distortion/boost question

[spenwah]

So I've been fooling around with my first pedal build, the Tweak-O from Small Bear. It uses diode pairs shunted to ground to create distortion. Today I added a switch to take the diodes in and out of the circuit. I expected this to turn it into a "clean" boost. I'm really pleased with its sound now, but to my surprise there is still quite a bit of distortion when I hit the switch. With the diodes out of the circuit, it's a subtler distortion that responds really well to my dynamics. With the diodes in the circuit, the tone is about the same regardless of the volume knob on my guitar.

Looking at the schematic here http://www.smallbearelec.com/Projects/TweakO/TweakO.htm, my switch removes C3, D1, D2 (and the third diode I added), and the pot R3 from the circuit. My question is, where is the distortion coming from after I remove the clipping stage? Is it the transistor?

I ordered all the parts to build the AMZ Mosfet boost last week. Should I expect it to be a bit "cleaner" than the Tweak-O sans diodes? Is there an easy explanation why?

Thanks

[newfish]

I'd say yes - the distortion is coming from the transistor.

From my meagre experience, the components you took out were providing a feedback loop - feeding a varied amount (through R3) of output *back in* to the circuit for more gain.

The diodes here (as always?) are the clipping part - while they don't provide any gain on their own, they *do* square the signal off - giving a 'harsher' tone (subjective description of tone here...) - so instead of your signal being a sort of rounded waveform, it's now got square shoulders.

Looking at the scematic, I'd say if you wanted to clean up the sound, try changing R6 or R5 (maybe to trim pots).
By changing the amount of current available for the Transistor to work with, you can vary the amoung of gain it will produce.

The best advice I can give would be to build a simple one Transistor booster on a breadboard and change a few of the components to see what each one does.

We're only dealing with 9 volts here, and a fried Tranny isn't the end of the world...

Best of luck with this buld -

[Caferacernoc]

Yup. The transistor distorts too. See the recent post about the "clean boost hoax". Some are cleaner than others. But most designs for distortion boxes want distortion so the transistor or opamp is set up for maximum gain. Then the diodes add more distortion and "shape" the distortion depending on what diodes are used, how many, symmetrical or asymetrical......

[earthtonesaudio]

Just wanted to add my two cents...
Yep, distortion coming from transistor.
Also:
The feedback loop from collector to base actually reduces gain.  More resistance (up to infinite) means more gain.  When the diodes conduct, they have some resistance that is probably a lot less than what goes through R1, R2, and C4.  That's why removing the diodes increases the level.  At lower voltages, the diodes don't conduct, and the threshold voltage is a rather abrupt change, which eventually causes the waveform to sound more distorted.

My suggestion to get a cleaner sound would be to remove C5.  Doing that will reduce gain drastically.  If you want it to be a little more "tweakable" then swap out R4 with a pot (value anywhere from 1k to 10k probably should work) and connect C5 to the wiper (like a Fuzz Face style "gain" control).

[MikeH]

It uses diode pairs shunted to ground to create distortion.

FYI- this isn't really true.  Take a look at a schematic for a Dist+ or Proco Rat and you'll see what you've described.  The gain control here just seems to be varying the amount of clipping (almost looks like it creates xover dist?) in the feed back loop (I think; anyone want to confirm?).

[earthtonesaudio]

The first part here (inverting clipper) is helpful:
http://www.bteaudio.com/articles/TSS/TSS.html

...You can just pretend the inverting op-amp is a transitor.

[spenwah]

Thanks for all the tips and pointers. I'd like the box to function as a switchable distortion/boost (although I'm pretty happy with it as a two voiced distortion right now), so I may try also removing C5 with the switch as earthtonesaudio suggested. Or maybe adding a second switch specifically for C5. Before I go ahead and do this, can you predict how this will affect the output level of the pedal? Does less gain mean less output?

What makes the volume decrease as I turn the gain knob R3? My impression is that as I turn up the gain knob, resistance after the clipping diodes decreases, and more clipped signal is mixed with the input signal through the feedback loop.  Is this accurate? (Why should the two signals be mixed instead of added for more output?) Also, what is the purpose of the ground connection between the 1meg resistors in the clipping stage? I thought this meant the diodes were shunted to ground, but now I see the difference after looking at the Dist+. What happens if that ground connection is removed, or if R1 no longer connects to C3 and R2 no longer connects to R3?

Thanks for all the learning

[Caferacernoc]

The reason the volume decreases as you turn the gain knob is because it's not really a gain knob in this pedal. That knob just turns on the diode clipping which also acts as a limiter as it clips off the tops of the signal for the distortion.