What does what?

[MmmPedals]

I built the following circuit

I am tweaking values to my taste. I would like to know what they do. the 2 caps on the switch sound like it controls mid range. Is that right? (I currently have it wired without a switch at 4n7).
What does rg1 do?
How would I go about lowering the treble? no matter what I do the treble control never gets above 1/4 up and sometimes its all the way down leaving me wishing for more.

[brett]

Hi
the 2 caps form a 'high pass' filter so that bass gets stopped. You can work out the cutoff frequency (which, unlike the 'cutoff' in the name, is where frequencies start to get tapered off) from fc = 1/(2.pi.R.C). There are calculators around on the web.
Regarding the treble, I note that the ratios of the 220pF caps and the 22n and 2u2 caps in the boost sections are quite low. I'd be tempted to reduce those caps to 2.2n and 0.22uF. That reduces 'leakage' of treble from the feedback loop (and reduces the gain of the treble). But I'm no expert at working out in my head what a messy feedback network like these is doing.
cheers

[MmmPedals]

Thanks for the info def. helpful.
What does rg1 do?

[amptramp]

Rg1 establishes the gain in the midband where the effect of the 22n and 220p capacitors is negligible.  The gain is 1+220K/22K or 11 for this stage.  In fact, the capacitors do have an effect, so you never quite get to the gain of 11 at any frequency.  The upper cutoff frequency is 3920 Hz as set by the 220K and 220pF and the lower cutoff frequency is 329 Hz as set by the 22n and Rg1.  The calculator for this is:

http://www.muzique.com/schem/filter.htm

Below 329 Hz, the gain drops to unity.  Above 3920 Hz, the gain drops to zero.

[ashcat_lt]

Hate to be that guy, but...

It's 330K in the feedback, so gain the pass band is 15.  Gain at high frequencies is 1 (unity).  It can't be less in a non inverting opamp.  Otherwise it's a good analysis of the thing as depicted.

It's drawn in "Mesa" mode.  Changing the resistor to do "Marshall" doubles the pass band gain and also doubles both cutoff frequencies.

Rg2 does about the same thing in the next stage.  The 15K shown means a bit more gain slightly lower cutoffs.  The 2u2 means a lot more bass gets gain.  Swapping to 2K for "high gain" increases gain the pass band to 165, and bumps the cutoffs way up.

[MmmPedals]

Tell me if understand this correctly. Both rg1 and rg2 conceptually are the same. raise the resistor and it will 1) lower the gain 2) raise the cutoff frequency Which to our ears means 1) more "drive" 2) more trebley tone (not taking into account clipping/compression)

[PRR]

There's a lot of bass-cuts here. That's why Marshalls and Mesas sounded different from Fender: they screamed.

If there is too much treble, re-re-check all your values and connections.

Then increase the 22n under Rg1. As said, this takes out the lows below 300Hz, leaving a thin tone.

As it is drawn, that cap goes to Vref. So just short it for bass response to DC. That's probably too much, in several ways, so try 2uFd.

Also try increasing the 47nFd in the first stage.

[ashcat_lt]

Tell me if understand this correctly. Both rg1 and rg2 conceptually are the same. raise the resistor and it will 1) lower the gain 2) raise the cutoff frequency Which to our ears means 1) more "drive" 2) more trebley tone (not taking into account clipping/compression)

1 is correct.  More gain in the pass band.  2 is not.  Frequency being inversely proportional to both r and c, raising r lowers f for less treble, or more bass.

[amptramp]

Oops, ashcat_It is correct - the upper cutoff is 2193.3 Hz.  There seems to be a lot of bandwidth limitation in this circuit, both for upper and lower frequencies.

[PRR]

> seems to be a lot of bandwidth limitation in this circuit

Two cascaded stages with gain of 15 each, 225 total, after a preamp stage, is gonna clip madly. And throw IM products all over the sonic stage. Narrow-banding to a couple or three octaves makes it less nasty. Apparenly a *good* amount of nasty, if both Mesa and Marshall used similar schemes.

[ashcat_lt]

Tell me if understand this correctly. Both rg1 and rg2 conceptually are the same. raise the resistor and it will 1) lower the gain 2) raise the cutoff frequency Which to our ears means 1) more "drive" 2) more trebley tone (not taking into account clipping/compression)

1 is correct.  More gain in the pass band.  2 is not.  Frequency being inversely proportional to both r and c, raising r lowers f for less treble, or more bass.

Actually I think my reply is not exactly acurate.  Your second #1 doesn't seem quite right.  Assuming that by "drive" you mean overdrive/distortion, then less gain should mean less "drive" if all other things stay the same.

I'm thinking you got this idea because the Mesa has, or should have, more distortion than the Marshall.  That part kinda confused me when I looked at the thing, but I developed an hypothesis on that.   The Marshall resistor does give about double gain in the pass band, but the lower cutoff is also twice as high, and starting to get way up out of the range of most of the fundamentals on the guitar - where the bulk of the signal actually sits.  So it's kind of amplifying more of less, if that makes any kind of sense.  This gives less overall distortion.  Or something like that.