Re: Description of circuit topology (Adcom GFA555-II) - tlenthe - Amp/Preamp Asylum

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Re: Description of circuit topology (Adcom GFA555-II)

In Reply to: My guess of what's going on posted by andy_c on March 27, 2005 at 12:13:28:

Hi Andy! Thanks for your input. I'm not sure I'm familiar with the term "CFB". Let me give you a thumbnail overview of the circuit:
Amp input (AC coupled and rolled off through RC low pass network) is to the base of one side of a differential pair with common emitter to a current source. (the other side is the global feedback point) with about 20X voltage gain.
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.
Output from the collector of this stage (actually at two different DC points to account for DC bias and AC signal offset voltages to the output stages), drives massive 3-stage darlington EF stage, one NPN for positive currents and on PNP for negative current. This connection is by wire from the input stabe board to two seperate high and low driver boards for each channel.
Each of the final output power transistors (4 in parallel) has 10 ohm base resistor and 0.22 ohm emitter resistor to balance the currents. The initial driver stage of these darlington stages has series 47ohms and 68 pf from base to collector/power rails. All three stages of each darlington are on one PCB.
One other potential oscailatoin source?... This amp does not have a regulated DC supply for ANY of the circuitry (another future tweak). Could this be related to the oscillation?
Does this summary correlate to your expectations? Are there any points of local oscillation which you would suspect based on this topology?
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.
I haven't traced the oscillations back through the circuit yet, having already mis-steped once and blown the drivers on one side by a slip of a test probe! The B+/- is about 80V, so I'm a little timid when it comes to poking around the circuitry...
Meantime, I have switched back to the lower cap speaker cables, but despite the osciallation, there were some noticable sonic improvements with the CAT5 high-cap cables, so I'd like to figure a way to make them work.
Thanks again! - Ted

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Follow Ups Full Thread

Topic - Adcom GFA555-II oscillation (long & technical) - advise needed - tlenthe 08:12:48 03/27/05 (10)

Solved!! (Adcom GFA555-II oscillation) - tlenthe 16:13:29 03/29/05 (1)

Hmmmm - chris_w 16:22:45 03/30/05 (0)

Fusible resistor blown? (long) - Charlie S in Maryland 16:00:00 03/29/05 (0)

More observations and another theory? - tlenthe 05:20:04 03/28/05 (0)

Why are you bothering? - Bare 18:16:29 03/27/05 (2)

:Why are you bothering... - Neil49 19:34:00 03/27/05 (1)

Re: :Why are you bothering...Theamp must be in the shop and have nothing better to do than putting down other inmate's - dimitri_P 23:53:06 03/27/05 (0)

My guess of what's going on - andy_c 12:13:28 03/27/05 (2)

Re: Description of circuit topology (Adcom GFA555-II) - tlenthe 16:48:15 03/27/05 (1)

Re: Description of circuit topology (Adcom GFA555-II) - andy_c 17:59:27 03/27/05 (0)

Follow Ups

Re: Description of circuit topology (Adcom GFA555-II) - tlenthe 16:48:15 03/27/05 (1)

Re: Description of circuit topology (Adcom GFA555-II) - andy_c 17:59:27 03/27/05 (0)

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