Tone control

[argonrepublic]

I have a Realistic SA-102 Amplifier. I snipped a circuit from the volume control that causes it to go into class B. I changed resistors so that I have more headphone volume at lower level. It has become an incredible headphone amp. The tape out is very transparent so I am running from my DAC to the SA-102. From the SA-102 tape outputs, I am running my Harman Kardon HK 770 for nearfield monitors. I am so happy with this. Now I am going to build a light speed attenuator to the tape outs.
My problem is that I want to use the physical location of the tone control for my tape out volume control. I removed the pot and it is 10A50K. What it actually measures is 42K and at center postition it measures 5K vs 37K. The amp still plays but I think it is too bright. It is my undertanding that the tone circuit on this amp is a high frequency attenuator. I tried a resistor fron 1 to 2 and 2 to 3 postions where I pulled the attenuator with no noticable change.
What do I ned to change to get It flat again?

[Mooly]

Sounds like a log pot to me. Would have to see the circuit to see how it works... it may even be in the feedback loop of the power amp... impossible to say

Refit it, set it to what you think is "flat"... the only way to know for sure is 'scope it and use a sig gen... and then remove without altering position and measure, then fit fixed resistors of the same value (ha... that won't be easy) to bring it back to how it was.

[Mooly]

So that means 2 resistors as a divider for each channel
Measure on the pot from the center leg to each end to get the values. And you must measure with it out of circuit if you are not already. If not then it may not be a log pot after all, your readings may be influenced by surrounding circuitry.

[argonrepublic]

I measured out of circuit. Also I noticed it is much steeper 1- compared to 2-3. I guess I will have to use scope. I have a scope sitting here that I never use. Guess it's time. Do I use a 20-20k sweep and watch for spikes toward high frequency?

[Mooly]

20-20K is fine. Amplitude should be constant through the sweep within pretty tight limits assuming the generator or source is at a constant amplitude throughout the sweep.
A square wave is a good test on any tone control network. It should be absolutely square with the controls flat, and testing at say 100 hz and 10khz. That's the most critical test of any. Any deviation from square shows a rising or falling response.

[argonrepublic]

Another very rudimentary question. Would my source be a great contributing factor? Like if I played my sweep CD from a Discman as opposed to playing it from my transport and DAC?

[Mooly]

It should be within tight limits from a discman... to within 1db or so at the frequency extremes... and probably much closer than that.
What I would do is refit the pot... that's as good as the amp gets, back to original... then run the sweep and look at it on the scope. That way you will have a much better idea of what's "normal" for your amp.
Look at the sweep too, direct from the line out on the discman so you are sure all is well first.

[argonrepublic]

Ah yeah. Now it makes sense. I guess I should have though of that. Just needed to be pointed in the right direction. I guess I'll have to wipe the dust off my neglected scope.

[Mooly]

You will soon figure out what's what... get the scope on it... and you shouldn't even need a load on the amp.
As I say, without seeing a circuit it's hard to give definite details. Normally you would check something like this at the output of the stage itself, maybe on the top of the volume control... but if the control is an HF cut it "may" operate on the power amp itself so you will just have to try it all and see.

[argonrepublic]

I ran one channel from CD to one trace, the other is in to amp with headphone output going to other trace. They are dead on with tone control absent.
Thanks for help.