For review: One pickup, PTB, volume, resonance cap switch

[David Mitchell]

With my still-unfinished rewiring project (largely delayed due to having a family ), I can't make up my mind about which controls and pickups I want to use. So now I'm planning to put in a PTB circuit to see if it seems useful, along with a volume control and a cap switch (I have a Redeemer buffer in place and want to be able to adjust the capacitance to change the pickups' resonant peak). Then I'll wire in the pickups I have, one at a time, and see what I like and where. (I want to reduce the amount of wiring for now, and avoid drilling another hole for a pickup switch — hopefully I'll be able to reduce the controls in the end, whether with a TBX pot set up for PTB, stacked pots, or just giving up a control.)

The wiring order is largely due to the physical layout, but I'll change it if I need to. The volume pot is actually a push-push pot, but I'm probably not using the switch at this point, unless I wire it to the boost control on the buffer.

Does this look like it'll work? Does back of the volume pot need to be grounded? Think I missed that....

[mikecg]

Hello David,

I've simulated your circuit, and here are the results:

























In the upper plot, all three curves are for the case with the treble and bass cut controls set to minimum cut - as you can see from the circuit diagram.
I have assumed a 10 megohm input impedance for the 'Redeemer' buffer - it might be higher - but it won't make much difference.
The black curve shows the 'un-switched' resonant peak with an amplitude of around +3 db wrt the base line, at a frequency of around 3.7 kHz.
For the two active positions of your 'peaking' selection switch:
The blue curve shows a peak at the same amplitude, at a frequency of around 1.5 kHz.
The red curve shows a peak at the same amplitude, at a frequency of around  2.5 kHz.

The middle plot shows the 'un-switched' case, with the value of the treble cut pot stepped in 5 equal steps from 250k to 0k. Simulation shows that, without the 100k pad, nasty things start to happen when the pot value drops below 100k, so the pad cures that problem!

The lower plot shows the 'un-switched' case, with the value of the bass cut pot stepped in 5 equal steps from 0k to 1 megohm. 

You will note that I have used some generic values for the humbucker inductance, DCR, and capacitance, and the 180pF includes a nominal 60 pf for the wiring. Also there is an additional nominal 100pF associated with the wiring to the onboard 'Redeemer' buffer. Your final capacitance values may be a little different - but hopefully not too different! R4, and R19, are resonance 'damping' resistors, and have been adjusted to set the amplitude of the resonant peak(s) to around +3 db. 

[David Mitchell]

Sept 10, 2023 11:33:54 GMT -5 mikecg said:

Hello David,

I've simulated your circuit, and here are the results:

SPOILER: Click to show

All three plots are for the case with the treble and bass cut controls set to minimum cut - as you can see from the circuit diagram.
I have assumed a 10 megohm input impedance for the 'Redeemer' buffer - it might be higher - but it won't make much difference.
For the two active positions of your 'peaking' selection switch:
The blue plot shows a response peak of about 4db wrt the base line, at a frequency of around 1.35 kHz.
The red plot shows a peak at the same amplitude but at a frequency of around 2.8 kHz.

As you have substituted a 250k pot for Leo's 500k treble cut pot, you may benefit slightly, by reducing the value off C1 to 1.8 or 1.5 nF.
And the two 10 Megohm resistors wired across the switch contacts, don't seem to have any obvious function - no doubt they are there for a reason?

So, to answer your question - yes, your circuit checks out.

mikecg, I did not expect a full circuit simulation! Wow, thank you!

• The 10M resistors are supposed to reduce popping. That part of the diagram is based on MattB's response to my previous diagram.
• I see now that it got cut off a bit, but the 250K treble pot is a no-load. I planned to just start with the stock capacitor, but I appreciate the suggestion for lower values!

I expected from my experimentation with GuitarFreak that the 220K parallel resistor (R4 in your schematic) would reduce the treble roll-off and also the overall volume (I was going for the former, not the latter), but I may be misremembering or misunderstanding something. 

For the record, I should give credit to sci4us for their diagram in the PTB thread, which I mostly followed for mine. If you see this, sci4us, thank you!

[mikecg]

Hello David,

Sorry to say - there is an important omission in my circuit, that I have only just spotted.

I forgot to add the pickup shunt capacitance, and this will invalidate the previous simulation.

I will edit it out - and replace it with a corrected version - shortly.

[mikecg]

I have completed the edit - I hope!

[David Mitchell]

mikecg, thanks for the further information and experimenting! Unfortunately, FYI, I don't read schematics well, but I have questions before I get to deciphering some of the details in your schematic.

Sept 10, 2023 11:33:54 GMT -5 mikecg said:

The middle plot shows the 'un-switched' case, with the value of the treble cut pot stepped in 5 equal steps from 250k to 0k. Simulation shows that, without the 100k pad, nasty things start to happen when the pot value drops below 100k, so the pad cures that problem!

What sort of nasty things happen? The pickup will in any case be "loaded" by the 500K volume pot, which I would expect to make the circuit electrically the same as using 1M pots when the 250K pot is in the no-load position, and till now I haven't heard of issues with the lower range of a 250K pot.

R4, and R19, are resonance 'damping' resistors, and have been adjusted to set the amplitude of the resonant peak(s) to around +3 db.

Here I think we have a misunderstanding because I didn't explain my design goals — or that I'm probably going to use Filtertron-type pickups, or perhaps a mini-humbucker. With my cap selector/peaking switch, I intend for one position to have the "natural," undamped resonance found with the capacitance of a typical cord, usually figured at 500pF; the other position to attempt to make the resonance of a Filtertron similar to that of a P90 (the parallel 220K and 2.2nF position in my diagram); and the bypass simply to allow me to hear the "direct" sound. I'll probably have to experiment with the values. In the long run, I'll probably switch to a larger rotary cap switch, but want to simplify for now.

Hope it makes more sense now!

[mikecg]

Aha - Now I think I know where you are heading with this!

Armed with your updated information, I have revisited the simulation. The first pair of circuits and plots, are for your 'bypass' switch position. Circuit 1, and plot 1, use your original circuit values, with typical filtertron pickup values, and show the 'nasty' resonant peak that occurs when the treble cut pot is at it's minimum setting. This is a well known 'feature' of the PTB circuit. Circuit 2 and plot 2 adds a series 10k resistor, and increases the treble cut cap to 6.8n, to damp the 'nasty' resonance, and produce a 'better behaved' treble cut control.












The second pair of circuits and plots, are for your '470p' switch position. Circuit 1, and plot 1, use your original circuit values, with typical filtertron pickup values. Circuit 2 and plot 2 adds a series 10k resistor, and increases the treble cut cap to 6.8n, to damp the treble cut resonance, and produce a 'better behaved' control range. With the 250k treble cut pot set to around 125k, the resonant peak at around 4.5 kHz, has an amplitude of around 2dbv wrt to the 0dbv baseline, showing a typical response for a filtertron pickup.














The third pair of circuits and plots, are for your 'P90' switch position. Circuit 1, and plot 1, use your original circuit values, with typical filtertron pickup values. Circuit 2 and plot 2 adds a series 10k resistor, and increases the treble cut cap to 6.8n, to damp the treble cut resonance, and produce a 'better behaved' control range. With the 250k treble cut pot set to around 125k, the resonant peak at around 2.2 kHz, has an amplitude of around 2dbv wrt to the 0dbv baseline, showing a typical response for a P90 pickup.