Simple but useful

[PBE6]

I just put together a little passive box I'm finding to be much more useful than it has any right to be:



This circuit manipulates the natural frequency response of my P-Bass wiring. Increasing the capacitor value moves the frequency of the resonant peak lower, while decreasing the resistance of the "load" pot reduces the height of the resonant peak. I put 11 capacitors on a 12-way switch (with one spot left empty) so I could cover a wide range. Found some very cool tones between 5-30nF, especially for feeding into distortion pedals!

I also ended up changing the capacitor on my P-Bass's tone knob from 47nF to 7nF and adding a DPDT switch to add either 10nF or 22nF in parallel. This approach turns the end of the tone knob travel into a resonant booster while the the traditional muted tone now lies just before the end of the travel. Also getting some great mid-forward tones with this mod! I much prefer the cap values being smaller than 47nF, the stock configuration always seemed so dull with the tone rolled all the way off (reggae thump notwithstanding).

[aron]

Very nice! We need more bass circuits.

[pupil]

So, I understand that increasing the capacitor values would decrease the impedance of this circuit. This is proved by the capacitive reactance formula. Because the impedance is reduced the transducers (pickups) now have a resonant peak that strikes at a lower frequency and sounds "less bright". Right so far?

Reducing the potentiometer resistance would decrease impedance just like increasing capacitor size would decrease impedance. And so enlarging caps sizes and lowering the potentiometer value achieve the same result of lowering impedance, do they not?  I wish to understand if it is true that decreasing the resistive load likewise decreases the height of the resonant peak and only when the resistance is lowered and not when the capacitors are made larger?

I guess I should stop typing and break out some components  

[PBE6]

You're right, as you increase the capacitance from a low value like 1nF to a higher one like 15nF the frequency of the resonant peak gets lower in frequency but larger in amplitude. But as you continue to increase the capacitance to something like 47nF or 100nF, the resonant peak continues to decrease in frequency but gets smaller in amplitude. The purpose of the "load" knob is to reduce the amplitude of the resonant peak for those cap settings where the peak is too obvious (with my P-Bass the predicted amplitudes can be almost 6dB for some of the middle cap settings).

When the "load" knob is set too low, it acts like a voltage divider and can kill the volume completely, but settings around 30k-50k seem to do a good job of taming the resonances while adding some welcome softness to my bright P-Bass tone.

[Gus]

A thread with p bass volume and tone sims
http://www.diystompboxes.com/smfforum/index.php?topic=93990.0

[PRR]

> enlarging caps sizes and lowering the potentiometer value achieve the same result of lowering impedance

Capacitor (and coil) impedance *varies* with frequency.

Resistor impedance does NOT vary with frequency.

Capacitors (and coils) store energy. Resistors only waste energy.

They do two different things in broad-band (audio) signals. Fun to play with.
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I do not see a need for the 10pFd. This is like 4 Inches of extra cable, which we know doesn't have audible effect, certainly not with common 6' (180pF) to 10' (300pF) cables and 50-500pFd (or more) on any input jack. There's also likely to be 30pFd of stray wiring capacitance in almost anything big enough to build.

The pot might get twitchy at extreme settings, also hard to get the same setting again after it has been bumped. An alternative is a 6-throw switch with fixed resistors like 470K, 220K, 100K, 47K, 22K, 10K. If a 12-way switch jumps to hand, similar values plus the in-between values (470K, 330K, 220K, 150K, etc).

The cap-switch may click/pop? If never turned while the preacher is talking, that's fine. If you need to sneak a change quietly, try 10Meg resistors on the switch lugs each cap to the next. 10Meg will hardly affect the audio; but it does give a bleed-path from whatever cap is in-use to the other caps, so any stray DC is the same on all and you don't have sudden DC-jump "pops".

[PBE6]

I put the 10pF ceramic capacitor in there just to prevent RF interference even if the switch is set to the blank position. I pinched the idea from some Jack Orman booster builds, but honestly I'm not sure if it's much different than stray capacitance at high frequencies.

Using a switch for the resistance is a good idea, I'm going to get some more.

The caps don't seem to click at present. I should have drawn a better picture, but all the caps have one end connected to ground so there shouldn't be any voltage build-up.

[PRR]

> The caps don't seem to click at present.

Sometimes you get lucky.

It may depend what is before/after this box.

> all the caps have one end connected to ground so there shouldn't be any voltage build-up.

Yes, but the other end has NO path for DC when not selected, but is pulled to ground through resistor and external devices when connected. The un-connected end "can" charge-up to any voltage.

If you don't need 10Meg bleeds now, great.