First attempt at coming up with something original. How'd i do?

[HeavyFog]

I've been getting bored of building just clones and whatnot lately so i thought i'd change that by coming up with something more "original". This is what i came up with after an hour or two.



What i was going for was sort of a "fuzz overdrive" type sound.I used a modified ehx muff fuzz for the front end, a fender tonestack for eq, and a simple boost at the end based of the lpb1 and rangemaster for a full boost. I an no genius when it comes to this stuff so the changes i made i made based off what i heard would change the sound. I brought the collector resistors on the front end down to try and reduce gain a bit (maybe i overdid it?) and i'm hoping i did the gain pot right.

Haven't picked transistors yet (i know resistors on the output stage especially the base bias resistors will need to change accordingly) but what i was thinking was maybe using two 2N5088 or 2N5133 for the fuzz part, and maybe a BC108c or a BC109b for the boost since i have a few i haven't used yet. Was thinking of changing the tonestack as well so the mids aren't overly scooped. I wouldn't mind changing pretty much all values and i'm looking for suggestions or ways to improve this, since chances are this probably isn't so great to begin with. I will be breadboarding it as soon as i get parts and confirmation that what i have works at least mildly well and isn't some disgusting mess.

Any ideas or opinions?

[merlinb]

First glance, looks good! You'll want coupling caps to isolate the input and output of tone stack though, or you'll get rustling sounds and the bias will wander when twiddling the pots.

A resistor in series with the PSU, and a protection diode, wouldn't go amiss.

[antonis]

What i was going for was sort of a "fuzz overdrive" type sound.I used a modified ehx muff fuzz for the front end, a fender tonestack for eq, and a simple boost at the end based of the lpb1 and rangemaster for a full boost.

And you have the f$&%ing nerve to call it "original".. 

I brought the collector resistors on the front end down to try and reduce gain a bit (maybe i overdid it?)

Gain of Q1 grounded emitter amp can't be changed because of lowering collector resistance leeds to raising collector current which leeds to lowering r_e (intrinsic emitter resistance) which leads to keep Rc/r_e ratio constant.. 
(another way to see it is Gain = - gm x Rc, where gm = 1/r_e

I don't know if altering Q1 collector resistance leeds to severe mis-bias of Q2 (don't know the original values) but, as long as, Q1/Q2 gain is primarily set of negative feedback path, maybe you should set Q1/Q2 overal gain from feedback resistors..

Also, without any calculation, 47μF emitter by pass caps seem a bit big to me...
(but, breadboading will show the truth..)

[HeavyFog]

IL definitely add some coupling caps to the tonestack. Probably some value between 0.1uf and 1uf. As for changing the gain of the fuzz stage IL fix the collector resistors. I'd like to lower the gain a bit to make it a bit more like an overdrive, and bias the 3rd transistor properly (bc108c or bc109c).

So far so good

[duck_arse]

some people building fuzzfaces (I don't know what made me think of that circuit) or the like, put a small value R in the Q1 E to ground, to reduce the gain a little. maybe you could gain-control the Q1, and run the fuzz pot flat out all the time. maybe. also, you might need a small pF value cap between Q1 b and c to snub the highest freqs. and an input pulldown resistor. and reverse that input cap (+ to the transistor).

[merlinb]

I'd like to lower the gain a bit to make it a bit more like an overdrive, and bias the 3rd transistor properly (bc108c or bc109c).

Try a resistor directly in series with the audio input (10k to 50k) this lowers the gain and has a really useful smoothing effect on the fuzz.

[HeavyFog]

Alright here's an updated schematic



Here's what i changed:
-added 100ohm resistor to emitter of Q1 to lower gain
- lowered emmiter bypass cap to 20uf
-added a 1uf and a 0.39uf coupling cap before and after respectively to the tonestack
-added reverse polarity protection diode
-determined transistors for q1,q2,q3

I don't think il cut out any treble with that cap on q1 since the tonestack should help sort that out. I just hope there's enough treble to begin with to make a two band tonestack worth while.

[Bill Mountain]

My only thought is that the last stage might load down the tone stack.  You may want to scale the tone stack down or maybe raise the impedance of the last stage to avoid loading.  Perhaps an FET boost?

[merlinb]

My only thought is that the last stage might load down the tone stack.  You may want to scale the tone stack down or maybe raise the impedance of the last stage to avoid loading.  Perhaps an FET boost?

Good catch. Remove the 47uF emitter capacitor to increase input impedance. Gain will drop of course, but you probably don't need as much as you currently have.

[HeavyFog]

That's a good point. IL take it out all together.

Also forgot to mention I raised the value of the collector resistor on q1

[antonis]

Remove the 47uF emitter capacitor to increase input impedance. Gain will drop of course, but you probably don't need as much as you currently have.

Also good catch...!!

To go a bit further, he can lower enough the resistors of voltage divider (for an even stiffer bias) and bootstrap the base from emitter with a resistor of aproximatelly the value of divider resistors in parallel and a capacitor of lower impedance compared to the bias resistors in frequencies of interest..

[PRR]

I don't recall THAT tonestack in any Fender. (Have seen it in other brands.)

If you steal directly off a Fender, you do not need a coupling cap IN to the tonestack. You need one coming out, unless you are going to a ground-biased tube or JFET.

A fine point: The input cap is drawn backward. The "+" side here must go to the transistor. The symbol for electrolytic cap has a "curved plate", which used to be the jar, then the can, and is supposed to represent the "-" side. Since nobody remembers this, I prefer to use an explicit "+" mark. (Which is odd because on most caps, the "-" is marked.)

[Kipper4]

I'd say your doing good.
You should have seen my first attempt iirc a preamp I called the Piglet. More like a pigs ear.
If it work great. Well done.
The way many of us learn is using those previously designed building blocks. Or stamps as RG terms them and pair them together to get to where we want to be.
The help the forumites offer on how to better pair them is invaluable for learning.
Such a good place and people.
Rock on.

[merlinb]

I don't recall THAT tonestack in any Fender.

Bandmaster, brownface Pro-Amp.

[HeavyFog]

IL watch out for that input cap when I breadboard it. Good point with the coupling cap il take the one out.  As for the tone stack I heard that this one inpaticular was nice and a bit different than the blackface tonestack. Wasn't fond of the baxandall stack so this one seems like a good option

[antonis]

IL watch out for that input cap when I breadboard it.

Maybe you'll have to take in mind the shunt-series feedback in combination with the not anymore grounded emitter, for it's value...

But, after all, breadboarding different caps should be easier than calculations...