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Re: Ampeg SVT 3 Pro output has DC

11/6/2005 1:04 PM
R.G.	Re: Ampeg SVT 3 Pro output has DC Now that I got that out of my system, it's nearly always a dead driver or a bias fault that re-kills solid state amps once they've been intially killed. In fact, I usually presume the driver transistors are dead until proven otherwise. It can also be a great help if you remember that the amp will actully work, albeit in a rough and ugly sounding way, with no bias at all on the output devices. So short the bias down to 0V and measure for deadness on the driver and pre-driver if any. If this is one of those amps with elaborate volt-amp area limiting circuits, you could have problems with that as well. It might be good to just assume you're going to have to test each component in the power section with a DMM. It's also worth remembering that a class AB audio amp has an output stage with a current gain of zillions and a voltage gain of just under unity. The amp will actually work into a 10K or so load resitor with no output stage at all, just the voltage gain stage.

11/6/2005 1:04 PM

R.G.

Re: Ampeg SVT 3 Pro output has DC

Now that I got that out of my system, it's nearly always a dead driver or a bias fault that re-kills solid state amps once they've been intially killed.

In fact, I usually presume the driver transistors are dead until proven otherwise. It can also be a great help if you remember that the amp will actully work, albeit in a rough and ugly sounding way, with no bias at all on the output devices. So short the bias down to 0V and measure for deadness on the driver and pre-driver if any. If this is one of those amps with elaborate volt-amp area limiting circuits, you could have problems with that as well. It might be good to just assume you're going to have to test each component in the power section with a DMM.

It's also worth remembering that a class AB audio amp has an output stage with a current gain of zillions and a voltage gain of just under unity. The amp will actually work into a 10K or so load resitor with no output stage at all, just the voltage gain stage.