If you want to directly see what is going on with a switched jack, just get an open style one like the panel-mount Switchcrafts.

This may be his handwriting 

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Look there's another TDA 2040

Anyhow no dice on this one either can't see jack

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But whatever message to self get a clue and a meter and start probing when the plug is plugged in

And get some compressed air and blow all those spider webs out 

I went to find a different input jack one that has some transparency 
so I could actually see the terminals making contact
What about my Crate

Nope

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Ok let's see I have a RAGE 108 but wait a minute what's this 

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I wonder if this amp has been to Canada ?!?!
 

Running to see if my Audition Chorus has it You cannot view this attachment.

Nope

Man what a trip I may have a super rare G1 modded Peavey Rage 108    

Quote from: Kaz Kylheku on September 15, 2026, 11:05:31 PMThis schematic cannot be right. It makes no sense.

Are you referring to the actual Peavey schematic (attached)?  What is wrong with it as far as the input jacks go?
You did not mention that when using the 'Lo' jack, we go through the R15 22K, but there is also R13 22K to ground (through closed tip switch of J3), so we have a voltage divider of R15 and R13.

The 3 or 6 terminal jacks are for TRS: tip ring sleeve (stereo) plugs, but can be used with standard mono TS type plugs.  With mono plugs, the ring contacts just become redundant sleeve connections.
With the 6 contact types, you have tip, ring, and sleeve contacts, plus a switched contact for each of those: tip switch, ring switch, and sleeve switch (TS,RS,SS).
When you insert the plug, you can see that the 'leaf' contacts of the jack are in contact with the plug, the switch contacts are no longer connected to any other terminals.  So the switch terminals are only closed when nothing is plugged in to the jack.


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

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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

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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

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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.

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.

[EDIT: nope, that is backwards. In an inverting stage, a taller feedback resistor raises the gain; taller input resistors shrink gain.]

Hey thanks man  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  so I grabbed it the next day I was gonna try it out and there was no input jack 

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

Like this
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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.