OT: distortion won´t run on 9v

zipzap · Posted: Mon Apr 02, 2007 5:02 am Post subject: OT: distortion won´t run on 9v

Hi people
this is not a real synthmodule, but a nice project. distortion can be used anywhere and i like this one. input expects quite weak signals, so for a modular some values in the first stage should be changed. output is around 1,2v for driving a normal mixing desc.
I promised a friend to build him a nice distortion box for his guitar (he plays 3 e-string guitar in a drum and bass band)
So i tried to do my own design, first time using discrete parts instead of opamps. i designed this using a 15v supply and it sounds great. but with 9v or 12v i don´t get much sound, only some "farting" when i hit the strings quite hard...
the scan is quite bad, i hope you can see anything.
i never really worked with transistors or fets, so most values are borrowed from other schematics or just guesses. i´ve used what i had laying around, guess some things could be improved. all decoupling cabs are 10uf elec. Propably to big, smaaler mkps would sound better i guess.

the function is like this:
first gainstage with LED distortion. sounds quite tubish, rather subtile at low input. the second stage uses 4 standard silicon diodes in the feedback. Perfect heavymetall. Switch 2 selects two modes. normal and a slight gainboost (highboost) with noisgate. that´s when the cab c7 is closed around the 1k resistor and the switch 2b over the diodes is open. sounds perfect.

Then there is switch 1 to select witch stage is going to the output buffer.
the supply goes through a gyrator because this is quite sensitive to noise from those cheap psus

c8 will be around 10-12nf, taming the highs a bit.

any suggestions? obvious mistakes? i wonder why this wont run on 12v.

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Uncle Krunkus · Posted: Tue Apr 03, 2007 3:08 am Post subject:

I'm not sure Zipzap, but I'd like to find out what others might have to say about it. Smile

bugbrand · Posted: Tue Apr 03, 2007 3:24 am Post subject:

I don't really know about the bit you've got on the power input --- is that a type of regulation circuit? What's the output voltage from that stage.?.

Other than that it sounds like a biasing problem maybe? (hmm, do I really know?!!?) I really don't know much about such projects, but did use a kind-of similar setup for a couple of disto-overdrives from www.runoffgroove.com -- similar use of JFET stages.

zipzap · Posted: Tue Apr 03, 2007 4:12 am Post subject:

the transistor in the supply with the resistor and cab is a gyrator - the active equivalent of a coil. can´t say much about the function, just used it here... i know this from hifi schematics where it will elliminate the last bit of hum in the power supply. they can also be build for high voltage when powering a tubeamp. much cheaper than a large coil.
i was powering the whole circuit from a cheap 12v psu that actually outputs 18v when powering this little circuit. it was very noisy. with the gyrator it´s quiet.
it drops the voltage about 0,5v.
In another situation i was powering a russian pencil tube from a 7815/7915 supply. because of the very little current demand i had a very highpitched oszillation in the supply. the gyrator solved it.

biasing. yes, i have heard that word before...
I have used opamps all my electronic life. ICs are so simple to use. Here i thought: well, lets build a fet stage and guess the values - see what happens. everything was working... and sounding good.
the link you posted is very good. this article i think will give me the knowlage to solve the problem. guess my drain resistors need some adjusting.
http://www.runoffgroove.com/fetzervalve.html

i´ll be back with more soon.

Just this - discrete circuitry is fun. really different feel than all that ic-lego.

zipzap · Posted: Tue Apr 03, 2007 12:25 pm Post subject:

ok, adjusting the drain resistors made the sound reapear.
But gain is lower and the distortion is not as nice as when running the module on higher voltage. maybe the fets are now clipping at some end... lot of things to consider when working with fets...
have to make some measuring tomorrow to see whats really going on.
maybe i´ll make a little setup with all trimmers to experiment.

goodrevdoc · Posted: Tue Apr 03, 2007 2:53 pm Post subject:

I remember something about higher dc voltages equating to cleaner sounds. There was some discussion on the diystompbox forum quite a ways back. IIRC, it was with regards to opamp circuits though. I would say try running it at 9V and see if you can squeeze some more dirt out of it.
-justin
EDIT: Looking at the schem again, it looks like you've got your diodes going from the input of the second stage right to the output. i would at least put them behind C10, to eliminate the DC component of the signal.

zipzap · Posted: Wed Apr 04, 2007 3:15 am Post subject:

they are behind c10.
with opamps it´s a little easier, since handling and sourounding is always the same, only the possible in/out swing is different (but it´s limited by the diodes in the feedback anyhow)
But here it seems to me that everything depends on everything, and that changing the voltage "shifts" everything around...