Adjusting a volume only one knob FF like circuit, Colorsound etc. like

[Gus]

Something I have been working on

Adjusting one knob volume only fuzz face like effects. This is biasing only.

The one knob fuzz face like effect can be adjusted for different transistors and tones, however a greater understanding of the parts interaction is needed.

Having the gain always set to the maximum in a one knob fuzz removes PART of the the Q2 emitter to Q1 base resistor value interaction with the passive guitar and cable.  This resistor supplies bias current to Q1 and is also part of the  feedback network, with a "stock" FF  the gain control has more of an interaction with the passive guitar and cable before it. 
Q2s emitter follows Q1 collector 1 VBE drop lower.  This forms an inverting amplifier basically the same as collector to base feedback/bias, when the gain control is at minimum the input is as close to a summing node as possible with the limited gain of Q1.  The gain control wiper is a capacitor to ground.  As the gain is set higher the "bottom" section of the gain control is AC bypassed to ground, when the gain is at max the gain control is fully AC bypassed and the emitter node moves the least amount.

An one knob fuzz uses a fixed resistor at Q2 emitter to ground with a capacitor bypass to ground so the bias/feedback resistor value is mostly removed from the feedback gain setting. This makes adjusting an one knob fuzz  biasing a little easier.

All the resistors adjust the biasing of a FF like circuit not just the Q2 collector resistor which is a bad one to adjust because it changes the output resistance of the fuzz.
It is good design to keep the total value of the Q2 collector resistor(s) to a max of around 10K and a minimum of what you decide you like maybe 2K minimum.  Fix this value and work with the other parts of the circuit.

I like about 2.3K(2K and 330) total value for Q2s collector resistor.  Next what collector current do you want Q1 to operate at sometimes you can find a Beta vs collector current graph.  You might want to run the device where the curve is the flattest. I like 10K to 15K to start for Q1s collector resistor.

Simple tuning
Set the output resistance with Q2's collector resistors(s)

Measure the transistors collector to ground voltages of what you built

Q2 emitter resistor value increase, Q1 collector voltage goes down, Q2 collector voltage goes up

Feedback/Bias resistor value increase, Q1 collector voltage goes up, Q2 collector voltage goes down

Q1 collector resistor value increase, Q1 collector voltage goes down, Q2 collector voltage goes up

The following has collector voltage readings from sims using LT Spice.  Two different beta range transistors were picked.  With circuit like this the 2N2222 and 2N5089 models often are very close to a build of the circuit.  You can change more than one resistor value.
So if you build a fuzz face like circuit and you want different biasing make a sim and change the values to adjust the biasing of the circuit.
When you adjust a FF like circuit with a gain control and volume control there is something else to think about.



2N2222 2N2222
R1   R2   R3   collector   collector

8.2k   150k   1k   1.95VDC   6.07VDC   
10k   150k   1k   1.83VDC   6.32VDC
15k   150k   1k   1.65VDC   6.73VDC   like this
10k   47k   1k   1.47VDC   7.12VDC
10k   100k   1k   1.66VDC   6.70VDC   like this
10k   220k   1k   2.06VDC   5.82VDC
10k   150k   1.5k   1.82VDC   7.21VDC
10k   150k   2.2k   1.82VDC   7.78VDC

2N5089 2N5089

8.2k   150k   1k   1.50VDC   7.09VDC   
10k   150k   1k   1.47VDC   7.17VDC
15k   150k   1k   1.41VDC   7.29VDC
10k   47k   1k   1.37VDC   7.39VDC
10k   100k   1k   1.42VDC   7.27VDC
10k   220k   1k   1.53VDC   7.02VDC   like this
10k   150k   1.5k   1.45VDC   7.78VDC
10k   150k   2.2k   1.44VDC   8.17VDC

[Gus]

The first post is for adjusting circuits like the Colorsound one knob fuzz, Meathead etc.  You should be able to adjust values to make a number of different Silicon transistors work

This a adjustment I like using PN2222s.  Yes a 100uf across the emitter resistor.
http://www.diystompboxes.com/smfforum/index.php?topic=112880.msg1044061#msg1044061

[Kipper4]

Thanks Gus
I'll breadboard it when I get time.

[PBE6]

Great thread, very useful. Thanks Gus!


Sent from my iPhone using Tapatalk

[Gus]

posted a LT spice .asc file http://www.diystompboxes.com/smfforum/index.php?topic=114594.0
It is for DC voltages.  This should help people understand that changing the Q2 collector resistor is not the only way to adjust a circuit.
Add the caps and other parts and save as another file for a more complete circuit

[Lino22]

Why is that when we increase the Q2 emitter resistor, the Q1c goes up? Thank you

EDIT: obviously because the Vq1c = Vq2e+0.65