exactly reach 0 V, but the output will be offset by a small voltage. The smaller the offset, the better.

Too much could indeed damage your speakers, although it would have to be in the V range to do that. Even a few mV will however have an influence. The output swing is reduced by the offset in one direction. The speaker, at least the woofer is kept off its center position by the offset, which can lead to higher distortion. By being offset, it is not aligned with the other drivers in a multi-way system, so the sonic balance can suffer. The constant power through the coil will heat it up, which increases its DC resistance, and reduces the current through it for a given amplifier voltage, reducing the sound pressure in relation to the other drivers, again affecting the sonic balance.

If you want to know the DC offset, measure the voltage at the speaker terminals, when no signal is present. An AC-coupled chip amp will have something between 1 and 10 mV. A DC-coupled chip amp will have more.

[karul]

thanks to all contributers