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In Reply to: Re: Description of circuit topology (Adcom GFA555-II) posted by tlenthe on March 27, 2005 at 16:48:15:
I'm not sure I'm familiar with the term "CFB"...The CFP acronym is for "complementary feedback pair", called a "compound pair" in the link below. Looks like your topology is a straight EF though, so you can disregard what I said earlier :-).
http://sound.westhost.com/amp_design.htm#op_stage
You mentioned this:
My initial concern was based on the fact that there appears to be virtually no frequency roll-off in the global feedback, so I assumed that was the likely culprit.
The high frequency rolloff of the feedback is provided by the 15 pF capacitor you mention here:
Output taken from collector of input side of pair (inverted) to a single PNP transistor configured as voltage gain stage. This transistor has a 15pf cap from base to collector.
You referred to an earlier post where there was a failure, and the amp is in a state where it's been repaired from that, I think? This seems like a remote possibility, but if that 15 pF cap failed as an open, you'd lose the overall frequency compensation and might get a 20 MHz oscillation of the overall feedback loop. Seems unlikely though, but if there was a big-time failure previously, ...maybe? Here's another thought. How about supply bypassing on those driver/output boards? I'm assuming there's a bypass cap, and the ground side goes back to a ground connection that's wired to the input board along with the signal connection? If that bypass cap went bad, it could cause local oscillation of the output stage from the inductance of the collector supply circuitry. If these are electrolytics, they'd be more prone to failure than low value ceramics or similar. Also, the fact that you've got 10 Ohm resistors in series with the bases of the output stage transistors is a good thing, and should prevent parasitic oscillation of an output stage that's healthy otherwise.
Regarding voltage regulation, that's not often found in power amps. Sometimes you'll see a diode in series with the supply going to the lower current circuitry, with a large cap to ground from the cathode on the plus supply, anode on the negative. That causes the lower current circuitry to hold its voltage better if the unregulated collector supply drops due to heavy loading and large signals.
That's all I can think of at the moment.
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Follow Ups
- Re: Description of circuit topology (Adcom GFA555-II) - andy_c 17:59:27 03/27/05 (0)
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