Pickup Simulation - AQUATAUR Musik

Pickup Simulation With An Audio Transformer
Is It Necessary For Driving A Fuzz Face With A Buffer?
last update: May 19, 2024

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Index

Introduction

The Guitar Pickup And Its Interactions

A Good Idea To Restore Frequency Balance And Why It´s Flawed

The Simplest Of All Solutions

Reference

Introduction

The fuzz face´s (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 FF´s 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 people´s opinion the way god wanted it.

Using a buffer does indeed change your face´s tone - for the better.

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The Guitar Pickup And Its Interactions

First, let´s have a look at the guitar pickup.

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 guitar´s 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 Q´s translate to high peaking, which almost always is perceived as unpleasant.

Let´s 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 user´s 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. It´s input impedance is very low, around 8kOhm.

At full volume setting (we are speaking of the guitar´s 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 pot´s 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 let´s 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 pickup´s second order tone-shaping function has gone away. It´s 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 pickup´s side. What about the FF´s 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 It´s 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 advent of the following circuit, which indeed simulates a pickup:

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.
Note the way the primary and secondary transformer side have been interconnected to achieve a higher inductance. This I have nor seen before in any commercial product, neither on any other site. 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.

But the idea is severely flawed for several reasons:

It does not take into consideration, that a buffer has modified the pickup´s frequency response towards brittle beforehand.
It 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 pickup´s impedance values, which will swamp all alleged resonant behavior from the pickup stone dead. There is nothing in a fuzz face's input that depends on resonance. Its problems stem purely from being driven hard by a low impedance.
It disregards the fact the guitar's pickup acts as a second order filter, and any subsequent "emulator" adds another two poles. You end up with a four pole filter with a steeper cut-off, which may create the impression that the circuit really does something. It may do something, luckily nothing destructive, but definitely not what you may think.
Particularly the mediocre inductance values that can be achieved by not using the series connection as shown above indicates that there may not be much resonance going on on the commercial devices seen.
Going the "pickup emulation way" either exhibits that the designer barks up the wrong tree in a failure to recognize the real cause of the problem - unless this move was deliberate. Deliberate in the way that it alleviates a problem without fixing the cause, but gets you up and running again quickly. This is a valid approach, but be aware that it introduces some resonant filter but not necessarily the one your guitar would introduce. This is in order if it sounds OK for you.
If you fix the problem at a later stage, then you tamed the shrillness only as long as you have the unit in question activated. Turn off the unit with the band aid in it and you have your shrillness back...

Agreed, if you don´t fix this brittleness at the point of origin, you have to fix the response lateron, but in all cases before the FF. The real problem is that a buffered pickup is no longer damped, that´s the source of the phenomenon. It likely needs a damping resistor in parallel (if the internal components are not sufficient) and a voicing capacitor before any buffering.

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.

So, this all really has nothing to do with the series input resistor we have inserted. This is an understandable, but radically erroneous view on the subject.

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The Simplest Of All Solutions

The good news is, treble can be controlled by the guitar´s 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 signal´s frequency response will be dominated by the control´s 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.

Trim 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.

Input impedance is about 1Meg.
HF tailoring is achieved by the input R/C
drive current limiting is achieved by the transistor´s collector load.

This circuit even has a small gain on it.

Besides all that superfluous 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.

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

May 19, 2024 update chapter "A Good Idea To Restore Frequency Balance And Why It´s Flawed"
Mar 15, 2021 initial release

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