Colorsound One Knob Fuzz issue

[PBE6]

I just built a Colorsound One Knob Fuzz from this schematic:


It works OK, but I'm having an issue with quieter notes and the note decay on louder ones - possibly a bias problem?

I made a few small changes to the schematic to start:
- added a 100R/1000uF power filtering section to the front end (will eventually be plugged into a 1 Spot, no battery)
- used 2n2222's for both transistors
- used 75R instead of 82R on the second transistor
- used 3.3uF instead of 10uF on the second emitter
- used 100nF instead of 220nF on the output
- used 250k instead of 500k on the output

My temporary power supply is giving 16.41 V at the plug. After filtering, it's 16.11 V.

The first transistor's voltages seem OK:
C - 2.09 V
B - 0.61 V
E - 0.00 V

The second transistor's voltages seem valid, but the collector seems a bit high:
C - 12.77 V
B - 2.09 V
E - 1.47 V

I've read that the second collector is usually biased closer to 1/2-supply for a standard fuzz, which would be about 8 V here. Is the Colorsound using a different bias point on purpose? Or is my fuzz just mis-biased? If so, what's the best way to correct it?

One other thought, could the issue just be an artifact of the high voltage?



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[PRR]

> I'm having an issue with quieter notes and the note decay on louder ones

What is the "issue"?

[PBE6]

Loud notes behave as I expect on the attack and sustain portions, but the fuzz dies out too quickly on the decay portion. Quieter notes don't seem to trigger any fuzz at all, but there's an abrupt transition to ratty fuzz on slightly louder notes.


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[PRR]

I would really try 9V and 10uFd first.

9V battery isn't a big investment. And 10uFd is already silly-small; what are you expecting with 3uFd?

The "half voltage" thing is OK when you are shooting blind, but isn't exact. Especially in a distorter.

And you will find that the Q2 C voltage will come down with supply voltage.

[PBE6]

Good points, I will try a battery tomorrow and see what kind of bias I get.

With regard to the bypass cap, doesn't it provide high gain for frequencies at or above the 1/2piRC cutoff, in this case 48 Hz? I thought that would be plenty low enough for guitar.


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[Gus]

The following threads might be helpful.  Run it at 9VDC with a battery. 
This is a good thread to read http://www.diystompboxes.com/smfforum/index.php?topic=114153.msg1059002#msg1059002
I tend to like about 1.6VDC and 7VDC with 2N2222 one knob fuzzes with a 9VDC supply
3uf is small I sometimes  use 100uf in a one knob

That 1/2 the power supply voltage at Q2 collector is not what you want with some effects

http://www.diystompboxes.com/smfforum/index.php?topic=113879.msg1063006#msg1063006
http://www.diystompboxes.com/smfforum/index.php?topic=113810.msg1056326#msg1056326

EDIT posted this in simulation http://www.diystompboxes.com/smfforum/index.php?topic=114594.0

A 2k and 330 is about the same total collector resistor value as 2.2k 82 and the 330 gives more output volume

[Electric Warrior]

Actually this circuit was originally biased to arout 1/2 supply voltage. The Colorsound version uses a smaller resistor on Q2's collector (2k2 instead of 4k7), which should bring up the voltage - and most importantly, volume. That's a good thing, as the original Jennings/Rotosound version was awfully quiet. I built a clone and left away the volume limiting resistor and it still barely reached unity gain.

In a silicon fuzz face style circuit, Q2's collector voltage depends mostly on Q1's hfe. The higher the hfe, the higher the voltage. Easy enough to tweak by selecting the right part.

[Gus]

Q2 voltage
http://www.diystompboxes.com/smfforum/index.php?topic=51351.5;wap2
I think this is the same look at the last post
http://www.diystompboxes.com/smfforum/index.php?topic=48885.60

[rutabaga bob]

For what it's worth, I built this one a few years ago using the schematic values and 2n2222s, and it sounded great!  Use 9 volts.

[PBE6]

I will give it a try!

Going back to my earlier question about the bypass capacitor size, I was reading RG's technology of the fuzz face article and he mentioned that the AC gain of the first stage is set by the AC feedback of the second stage through the 150k resistor. In my case, all the AC above 48 Hz should be heading to ground through the cap, so there's not much feedback and the first stage will have a high gain. Making the cap bigger just pushes the cutoff frequency lower, but shouldn't 48 Hz be low enough? Why is a larger bypass capacitor a good thing?


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[PRR]

> all the AC above 48 Hz should be heading to ground through the cap

Gain won't max-out until Z(cap) is similar to *emitter internal resistance*, not the bias resistor. There is a range where "some" of the AC is "heading to ground", a range of rising gain.

Emitter internal resistance is 26 Ohms at 1mA current. Here you are running near 1mA. On that assumption, 26r with 10uFd is 640Hz; 2KHz with 3uFd.

But-- we must also add the impedance seen at Q2 Base (around 10K) divided by hFE. This is maybe another 50 Ohms. So 75 Ohms total. 200Hz for 10uFd, 700hz for 3uFd.

These may be "appropriate" values for a fuzz. But I wonder, if you go too far with this low-cut, if it will be working on harmonics more tnan fundamentals. As harmonics phase crazily on pluck and decay, this may give unexpected sounds.

[PBE6]

Ah! Very interesting, thanks Paul.


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[PBE6]

So, I ended up replacing the 3.3uF cap with one of similar dimensions I had handy, which happened to be a large 220uF cap, and...bingo! Works like a charm, even with my high(er) voltage power supply. No more abrupt transitions, just funky fuzz from the bottom to the top.

I guess the cap really was too small to kill all the AC feedback to the first transistor as PRR pointed out, so I was inadvertently putting on the brakes for smaller signals. Interesting! Glad it's fixed though.

Thanks all for your help!


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