Cheap DIY Distortion and Compressor and Fuzz kits

[Marcel]

Thanks Doc, posting voltages would be a great help.

I've checked all the voltages and they seem okay, but that doesn't mean they are correct. Both the B-E voltages are at a fraction under 0.3V, however the collector voltages indicate either saturated or cut-off. With these transistors I might have t try a resistance wheel to Q1 base to find the right value to get Q2 collector at 4.5V

[DrNomis_44]

Thanks Doc, posting voltages would be a great help.

I've checked all the voltages and they seem okay, but that doesn't mean they are correct. Both the B-E voltages are at a fraction under 0.3V, however the collector voltages indicate either saturated or cut-off. With these transistors I might have t try a resistance wheel to Q1 base to find the right value to get Q2 collector at 4.5V

You could also try replacing the collector resistor (typically an 8k2) for the second transistor with a 10k trimpot, this will allow you to easily set the collector voltage to around 4.5V, the smaller value resistor (330 ohms or 470 ohms) can then be increased to 1k to increase the output level a bit.

As a sidenote, the collector voltage on the second transistor plays a big part in determining the sound you get out of a Fuzz Face, if it is around 4.5V, the result is a smooth squarewave sounding fuzz tone, below this the Fuzz Face tends to sound a bit sputter, which some guitarists tend to like, above 4.5V, the Fuzz Face gets really smooth, but you also lose a bit of sustain. most of the commercially manufactured Fuzz Faces tend to be biased so that they clip the signal asymmetrically, I seem to remember playing around with the Fuzz Face circuit, after breadboarding it, and managed to take some pics of the waveforms I saw on my Oscilloscope, I think I still have the pics on one of the slave HDDs in my studio PC, I'll post them here as reference.


I managed to find this one, if your Fuzz Face build is working and biasing correctly, you should see a waveform like this on the first transistor's collector, the waveform on the second transistor's collector should look like an inverted and more sharply clipped version of this waveform (assuming that you've set the Fuzz control to maximum):




Here's an interesting article about the technology of the Fuzz Face:

http://www.geofex.com/article_folder...ace/fffram.htm

[Marcel]

The exact results I got Doc are a bit hazy as I did measure lots of things lots of times, however 8.5V seems to stick in my head as the collector voltage on Q2. And I can't be sure but I think it was set there pretty hard.

When I drove the FF with the Comp I did get a asymmetrical waveform similar to the picture you showed. Mine was much more squared off in shape, and the squaring off varied slightly depending on the frequency. Lowering the signal generator input level could result in a sine wave pulsed shape, and with full gain and enough drive will result in a near on full square wave. So I'm sure the circuit works, but as you say it needs the right bias and where to set that bias to is the million dollar question. Seems like a Q2 collector of 4.5 to 8V would be the optimum range.

[DrNomis_44]

The exact results I got Doc are a bit hazy as I did measure lots of things lots of times, however 8.5V seems to stick in my head as the collector voltage on Q2. And I can't be sure but I think it was set there pretty hard.

When I drove the FF with the Comp I did get a asymmetrical waveform similar to the picture you showed. Mine was much more squared off in shape, and the squaring off varied slightly depending on the frequency. Lowering the signal generator input level could result in a sine wave pulsed shape, and with full gain and enough drive will result in a near on full square wave. So I'm sure the circuit works, but as you say it needs the right bias and where to set that bias to is the million dollar question. Seems like a Q2 collector of 4.5 to 8V would be the optimum range.

Yep, I agree with Q2 being biased anywhere from 4.5V to 8V, based on my experience from breadboarding the Fuzz Face circuit using the standard resistor/cap/pot values, to my ears, any Q2 collector voltage from about 4.0V to about 6.0V tended to produce a pretty good sounding Fuzz tone, for Silicon transistors the sweet spot seemed to be about 4.5V, and for Germaniums, it seemed to be about 6.0V, that's assuming that Q1 had an Hfe that measured around 82, and Q2 had an Hfe that measured around 120, I noticed that if you substituted a transistor with a higher Hfe for Q1, that increased the amount of sustain you got out of the Fuzz Face circuit.

The trick is to measure Q2's collector voltage with your multimeter probe directly on Q2's collector solder joint rather than where the 10nF output cap joins at the junction of the 470 ohm and 8k2 collector resistors.

[Marcel]

Ah ha !!!

I clicked on that link you posted Doc.... It is amazing how your perception of a circuit can change when it is drawn differently... and the explanation sure is a great help too...

[Marcel]

I had an Ah ha! moment there Doc.
Clicked on the link you posted -- It is amazing how your perception can change if a circuit is drawn even slightly differently.... and the explanation certainly helped too...

[DrNomis_44]

I had an Ah ha! moment there Doc.
Clicked on the link you posted -- It is amazing how your perception can change if a circuit is drawn even slightly differently.... and the explanation certainly helped too...

No worries, I like it when those little "Ah ha!" moments occur, especially when you've been working on something for a long time, makes it all worthwhile.

The Fuzz Face circuit is a deceptively simple circuit that functions in a pretty complex way, I think I've probably spent a lot of time analysing how it works from an electronic theory point of view.


There's nothing like having a decent and clearly drawn schematic available to help when it comes to faultfinding an electronic circuit.


Whether through deliberate design, or perhaps a happy accident, the way I see it, the Fuzz Face circuit appears to have been designed to behave in a non-linear way on purpose, normally a circuit designer would try and design the circuit to produce the least amount of distortion possible, the original designer of the Fuzz Face circuit is probably unknown.

[Marcel]

After reading that link, and stabbing my Fuzz kit with a DMM it didn't take long to work out that something was darstardly amiss with the installed AC128's. Nil volts across the B-E junction, Collector voltages (Wrt E) at saturation on Q1 and 1V on Q2. So out they came, to be tested on the DMM semiconductor mode. Yep, On both Q1 and Q2 the B-E and B-C junctions tested correct in both forward and reverse, and then I tested E-C and found their weakness. A weakness of 92 ohms in both directions on Q1 and 118 ohms on Q2. These guys don't need resistors to bias them up, the internal leakage does it all for you and to hell with your intended purpose or circuit.

I had on a previous occasion bought various Silicon transistors from Jaycar so I though maybe I could swap a pair of those into the pedal if only to prove a point. I found two 2N2907 PNP types that have a hfe of 75 at 10V/0.1mA so gave them a try. Voltages this time look far better - Q1 B-E at 0.57V and Q2 E-C at 3V which is much closer to the ideal range.

Unfortunately when I looked at a signal through the pedal on my TFT Cro I saw a horrible signal that didn't resemble al all the signal going in. Connected up an amp and was greeted with a woeful motorboating burble that refused to quit. Long story short I had made a rookie mistake - I was using a plug pack that has very limited filtering on the 9V out and with only the FF as a load the incoming 9V was basically just crap, adding a 2nd pedal to add load to the plug pack cleaned up the 9V and the FF.

Inspired I connected my test guitar and flicked the strings.... Yep! It's 'Fuzzing'...!... Not the best fuzz, better fuzz on the A and E strings than on the D and G strings. Quite dynamic, the fuzz depth is highly dependant on how hard the strings are played. Picking the test guitar up and with the pedals guts all over my bench I played a few notes, for about an hour.....just to get a feel for it... you know... sorta thing....

So the hunt is on.... for an even better replacement to the AC128's that let me down... Particularly as I'm not keen on spending lots of money on hundreds of AC128's to test so as to find the two 'good ones' needed ... the Jaycar 2N2907's Silicon PNP's will do until something better is found.

[DrNomis_44]

After reading that link, and stabbing my Fuzz kit with a DMM it didn't take long to work out that something was darstardly amiss with the installed AC128's. Nil volts across the B-E junction, Collector voltages (Wrt E) at saturation on Q1 and 1V on Q2. So out they came, to be tested on the DMM semiconductor mode. Yep, On both Q1 and Q2 the B-E and B-C junctions tested correct in both forward and reverse, and then I tested E-C and found their weakness. A weakness of 92 ohms in both directions on Q1 and 118 ohms on Q2. These guys don't need resistors to bias them up, the internal leakage does it all for you and to hell with your intended purpose or circuit.

I had on a previous occasion bought various Silicon transistors from Jaycar so I though maybe I could swap a pair of those into the pedal if only to prove a point. I found two 2N2907 PNP types that have a hfe of 75 at 10V/0.1mA so gave them a try. Voltages this time look far better - Q1 B-E at 0.57V and Q2 E-C at 3V which is much closer to the ideal range.

Unfortunately when I looked at a signal through the pedal on my TFT Cro I saw a horrible signal that didn't resemble al all the signal going in. Connected up an amp and was greeted with a woeful motorboating burble that refused to quit. Long story short I had made a rookie mistake - I was using a plug pack that has very limited filtering on the 9V out and with only the FF as a load the incoming 9V was basically just crap, adding a 2nd pedal to add load to the plug pack cleaned up the 9V and the FF.

Inspired I connected my test guitar and flicked the strings.... Yep! It's 'Fuzzing'...!... Not the best fuzz, better fuzz on the A and E strings than on the D and G strings. Quite dynamic, the fuzz depth is highly dependant on how hard the strings are played. Picking the test guitar up and with the pedals guts all over my bench I played a few notes, for about an hour.....just to get a feel for it... you know... sorta thing....

So the hunt is on.... for an even better replacement to the AC128's that let me down... Particularly as I'm not keen on spending lots of money on hundreds of AC128's to test so as to find the two 'good ones' needed ... the Jaycar 2N2907's Silicon PNP's will do until something better is found.

I just remembered that I have a whole heap of 60's Philco Surface Barrier Transistors in a couple of boxes in my old flat, I could grab them next time I'm at the old flat and then send some to you in the post if you like, I've tried some in a breadboarded Fuzz Face circuit and they do seem to work pretty well, and from what I've read online they are Germaniums, they don't have any part numbering on them though.

One thing you can do to cure the Motorboating is to put a bypass cap from circuit ground to the supply voltage rail, any value from about 47uF to 470uF should work fine.

[Marcel]

I've got box's and box's of all sorts of caps Doc, I was just too lazy to get off my arse and get one, it was simpler to plug in another pedal that was already sitting on my bench.... but I know what you mean, and it's probably a good thing to do any way to every pedal build to keep the 9VDC playing nice under any circumstance.

Those SBT's could be the go. Not many Germanium transistors available any more. All the major suppliers don't seem to stock them either. Let us know if you do find them. I know how easy it it to loose things, I've only had my big shed for 8 years and I'm always finding stuff that I thought I had lost or loosing stuff that I had in my hand only yesterday....

Even though the 2N2907's are silicon they still do a pretty decent job at creating fuzz. For the reasons given in the article you linked Doc their hfe is in the right range and their other characteristics like Max V and frequency response are reasonably similar or better to the published AC128 values. I didn't measure their hfe and I only had two of them so it was always going to be pot luck at getting a positive result. At uA currents the hfe is 75 but at mA currents the hfe increases to 100 so it is definitely in the ball park on that front. On some fronts I'm suspecting the whole Germanium thing might be a bit of a mental phurphy played out by some guitarists as these 2N2907's are doing a commendable job. Knowing now the FF theory from the article I can see how the BC108 NPN crowd grew their legs, so to speak...