Pickup Simulation With An Audio Transformer
Is It Necessary For Driving A Fuzz Face With A Buffer?

last update: March 15, 2021

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Index

Introduction
The Guitar Pickup And Its Interactions
A Good Idea To Restore Frequency Balance And Why Itīs Flawed
The Simplest Of All Solutions
Reference

Introduction

The fuzz faceīs (hereafter: FF) notoriously low input impedance (around 8kOhm) leads to all sorts of problems due to impedance mismatch. Unfortunately, using a buffer without further thinking alters the FFīs tone. There are several attempts trying to cure this, and this article is an step towards a practical solution.

Ideally, we want a subcircuit that emulates the sound of a FF connected directly to the guitar, which is in many peopleīs opinion the way god wanted it.


Using a buffer does indeed change your faceīs tone - for the better.
Knowledge is power.

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The Guitar Pickup And Its Interactions

First, letīs have a look at the guitar pickup.

guitar pickup simulation
Pickup model circuit with typical guitar electronics, typical cable and fuzz face load
A guitar pickup is a second order resonant circuit, connected to the guitarīs control circuitry and a load comprised of cable capacity and termination impedance. Note that the volume pot is drawn as two resistors.

The interaction of internal pickup parameters with load, particularly capacity and FX device load, determines if the resulting tone is brittle (buffer without voicing), obnoxious (high peaking), dull (heavy load), pleasant (ideal load and voicing), or anything in between. High and untamed Qīs translate to high peaking, which almost always is perceived as unpleasant.

Letīs look how all that impacts tone.

Guitar > tube amp

A commercial guitar will most certainly be voiced in a way that it sounds pleasant into the average userīs sound chain, which includes a tamed resonance. A behaved system therefore does not exhibit much ringing.
  • balanced and pleasing tone
  • normal volume control travel
  • treble deterioration over volume control travel (unmodified vol control)
Guitar > fuzz face

Now we are speaking of a behaved load too, but a FF does not belong to this category. Itīs input impedance is very low, around 8kOhm.

At full volume setting (we are speaking of the guitarīs volume control), this load will be in parallel to the pickup, which will inevitably flatten any existing resonance. After even a small deviation from maximum position, the volume potīs series resistance (in the picture above called RvolA1) will take over and together with any alleged load capacitance flatten all ringing stone dead. The total circuit will thus appear as a smooth second order rolloff filter with no peaking.
  • unnaturally fast volume decrease over travel
  • treble deterioration over volume control travel, but probably swamped in the unnatural travel.

Guitar > close buffer > fuzz face

Now a buffer comes into play. A buffer on the end of a long cable is useless, because it does not eliminate cable capacity. So letīs put a buffer right next to the guitar. The pickup immediately resumes its no-load condition (meaning: no load capacity) and starts to extend its frequency range far up. This will sound unnaturally bright and, since some small capacity is always there, exhibit some peaking. So drive the FF with this signal, and you will have the impression that it sounds unnaturally bright. And it does, but not because the pickupīs second order tone-shaping function has gone away. Itīs still there!
  • brittle tone with peaking
  • FF sounds unnaturally bright
Ideally, a careful design will bring the resonant frequency down to a pleasing range by installing a voicing capacitor, while taming any alleged peaking with a carefully chosen parallel resistor. Look at an active bass, they all have this...
But you must not add any capacitance after the volume pot, since this will throw us back to scratch. It has to be added before.

Up till now we have only talked about the pickupīs side. What about the FFīs side?
A FF essentially has a current driven input. Use Logic, Watson. Such low impedance draws current. What has a pickup to impede this? A typical Strat pickup, 6-8k. And guess, yes, if the FF is driven hard (low impedance) it will sound driven hard.

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A Good Idea To Restore Frequency Balance And Why Itīs Flawed
 

This observation lead to the impression, that the FF sounds unnatural, because of driving it through a buffer with a simple series resistor to fake internal impedance.


"To simulate the response of a guitar pickup, it is often recommended that some
resistance should be added in series with the input of the effect. Typically 10k to 15k is equivalent to the resistance of a guitar pickup and while this does change the source impedance, its response is flat across the frequency range unlike that from the guitar.
"
 

    - Jack Orman (AMZfx), Guitar Pickup Simulation [1]

This lead to the invention of the following circuit, which indeed simulates a pickup:

Coil simulation
Pickup emulation circuit:

A circuit trying to emulate the way a guitar pickup perceptively drives a fuzz face goes back to a Jack Orman design from 2002
[1], and many have adopted it [2]The values shown lead to resonances you would typically expect from humbuckers and single coils.
N
ote the way the primary and secondary transformer side have been interconnected to achieve a higher inductance. This I have not seen before in any commercial product. Although the primary and secondary inductance values are small, they give a much higher value in series, unless you mix up the winding sense [3]. The working of this has been verified. The heavy FF load will yield a damped response as above.

But the idea is flawed. To my understanding, it does not take into consideration, that a buffer has modified the pickupīs frequency response towards brittle beforehand.
It also does not take into consideration, that as soon as you deviate your volume control even at the slightest from its maximum position, the series resistance (in the drawing above called
RvolA1) easily takes on resistive values that are several decades beyond the pickupīs impedance values, which will swamp all contribution from the pickup stone dead.

Agreed, if you donīt fix this at the point of origin,  you have to fix the response lateron, but in all cases before the FF.


You eliminated the expected frequency behavior by inserting a buffer without understanding what it does. If you then try to restore it with a similar circuit, the cat bites its own tail.


To my logic, this has nothing to do with the series input resistor we have inserted.

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The Simplest Of All Solutions
 

The good news is, treble can be controlled by the guitarīs tone control, which eliminates peaking too for the majority of its travel.
If felt necessary, an additional filter cap after the 10k driving resistor yielding a 3kHz rolloff (about the frequency range of a well voiced strat pickup) would suffice.

If you consider all this, a 10k drive impedance will work fine - the original frequency content is still there. This is, where the flaw is in the above thesis.

So, lets sum this up:
  • A pickup with a FF directly following the guitar will never exhibit any ringing, so any additional simulation must not introduce ringing or it will introduce an unnatural sonic component.
  • Any additional pickup simulation circuit must be a damped circuit and provide no more than a gentle high rolloff.
  • After tweaking the volume control, the signalīs frequency response will be dominated by the controlīs series resistance
  • The emulation circuit will only be a true model in one case: full volume
  • If the treble range is adjusted,  a moderate series resistance (possibly with a capacitor) will perfectly work for driving the FF softly in a comparable manner.

Conclusion: a 10k series resistance will work perfectly as simulation.

Now you will invariably ask: "why do I need another buffer on the input? I already have a buffer at the guitar."
This is true for the most basic setup guitar > buffer > fuzz face. What if you change the sequence? If you insert a wah pedal before the FF, will the wah be happy with the 10+8k input impedance? No. We do want our effects to have a high input impedance and a low output impedance to enable all sorts of interconnections reliably.

Any input buffer will serve for this current drive mechanism, be it discrete or integrated. A voltage follower of any sort just needs a 10k series resistance, a discrete amplifier may use a 10k collector (resp. drain) drive.

The input impedance should be in the 1M region.

fet
                                bufferTrim R2 for 4.5V at the source.
Input impedance is about 800kOhm.
HF tailoring is achieved by the 4k7 input resistance and the j-fets high G-S capacity.

The voltage-to-current operation is achieved by the 10k series resistor.
transistor buffer
Input impedance is about 1Meg.
HF tailoring is achieved by the input R/C
drive current limiting is achieved by the transistorīs collector load.

This circuit even has a small gain on it.

Besides all that rigmarole, practical experience proved that the transformer is susceptible to hum intrusion. However, this can be remedied by a 90° flip of the casing or the hum source.

So I will not likely use this circuit in a future build. Fortunately, even on an existing board, the whole arrangement is easily replaced by a 10k resistor or something in that range. From now on, you will look differently at that humble resistor.

10kresistor, the wonder weapon
Give praises to the magic output impedance increaser

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Reference

[1] AMZfx: (Jack Orman) Guitar Pickup Simulation,  http://www.muzique.com/lab/pickups.htm
[2] AIONFX: Solaris Germanium Fuzz building documents,  https://aionfx.com/project/solaris-germanium-fuzz/
          
and others like the Proteus. Also see EQD eruptor (clone: PCB Mania Volcano device)
[3]
Stack Exchange, How do I use a transformer as an inductor?
     https://electronics.stackexchange.com/questions/33860/how-do-i-use-a-transformer-as-an-inductor     

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Update History
  • Mar 15, 2021  initial release
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