Passive Treble and Bass control system - cap interaction?

[jimboyogi]

Hi all,

I've been looking at the the G&L PTB system, and trialling it in a Ric 330 on the neck pickup circuit. I've modified the circuit by placing the pickup switching after the volume control.

However it doesn't seem to behave exactly as expected WRT tone response, particularly the treble bleed. Even using the very low value treble bleed cap (200pF), I'm still losing high frequencies when I turn down the volume pot.

I'm wondering if there is any interaction between the bass cut cap and the treble bleed cap that may be causing this? To my layman eyes the two caps are in series between pickup and output, so they will act as a smaller value cap?

S-500_schematic_blockdiagram.pdf

[Chuck H]

Well, there is no bass cut cap. The bass circuit is a high pass filter and the bass is reduced by resistance. As you noted, these are passive controls. That means they can only subtract. Turning up the bass pot gives full response as if the bass pot wasn't there. In other words, anything other than full up on the bass pot is cutting signal that would get through in a more typical wiring scheme. The treble pot is close enough to a typical tone control that it's operation shouldn't be posing many questions. Same goes for the "bright" cap on the volume control. I think the problem your having is related to C3 which is a constant treble bleed. It's probably intended to "balance" the tone with the bass circuit, but, being passive controls it can only do that by subtraction. So, with the bass pot set at "nominal" (5?) and C3 in place you have a balance of bass and treble frequencies, but that doesn't mean that it's not a result of both being cut out of the signal path AND you have a mid heavy signal. Interactions with specific pickup resonances not withstanding. I'd snip out C3 and throw it at a car.

[jimboyogi]

Thanks for your detailed reply Chuck!

My sincere apologies, I forgot about C3 on the schematic. I am not using that cap in my wiring, I'm looking for all the treble I can get
IMO C3 is there to counter the very bright G&L MFD pickups, mine are more standard alnico's.

[pdf64]

I suggest to increase the value of c2 bright cap; 1nF would probably act to make the tone too bright at low volume control settings, though that may be a useful effect.
The action of any particular value will depend on the rest of your input circuit, primarily the cable capacitance.
Personally, with a 250k control and a very low capacitance cable such as GeorgeL, I find that it's not a problem for me, and I don't have any kind of treble bleed.
Note that there are other treble bleed circuits that use a resistor in series or parallel with the bright cap, so you may find that they work better for you.

[jimboyogi]

Thanks pdf64. Just now I did exactly this - increased the bright cap value to 1nF. It actually seems a lot brighter than with the 200pF there, the knee on the 200p must have been so high it wasn't very effective at guitar frequencies in this circuit. There's still a slight dulling of HF when I turn the volume pot down slightly from full, then a bit lower the HF seems to return to the balance of volume. At lower volume settings the treble is emphasised, which is good for how I want to use the guitar - using the volume (and bass cut) to control amp overdrive.

I have used treble bleed circuits with resistors in parallel or series to the cap before, they all have their best uses

As a further example of this possible cap interaction effect; the bridge pickup uses the same circuit as the neck pup, but without the "bass cut" circuitry. I have tried a few different values of treble bleed cap across the bridge pup volume pot - all have worked very much as expected, with no loss of HF as the volume pot is turned down. Yet the neck pup circuitry including the "bass cut" has had this HF loss at near full volume, trying a range of different bright cap values.

I began this thread for a bit of a theory discussion about the possible interaction of the caps. I have googled a fair bit about this, but I haven't seen it discussed. Perhaps because it doesn't matter, but I thought I might as well ask the very knowledgable members here. Thanks to all who have responded, and to those who have taken the time to read and/or think on the question.

[Dave H]

I think the problem your having is related to C3 which is a constant treble bleed. It's probably intended to "balance" the tone with the bass circuit, but, being passive controls it can only do that by subtraction.

I think C3 is there because the bass pot isolates the pickup from the cable capacitance. There would normally be a resonant peak caused by the pickup inductance and cable capacitance which increases treble response. The bass pot resistance will damp the resonant peak making it sound dull so I'm guessing C3 (1n) was added to mimic cable capacitance (my cable measures 680p).

When the volume is turned down the added resistance from the cw end to wiper will also damp the resonant peak. The cap (and resistor) across the pot is to compensate for this.