Creating a low-impedance input stage for guitar volume knob sensitivity

[famousbirds]

So - I've been experimenting quite a bit with a Fuzz Face on my breadboard recently, and one aspect of it I particularly enjoy is the extreme cleanup available by rolling down the volume knob (if the FF is first in the signal chain).  I found this article very helpful in understanding why this occurs - it's not simply a reduction in input signal from the guitar volume knob.  Instead, the low impedance of the FF input interacts with the large series resistance from a volume knob on 7, creating a voltage divider that reduces the signal far more than the guitar knob voltage divider.

In short, I love it.  Since I mostly play jazz, clean tones are everything.  I can set my bypassed rhythm sound at 7, and kick on the FF for overdriven rhythms, while still having all-out-fuzz within a quarter of the knob's rotation.  Perfect, rhythm tones are at 7, and lead tones are at 10, regardless of whether I have dirt on or not.

I'd like to replicate this effect with other dirt boxes.  Here are my questions:

1) Can I simply lower the input impedance of the input to achieve this effect (assuming the dirt box is first in the signal path)?  I understand low impedance inputs can cause a treble rolloff from loading the pickups - I can live with this.

2) What's the best way to implement this?  I read RG's "What is impedance?" article, and it seems I could just get away with placing a small resistor R to ground after the input cap.  Assuming the other elements contributing to input impedance are of reasonably high value, input impedance should basically be equal to R.  I imagine I'd have to increase the size of the input cap to compensate.  Anything wrong with this approach?

[Gus]

You need to do more reading there is more to the FF than what you posted.   Think of the first stage of the FF as an imperfect inverting op amp circuit with an imperfect summing node.  The feedback resistor from Q2E to Q1b and the open loop gain of Q1 and the source Z feeding the FF set the gain/EQ of the first FF stage.

If you read about opamps the gain is very high so the input - node is a summing junction.  The gain is the feedback resistor / source Z and signal.  The open loop gain is OFTEN so high you don't need to worry about adding that part of the math you often can just use the feedback elements for the math.  Now the gainbandwidth product matters with circuits like the D+ that use 741 opamps because the gain of the opamp is "running out " with the gain control turned up, do the math for the gain at max and look at the open loop gain vs frequency of a 741

The first stage transistor is often the one you select or adjust with a FF circuit you want the mix of the lower transistor gain and the inverting stages interaction with the source(the guitar pickups, pot(s) and cap(s).

Look around this site myself and other have posted about FFs and inverting opamps.

[joegagan]

welcome.i agree with you about the ability to use the fuzzface for rhythm as well as lead, i love it.

your link is messed up, here it is , fixed.
http://user.eduhi.at/aquataur/aquataur/musicstuff/pokerface-theory.html#Anamnesis_of_environmental_facts

[famousbirds]

You need to do more reading there is more to the FF than what you posted.   Think of the first stage of the FF as an imperfect inverting op amp circuit with an imperfect summing node.  The feedback resistor from Q2E to Q1b and the open loop gain of Q1 and the source Z feeding the FF set the gain/EQ of the first FF stage.  If you read about opamps the gain is very high so the input - node is a summing junction.  The gain is the feedback resistor / source Z and signal.

Interesting, ok, this makes sense.  So in addition to the input-impedance-voltage-divider effect I described, there is also the fact that gain of the circuit itself is inversely proportional to the input impedance.

I'll have to think about this more once I get home... I'd like to develop an input stage that behaves in this manner, without the possibility of distortion, to use a volume-knob-aware frontend for dynamic dirt pedals.

The first stage transistor is often the one you select or adjust with a FF circuit you want the mix of the lower transistor gain and the inverting stages interaction with the source(the guitar pickups, pot(s) and cap(s).

Yup, I spent a long time measuring gain and experimenting with my transistors, eventually selecting my lowest-gain 2n3904 for Q1, as it exhibited the best cleanup tendencies.

[famousbirds]

welcome.i agree with you about the ability to use the fuzzface for rhythm as well as lead, i love it.

your link is messed up, here it is , fixed.
http://user.eduhi.at/aquataur/aquataur/musicstuff/pokerface-theory.html#Anamnesis_of_environmental_facts

Good catch on the link...  also I played around with your input-blend cap idea from the Easy Face - very cool!  I noticed in addition to setting pre-bass, it also provided some smoothing in the middle position, similar to adding a series resistor (or pot) to the input.  Neat side effect...

[PRR]

> placing a small resistor R to ground after the input cap.

Put it before the cap so the bass response does not change.

Try 33K. That's probably too low, go up until happy.

You can even permanently mount a resistor in the guitar, though it is probably a bad idea to modify a guitar in a strange and hidden way for one user's preference.

> gain of the circuit itself is inversely proportional to the input impedance.

Only for THAT circuit (inverting opamp). Most amplifier stages work differently, don't over-generalize or you will be confused another day.

[pocket]

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[famousbirds]

> placing a small resistor R to ground after the input cap.

Put it before the cap so the bass response does not change.

Try 33K. That's probably too low, go up until happy.

Makes sense.  I imagine I can just use this as a quite-effective pulldown resistor as well.

> gain of the circuit itself is inversely proportional to the input impedance.

Only for THAT circuit (inverting opamp). Most amplifier stages work differently, don't over-generalize or you will be confused another day.

Oh, of course; it's just helpful to understand the FF a bit better.

[aquataur]

Good to see that may work is slowly catching ground

In short, I love it.  Since I mostly play jazz, clean tones are everything.  I can set my bypassed rhythm sound at 7, and kick on the FF for overdriven rhythms, while still having all-out-fuzz within a quarter of the knob's rotation.  Perfect, rhythm tones are at 7, and lead tones are at 10, regardless of whether I have dirt on or not.

An interesting approach.

I'd like to replicate this effect with other dirt boxes.  Here are my questions:

[...] and it seems I could just get away with placing a small resistor R to ground after the input cap.

If you use a resistor in the 3-5k region you are at home. This should replicate exactly the conditions a typical guitar volume pot meets when a stock fuzz face is plugged in - but only if you don´t have the guitar buffered. In this case the buffer could care less about a load like this.

It´s simple, make yourself a plug-in adapter with the resistor installed. In this case you can remove it if you don´t like it.
And let us know if this made things better

have fun,

-helmut