Colorsound Overdriver/Powerboost circuit "analysis"

[nocentelli]

http://www.beavisaudio.com/schematics/Colorsound-Overdriver-Schematic.htm

I've been playing about with this on the breadboard and have a few questions: First off, how does the EQ section work? At a glance, it looks Baxandall-esque, but there are no bass caps across the bass pot terminals, nor is there any connection to ground. Instead, the bass+treble lug1s are connected to the Q3 collector - Is this a form of negative feedback?

There is a 150k resistor from the Q2e to Q1b, which i assume is for "voltage feedback biasing" like the fuzzface, but there is also a 12k resistor from the output of Q2 back to the emitter of Q1: What does it do? It looks like some kind of "feedforward", maybe blending in clean signal with the output of Q2...... 

[PRR]

R5 R6 C6 and Vbe(q1) set the _DC_ bias point.

R3 VR1 set the _AC_ (audio) voltage gain.

No caps on Bass pot means this knob affects "all" frequencies, over-ridden in treble by the treble network.

[nocentelli]

Thanks! And C10?

[Electric Warrior]

I've been playing about with this on the breadboard and have a few questions: First off, how does the EQ section work? At a glance, it looks Baxandall-esque, but there are no bass caps across the bass pot terminals, nor is there any connection to ground.

There is a cap across the bass pot terminals.

[nocentelli]

Ah, ok, it's missing from these two schematics:

http://www.beavisaudio.com/schematics/Colorsound-Overdriver-Schematic.htm
http://fuzzcentral.ssguitar.com/schematics/overdriverschem.gif

[R.G.]

I've been playing about with this on the breadboard and have a few questions: First off, how does the EQ section work? At a glance, it looks Baxandall-esque, but there are no bass caps across the bass pot terminals, nor is there any connection to ground. Instead, the bass+treble lug1s are connected to the Q3 collector - Is this a form of negative feedback?

It's drawn incorrectly. It's a feedback style Baxandall. Q1/Q2 are a feedback-stabilized amplifier running at a fixed gain. Q3 is the amplifier supplying the gain for the Baxandall. And 8-pin DIP could be used to re-engineer it.

[PRR]

> Q1/Q2 are a feedback-stabilized amplifier running at a fixed gain.

User-adjustable from 2.2 to "infinity". Except at the limit, a very solid stable stage, no "magic", opamp could do the same.

> And C10?

You said it looks Baxandall-esque, so I assumed you knew a Baxandall. C10 couples signal but blocks DC back into the tone network/pots. As R.G. is saying, that's a great place for an opamp. So the messy discrete-BJT wire-up does reduce to an 8-pin chip and maybe half the resistors; a few less caps if you really go minimal.

[Gus]

A thread
http://www.diystompboxes.com/smfforum/index.php?topic=93814.0

[intripped]

my powerboost clone doesn't sound good when gain is turned up: kind of sputtering distortion, clearly hearable when the notes decay.

searching around the net i've discovered that this is very common and it could be because of misbiased transistors - the suggested solution is to change the value of the collector resistors in Q2 and Q3, in order to set the collectors voltages around 1/2 the supply voltage (4,5V if you're feeding the circuit with 9V)

already tried this solution, but it doesn't work very well: there is a little improvement, but the sputterying is still there.

i'm just making a guess here, but i think that changing the DC bias point of Q2, and moving it more towards the saturation zone, could solve the issue (i think that the sputterying is due to the AC signal hitting the cut-off region of Q2, when the signal amplitude is increased with the gain pot).

does this guess make any sense to you? and what components should i modify?

thanks

[Davelectro]

Maybe I'm wrong, but I think the problem with this circuit lies in the power supply, since it was actually designed to be operated at 18V. The only thing they changed for the 9V version was the color of the enclosure.  

In my experience, biasing Q2-Q3 at 4.5V (or 5V, as in the original factory schematics) won't help at all.

The "overdrive" zone is kinda ugly, but the clean range is great and at max gain you get a magnificent fuzz. Powerful yet articulate.

[deafbutpicky]

I've come across some schematics where the electrolytic cap on Q3 is drawn in the wrong
direction, (Beavis Audio and madbean cherrybomb for example), so make shure it's + is on
the collector side. Had the same problem and symptoms and this solved it.

[intripped]

thanks for your replies!

i'm supplying my circuit with 18V (well 17,4V actually) through a charge pump, so it's not a power supply issue
...and maybe the pedal is exactly like this: ugly "overdrive" zone and great clean boost and fuzz

i've also checked the orientation of that electrolytic cap on Q3 and it's correct: + to the collector

anyway, i would like to give a go on changing the DC bias point on Q2 ...but i'm not big on theory (not at all!) and also Q2 has not the typical biasing net, so i definitely need some help

R5 R6 C6 and Vbe(q1) set the _DC_ bias point.

what resistor should i increase or decrease in order to move the bias point of Q2 towards its saturation region?

[yeeshkul]

Guys would a happy owner of an original 18V Power Booster kindly help me with the collector voltages?

[Electric Warrior]

Battery: 18.84V
Q1 C 6.43V B 3.92V E 3.54V
Q2 C 11.53V B 6.44V E 5.82V
Q3 C 10.19V B 2.77V E 2.23V

Since the factory schematic calls for 5V for Q2 and Q3's collectors for the 9V version, I guess they were aiming for 10V with 18V.

[thehallofshields]

> Q1/Q2 are a feedback-stabilized amplifier running at a fixed gain.

User-adjustable from 2.2 to "infinity". Except at the limit, a very solid stable stage, no "magic", opamp could do the same.

An Opamp would really clip the same as a BJT here?
What would the configuration look like?

[R.G.]

> Q1/Q2 are a feedback-stabilized amplifier running at a fixed gain.

User-adjustable from 2.2 to "infinity". Except at the limit, a very solid stable stage, no "magic", opamp could do the same.

An Opamp would really clip the same as a BJT here?
What would the configuration look like?

>>> Except at the limit <<<
Clipping is by definition at the limit. Of course an opamp would not clip like a BJT. The power boost overdriver is intended to beat an amplifier's input tube grid solidly about the head and shoulders. It's not intended to make distortion on its own so much as to overdrive something after it.

The configuration would look like one opamp replacing the Q1/Q2 interlocked gain stage with a fixed gain, and another opamp replacing Q3 for the Baxendall. The biasing and gain setting resistors would be entirely different, chosen to make the opamp stages have the same gain as the transistor stages they replace, and to be biased in the middle of the 18V power supply. It's a redesign to the intent of the original circuit, not necessarily preserving anything of the actual original circuit.

[Paul Marossy]

I'm going to be building the Overdriver just for fun. Doing a little bit of research, I have found the following discrepancies between the original factory schematic and some of the ones on the interweb:

1 - Bypass cap on Q1 is 25uF, not 10uF
2 - The coupling cap on the collector of Q1 is 6.4uF, not 4.7uF
3 - The 0.01uF cap across the bass pot is missing on the non-factory schematics
4 - The cap from collector to base on Q1 is 200pF, not 500pF
5 - The coupling cap on the collector of Q3 should have the positive terminal of the cap going to the collector AND this cap should be 25uF, not 4.7uF

According to the factory schematic, there should be 5V on the collector of Q2 & Q3 and they should have 1V on the emitter. I don't know how many of these few items may contribute to the sputtering sound some people have reported.

I'm going to build mine with as close to the factory specs as possible and find transistors that will have the same measurements, well as close as I can get anyway.

[PRR]

> 3 - The 0.01uF cap across the bass pot is missing on the non-factory schematics
> 5 - The coupling cap on the collector of Q3 should have the positive terminal of the cap going to the collector..

Yes, yes. The bass pot is not a bass control without the cap. Useless knob. The + side of that cap is surely the Q3 collector side; this could explain sputtering (cap breakdown).

These mistakes should be fixed. Thanks for highlighting them.

> 1 - Bypass cap on Q1 is 25uF, not 10uF
> 2 - The coupling cap on the collector of Q1 is 6.4uF, not 4.7uF
> 4 - The cap from collector to base on Q1 is 200pF, not 500pF
> 5 - The coupling cap on the collector of Q3 ...should be 25uF, not 4.7uF

These are less critical. If this were a studio mike preamp Q1 Ce would be large to allow full bass at maximum gain. I'm less sure that guitar needs 55Hz response at high gain. #2 (you mean Q2?) should be much more than 2uFd (to pump R7) but 5 or 7u is whatever falls to hand. I could argue that C10=C5, or C10>>C5, and don't think it matters much. C4 limits gain to 1.1MHz or 2.8MHz, and even my dog does not care.

[Rob Strand]

I've got this original (which doesn't need the "fixes")





The "other" schematic has some translation errors.

Note also some schematics have the electrolytic on TR3 around the wrong way.  The -ve should be towards the tone control for both electros.

I'm assuming the bass-cap as 100n is correct.
[Edit: One PCB shows 100nF.  It has two 100nFs on the board.]

Some 2M2 de-pop resistors wouldn't go astray on the input and output.
Actually my factory schematic is missing the 100k log volume pot at the output (so no 2m2 required on the o/p).
[Edit:  Sorry unit is only 3-knobs.]

[287m]

Rob, correct me
in colector Tr2, 6.4u? or my eyes plays trick on me?