Solutions for 'fizzy' decay from diode clippers?

[chptunes]

Hey folks.. I've been tinkering with my Classic Mini Booster (see the 'Pictures' area for more details on my build).. I wanted to add some soft/subtle diode clipping just before the Master Volume at the end of the circuit.

I tested several types and arrangements, but I settled on a pair of IRF510 MOSFETs because of their soft and sweet character.  They are connected for symmetrical clipping, like Mr. Orman's first diagram in this article: More Diode Clipping.

I like the soft character, but I am also hearing a fizzy/raspy sound on the decay of notes (especially while playing power chords).  I've tried to soften the fizziness by adding a 10k Saturation control (series resistance before the clippers), but the fizzy decay is still there.

Is there another method of 'tuning' out the fizz of this arrangement, or should I look elsewhere for soft/smooth clipping?

[Mark Hammer]

If you look at a scope trace of the unclipped signal, you will see that there are plenty of intermittent high-frequency peaks during the decay phase of the note, that make its amplitude rather unpredictable in some respects.  It is those unpredictable peaks that also result in audible envelope ripple in any effects that use an envelope detector to control things.

Since they are high-frequency components, sometimes the best way to reasonably assure a reasonably consistent amplitude (and less fizz) is to lowpass-filter the signal prior to clipping.

[chptunes]

Thanks for the tip sir.. I'll try it tonight.  What cut-off frequency is suggested?  Maybe 7 kHz, 5 kHz..

[Earthscum]

One thing you may try is putting a 1k before and after the clippers in a T configuration and trying different caps across those to pass the higher transients rather than clipping them.
As well, sometimes a resistor paralleled to the clippers will soften things, or a smallish cap parallel to the clippers. The DS1 uses a cap parallel to the clipper diodes to tame the top end. This does the lowpass filtering that Mark mentions.
I am not sure if you are using a single clipper or a pair but a good way to add softer clipping is ising asymetrical clipping. If you are using a pair of mosfets you might try replacing one of them with a red LED and see what the results bring. Also, you do have them decoupled from DC, right I found that a forward bias will cause one diode to persistently turn on and off, but it was close enough to ground that it acted like a resistor and just passed a fizzy sound to the next stage. This was with a schottky, but it's good to double check. in my case, I had a bad cap from a grab bag that ended up being a reroll fake, on top of leaking.

[Mark Hammer]

As well, sometimes a resistor paralleled to the clippers will soften things, or a smallish cap parallel to the clippers. The DS1 uses a cap parallel to the clipper diodes to tame the top end. This does the lowpass filtering that Mark mentions.

precisely!

[pinkjimiphoton]

i know this sounds counterintuitive,

but try a guitar-style simple tone pot in parallel with the input signal...did wonders for my 386 based stiff hippy overdrive, no fizzys at all.
put it right in parallel with the input to the board, try say, a 10k pot and an .022 cap...you can ground the cap right to the back of the pot.

[Caferacernoc]

When you are done experimenting please post whatever works best for you!

[DDD]

A simple way to smoothe diode clipping is to add a capacitor in parallel with diodes.
Begin with 1nF, maybe 2,2nF to 10 nF will do the work decently.

[chptunes]

First.. thanks for all of the helpful input so far.  You've helped me to get "outside the box" a bit.. I'm still just a couple of years into this 'diy' stuff.. lots more to learn.  After starting with just a pair of IRF510 trannies and listening.. adding a component.. listening.. changing values.. adding more components.. the "fizzy decay" is canceled:



I am surprised by some of my findings.  I had made incorrect assumptions about what values would work best in this situation.  For instance, I didn't expect to like such a large Cap (.047 µf) for the Low-Pass section [note: I did try some small value Caps without a Resistor, but the "fizz" was still present].

This asymmetrical (+2v/-0.7v) MOSFET clipping arrangement is very soft and subtle, only adding some harmonic character and a little low-end chunk.  A subtle effect was my goal, since the Classic Mini Booster already has a sweet and fat tone.  I can use picking attack and guitar volume level for more or less clipping effect.  This Mini Booster is 'always on' (no bypass switch).

[chptunes]

..took a fresh listen and did a little more fine tuning today.  Changed the 10k Resistor to a Pot temporarily, then adjusted to taste and replaced with 4.7k Resistor.  The tone is a bit warmer, and still very nice.

Now the .01 µf Cap isn't completely necessary for canceling the fizzy decay.. so I tested some lower values, and decided to just remove it for now.

One or two more observations that I can share..  As suggested by Earthscum above, I do think that the asymmetrical MOSFET clipping arrangement is softer and more subtle than the symmetrical arrangement.  Also, my Mini Booster has a nicer tone when the Master Volume is cranked past 1 or 2 o'clock.. I didn't notice this characteristic before performing these modifications.  It's not an Overdrive trait that I'm noticing either.. just a more 'open' and 'breathy' tone above 1 o'clock or so.

Thanks folks.