Heavy Metal Pedal Prototype

[RDV]

My first stab at designing and partially tested.  :shock:
Willed it to work!
Thanks Alf, & R.G. for hints and parametric circuit respectively.



Regards

RDV

[David]

Hmmm...

FIVE op-amps and 2 stages of hard diode clipping, huh?  Where is your overdrive taking place?  If this thing works, I'll bet it'll scream!

[Paul Marossy]

Looks interesting. Hard clipping in two spots, eh? If it were me, I might use one of those diodes pairs in the first opamp's feedback loop and keep the other ones at the output of the second opamp.

The parametric EQ thing looks like a cool idea. Is the opamp at the end intended to be an output buffer, or to compensate for the tone control section?

Maybe someone here can breadboard it and tell us how it works?! I would, but I just don't have the time...

[petemoore]

I'm strongly inclined to want to try that Parametric Bass Notch booster on some already made clipping devices.
 The Concept certainly makes sense.
 I don't quite follow how it works though...does the bottom OA/filter...N/M...How does it work, basically...?

[spongebob]

I'm sure the second opamp needs a resistor/cap to ground (or Vb?) from the inverting input, like on the first opamp. How much gain is the second stage supposed to have?

And the diode clipping sections have +Vb DC offset, not sure if this would work. I think I will draw this beast up in SPICE and see how it behaves!

[RDV]

http://www.massenburg.com/cgi-bin/ml/parametric.html

[will]

Hi RDV,

And the diode clipping sections have +Vb DC offset, not sure if this would work. I think I will draw this beast up in SPICE and see how it behaves!

Spongebob is correct, The 1st set of diodes need to be connected to Vb or you need to add another coupling cap just after the diodes. Otherwise 1/2 your signal will be totally clipped.

I wonder if it would be interesting to put the parametric eq before the clipping.

A big tonal component of the distortion is the opamps, based on MXR Dist+ experience. You might consider temporarily replacing the 100K in the feedback loop of the 2nd opamp stage with a 500K trimpot. Then you can play with the gain of the second stage until you are happy with the sound.

Good luck!

Regards,
Will

[Mark Hammer]

Note that the large amount of gain in the first clipping stage will peg the level close to whatever is the maximum signal permitted by the diodes.  Part of what that means is that even a modest amount of gain in stage 2 will "reclip".  So why limit yourself to Si diodes?  Stick in a pair of red LEDs instead of the diode trio and you will still get the benefits of a second clipping stage but will harvest a much heftier output level for punishing your amp and adding a "third clipping stage".  Worst case, stick in a single LED and a pair of Si diodes to maintain your asymmetrical clipping with a higher overall threshold.

Note that the 56pf cap in stage 1 limits HF rolloff at around 2.8khz, and the 330pf cap in the second stage limits HF rolloff at around 4.8khz.  Note that the .001uf cap you have in parallel with the second set of diodes rolls off high end around 16khz.  If you were not adjusting the tone downstream, I would agree with your choice of stage 2 rolloff, but the various rolloffs seem to be at cross purposes here.  

Given the reticence in your comments, this is obviously a first stab at a design using building blocks from other pedals.  You are to be congratulated for taking that step, and I imagine it feels VERY good...as it should.  On the other hand, what works just great in isolation in one pedal may conflict with something that also works great in isolation in another pedal.

So here are some suggestions.  I'm a big fan of cascaded rolloffs but for a design intent on catering to metal tastes, you don't want to roll off too much too soon.  So, drop that first feedback cap down to 33pf for a rolloff at 4.8khz.  Of course the clipping will add harmonic content beyond this point, but the idea is that you don't want too many harmonics of upper harmonics early in the game.  The cap in parallel with the diodes can stay just to keep these high gain stages from doing something foolish like oscillating.  

For stage 2, drop the feedback cap down to 220pf for a 7.2khz rolloff.  That will keep things sane but still permit some sizzle if you want it.  The .001 cap in parallel with the second set of diodes can go.  It really isn't doing anything terribly constructive, although note that when the tone pot is minimized to zero ohms, that cap and the tone cap parallel to form a .0043uf capacitance which forms a lowpass filter with the 10k resistor immediately preceding the diodes with a rolloff of 3.7khz.

Clearly, the .001 cap and .0033 cap aren't doing a whole lot of tonal changing, and my guess is that your tone control currently works largely as a "defizz" control, rather than a serious treble-altering device.  So, t'wer I, I would omit the second .001 cap, and replace the 10k tone pot with a 50k pot and 2.2k resistor in series.  This will provide a max treble rolloff at around 22khz (essentially no rolloff to speak of) and a min treble rolloff around 945hz, which will produce a nice round sound.  You can even stick in a 3900pf cap for a rolloff range of 780hz - 18.5khz.  Since the other suggested cap changes will offer up more treble to start with, you'll notice the difference a LOT.

These changes (diodes and caps) in tandem with your bass boost/scoop control should be able to deliver a very nice broad range of tones from  gronky resonant grind to big-scoop searing metal.  

One final note, as the article at GEOFEX shows, you can make a gyrator section with a single transistor.  This means you can declutter the board by using a single quad op-amp or a pair of duals, and don't have to stick 5 op-amps on there.

Congrats.

[RDV]

Mark:

Thank you for your kind comments. My purpose(besides trying to finally come up with something of my own) is trying to come up with a real grinding, heavy sound that I haven't yet found in my DIY builds so far. I feel that many have enough distortion, but not the right EQ. I should probably try your Chaos & Roseyray, but I want to do it myself(with help, of course :wink: ). I spent all night finishing & debugging my Easyvibe(which I'm naming "Bridge of Thighs" ), which I finally got done(cold solder joints on a cap kept it from phasing). I printed out your post & studied it & revised my schem. Oh, and I am just going to use a pair of duals.

I wouldn't try it out yet, everybody!!

Thanks Mark, Alf, & all.

Goodnight

RDV

[spongebob]

Good morning!

I have just finished the circuit in LTSpice, here are some observations:

- A coupling cap (1uf or so) is needed right before the 100K input resistor at IC1b
- The EQ stage isn't working, the signal get's totally screwed up, I don't understand what's going on there anyway :cry:

Here are a few plots:

This is the input signal, 150mV sine wave, 333 Hz


This is after the first diode clipping stage, as seen by the noninverting input of the second opamp:


And that's the output of the second opamp, there's not much left of the original signal, probably this stage has too much gain right now.


And here is the frequency response plot (excluding the eq stage as it is not working correctly).


Can anyone advise on the EQ section? The signal goes completely out of whack there...

[RDV]

Spongebob:

Which schem did you simulate? The original or the latest? Cause I've changed it 3 or 4 times since I originally put it up.

RDV

[spongebob]

After looking at the geofex page I noticed that the last opamp only needs a 2k7 resistor from output to inverting input!  In addition, the EQ stage must be biased to Vref, so put a 100K / 1uF combo before the 2K7 resistor at the start of the EQ section.

Edit: You can grab the LTspice circuit file I made from here. If you don't have the potentiometer lib installed, unzip this file into your LTspice directory. LTspice itself is available for free from here.

[Mark Hammer]

RDV,

Nothing special about my two fuzz contributions except that they share double-clipping in common with yours and try to provide a broad range of sounds from a single 3-knob pedal.  I like them, but then I imagine we probably all like those things we feel we had a hand in dreaming up, even if all our "design" amounts to is adapting some stock things we saw while sifting through schematics on the busride home from work (nothing autobiographical about that :wink: ).

I guess the thing to remember about multi-stage clipping pedals is that they are signal-sculpting[/i] devices.  Clipping something twice is not just doing the same thing over again.  The signal received by the second stage is not the same thing as what the first stage got, either in terms of bandwidth or emphasis within that passband.  So the thing to guide one's thinking in terms of tweaking or shaping the circuit is to think in terms of "What am I working with *here*, at this point in the signal path?" , and conceptualize the entire effect as if it were several pedals under one chassis.  

This has implications for tone shaping, pre-emphasis, de-emphasis, level-setting, diode selection, etc.  For instance, suppose you stuck the gyrator/EQ section *between* the two clipping stages.  What are the implications?  For one thing, it would permit you to adjust the extent to which different parts of the spectrum double-clip.  So, goose the low end with the boost/cut control, and now even modest gain settings in the first stage will result in heavy clipping of certain parts of the spectrum, while leaving others with just a hint of dirt.  Conversely, notch out a band between the first and second clipping sections and not only have you changed the tonal balance, but you may have placed some parts of the spectrum well below some critical level needed for re-clipping. (While it occurs to me, some experimenting with what goes between transistor stages 2 and 3 on a Big Muff is worth thinking about, since it is the granddaddy of all double-clippers; "Tone - its not just for output anymore").  If you *want* that to happen (i.e., touch of dirt for most notes but serious overtones for selected notes, not unlike use of a wah), then one starts thinking about how much gain needs to be applied in stage 2 and what the sensible choices are for what the diodes need to be in order to set clipping threshold.

If one sticks a gyrator/EQ section *after* all the nastiness has occurred, then that control does not shape the quality of the clipping, or where the clipping occurs most, but rather shapes what part of the clipped signal you *hear* the most.  That's not good or bad, just a different outcome.  

If you've seen any of the schematics for the 600 or so distortion pedals that DOD and Ibanez have out (like the "American Super-Metal Disemboweller", the "Tinitis Driver", and the "Temporal Lobe Destroyer..XL" pedals -  :wink:  ), you may note that many of these have gyrator sections both before AND after a clipping section.  In some cases the gyrator section is fixed.  It may not strike you right away as a boost or cut because there are no pots, but one educated look and you immediately recognize the component configuration seen in your bass control subcircuit, just with fixed resistors mimicking the boost/cut control set in fixed position.  

Some of us have been waxing poetically about the virtues of combining preclip and post-clip EQ for years now.  Akai took it to the hilt with their pedal that had dual 7-band EQs, one before and one after the distortion stage.  Not a big seller if I remember correctly (which is probably why a few folks here picked one up cheap from MF or Zzounds or whoever had em on sale), probably because most players don't wanna feel like they're designing a house when twiddling fuzz knobs.  

Still, something that alters spectral balance along the way, at least in a simple and musically valid way, is a nice thing to have.  In your case, something like a one-switch-plus-pot pre-boost may be right up your alley.  That is, a frequency-sweepable gyrator circuit with preset boost.  One pot sets where the boost occurs between stages, and the other either engages the boost or lifts it.  If one had a 3-position on-both-on SPDT toggle, you could actually mimic the two extreme and middle positions of a pot for boost/flat/cut.  That, *PLUS* a treble rolloff and bass/mid boost cut control on the output, would provide extremely flexible tone shaping. and a pedal you'd never tire of.  Your existing design has 5 knobs/pots.  What I'm suggesting would make it 6 and a toggle switch.  If the inter-stage resonant boost and the post-clip boost are set to the same centre frequency, that will likely deliver ridiculously easy singing feedback for the note range you select.

[Ansil]

hey rdv man  for asy clipping man with leds try using the standard red led on one side and using a larger different color on the other side.  i use a 3mm red and a 5mm green for most of my clipping there, and u can tell a difference. a lthought i like two green 5mm too..

[Paul Marossy]

I've tried the green/red LED combination before. I think that it sounds pretty good.

[Ansil]

I've tried the green/red LED combination before. I think that it sounds pretty good.

thanks... 100 people can't be wrong lol  well i guess they could look at the whole jim jones cult purple koolaid thing.. hmmmmm  never mind  

[RDV]

O.K. I've implemented pretty much all the suggestions for improvement.

Will it work now?

I guess I'm gonna perf & see! :shock:

RDV

[Davide]

it looks great !
if it works i'll build it immediately ! i swear !  

(i was going to try building a dod death metal pedal.. it will wait)

[Paul Marossy]

"O.K. I've implemented pretty much all the suggestions for improvement.

Will it work now?

I guess I'm gonna perf & see!"

Cool. I am interested to know how it turns out.

[Brian Marshall]

it will work.  the diodes go th vb, and so does the bias risistor.  as long as the cap is sufficiently large in the voltage divider.  47uf should work

Hi RDV,

And the diode clipping sections have +Vb DC offset, not sure if this would work. I think I will draw this beast up in SPICE and see how it behaves!

Spongebob is correct, The 1st set of diodes need to be connected to Vb or you need to add another coupling cap just after the diodes. Otherwise 1/2 your signal will be totally clipped.

I wonder if it would be interesting to put the parametric eq before the clipping.

A big tonal component of the distortion is the opamps, based on MXR Dist+ experience. You might consider temporarily replacing the 100K in the feedback loop of the 2nd opamp stage with a 500K trimpot. Then you can play with the gain of the second stage until you are happy with the sound.

Good luck!

Regards,
Will