Passive Bass Control

[Passaloutre]

This question is about a guitar wiring circuit, but I suppose it could also be used in a pedal, so hopefully it's appropriate here. You guys have taught me a lot about fundamental electroncis, so I figured I'd ask here.

I have installed the following circuit in a guitar, and I really like how it works. The guitar is a hollowbody with slightly dark pickups, so it's nice to be able to roll off some bass.




I would like to install the circuit in another guitar, but there's a slight complication. Because of the layout of the guitar, it would make wiring a lot easier if I could put the bass control after the volume control (the pot placement puts the bass knob far away from the volume and tone controls, and close to the output jack. I suppose I could send wires back and forth across the guitar if I have to, but I'd prefer to keep it simple.

So my question is: does the frequency response of the above circuit change if the bass control comes after the volume control? My intuition is that the corner frequency of the RC filter formed by the bass control is a function of the resistor that comes after it. At least, that's the way an RC high pass is always drawn. So in the drawing above, we can calculate the frequency of the filter with the bass capacitor and the volume pot values. If I put the bass control after the volume control, is the filter frequency now determined by the input impedance of the amplifier (or pedal) it's plugged into?


On the other hand, the following diagram shows the same bass control after the volume controls, so maybe it doesn't matter?

[antonis]

Green arrows are the upper part and red arrows are the lower part of the voltage divider..
(LOAD is just next stage input impedance)..

[radio]

Your first diagram reminded me of the G&L Legacy wiring and a similar question rouse up in their forum

So I took the liberty to give you the link of their thread:

http://www.guitarsbyleo.com/FORUM/viewtopic.php?f=115&t=11704


Edit:

Sorry you want a volume per pickup so ,they ll be between the pickup and the switch,and no master volume

[iainpunk]

placing it after the volume control doesn't do it justice.
the cut off frequency becomes dependent on the impedance of the next thing in line.
most pedals have a high input impedance, thus it cuts less bass than it would in the original schematic, i'd go for the longer wires back and forth, or change the volume pot to be the one that's far away.

cheers, Iain

[radio]

I stand corrected Ian! Didn't think of this rather obvious issue

[Rob Strand]

So my question is: does the frequency response of the above circuit change if the bass control comes after the volume control? My intuition is that the corner frequency of the RC filter formed by the bass control is a function of the resistor that comes after it. So in the drawing above, we can calculate the frequency of the filter with the bass capacitor and the volume pot values. If I put the bass control after the volume control, is the filter frequency now determined by the input impedance of the amplifier (or pedal) it's plugged into?

Your intuition is right on the mark.

The original circuit (from the G & L basses) is a shelving or step equalizer which cuts the bass.   At high frequencies the cap bypasses the circuit but at low frequencies the 1M pot and the 250k output pot form a voltage divider.   Actually the amp impedance is in parallel with the 250k but I'll ignore that for not.   The how much cut you get depends on that voltage divider.  For 1M and 250k the maximum attenuation at low frequencies is   250/(1000+250) = 0.2 = -14dB.  The important point for a fixed bass pot value is the amount of attenuation is determined by the 250K volume pot.     

If the load was increased to 1Meg then the attenuation would only be 6dB.   You would need to increase the pot to 4M to get the same amount of attenuation.

The next problem is dependency on the amp impedance load.   The amp or pedal impedance could vary between 470k and and 1M, perhaps more.  Without the 250k volume pot the amount of attenuation would vary depending on the amp.    For the 250k pot case, the 250k pot dominates the impedance on the divider and reduces the effect of the amp impedance, 250k parallel 470k = 163k and 250k parallel 1M = 200k.   

The shape of the response is like,



The upper frequency where the low-cut starts is determined by the 1M pot in parallel with the 250k in parallel with the amp load.
f_upper = 1/(2*pi*R2*C)  R2 = 1M pot parallel 250k parallel Amp impedance.

The lower frequency where the low-cut levels off is determined by the 1M pot,
f_lower = 1/(2*pi*R1*C) R1 = 1M pot

The ratio of the upper and lower frequencies is very much related to the amount of bass cut.

As you vary the bass control both the lower and upper frequencies move up and the amount of low frequency attenuation gets reduced.

So one thing about the G&L basses is they use a pickup selector switch and a *single* master volume control.   They have done that in order to implement the bass treble controls.    In your case (diagram) you have two volume controls.   You can't actually do the G & L wiring unless you have a dual Bass and Treble pots.    BTW, the Bass pot actually needs to be after the Treble otherwise the Treble pot messes with the Bass - it might work but I suspect some odd results.

Anyway in short you are stuck with the single bass control feeding the amp impedance at the end.   That means the bass control won't have the range of the original G & L circuit.    You could help it a bit by adding a fixed 1M resistor from the output point to ground.  That will give you a bit more range out of the bass control (as well as hiding some of the amp variations).   The downside of that is a bit more pickup loading.   You could do an experiment to see the effect.


EDIT:
The way around it might be a parallel inductor with a series pot.   Not ideal but an option.   Check out some old 60's and 70's guitars which had rotary switches.   I think the Gibson L6S was one.  Not variable bass but they have an inductor.

Here,


You would need to pull the series cap.

[Passaloutre]

Thanks for all the help. I only have the one volume control, so I'll probably just do it like the first diagram and deal with the back-and-forth wires.