Solid state feedback circuit

[Carbsandcaps]

I know this has been beat to death but I’m curious about feedback loops in solid state amps. I know in tube amps in general the feedback connects to the preamp tube’s cathode but how about in solid state? I’ve seen some schematics where it connects to the emitter of a preamp transistor and others where it goes directly to the input through a large (100k) resistor. I also hear that feedback is absolutely necessary in solid state amps. I know in tube amps you can use it or not and I’m not trying to start the old feedback vs non feedback argument but I’m just trying to understand how the signal flows in the solid state amp.

 

[audiotemp]

The vast majority of present day solid-state amps route global feedback through the inverting input of a long-tailed pair (diff amp) at the input.

Global feedback routed to the emitter of a single input transistor was used early, when transistor amps were built in configurations mimicking tube circuits, and later in cases where only a small amount of feedback was going to be used. Avoiding a diff amp is beneficial in that case.

Higher levels of feedback are generally required if a diff amp is present, either tube or transistor.

 

[Carbsandcaps]

Ya most of the schematics I was looking at were older amps. Some of the newer ones I can’t even find the tap off the speaker output I’m wondering if they’re local feedback.

 

[gadget73]

Most do come from the output, but its maybe not directly at the speaker. Sometimes there is a filter choke or speaker protection and whatever else between the amp output and speaker terminal. Most of the time you'll find that the feedback comes from the point that the two output transistors come together which is where the speaker output ultimately comes from. In cap coupled amps, many of them will feed in from the speaker side of the output cap to compensate for any loss in the capacitor itself. Those haven't been much of a thing since the 70s though.

Have a particular amp in mind? Schematics answer a lot of questions.

there are some early ones where the feedback comes in to the emitter or the base of an early transistor but those are often very early designs that look more like tube amps in terms of circuit design. By the early 70s most things had settled on a differential pair up front and the feedback comes into that.

 

[Carbsandcaps]

Well this is the imperial that I’m working on now. It uses an interstage transformer instead of the a differential pair so the feedback goes back to a driver transistor

But then this is a kenwood ka-907 that I was just working on and I have no idea where the feedback goes.

 

[gadget73]

this one is hard to follow because its across a couple sheets, but here is the input side. It comes into the diff pair, the highlighted 390 resistor is the divider to ground to set the total amount of feedback. Looks like it has two inputs though, through R51/C41 and through R53



This is the other end of that circuit



The wiring to J25 and J27 are those two resistors. One to each side of the output relay it looks like, so the signal is in parallel. R69 goes over to what I presume is the protection circuit control IC on the thermal detection pin. Don't quite know how that works or why they did what amounts to parallel paths but it must have something to do with the protection circuit. its probably talked about in the service manual sections I didn't read.

so fairly conventional circuit-wise, just a PITA to follow because of how the schematic is laid out.