Please Help me to understand this CM Plexitone tone circuit

[reubster]

Hi There

Great site I'm learning heaps, but I'm currently going around in circles trying to understand how this tone circuit works.
Specifically, how the RC filters work within an inverting I/P  feedback loop and how to calculate the frequency.
Not sure if I can attach diagram, but the circuit is pretty simple to describe:

The output from the clipping stage [diodes in a FB loop] feeds the tone circuit:

Tone Circuit
a 4K7 Resistor in series with 470nF cap is  connected to the Inverting input.
Non inverting input connected to ground.

In the tone circuit's feedback loop [o/p back to  inverting i/p] there is a 22k resistor.
Paralleled across the 22K resistor is  a 5K tone pot in series with a 68nF cap.

The tone cct output drives a 5K log [volume pot]

Any suggestions / links etc would be most appreciated
Cheers[/s]

[reubster]

bump...any comments re the tone cct?.
BTW I'm not sure why some of the text is struck out, I trust it can be read by all.

[slacker]

I'm not sure about the maths, but I simulated it and it looks like when the pot is at minimum resistance you get a boost around 100Hz and everything above that is rolled off at 3dB per octave. When the pot is at maximum resistance there's still the boost at 100Hz but the rest of the response above that is basically flat. So it's like a treble control with an inbuilt low end boost.
Making the 68n cap smaller seems to move the boost to a higher frequency and widens the range of frequencies that are boosted.

Obviously in the actual circuit you might get a different response than just looking at the tone control in isolation. For example the low end boost might be there to make up for some bass cut earlier in the circuit.

[reubster]

Thanks Slacker

I cant really see how/where the 100 hz boost is created  from this circuit  but I can sure hear it....hence my post .
This pedal simulates a marshall + quad combination really well....right down to that 100hz hump....just as you measured.
[Quad boxes produce this kind of low end hump]

Your simulation explains why I love this pedal with all my 1 x 12" & 2 x 12" open back amps, and also when using it in a bedroom at low volumes, but also why I find it too bottom end heavy when I drive my JTM + QUAD.

I love the overdrive characteristic of this pedal and the  the high fidelity sound it produces,
What I'd like to do is be able to switch out the 100Hz bump for my marshall + quad applications......If any one can point out where/how  to do this I'd be most appreciative.
Also,
I'm pretty close to finalising a vero layout for it if anyone is interested.

Regards
Reuben

[slacker]

I've noticed quite a few distortion pedals that have a low end boost, like you said I guess this is so they sound good at bedroom levels. Makes them way too muddy at gigging levels though.

I had a quick look and it seems like making the 470n input cap smaller reduces the hump, changing it to 220n, roughly halves the peak. I don't know what that will sound like though. You could try making it even smaller, but you might loose too much bottom end that way.

Can you post a link to the schematic that might help.

[reubster]

Hi Slacker
FYI Schema attached......http://img368.imageshack.us/i/plexitunarev3jd5.gif/

By my calcs, the roll off in the clipping stage is 1540Hz [via R4 & C11], so you are right when you suggest that the tone is compensating for some low end rolloff. To my ears there is still an overall low end boost, but it is remarkedly clean, I guess these low freq's are just not being clipped.....very tubescreameresque.
As I said I'm not sure how the tone stage works so I did the old fashioned pen and paper method and calculated the Xc for the 68n cap for a bunch of frequencies from 50 - 5000Hz.
Then I calculated the parallel combinations with the 22k resistor for both tone pot = 0 and tone pot = 5K.
Then I worked out the opamp gain, using these values with the 4k7 resistor.

This showed a fixed gain of around 3 for frequencies around 50Hz, the gain decreased as the frequency went up, but decreased to a lesser degree as the tone pot was set higher.....This seems in line with what you simulated.
Increasing the 68n cap, will change [slow] the rolloff response of the tone pot, but this cap value will not significantly alter the bass frequencies.
So I can now see that playing with  high pass effect of the input cap [470n] will have some bearing on this, which is exactly what you suggested.
The way this whole circuit works reminds me of the old RIAA equalisation....first you take away, then you add it back.

Over the next couple of weeks I'm going to have a play with the cap values you suggest, perhaps add a switch to select the appropriate voicings for my stack and combo amps.
I think I'll add second switch to increase C11 in the clipping circuit, to give me bedroom & stage overdrive voicings...