Would this work?

Take a regular voltage-follower buffer, but make the input impedance low, so it loads the guitar.  Then place a fuzz after the buffer.  Do you still get the nice cleanup when you lower the guitar volume?


(New dads don't get much time to play on the breadboard!)

Thanks!
-Alex

Well, the whole point of a buffer is to provide current drive for a high impedance source (high Z in, low Z out). If you need to make the impedance higher (load the pickup more) it's easier just to use a series resistor.

I think there was a thread on here once about a variable input impedance buffer, but I can never find it when I search...

Actually, I had that backwards...

Adding series resistance will raise the impedance and can be used as a trick for a low impedance source. Not sure why you want to load the pickup more but some parallel resistance from sig to gnd should do the trick.

Clean up is not about low input resistance.  Controlled pickup loading to change the frequency response is something people use in circuits like a RM.  I think of clean up as using all of the gain or volume control.

Clean up is more about the design of the gain stage(s).   Make a gain stage(s) that just clips or sounds good to you with the guitar volume at max and the gain stage control at max is a good starting point, then you make the gain control work down from there there is a good chance the control pot will have good interaction with the player and guitar volume control also a the taper of the pot can help.
 
I am guessing people might confuse what is going on with a FF in the first gain stage and think clean up is due to low input resistance.
  The input is a IMPERFECT summing node that has its gain controlled by the feedback R from Q2E to Q1B and the openloop gain of the transistor and the guitar, bass connected to the input.  That is why the feedback R is sometimes adjusted (you need to keep in mind it has a dual function both gain setting and bias for the first transistor).  Think of a inverting opamp circuit.  The FF feedback R is the out to - input resistor and the guitar is the input resistor to the - input.  The FF sound is partly about the first transistor openloop gain R.G.s writeup might help Q2E is Q1Cs - Q2 Vbe drop and is inverted from the input signal, so the transistor is a imperfect inverting opamp like circuit.   That is one reason people select transistors for FFs

QuoteControlled pickup loading to change the frequency response is something people use in circuits like a RM.

I also think people's perception of "clean" is colored by the frequency response. When there is more treble, people perceive it as "clean" even if it may have considerable distortion. When you lower the guitar volume, the loading of the pickup (and the effect of the bright cap, if you have one) comes into play and produces a more trebly sound, which reinforces the perception of "clean".

Good point maybe we need to have a better definition of clean

To me most fuzz boxes with a buffer in front sound like crap.
For a better clean up I recommend to alter the 1st stage of the fuzz circuit.

JHS

I re-read the "technology of the fuzz face" at GEOFEX and now I'm pretty sure this idea won't work.  Ah well.

I think the question here is (but I don't want to speak for Alex): how to make the fuzz face as stable as possible whatsever is plugged to it (guitar, effect, guitar inboard preamp, etc.), gain or frequency wise.  Is it changing the biasing method to a stable(r) one, or an other method in which Q1 sees always the same impedance in front of it.  That is, making the front summing node voltage at the input a constant (?).

Is that the question (perhaps I am completely wrong) ?

Put a series resistor on the input of your fuzz face that can be switched in and out. Switch it in when you are using it with a buffered line. You'll have to experiment with what value of resistor works. (I used a 47k on the output of my wah to get it to work with the fuzz face and it worked pretty well.)

Alternately, you could put a switchable pickup simulator on the input, like Jack detailed at his AMZ site. I'm not sure how necessary the inductor and cap is in this instance though.

I'll have to think about engaging that AMZ transformer or resistor more.

One drastic example that come to mind (where the resitor or transformer will perhaps not work) is the Fuzz Face used with the mid booster in the PowerHouse Strat.  When you play with the inboard mid booster knob the output volume of the Fuzz Face goes (with the rotation the mid knob on the Strat):   -> high vol. -> low vol -> then at full mid boost the volume rise up again (with frequencies content changes also of course).   So the two circuits have effects on each other.

So for me, the switching of a compensation circuit is not ( (?) i'll have to try ) helpful in that situation.
That is why I posted the message before and why the post of Alex seems pertinent.

I love to over-complicate things...

Couldn't one construct a unity gain buffer with a knob that controls its effective input impedance?

The inverting op-amp presents an input impedance equal to the series resistance on the input (the inverting input is at virtual ground, so the input "sees" the input resistor to ground).

If one used a dual (stereo) potentiometer (say, 500K) and wired it so that one of the pots controlled the input resistance and the other controlled the feedback resistor (so that the two resistances are always the same), this would always produce unity output but the input impedance would change.  I'd include a fixed input resistance and an equal fixed-value resistor in the feedback loop to make sure that the circuit never sees both as zero...

like... 


The pot value sets the input impedance, while the gain remains fixed.  Haven't tried it with a guitar yet to see if it sucks tone the way it should, but it might be a way to overly complicate a situation looking for a simple solution 

Don't forget the passive guitar or bass forms an EQ because it is part of the input feedback to a FF.   The 3tran uses a 10K between the offset  EF and the FF type section and a 47K feedback R.  No it does not sound like a "standard" FF type circuit.

Quote from: brett on August 02, 2007, 09:09:31 PM
This topic comes up many times, and Gus or RG or I or any one of many others have answered it again and again.
I think that Gus is saying something so simple that it isn't being believed or maybe the importance of the statement isn't obvious when he says

Sometimes the guitar or bass is part of the circuit.

(Please note that I'm not trying to be rude or obnoxious here, I'm just trying to convey the importance of the statement. Maybe we could call it "Gus' law" or something.).

If we think about how easy it might be to emulate the electronic characteristics of a pickup further down the chain (with inductors (ie coils and transformers), resistors and capacitors), we find that it is not easy (because of the electronic complexity of the pickup).  That's not to say that some good tones can't be achieved with these other circuits, but it is unreasonable that simple ones can emulate the originals (such as pickup -> fuzzface).

Furthermore, as best as I understand them, the input stage of effect pedals are usually simple in electronic terms (usually dominated by a fairly constant resistance, whith maybe a little capacitance).  Therefore, they are easier to emulate than the pickup.  For example, a useful approximation of the first stage of the fuzzface is a 22k resistor.  So a buffer with 22k of input impedance will give some (but not all) of the tonal changes seen at the input to a typical germanium fuzzface.

All comments and feedback are welcome.
cheers

OOops...  Didn't know there was the Gus law...   
I forgot about the search button.

Great post.  Thanks for finding that, Frank.

Quote from: PerroGrande on January 07, 2009, 02:12:30 PMHaven't tried it with a guitar yet to see if it sucks tone the way it should

We have reached a milestone at DIYstompboxes- creating unity-gain opamp circuits with the sole purpose of SUCKING TONE! 

Hi
An op-amp distortion circuit with input impedance and gain (and frequency response?) that follows the signal envelope would be a lovely thing.
Maybe not simple, but potentialy quite lovely.
cheers

Quote from: brett on January 08, 2009, 12:40:15 AM
Hi
An op-amp distortion circuit with input impedance and gain (and frequency response?) that follows the signal envelope would be a lovely thing.
Maybe not simple, but potentialy quite lovely.
cheers

Envelope detector drives an LED, and the input bias resistor to gain stage is an LDR... maybe.  Neat idea.

Quote from: Alex C on January 07, 2009, 09:20:21 PM

Quote from: PerroGrande on January 07, 2009, 02:12:30 PMHaven't tried it with a guitar yet to see if it sucks tone the way it should

We have reached a milestone at DIYstompboxes- creating unity-gain opamp circuits with the sole purpose of SUCKING TONE! 

Well, geez...