Tuesday, 1 January 2013

Runoffgroove Peppermill

Adding to the Runoffgroove library.
From ROG:

The idea behind this project is to create a natural sounding overdrive sound using minimal components. It should also be suitable for a booster for an already driven amp.
*snip*
Please take note of the transistor pin voltages shown below, specifically the source of the J201 JFET. It may be necessary to audition different J201 for the best sound. MPF102 and 2N3819 will not provide the desired sound in this circuit. Various J201 may have a range of source voltages, even when the drain voltage is set at 4.5 volts. Having a source voltage near 115mV should produce results that are very similar to the sound clips.

The LED serves as protection against potential damage to the MOSFET caused by static electricity (thanks to Joe Davisson for this idea). A 9.1v zener diode may be used as well (as seen in the Jack Orman's AMZ Mosfet Booster), with the same orientation. Jake Nagy's Blue Magic predates the Peppermill and exhibits a similar topology. However, the development of the Peppermill was not influenced by the Blue Magic design and any resemblance is coincidental.



17 comments:

  1. 2013 is off to a flying start!
    I like the idea of this minimal parts light overdrive, will give n00bs like me a chance to poke around with values and see exactly what does what. The videos I found on YT show a high cut and somewhat of a bass boost (though I listened on laptop speakers, so there's really no telling) would changing some of the caps to a lower value help with clarity?

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  2. Man, is vero-layouts a hangover cure or something? :D

    Thanks for this Mark, can't wait to try it!

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  3. I have tried to build this 3 times from different sources (once on perfboard - what a nightmare) without success. Until now. Thanks, guys. One question: is the 2.2n cap the output cap? Can I bump it up a bit to get a little more bottom? I have a Blues Jr, so it has room to add some bass without getting tubby. Thanks again

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    1. If you want more low end, you should up the 47n and/or 22n.
      +m

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    2. I think I had the wrong idea before, on subsequent auditions, it's the harshness of the high end that I want to minimize. I might try a few different J201's to see if that's the problem.

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    3. This question was made a long time ago but in case there's someone else wondering, I've been using a 50k B pot in place of the 8k2 resistor as a tone pot. The higher the resistance here more high-end gets rolled off. I'll eventully set on a switch with a 22k resistor for bright and a 47k for dark...8k2 is mostly too bright for me and 22k is enough to mellow it up some.

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  4. None of my J201s get close to 115mV source. Would using a trimmer for the source resistor be advisable?

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  6. Just built this - nice sounding 'low-verdrive'. None of my J201s came close to 115mV though. The lowest I could find was 140mV, which sounded nice. It seemed the higher the source voltage, the less drive was available, with almost no distortion at all from a J201 with a 300mV source voltage. Is that how you'd expect it to work?

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  7. Am I able to replace 50k trim with a 50k Lin pot?
    If so, what lug goes where? Thank you!!

    Jarrod

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    1. Sure you can, but I'm not sure if I would really advise it...the bias trimmer here is more like a set and forget, it's not really like a fuzz face kind of thing in which low bias (in that range of 3k-4.5k) settings still sound cool...just my opinion. And you probably want to use lugs 3 and 2 of your pot to lower left lug and upper lug, respectively...but I may be mistaken and the order should be reversed hehe :)

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  8. How are you guys figuring out the source voltage?

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    1. What do you mean exactly Steve, getting it closer to that what is specified by RoG? If that's it, see that 1k resistor going to the source of Q2? you can either increse its value or decrease it and take some measurings to see if you are getting closer to 114-115mv. If I can remember, I had that 1k resistor soldered and the voltage on Q2 was a little high... 200mv maybe.. then I tried a 330R and it got down to 120mv (that's what I have on now). Rebiased the trimpot to ~4.5v... It does increase the crunch factor without changing the overall character of the effect.

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  9. I'm also having trouble with my build. I did it on perf, but none of my questions are related to layout. I've checked and triple checked, and it's correct. The symptom is that the Gain knob does essentially nothing, or is REALLY subtle to the point of being almost undetectable. Volume works just fine, and the pedal gives a really good clean boost as it is, but unless I'm mistaken, there's supposed to be some hair available. I have tried several different J201, and the closest I can get any of them to 115 mV is 109 mV with the trim pot maxed out (9V). I'm going to try Iago's suggestion of swapping the source resistor for a different value. If lowering the value made the voltage go down for Iago, raising it should make the voltage go up for me, right? I'll try a 1k5 and see where that gets me.

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    1. Quadruple-checking the layout worked. I had a 1M resistor between lug 1 of the gain pot and ground instead of 100k. I put a 110k in parallel, and it's wonderful now--even though I still get only 109 mV at the source of Q2. Lots of nice bark when I dig into it.

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Should all layouts be verified before publishing? This would mean less layouts but more peace of mind?