#1
I'm trying to add a pot so I can control the gain level on my Yamaha GA-10, since when you push the channel button, it's at a set gain, which makes the amp not very versatile gain wise.

It has two diode stages, one that has two right next to each other, with a resistor in front of them, but that wasent it, it controlled the volume more than gain. The second one goes like this, resistor-capacitor-zenor diode-capacitor-zenor diode-resistor. I put a pot in the last resistor, and got no change when turning the pot, haven't tried the first resistor, but idk it'll work either.

How would I identify the resistor that controls the amount of gain?

Idk if this picture helps any, but I've replaced all the diodes with germanium ones, hence the wires running everywhere.

#2
I don't see any discrete transistors and only the one opamp. So that opamp should be connected to the resistors that will control the gain. Now might be a good time to buy "The Electronics Self-Teaching Guide" or something similar.
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#3
That's a good point, this amp doesn't have any transistors, other than the output transistor. So, one of the resistors leading into the OP amp should control my gain?

Do you think it would be R10(the one closest to the switch)? Or which leg of the OP amp would be the gain control?
#6
Do u have a schematic for the thing? Or can you draw one? I can't (and won't) figure out anything from that picture...
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#7
hmm did changing the diodes have any effect? i have one of these, don't even use it for practice because it sounds so shit, it'd be cool if you get any improvements
#9
^ Yeah, I just put in a 2134 OP amp in it's place, sounds pretty good. The diode change had a pretty decent effect, gave it more warmth, and cleaned up the gain, still has a harshness to it though.

^^^ I can't find a schematic, anywhere, unless I want to pay money for the service manual, which I'm not... I just need a basic clue of where to start looking for the gain control resistor, cause then I'll just start poking around till I find it.
#10
If youre using a OPA2134 check for the resistor between pin 1 and pin 2 or pin 7 and pin 6. You may have to trace the track work until you find it / them.

If you want to increase the gain, increase the resistance value. Try a small value pot in series with it first just to test before going the whole 9 yards. Problem is, doing this will bring the amp to saturation a lot sooner.

Putting a pot across the resistor won't work as it will only act to reduce the gain (like a pseudo volume control)

The OPA has two amps in it. Without knowing the complete circuit, one (or possibly both) of them can provide gain factor.
Last edited by Phoenix V at Sep 15, 2011,
#11
Originally Posted by kurtlives91
You need to decrease the feedback resistor in the op-amps NFB loop. Lower the value.
TS, the NFB (Negative Feedback) loop should be connected to two pins of the opamp. I cant be bothered to look up the input and output pins, but it's probably the ones Phoenix mentioned.
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#13
U should increase the gain of the op amp stage before your diodes, so the amp gets pushed to diode saturation, but diode limits what enters tha next amp to keep it relatively clean. But op amp stages have infinite capability of gain (on paper) so watch out don't saturate the opamp, unless that's what you're looking for.
Call me "Shot".

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