A Christmas Present

[mac]

I made some changes and uploaded the Layout and PCB in my gallery, mac --> my circuits
Please check it and tell me if you find some errors. If OK then I'll post the images here.

- Fuzz pot wiring is different
- Q2 by-cap can be 2u2 or 4u4
- NPN or PNP, and Silicons

mac

[nocentelli]

Just tried the original schemtic with Si BJTs (2N2222s and BC107s - the lowest gain I have). I increased the collector resistors to 15k which gave very close to 4.5v with any transistor I tried. I added an input cap blend bass control in front and a simple treble roll-off at the end and it sounds great! Many thanks.

[Kipper4]

Care to knock up a schematic of the changes you made please nocantelli please?
thanks

[Gus]

Just tried the original schemtic with Si BJTs (2N2222s and BC107s - the lowest gain I have). I increased the collector resistors to 15k which gave very close to 4.5v with any transistor I tried. I added an input cap blend bass control in front and a simple treble roll-off at the end and it sounds great! Many thanks.

I would adjust one or both voltage divider bias resistor values and not the collector value.

Don't get stuck with the 4.5VDC collector stuff you read on the web.  Try lower and higher with circuits like this.  Did you read my post earlier?

[nocentelli]

This?

I sometimes like the collectors at a lower voltage I have a two stage fuzz (+9VDC, 2n3904s)with the collectors at about 2.5VDC

I'm not hung up on 1/2 supply as the ideal collector voltage, i tried a range of voltages on the collectors: around 4-5 volts sounds good, so I just posted 15k for 4.5v to give people an idea of what to expect with silicon. I also tried the 33k instead of 22k for the base-> ground that mac already suggested. You get the same voltage, i.e. b=22k/c=15k and b=33k/c=10k both yield 4.4-4.7 with all the silicon transistors, and it sounds the same. I know you've mentioned before it's not the best way to alter bias, but I forget why (stability? noise?). I will try to rig up a switch to see if it's actually the same sound - Should be possible with a 6PDT rotary to add/remove parallel resistors on all three BJTs.

Here's what I have on the breadboard, I think it sounds good:

[Kipper4]

Cheers dude. Appreciated

[mac]

This are my Layout and PCB, unverified,






Leo,
Either way will sound the same, but 15k at 4.5v will draw a little less current.
I set Q2 at 6v.
The treble cut is a nice addition.
I used B to C 100pf caps instead, and 2n2369.

mac

[Gus]

This?

I sometimes like the collectors at a lower voltage I have a two stage fuzz (+9VDC, 2n3904s)with the collectors at about 2.5VDC

I'm not hung up on 1/2 supply as the ideal collector voltage, i tried a range of voltages on the collectors: around 4-5 volts sounds good, so I just posted 15k for 4.5v to give people an idea of what to expect with silicon. I also tried the 33k instead of 22k for the base-> ground that mac already suggested. You get the same voltage, i.e. b=22k/c=15k and b=33k/c=10k both yield 4.4-4.7 with all the silicon transistors, and it sounds the same. I know you've mentioned before it's not the best way to alter bias, but I forget why (stability? noise?). I will try to rig up a switch to see if it's actually the same sound - Should be possible with a 6PDT rotary to add/remove parallel resistors on all three BJTs.

Here's what I have on the breadboard, I think it sounds good:

You need to think about Ohms law and Ic vs hfe and a gain stage output drive to start.  There is more to know if designing a gain stage like this

[Kipper4]

Please expand on this Gus.
It's something i would be more than willing to learn.
Thanks

[dwmorrin]

I know you've mentioned before it's not the best way to alter bias, but I forget why (stability? noise?).

Regarding changing the collector resistor, this changes the gain and output impedance of the stage, in addition to the quiescent collector voltage, within a certain range.  Make it too big, and you'll saturate the transistor and make the output impedance unreasonably high.  Too small, you lose your gain, and may bring the voltage too close to V+ to have any ac swing.

The voltage divider to the base are the textbook "bias" resistors.  They factor in dc stability.  The rule of thumb is their two values in parallel should be 1/10 the input impedance at the base of the transistor.
Put more plainly, lower bias resistors = more stability.  Unfortunately, this also means that they will load the input signal.  So a compromise has to be found, or a trick like bootstrapping.

If using 2N2222 transistors in low current circuit, the datasheet suggests a worst case beta of 35.  The 3k3 emitter resistor gets multiplied by beta for the input impedance, so 115,500, or 115.5k is the worst case, dc input impedance.

The 100k and 22k divider is about 18k in parallel.  Comparing 115.5k vs 18k, you're just over the recommended 1/10 rule, and this is a worst case scenario, so the circuit should be stable.

4.5V at the collector is great for class A, clean boost... you're just giving yourself maximum swing room before hitting the walls.  If you're going for distortion, all the rules go out the window.  Plus, 4.5V is just a dc voltage reading... doesn't convey any "distortion" information.  A scope measurement is much better to see the symmetry or asymmetry of your distortion, should you desire to tailor this effect.

[mac]

For DC stability I make the base current less than the current through the voltage divider resistors, 1/10 or lower.
AC is another story. The bycaps (bypass caps) make the input impedance very low so I compensate with a series resistor of about the same value of the previous collector resistor.

mac

[mac]

A couple of samples.
Guitar to TH to Laney LC30II clean chn to dell mini mic... sorry for pc vibrations
Fuzz at 3.

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH1.mp3.html

Talking about ZZ and rush

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH2.mp3.html

mac

[kingswayguitar]

A couple of samples.
Guitar to TH to Laney LC30II clean chn to dell mini mic... sorry for pc vibrations
Fuzz at 3.

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH1.mp3.html

Talking about ZZ and rush

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH2.mp3.html

mac

thanks
nice clips
would sound divine through a super lead or bassman!

[jrod]

A couple of samples.
Guitar to TH to Laney LC30II clean chn to dell mini mic... sorry for pc vibrations
Fuzz at 3.

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH1.mp3.html

Talking about ZZ and rush

http://www.aronnelson.com/gallery/main.php/v/mac/sound+demos+of+my+circuits/TH2.mp3.html

mac

Sounds GREAT, mac!!!