"As Seen On TV" Crunchy Boost

[rring]

I am sure my offering of "yet another distortion"  is emotionally overwhelming for all of us here on the forum.....but what the heck..I really like this circuit.

I cooked  this up over the weekend and made a pedal out of it. Is pretty simple to make and has a nice wide range from simple boost to medium overdrive. The tone control alters feedback so it provides treble boost and cut. It uses a couple junkbox transistors.
Lots of Mods possible...which I discuss in more detail at www.circuitsalad.com.

[Cozybuilder]

 I like it!!!! 

[idiot savant]

cool tone control!

looks a bit like the old Garnet:

http://amps.zugster.net/articles/tone-stacks#Garnet

[rring]

yes pretty much the same thing. The tonal response is a little different with the wiper going to ground but not much. My permutation has less treble cut at the low end of the control.

[tca]

Sounds good! What's the amp?

Cheers.

[rring]

its one of my various stomp amp designs....that one has a line out and boost footswitch.

there is a post about it here: http://www.diystompboxes.com/smfforum/index.php?topic=108799.0

[Kipper4]

Sounds great Ray good job.
This is I believe the first time I've seen feedback in a bjt circuit.
It looks so simple. Yet you seem to be using both emitter with tone shaping and collector feedback loops.
It must have taken a good amount of work to work out the biasing. Not a skill I could see myself ever doing.
Thanks for sharing
Marvellous.

[rring]

Yes the collector feedback is called shunt feedback and ensures you are always biased in the active region ..although not optimally..it also lowers Z on input and output(not much here)...R3 was chosen to reduce gain a little so that larger signals would not clip in the first stage. You can reduce this value some to increase the gain a little and or make it so the first stage clips a little.

The biasing:

I just started with 1 meg on the first stage to the base and chose a collector and emitter resistor combo that gave enough gain but still biased me near mid supply. R12 helps isolate the clipping diodes from distorting the output of the first stage when they start conducting. You can omit that resistor and it will change the sound some. For the biasing of the last stage.. I just had a bunch of 50k pots so I chose a collector resistor and emitter resistor such that I could get a good range of tone adjustment and then just set R9 until I was near mid supply.

My over all point is ...I chose some values based on experience and just tweaked the others until I got near 4-5v on the collector..So some rules of thumb and some experimenting will get you there pretty quick - with all the feedback and the generally consistent characteristics of BJT's these values should work with  any beef stew BJT.

You can use 2N7000 mosfets also. On the second stage just make R9 100K. On the first stage - I would make R4  2.2Meg and tie the high side of this resistor to the high side of the drain resistor(eliminating the the shunt feedback) and then add a 1 meg to ground from the gate (formerly the base).

[canman]

I like it!  And really nice playing, I could listen to that jam all day!

[rring]

Yes I rarely let a "song" interfere with my guitar playing -- I am wholesome, upstanding psychedelic meanderer. Consequently, a common mantra from my wife and kids is..."turn that down".  Years ago I did a show with a jam band where we played a 20 minute version of the theme song for the Brady Bunch...just to be clear - I am not bragging - I  am asking for forgiveness.

[Kipper4]

Yes the collector feedback is called shunt feedback and ensures you are always biased in the active region ..although not optimally..it also lowers Z on input and output(not much here)...R3 was chosen to reduce gain a little so that larger signals would not clip in the first stage. You can reduce this value some to increase the gain a little and or make it so the first stage clips a little.

The biasing:

I just started with 1 meg on the first stage to the base and chose a collector and emitter resistor combo that gave enough gain but still biased me near mid supply. R12 helps isolate the clipping diodes from distorting the output of the first stage when they start conducting. You can omit that resistor and it will change the sound some. For the biasing of the last stage.. I just had a bunch of 50k pots so I chose a collector resistor and emitter resistor such that I could get a good range of tone adjustment and then just set R9 until I was near mid supply.

My over all point is ...I chose some values based on experience and just tweaked the others until I got near 4-5v on the collector..So some rules of thumb and some experimenting will get you there pretty quick - with all the feedback and the generally consistent characteristics of BJT's these values should work with  any beef stew BJT.

You can use 2N7000 mosfets also. On the second stage just make R9 100K. On the first stage - I would make R4  2.2Meg and tie the high side of this resistor to the high side of the drain resistor(eliminating the the shunt feedback) and then add a 1 meg to ground from the gate (formerly the base).

Thanks Ray excellant explanation
Saves to favorites
Rich

[rring]

I Haven't carefully optimized the circuit by any means..perhaps someone can tweak some stuff and get some interesting sounds out of it. For example you could add  small cap across R6 and get some treble pre-emphasis or reduce R3 and by pass R6 and get pre-distortion, most likely non symmetric. The tone control  values may not be the best curve for guitar...if  anyone bothers to fiddle with it, hopefully some variations or improvements will result.

[rring]

Just an FYI...I tried this on a number of low output single coil pickup strats and found I needed greater range of drive...so I am posting a modded version of the schematic using two diodes, a little higher impedance on the input(larger emitter resistor) and less loading on the first stage from the second.

[Kipper4]

Cheers Ray this is definitely on my list

[Kipper4]

Debugging this right now

the single coil version

substitutions
1n914 instead of 1n4148s
addition
a reverse polarity protection diode from +9v to gnd

power supply
9.1v wallwart
after the 4148 or in my case 1n914 = 8.45v dropping approx 0.6v as I would expect


Q1    Hfe 275
C 4.5v
B 1.4v
E 0.8v


Q2  Hfe 260
C 4.2v
B 1.5v
E 0.9v

I doubt it makes much differance but I used linear pots for the tone and drive
and log for the level

problem
less than unity gain let alone any boost

[Kipper4]

Sorted
I had my C10 soldered to the wrong node.
Just a layout problem.
Works a treat now to test it on my proper amp.

[Kipper4]

Its all working as it should now.
This is a really nice booster with a classic rock/punk type crunch.
I've only built the single coil version so far but im liking it.
Sounds good with a solid stae amp and even better with a tube amp on the edge of break up.
Such a simple circuit too, kinda reminds me of the crunch box/box of rock.
Rich
I have a verified perfboard layout too now
edit BTW the transistor voltages stand.