Guitar Pedal

Dynamic Fuzz Guitar Pedal Circuit

I sometimes hack together a music device for friends or family as Christmas or Birthday pressie. Here’s one from a couple of years ago that I made for my wee bro. I’ve dug out the schematic to remind myself how it works, as I’m looking at using the THAT4315 ic for a new design.

The concept was inspired by a ‘moosapatamus’ blog post about an RA Penfold circuit (link at bottom). It’s a fuzz effect that is shaped by an amplitude envelope that tracks the pre-fuzz input. This is cool because it allows you to set a high overdrive and manipulate the volume of the fuzzy sound with your pickin hand. As a bonus the circuit is also an expander / noise gate – a useful feature in a high gain circuit. For a bit of fun I added the option to slew the attack and release times up to the order of seconds. The attack control is cool for auto-volume-swell weirdness, the release control not so much!

See the (deadTreeCAD) schematic above for my take on the circuit, based on the THAT4315 compressor-on-a-chip with an onboard VCA and RMS envelope detector. The 4315 is suitable for the standard 9V supply and generally well matched to this circuit. THATcorp have a good selection of app notes and some 9V guitar pedal specific designs (linked below) – I may have borrowed an idea or 2 from the THAT4316 ‘1 knob squeezer'(!) design guide.

The components around U4C(what happened to U3?!) provide a high impedance, ac coupled buffered input stage with 6 dB gain. The signal is then sent to both an overdrive stage for the clipping and an envelope detector that will generate the control signal for the VCA.

In the audio signal path – U4C is a typical ‘soft’ limiter with variable gain to in the feedback loop to push more volts through the clipping diodes. As always diode choice and quantity are ripe for experimentation – I’ve gone for cheap n cheerful 1N4148s with asymmetric clipping here. The distorted signal is sent to the 4315 VCA – the VCA is current output so U4D is required. The output is ac coupled to remove the 4.5V bias voltage and the output impedance is set by the components following U4D.

In the control signal path – The 4315 extracts the rms envelope of the conditioned input audio signal. The 6k8 Ohm resistor sets the threshold to… The 22u cap hanging off the timing capacitor should be placed close to the CT ground junction to help avoid cv signal coupling with the audio signal. The 2u2 timing cap gives a time constant of approximately 8ms which is good and fast for responding to transients however for low frequency steady state signals, there is a risk of excessive distortion due to ripple in the rms output modulating the audio input – a 10u cap here would be a better middle of the road value, maybe 22u for bass duties. The best approach is an NLC (non-linear capacitor) – a circuit that presents a frequency dependant capacitance to achieve fast response (low capacitance) to higher frequencies and low ripple for lower frequencies, check out the thatcorp pedal link for more info on this.

The rms envelope output is processed by the circuit around U2A /U2B – this provides control of the attack and release rates of the control signal. I think I first noticed this approach on a that4301 compressor circuit credited to Harry Bissel but can’t find the schematic – do check out the Harry Bissel ‘Morph Lag’ for a neat version with lin/log response control and inverted cv out. Anyway, it works like this: because the output of 2B is fed back to the inverting input of 2A, as the input signal goes more positive the 47u cap gets charged up via the attack pot. When the input is negative with respect to the 2B output the cap discharges through the release pot – simple as that! The ‘steering’ diodes provide some degree of independance between the attack and release pots. Most modern op-amps will work here but the slew rate must be greater than ~1 MHz for the circuit to function correctly.

The 4315 VCA gain control is 6mV/dB and I didn’t find that any additional Control voltage processing was necessary after the lag circuit for decent sounding results.

The THAT4315 can be made thru-hole by attaching to a 16 pin qsop adapter. The component choices look to be a bits box special!

Suggested Improvements

Post clipping EQ control – clipped signals are rich in harmonics and really shine when you can shape those harmonics to taste – very little effort was put in to optimising the frequency response or offering tone control here. Check out Electric Druid’s design walkthrough for the ‘Hard Bargain’ pedal for some cool fuzz tone control ideas.

Timing Cap – use 10u here or better still an NLC circuit a la 1 knob squeeeezr

Capacitor in U4C feedback loop – IIRC I added a capacitor here to attenuate higher frequencies at high gain settings (cutoff down to ~5 kHz at full ‘Drive’ pot setting).

Output Volume – always useful!

Effects send/return – to make best use of the noise gate feature and to maximise control over the VCA, the pedal is best employed early in the signal chain. Adding an effects send/return loop in the audio signal chain would better suit larger rigs.

CV input – for optional volume pedal or feed the vca an lfo for tremolo.


Penfold Fuzz Circuit (moosapatamus)

Electric Druid ‘Hard Bargain’ design

THATcorp pedal page

Harry Bissel Morph Lag


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s