• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Marantz 8B

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boywonder

boywonder

Member
Joined 2008
I'm no expert, but it appears that someone was playing around with the global feedback/open loop gain/input sensitivity.

My guess is that Variable C is to tune the feedback for optimal square wave response.

There are plenty of folks here that can give you a definitive answer, like Eli D, SY, KevinKr, DF96 and many others.......
 
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SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
boywonder said:
My guess is that Variable C is to tune the feedback for optimal square wave response.
Click to expand...

Exactly right. I don't know why the 4k7 was clipped out, but I'd put it back in- it's part of the feedback network.

To adjust feedback, you need a scope and a square wave generator- a voltmeter won't tell you much. Details on how to do it can be found (among other places) in "Building Valve Amplifiers."
 
boywonder

boywonder

Member
Joined 2008
Rws62 said:
Thanks for the help. I guess I'll finish capping ,replace the resistors needed hook it up and listen to it and go from there.If it's not right ,will it be obvious?
Click to expand...

Maybe, maybe not. Sub-optimal square wave performance can mean ringing/overshoot, loss of low freq/high freq, etc. Search for square wave testing, I'm sure you will find something.

Rws62 said:
Any suggestions on what scope and square wave generator to shoot for?
Click to expand...

Well, any decent scope and func generator will work. The test is quite straightforward; you input a 10Khz square wave into the amp and connect the scope probe to the speakers outputs, and adjust that cap for the cleanest, sharpest square wave you can get, and it may not end up as clean and sharp as you hope.

Some basic results are:

Rounding of the leading edge of the wave means loss of high freq response

Ringing/overshoot means underdamped circuit and can manifest itself in poor transient response, harsh high freq response, etc.

tilted top of the sq wave means loss of low freq response IIRC.

Look at posts 79-96 in this thread for an idea of scope waveform.

http://www.diyaudio.com/forums/pass-labs/209627-aleph-30-best-choice-c1-c2-c3-brian-gt-pcb-2.html

Although this is a solid state amp, dialing in the feedback compensation is the same idea. I started with a lot of ringing/overshoot, and by tweaking the feedback comp cap value (In you case a variable C, although you could just solder in different values upon test). My final waveform has some rounding on the leading edge but sounds fine, at least to my ears.
 
E

Eli Duttman

Member
Joined 2004
As for the o'scope, get something with at least 100 MHz. bandwidth. The good stuff does not come cheap. Even though the audio band is 20 Hz. to 20 KHz., square waves contain overtone energy extending, in theory, to (sic) infinity. Prof. Fourier and his theorem strike again. Lots of high order overtones are needed to get good "shoulders" on a square wave.

A 1 KHz. square wave must be "perfect", in a properly adjusted amp. The look of a 2 KHz. square wave needs to be very good too. Small ringing and/or overshoot errors are tolerable, when a 10 KHz. square wave is applied. The realities of O/P trafo limitations become visible. Adjust for the "best" look. The age old tale of practical compromise gets "trotted out".
 
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