Ampeg BA115

[lowell]

Repairing one of these and also seeing if I understand the PA theory as well.

Q1/4 look to me like they are in common base configuration. Is this right? And this provides more voltage gain than a common collector arrangement right? Also, current gain is less than unity from what I remember, but this doesn't really matter in this amp because there is no gate current in power MOSFETS right? So we need not drive them with a low Z source?

Common base arrangement is non-inverting so it looks to me like the global feedback in this amp is POSITIVE feedback. Is this correct? And if so how is this not an oscillator?

[soundguruman]

Post the schematic.

[lowell]

Didn't want to infringe, but it's already here:

http://music-electronics-forum.com/a..._schematic.pdf

[loudthud]

Assume a positive going input at TP12. That makes the output of U5B go positive (TP10). That increases the current through R35 and Q1. This makes more voltage across R31 and turns Q5 and Q8 on harder pulling the output negative. This negative output comes back through R19 to the input of U5B. Even though the plus input of the opamp is used, the overall amp is inverting.

[lowell]

LT not sure you're looking at the right schematic? U5 is the 1st opamp in the preamp. I'm speaking of the feedback in the PA, pg. 2. I also do not see ANY test points on this.

[loudthud]

OOps, I was using the BA115HP schematic from reply 8 in this thread: http://music-electronics-forum.com/t26255/

[soundguruman]

Didn't want to infringe, but it's already here:

http://music-electronics-forum.com/a..._schematic.pdf

Dude, jump in any time.

[lowell]

Edited the schem link It's good now.

[Jazz P Bass]

Looks to me as if the feedback path is through OC1 (optocoupler).
Thar feeds the inverting input of U3 A.

[soundguruman]

The only thing I remember so far is that the optocoupler goes bad.
after another beer, perhaps more.

[lowell]

Ahhh... didn't even SEE that. Ok so there's some feedback. Btw I fixed the amp already. Shorted MOSFETS and drivers. I'm just studying this PA for fun.

What about the PA output's connection back to the non-inverting input of U3B? Looks like positive feedback! ?

The optocoupler limits gain of U3A when the PA output goes above (or below) 70% of the high v rails (plus about 1.8v for the 3 series diodes). Seems like limiting to me... and I don't see the typical "limiting" bjt's in this amp so that makes sense. R6/(R4+R6) is a voltage divider of 70%. Same goes for R7 and R8.

[soundguruman]

I usually jumper the opto out of the circuit entirely.
Otherwise, it just fails again, in the middle of the show.

[lowell]

This thread is not about fixing this amp, or what tends to be wrong with these amps... It is about the theory of how this PA works. I'd like it if we can stick to the subject at hand. Please don't take this the wrong way.

[lowell]

RG, Enzo, Mark, JPB, Steve C, LT,

I've fixed a lot of these. Do you guys perform any mods to make these more stable? I was thinking some flyback diodes across the output MOSFETS, and possibly high frequency snubbing. Just a thought.

[loudthud]

MOSFETs have the diodes built in as part of their structure. No flyback diodes are needed. I've never worked on one so can't comment about any HF issues.