Solid State Guitar Amp Forum | DIY Guitar Amplifiers

I was looking at a peavey schematic VSS Audition Chorus specifically the input jacks region

I wanted to draw it out and figure out the paths of each

Nothing plugged in

IMG_20260914_185937471 (1).jpg

Plugged in to low

IMG_20260914_190010313 (1).jpg

Which looks reasonable because a circuit is completed

Now if I plug into high I'm not sure what's going on

IMG_20260914_185945220_HDR (1).jpg

....now...of course the rest of the amp is downstream and there are a couple of grounds chassis and earth in that vicinity

Just wondering what the signal path is  :grr

You've got your drawing of the plug wrong.  The tip goes to the part of the jack that looks like a check mark.  The arrow is a switch contact of the jack, it does not contact any part of the plug.

Like this
input jack.jpg

Hey thanks man  :dbtu: I was gonna back up and take another look at this but I remembered having a couple board mount input jacks in a tool box so I was gonna scare em up and insert a cable so I would be able to see how everything fit in

The reason I have em is I had a couple extra that I ordered from this experience I went back and looked and the thread is still there

https://www.catfish1.com/threads/soldering-on-a-circuit-board.242688/?post_id=3335736&nested_view=1#post-3335736

One time I was coming home from work and there was an amp on the curb  xP so I grabbed it the next day I was gonna try it out and there was no input jack  :grr

But hey man thanks for taking the time to correct the drawing  :dbtu: it's very helpful

If we plug into the "high gain", the switched "lo gain" jack ensures that the signal goes throug both 22k resistors in parallel. So we have effectively an 11k resistor.

If we plug into "lo gain", the switched jack opens the circuit through R13, so the signal goes only through R15: so we have 22k.

These resistances form a voltage divider with the 100 pF C16, creating a low-pass filter.

You can calculate the frequency response here, using the Okawa Denshi online calculator (http://sim.okawa-denshi.jp/en/CRlowkeisan.htm).

You will see that regardless of which input is used, the audio is nearly flat all the way to 20 kHz, losing only a fraction of a decibel.

The 470k impedance set up at the op-amp is a negligible load on this voltage divider, so we can pretty much ignore it.

This schematic cannot be right. It makes no sense.

Furthermore, the topology is reversed. If we keep the topology as-is but choose differentt values for the resistors and capacitors, can make the low pass filtering more aggressive. But the "high gain" input will be more aggressive than the "low gain".  For instance if we change the capacitor to 1 nF, then the "high gain" input  rolls off -3dB at 7 kHz, whereas the "low gain" is flat past 10 kHz, rolling off -3dB at around 13 kHz.

Easter eggs like this are not uncommon in schematics for music gear released by the manufacturers.

The resistor values would make sense if the input were an inverting op-amp stage.  The 22k resistance of the high input would have double the gain (6dB) advantage over the 11k resistance in the low input. I have a slight gut feeling that that might be the easter egg: redrawing the inverting op-amp stage as noninverting.

I got home from work and straight away went out to the garage and got those input jacks

IMG_20260916_052009174 (1).jpg

Unfortunately they are all closed up so can't see diddly when the plug is plugged in

But it doesn't matter it's clear what's going on

IMG_20260916_052218624 (1).jpg

So the one above with six solder terminals I guess it has 2x sleeve (ground) connectors for whatever reason

And the PCB mount only has input and ground

IMG_20260916_052233181 (1).jpg

 xP