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previous: Preben Hansen I have read RG's technologi docume... -- 3/6/1998 12:51 AM

Re: Fuzz Face for the people!

3/6/1998 4:25 AM
MHelin	Re: Fuzz Face for the people! Preben: One topic affecting the tone seames to be the input impedance of the FF and the load of the input from the guitar PU. I have seen a schematic, the JSH-fuzz somewherw on this net. It uses an emitter-follower at the input. I will try that some time in the near future. Any comments/suggestions ? Input impedance of FF is about hfe*25/Ic. If hfe=84 and Ic .25 mA, it's about 8.4k, which is not much, as that 8.4k is quite typical output impedance for a guitar PU, and at PU's resonance frequencies it's much more (about 50k or so). A rule of thumb for the input impedance is ten times the output impedance, that's why the 500k is quite usual. However, the Ic is 0.25 mA only when the transistor is conducting and the base voltage is then most negative (with PNP Q1). When the transistor is not conducting, there's just the base current of Q2, which is not much, and the input impedance is much more, I'd say it's close to 100k (the feedback resistor is there). So during negative phase input impedance is low and during positive phase it's high, and this is one way to see how the asymmetric distortion is created. If you add an emitter-follower or other buffer circuit between PU and FF, you'll get rid of this asymmetric input impedance and loose the tone, so it's better not to use any input buffers. Instead the recommended mod for the collector resistor (33k --> 5k6) is good, as it lowers the input impedance (for the negative phase). It also lowers input impedance for the positive phase, and to be able to adjust it for your guitar PU's, it's better to put there a pot. 50k pot could be fine for humbuckers too, maybe a logarithmic one as it makes it easier to adjust at the lower end. Put a 1k resistor in series with it in order to avoid accidents (or how much current these ge transistors can take, 5 mA must be safe, so 9V/5ma = about 2k). This is the tone control for FF , -Mikko

3/6/1998 4:25 AM

MHelin

Re: Fuzz Face for the people!

Preben:

One topic affecting the tone seames
to be the input impedance of the FF
and the load of the input from the
guitar PU.
I have seen a schematic, the JSH-fuzz
somewherw on this net. It uses an
emitter-follower at the input. I will try
that some time in the near future.
Any comments/suggestions ?

Input impedance of FF is about hfe*25/Ic. If hfe=84 and Ic .25 mA, it's about 8.4k, which is not much, as that 8.4k is quite typical output impedance for a guitar PU, and at PU's resonance frequencies it's much more (about 50k or so). A rule of thumb for the input impedance is ten times the output impedance, that's why the 500k is quite usual.

However, the Ic is 0.25 mA only when the transistor is conducting and the base voltage is then most negative (with PNP Q1). When the transistor is not conducting, there's just the base current of Q2, which is not much, and the input impedance is much more, I'd say it's close to 100k (the feedback resistor is there). So during negative phase input impedance is low and during positive phase it's high, and this is one way to see how the asymmetric distortion is created.

If you add an emitter-follower or other buffer circuit between PU and FF, you'll get rid of this asymmetric input impedance and loose the tone, so it's better not to use any input buffers. Instead the recommended mod for the collector resistor (33k --> 5k6) is good, as it lowers the input impedance (for the negative phase). It also lowers input impedance for the positive phase, and to be able to adjust it for your guitar PU's, it's better to put there a pot. 50k pot could be fine for humbuckers too, maybe a logarithmic one as it makes it easier to adjust at the lower end. Put a 1k resistor in series with it in order to avoid accidents (or how much current these ge transistors can take, 5 mA must be safe, so 9V/5ma = about 2k). This is the tone control for FF :-)

,

-Mikko

Replies:

MHelin I wrote:How... -- 3/6/1998 8:36 AM
R.G. >Input impedance of FF is about hfe... -- 3/6/1998 8:42 AM