Please help me understand the input stage of a power amp

[deuce42]

Hi guys. I am wondering if you guys could help me.

I have an old rackmount power amp that I cant find schematics for. I tried to trace the circuit however and it is almost identical to the power amp stage of a soldano slo like the schematic Ive posted. I'm not crazy about the sound of my power amp and thought I'd try to modify it a bit.

Looking at the schematic attached, when I trace my power amp's circuit, it is almost identical except on my amp the input jack comes off the middle pin of the volume knob in this schematic. I would have thought that the input jack would need a input stage, i.e have all the circuitry from effects return jack in this schematic but without the tone stack thats sitting in between the input stage and volume knob.

Could you guys explain to me what the difference is between having a input stage like the effects return jack rather than simply hanging the input jack straight off the volume pot in the schematic and working backwards?

Thanks very much indeed

[martin manning]

I believe your amp is expecting to see a line-level input signal from a separate preamp.

[deuce42]

Yes I get it, so the input stage from the effects return of a normal guitar amp has a "preamp" where as my amp doesn't need a preamp stage because using an external guitar preamp drives a line level into it.

Why then do people often plug external guitar rack preamps into effects return jacks of amps then? Are they effectively sticking an external guitar preamp into another "preamp", ie the input stage of the effects return? In this respect are the effectively pre-amping another pre-amped signal?

Apologies if this is such a stupid question

[JazzGuitarGimp]

When you say you don't like the sound of your power amp, two things come to mind:

- What preamp are you using going in to it?

- Have you checked the bias of the output tubes?

CHeers,
Lou

[tictac]

Why then do people often plug external guitar rack preamps into effects return jacks of amps then? Are they effectively sticking an external guitar preamp into another "preamp", ie the input stage of the effects return?

They do this to use the return as a power amp in when you want to use only the power section of the amp...

TT

[Flynscot]

Precisely as TT says-

The effect return is between the pre and power amps. When you jack an external preamp into an effect return, the amp's onboard preamp is out of the picture and only the external preamp is feeding signal to the power amp.

[deuce42]

Ok but I'm still a bit confused and I'm sorry to be so such an idiot -

I get that by patching into the effects return they are bypassing the amps own preamp and going directly to its power amp- but what is the difference between patching into the effects return, and (if it were possible) patching the external guitar preamp from a jack directly connected to the master volume knob? Ie, does the circuitry between the effects return jack and the master volume knob have any effect on the signal or tone? I guess put another way, by tapping into the effects return, are you going directly to the power amp in the same way that patching from the master volume control does?

I hope I'm asking my question correctly. In another circumstance, imagine if I were going to build just an external power amp to use with a dedicated external preamp - would I build the circuit from the effects return jack onwards or from the master volume knob onwards?

Thanks once again guys

[martin manning]

There are multiple ways that effects loops can be done. They range from nothing but a couple of jacks that allow breaking out and returning the signal between the preamp output and power amp input, usually just before the master volume, to having both send and return buffer stages to match impedance and signal level.

[deuce42]

, to having both send and return buffer stages to match impedance and signal level.

Ah ok, so if I were to build a new stand alone power amp, would I incorporate something like the components between the effects send and volume knob for impedance and signal level matching or is this not necessarily worthwhile or that beneficial to bother?

I will mate it with an external tube guitar preamp, although in my quest to make it sound as pristine amp as possible, I'd like to eliminate as many unnecessary components to keep the signal path as pure as I can. Its my understanding of Ken Fischer and the like's opinion that as few components means less ways to bleed away signal tone.