Originally Posted by Sausage
A simple google search of "Fuzz Face input impedance" Should lead you to many links that can explain it in much better terms than I could ever hope to. Perhaps you should read them and post your theories. Here is one result for example.
Pickup loading. This is a very important aspect in the creation of a great fuzz sound, but not one that people usually think about. Most fuzz pedals typically have very low input impedances.
I don't think the input is 100k as you have suggested. However most other pedals seem to have 100k input impedance.
There seems to be confusion with terminology as to what the input of the Fuzz is seeing and what the actual input of of the Fuzz circuit is. For example A passive guitar puts out about 10k which is considered high. However that same 10k is considered low for a pedal like the Fuzz Face, because pedals normally have 100k. Perhaps you can clear this up.
I don't think it is his Lehle that is causing such a drastic drop in the signal going to his interface. And I am guessing that it is the Fuzz that is loading down his guitar. As I suggested if he replaces his Fuzz with any other 9 volt stomp box I am pretty sure he will not have a problem. I suppose the only way to find out is if the OP tries it and posts his results. Thanks.
An even better simple google search for Fuzz Face Schematic revealed the true answer. Fuzz Central -- Arbiter Fuzz Face
Look at the schematics and you will see that the input signal in the original model comes in on the wiper of a 500,000 ohm pot. This means that the input impedance changes and this changes the "load" the guitar pickups are looking into. With the pot all the way ON the input impedance will be 500,000 ohms (high impedance). When the pot is turned down it can lower the impedance all the way down to lower than the output impedance of the guitar pickups.
On later reissues they lowered the pot value to 100,000 which puts an even greater load on the output of the guitar and probably explains a lot about why the reissues don't sound like the originals, they load your guitar down more.
Lets assume a reissue Fuzz Face with an input impedance of 100,000 ohms (when input control is wide open) and lets consider a Jensen 12:1 direct box transformer with an input impedance of 140,000 ohms, when you parallel these inputs (by going thru the jacks on the DI box) the impedance the guitar will see is going to be much lower than 100,000 ohms (approx 58333 ohms) and the lower it goes the more the tone of the guitar pickup will be changed for the worse. So if the input level control on the fuzz was only about 10% of the way ON and we assume it's a log type pot (approx 20% of total pot value at 10% excursion) then you would be paralleling 20000 ohms and 140000 ohms for a total load to the guitar pickups of approximately 17500 ohms, that's a very low impedance for a passive guitar pickup to look into, the voltage output will collapse significantly and the tone is going to have a lot of high frequency removed as a result.
Here is the formula for combining two impedances: Parallel: RT = (R1R2)/(R1 + R2)
Here is more engineering data to consume if you want more knowledge: http://pr.erau.edu/~newmana/imped.html
Here is this from the Seymour Duncan guitar pickup web site:
Pickups can generally run from 2.5 K to 20 K DC resistance on average. 726-750 - Seymour Duncan
A guitar pickup is a very feeble producer of electricity, it makes voltage but not current. For the pickup tone to NOT BE CHANGED by the load it's connected to the input impedance of the load should ideally be ten times the output impedance of the guitar pickup.
You don't need to get a degree in electrical engineering to learn a few things about combining impedance and loads, John Worams "Recording Studio Handbook" was dealing with these issues back in the 70's.
No clue where the fuzz builder got his impedance ideas but I'm always open to learning.
From the Boss website, I looked at the classic DS-1 Distortion pedal and found the input impedance to be 470,000 ohms (high) and the output imedance to be 1000 ohms. 1000 ohms is low in terms of tube amps but rather high in solid state world, I'm guessing they specified 1k because they want you to not drive it into anything lower than 10K (which is 10x the 1k output impedance). Tascam did this a lot with their specs, they would show the output impedance to be 10k even though if you looked at the schematic you'd see it was actually much lower, they kept the spec high because the unbalanced -10dBv outputs were incapable of driving into the +4dBm inputs of pro equipment made in the 70's and 80's.
The original Tube Screamer (designed in 1979-1980 by S. Tamura, a Japanese Engineer working for Nisshin Onpa/Maxon) has an input impedance of approximately 500000 ohms also. Note that a high output Seymour Duncan humbucker with an output impedance of around 14000 ohms would be very happy driving into 500000 since 10x the 14k pickup output impedance is only 140000, far below the 500k input impedance.
If you plug two stomp boxes (that have 500k input impedance each) in series and if they don't have true bypass switches then the combined impedance the guitar pickup will see is still 250k which is higher than the 140000 we calaculated previously. Now, plug four of these stomp boxes in series and the combined load to the guitar drops to 125k, now we're below the 140k and we're going to start hearing rolloff in the highs and overall less clarity from the pickup. Hope that helps you understand "loading down the pickups".
All this impedance stuff is moot if you have a guitar with active pickups on it, in that case the solid state output will natively be very low impedance and probably be brought up by a series resistance of some sort. An active pickup should have no problem driving a load impedance as low as 10000 ohms, perhaps lower.
Good luck and good music to all.