XOTIC BB Preamp Versions Question

[lars-musik]

A friend of mine is a huge fan of the XOTIC BB Preamp, so I looked into the schematic to see if it's feasible  to squeeze one into a 1590a. After some research I realized that there are some differences in the existing schematics - obviously there's a version with a dual-gang volume pot and one with a standard single gang. One part of the dual gang potentiometer is inserted along with a cap and a resistor behind the second op amp stage. This assembly is missing in the other (Ver.2 ?) version of the pedal completely. It looks like the version with the dual gang also utilizes a double back-to-back diode configuration, now at least what that does, I know. But that indicates that different sounding pedals with the same name (at least regarding the clipping behaviour) exist.

I'd like to make these differences switchable for testing purposes. But before I do that, I'd like to understand a bit more of what is going on there.

So here is/are the question(s):
What are the parts in the 1µF+10K+variable resistor assembly (encircled in red) doing? How does the cap size 1µF) affect the tone? There's another schematic somewhere floating through the web, that rates the resistor across the pot at 22K (giving a max resistance of 18K instead of 33K here and probably also changes the taper.....).
And finally: is the orientation of the potentiometer in that assembly correct (I read the Secret Life of Pots, but still my brain is not really catching up)?

Thanks for your input!

[lars-musik]

To close that case, here are the complete drawings.

To switch from version 1 to version 2, I included a double back-to-back vs. single back-to-back diode switch and two toggles to (1) switch the before mentioned assembly in and out and to (2) invert the lugs of the potentiometer. Strangely I don't hear much of a difference between the two potentiometer orientations.

For those interested, here'a a 1590a layout with ungainly jumpers.

[287m]

woah!!
nice arangement for BBpre in 1590A Lars!
i just make layout for my friend, the perfguy. but still not happy because too close with 3pdt
just wait for mini 3pdt and lumberg jack

[lars-musik]

Thanks and good luck.

Look, this is the "testbox" with all the switching options. It will be fun to shrink all this into a 1590a.

[samhay]

Regarding the top schematic, the extra pot is a 60-160ish Hz bass-cut control (I guess you knew that).
But, if you turn the pot fully cw, this is effectively out of circuit*, so I don't see any point in making it switchable unless you want to make the setting a pre-set.

* The R13 1k isn't out of circuit, but this is in the noise and C13 (1u) || C8 100n is not much different from 100n
In any case, I would omit C13 and R13 as they are not necessary.

[lars-musik]

Hi Sam,
thanks for the clarification. No, I didn't know that this is a bass cut (same as high pass?)... I am still very much at the steep slope of my learning curve, so your input is more than welcome.


What sense does a bass cut make when it is paralleled with the volume control (it is one part of a dual gang pot)? I don't get it. Could you briefly explain what parts exactly belong to the bass cut?

I made the whole thing switchable because I thought that if I don't understand what's going on there in theory I might do so if I just listen to it (which I didn't).

Thanks!

[samhay]

>(same as high pass?).

Yes - the pot resistance || R14 22 or 47k? and in series with R13 (1k) and R15 (10k) works with the series capacitor(s) to form a high pass filter. For further reading, look at how you make a high-pass filter using an inverting op-amp and remember that you have to add up all the series resistance (resistors) and capacitance.
Basically, the corners are 11k vs. 91n = 159 Hz and 11k + 18k vs 91n = 60 Hz
(91n = 100n || 1u and 18k = 100k || 22k; if a 47k resistor is used instead of 22k, you get 32k and f = 40 Hz)

Oops - didn't realise that this was part of a dual-gang pot. In that case, making it switchable is quite worthwhile, at least for testing. I guess the ideal is that you roll off the bass as you turn down the volume (and vice versa), so it works like an active treble bleed. You might try using separate pots for the filter and volume to see how they interact.

[lars-musik]

All right, I get it. So an approach for testing purposes could be to omit C13 and R13 and decouple the dual gang pot by replacing the half pot in question including R14 with a 50K log pot. I am going to find me a nice high pass calculator to track the corner frequencies involved.

[samhay]

Sounds good.

I should have mentioned that the series resistance also determines the gain (with R16, 47k). The changes suggested shouldn't make any difference, but the dual-gang volume pot means that as you turn the pot up, you increase the gain of this stage (which should be clean anyway) and turn up the output volume at the same time. They may have done this to prevent the last (Bax tone stack) stage from being overdriven. As such, if you find that you get ugly op-amp clipping when you turn the bass cut up, reduce the value of R16 a little.