Hi
Thank you for taking interest in the Sparkle Face project. Funny the last time I made a Fuzz Face circuit with germanium transistors was back in 1982 when a friend obsessed with Jimi Hendrix asked me to build him one, which I did.
Anyway earlier this year I found in a drawer a left over pcb from 1982 but one that housed BC108C’s. I felt compelled to get that working and made a pedal that went to a friend. Anyway it got me thinking that I have some fine germanium transistors
of the last generation when the germanium transistor finally rivaled silicon but alas became too expensive for commercial use and were mostly used in early computors and snuck into some power amps but as years went by they were replaced by all silicon.
The point is there’s no magic just parts and the way you use them. So these transistors- and really a fuzz face circuit totally relies on its transistors- that I am using for this have low leakage in fact not bad at all compared to silicon and medium gain, no pop corn noise
and generally fairly good noise levels comparable to silicon and low temperature drift also comparable to silicon, but they are germanium transistors and have as such a lower Base to Emitter impedance referred to as hie in the four h parameters, and fairly low transit frequency.
For use in a fuzz pedal or actually for use in amplifier in general, these two parameters give rise to lower order distortion than silicon and the transfer function that is not as sharp. Now it’s a fuzz and yes you can get near perfect square wave with an emphasis on ’near’.
For musical use and as fuzz I have focused on getting the traditional sort of warm fuzz sound meaning one that does not make compensation for the various strength of the strings plucked- of course that makes the wound string thick of buzz while the unwound strings can
get a more violin or organ type of tone.
Because of this certain playing styles are suggested and so I have kept to this and not made sound more modern other than ever so slightly restricted broadband noise.
Sparkle Face has three controls
Volume and for this I have used a value similar to a guitar pot at 500K Log and this so that the traditional filter effect with an orange spiral cable can be achieved.
Saturation and this is a control that alters what the guitar sees as in either a low impedance current input or a slightly higher voltage input- what this means for the guitarist and the sound is that some of the more complex distortion patterns can either be had or turned down with this control and since
the input can be made into a voltage input the unit can go after other pedals- of course then the interaction with guitar pick up and the low current impedance would be non present- actually what this means is that as a current input gain of circuit depends on what the source impedance is and if it is a guitar pick up then the
most gain will be had when impedance of pick up is at its lowest which is its DC resistance. As frequency rises so does impedance of pick up and thus at higher frequencies there’s less amplification. This can also be viewed as a series coil feeding into a low shunt impedance and that forms a lowpass filter.
When guitar volume is turned down output impedance of the guitar rises swiftly and is at worst case at 8 with a 500K Log pot 250K and thus gain of circuit is reduced drastically and total fuzz level drops even further as the volume control on guitar is turned down due to the voltage division.
As crude as it is then this circuit makes a lowpass filter at the input due to pick up loading and a lowpass filter at the output via the impedance the Volume control forms as source and the shunt being the capacitance in the following cable. For those inclined to compute worst case output impedance is 250K
and the typical capacitance in an (orange) spiral cable would at worst case be 2000pF.
I recall early reading on fuzz pedals and fuzz sounds back in the 70’s that the principal would be to lowpass filter before intense clipping and lowpass filter after. As can be seen the circuit does just that while in a somewhat uncontrolled and crude way.
Fuzz control and this sets the total gain of the circuit from something controlled to the maximum the chosen transistors can give at in this circuit hfe1 time hfe 2- only the current input restricts maximum gain when feed by a guitar as explained above.
This is a modern design though so it has standard polarity DC inlet and a pilot LED and it’s true bypass and is housed in a typical floor pedal format like Hammond 1590B.
Anyway more on how this circuit works as a pedal
Due to the choices made the Sparkle Face has fairly decent noise levels for being a fuzz of this type.
As a model for the sound I envisioned a new shipment of fuzz pedals to a hip guitar store in the 1960’s and the privileged ones being there before opening hours picking the raisins out of the cake- now what would the best one sound like and play like if I had that opportunity to pick my favourite before opening hours?
I’d like mine to be one that could sound even but also broken at will as in a sound you can control with your hands ( this is where the Saturation control comes in btw) and I’d like when volume control turned down to get a slightly distorted sound even to a shade I might rather use instead of a dead clean sound.
I’d also like to able to use the fuzz pedal after wha and if I could wish anywhere I could think of putting it. I’d also be content without tone controls if the sound is balanced.
More later have fun
BJ
