What makes input impedance of the FF so low?

[yeeshkul]

Guys what makes the input impedance od FF so low? Is it the base-emitor "diode" of the first transistor that goes straight to the ground and it is parallel to the input? By the way how do we get the exact resistance of it?

Thanks Y

[R.G.]

The Technology of the Fuzz Face; GEO (http://www.geofex.com)

It's a real useful thing to read all of the "Technology of... " series if you're trying to understand effects.

[brett]

Hi.
As usual, good advice from RG.

Here's a quick and dirty runthrough of the input impedance story...

You got it right about no emitter resistor.  That first base-emitter junction is forward biased.  If it weren't for the fact that most of the emitter current comes from the collector rather than through the base, it would be too low to be useful.  The resistance through the base is equal to the hFE times the emitter resistance.  In this case there seems to be no emitter resistor.  But there is emmiter resistance due to the base-collect junction carrying current.  The effective resistance in ohms is 0.025/I, where I is the emitter current in amps.

In the fuzzface, the hFE of Q1 is usually 60 to 100, and the effective emitter resistance is about 250 to 350 ohms, making the input resistance about 70 x 300 = 21 k.  This varies by plus and minus 5k with normal variation in hFE and leakage.  With changes to the biasing, so that more or less emitter current flows, the base impedance will vary again.
cheers

[MKB]

You can easily increase the input impedance of a Fuzz Face by adding a common drain FET stage to the input.  I really like Fuzz Faces set up like this, it really helps humbuckers sound better with them as a 20K or so load on a humbucker sucks all the life out of it.  In this configuration it usually helps to have the pot mod between the FET buffer and the first Fuzz Face transistor stage.

[yeeshkul]

Brett thank you for the perfect answer!

R.G. i have read the article on FF circuit about 5 times there and back, but there are some points i need to have explained to understand the whole thing well, without making a twisted perception of how that circuit works, based on my own bad explanation of the occurrences  .

I also have another trouble with approach to the 50k input dump modification, which looks like both - empowering the input impedance of the FF circuit as well as empowering of the output impedance of the plugged guitar (voltage source - current source), that i noted in my last thread ( http://www.diystompboxes.com/smfforum/index.php?topic=53680.msg408844#msg408844 ).

Thank you guys!

[Gus]

MKB check the 3 transistor fuzz in schematics