Almost done with my project, (edit: no longer) confused about something...

[mg.audio]

Alright, so I'm working on a Rat circuit for bass. One last mod I'm trying to tweak is where a SPDT toggle switch will select the pre-gain high freq. response. I'm using the schematic from tonepad:

http://www.tonepad.com/getFile.asp?id=89

First, changes that I've made so far: 1. Ruetz mod (eliminating the 2.2uF cap and resistor and replacing the 560 ohm resistor with 47 ohms to the 4.7uF cap), 2. Asymmetrical clipping with LED's (LED on one side, LED + silicon diode on the other side), and 3. I increased the .022uF caps at the input and output to .1uF. Also, I'm not using the filter pot at all. It's just going to have gain and volume pots and a voice switch.

I'm really happy with the sound - it's very intense yet balanced with excellent bass response. One thing I've always liked about Rats is the sound I get by turning down the tone knob on a P-bass in front of a Rat pedal with the filter knob at min. This is why I'm trying this last mod. Here's where I need some help - Looking at the schematic and going with my basic knowledge, at the input I see a high-pass filter, followed by a low-pass filter via the .001uF cap just before the LM308. As I understand it, increasing that capacitor should send more high frequencies to ground, which ought to give me the same effect as rolling off the treble on my bass. right? So after trying a few different values, I'm a little confused. Right now I'm switching between the original .001uF and a .01uF cap, and I'm not getting any reduction of high frequencies. Any ideas/clues?

...I'm wondering if it has anything to do with the Vb coming thru that 1M resistor - what's that for, anyway?

[Seljer]

That low pass filter is there mainly to knock off radio frequency interference.

With a 1k resistor and 0.001uF you get a cutoff frequency of 159kHz

f = 1/ (2*pi*R*C)

With a 0.01uF cap, you lowered it down to 15khz, still above your typical guitar frequncy range. You're probably going to have to go to for something in the range of 0.047uF it 0.22uF to get 'rolled off tone control' type results.

[mg.audio]

Ah, that's a fancy little formula there. Does that work with any passive R/C filter, or does the placement within a circuit have an effect on the math?

[ashcat_lt]

Yep, it works for all passive RC filters, both high and low pass.  Also works for RL filters.  The thing is, though, that you need to take into account all of the Rs and all of the Cs (or Ls).  In this case, that 1K is in series with the resistance of the pickup, which affects things quite a bit.

That 1M resistor adds 4.5V to the guitar signal.  Otherwise, it would be asking the opamp to follow it through 0.  The opamp's output can't possibly be less negative than the bottom of the battery (actually usually about a diode drop short of the bottom of the battery).  The bias voltage makes it possible for the opamp to amplify the bottom half of the waveform. 

This resistor also basically sets the purely resistive portion of the input impedance of the box.  Making it smaller will start to sound like turning down the T control on the guitar. 

[GGBB]

If you want to approximate as closely as possible the sound of turning down the tone pot on your bass, why not just toggle a cap (PBass is .047u I think) from the input to ground.  If you want it to sound like it's only part way down, you could add some resistance in series with that cap.

[mg.audio]

I thought about that, but I also thought that differences in input impedance might alter the resonance of that cap, depending on whether this would be used with other pedals in front of it. So, I figured I would just try to utilize the high-pass filter that's already built into the circuit, and I'm always up for a challenge.

[mg.audio]

That 1M resistor adds 4.5V to the guitar signal.  Otherwise, it would be asking the opamp to follow it through 0.  The opamp's output can't possibly be less negative than the bottom of the battery (actually usually about a diode drop short of the bottom of the battery).  The bias voltage makes it possible for the opamp to amplify the bottom half of the waveform.

Is this analogous to the rectifier in a tube amp?

[ashcat_lt]

That 1M resistor adds 4.5V to the guitar signal.  Otherwise, it would be asking the opamp to follow it through 0.  The opamp's output can't possibly be less negative than the bottom of the battery (actually usually about a diode drop short of the bottom of the battery).  The bias voltage makes it possible for the opamp to amplify the bottom half of the waveform.

Is this analogous to the rectifier in a tube amp?

No.

Any passive filter at the front of this circuit will be sensitive to the source impedance.  If you really want to make sure that it cuts off at a given frequency no matter what is plugged into it, you'd want a buffer before the filter.  That's pretty easy to tack on.

I was going to say that you could increase the size of the cap across the feedback path, but the effect would then depend on the gain pot setting.

[GGBB]

I thought about that, but I also thought that differences in input impedance might alter the resonance of that cap, depending on whether this would be used with other pedals in front of it. So, I figured I would just try to utilize the high-pass filter that's already built into the circuit, and I'm always up for a challenge.

Impedance is mostly beyond me but... Isn't the input impedance the pedal itself and therefore non-changing?  And even if it isn't, how is a cap on the input of the pedal any different than a cap on the output of the bass?  The output of the bass is the same point as the input of the pedal is it not?  A length of cable is all that separates them, which I realize has resistance and capacitance but in the real world would that make a significant difference here?  I suppose if you're putting other pedals between the RAT and the bass, then the output impedance of the pedal before the RAT would be different than that of the bass itself (likely lower), which would affect the resonance frequency of the tone cap, so you'd need to find a cap value that works in that situation.  My guess would be that this is still going to sound a lot more like turning down your tone control than inserting a low pass filter somewhere within the pedal, but again, this is somewhat beyond my knowledge set.