Hello everyone. I'm new here and fairly new to electronic projects as well. I'm hoping that someone can tell me ho to tame this circuits output. I have the circuit board from the MXR Micro Amp (1981\82 I think) installed in a guitar. The boost is 26db. Which is just too much. The circuit board is wired up to it's own volume pot but the usable range due to the high output boost is less than 1/4 turn. The circuit board is difficult to get to but not impossible. I'm interested in hearing all ideas but I'm wondering if there is any way to lower the output without changing components on the circuit board?
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This appears to be an op amp circuit. Gain is Rf/Ri. If you can find Rf, you can lower it by paralleling that resistor with another stacked on top of it. I can't read the circuit PN to determine which it is. If it is a NE5534 then the feedback resistor is between out and minus in, or pins 6 & 2. 1 is the pin with the dot, the count counterclockwise from that one when looking on top. (clockwise on the bottom). An LF355 has the same pinout in DIP package. Parallel combination of two resistors is R1*R2/(R1+R2).
I have this schematic but I'm not 100% sure it is the same year. It says TL601 for the chip. It is supposed to be a very clean boost.
I zoomed in on the picture above it says
TLOGICP?
I zoomed in on the picture above it says
TLOGICP?
Ok, so YOU have it.
Care to share it here?
In any case, the MXR MicroAmp has a gain control, where is yours?
FWIW and if I don´t remember wrong it´s basically an MXR Distortion+ without the clipping diodes.
It will clip anyway if driven hard and it wil definitely make the Guitar Amp input stage clip.
Post the schematic and we´ll tell you how to add the missing Gain pot.
Yeah, same pinout, 6 & 2 is Rf. Parallel the 56 k with another 56 k to halve the gain. Use parallel resistance formula in post 2 to calculate other Rf for other gains. Ri is 2 k at minimum pot value & 502K at maximum pot value.
Reason I don't suggest pulling the feedback resistor & replacing, sometimes that pulls the lands off a cheap PC board like this. Just tack solder the resistor leads on top of the old leads. If this box gets kicked arounds, bend hooks in the additional resistor leads and hook them under before soldering. I do 300 lb organs, there is never a shock problem causing parts to fall off on those.
Reason I don't suggest pulling the feedback resistor & replacing, sometimes that pulls the lands off a cheap PC board like this. Just tack solder the resistor leads on top of the old leads. If this box gets kicked arounds, bend hooks in the additional resistor leads and hook them under before soldering. I do 300 lb organs, there is never a shock problem causing parts to fall off on those.
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If the pot is 500k REVerse Audio as noted, then gain at 0 is hardly-any, gain at 5 is 2, and gain at 10 is 25.
If that first half-turn from gain of 1.1 to gain of 2 is not all good, then maybe you don't need a boost at all. Disconnect the pot and use it as a simple buffer.
If that first half-turn from gain of 1.1 to gain of 2 is not all good, then maybe you don't need a boost at all. Disconnect the pot and use it as a simple buffer.
Indeed. There are hot humbucking pickups that will spit out 2 volts peak-to-peak on chords, and a TL061 powered by 9 volts probably won't swing its output much more than 6 volts peak to peak. In this case, anything more than a gain of two (+6 dB) will start to clip the TL062, and this is not a good kind of clipping - it sounds harsh and buzzy.
Great idea. This can be done (i.e. disconnecting the pot) by snipping through either end of the 2k resistor, or snipping the wire that runs from one end of the 2k resistor to the 500k (gain) pot.
-Gnobuddy
Gus thank you so much for the responses. I will try some of the ideas and report back if I find something working well.
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