Hello everyone!
Big fan of the KG250 amps, cant find the original and also probably cant afford it, so im building an entire preamp section of the amp excluding stereo effects.
My question is about the compressor effect integrated in the clean channel witch i believe i marked red on the schematic. Can anyone give brief summary on its building blocks, how they work and what they do? Trying to figure out what exactly do i need to keep in this schematic for my project? Part of the U14 (circled yellow) might be an some kind of splitter for stereo signal if i understand correctly, in whitch case i dont need it? Main signal output to power amp is marked E after the FX loop?
Plz. corect my mistakes and THank you!
(https://i.postimg.cc/5HnpYf4T/gk250rlschemcomp.png) (https://postimg.cc/5HnpYf4T)
Welcome Hooligan!
The gist of it is that U1/353 is set up as a non-inverting amp. The JFET Q1/J113 acts as a voltage-controlled variable resistance aross R15 (along with R15A). This changes the gain of the U1 stage. U3 is a half-wave rectifier. C9 and C11 and the associated bits provide smoothing of the half-wave output and make something that's reasonably close to the envelope.
What's interesting about this is that there's basically an entire second copy of the same (or very similar) circuit consisting of U13/353 and Q4/J113. I'm not entirely sure what that's for, but if I had to guess, I'd say it was a noise gate - so unlike the compressor stage that is *boosting* quiet signals to make them louder, that part is *squashing* anything below a certain level to make sure that no noise reaches the far end of the amp. I haven't checked this hypothesis though, so that's all it is.
HTH
Thank you very much! :o
Thats huge help!
Quote from: ElectricDruid on December 22, 2024, 08:42:03 AMWhat's interesting about this is that there's basically an entire second copy of the same (or very similar) circuit consisting of U13/353 and Q4/J113. I'm not entirely sure what that's for, but if I had to guess, I'd say it was a noise gate - so unlike the compressor stage that is *boosting* quiet signals to make them louder, that part is *squashing* anything below a certain level to make sure that no noise reaches the far end of the amp. I haven't checked this hypothesis though, so that's all it is.
It's a while since I've looked at these amps.
The second copy looks a like a noise gate to me.
Strong input signal => large negative signal on precision rectifier
=> That will turn off JFET Q1 ie. compress ; as expected
but, also
=> Output of U13 will be high (due to inverted input in Schmitt)
Output D12 rectifier will approach zero volts.
That turns JFET on JFET Q4 and U14 will the be high gain.
With no input U13's output will be negative and reduces the gain of U14.
That action and the threshold due the Schmitt implies a noise gate.