Crunchbox/Angry Charlie: Lower Compression

[ct_anthony]

Hi all!

I got a question regarding a crunch box/angry charlie clone... How to lower to compression of this drive? The JHS AT for example is described as having lower compression and newer Crunchboxes have a toggle for this... I'm not talking different diodes for clipping... I think it is something like lowering the gain resistor in the second opamp loop: 1M (Crunchbox) vs 680k Guv'nor... Is this correct or do you think "they" do something different?

Schematic e.g.: http://revolutiondeux.blogspot.de/2012/01/mi-audio-crunch-box.html

Regards,
Anthony

[Mark Hammer]

I don't know what "they" do, but the compression comes via the clipping.  The point at which clipping sets in is given by the LEDs used as the clipping element.  The value of the stage 2 feedback resistor merely sets what the maximum gain of that stage will be.

Using a pair of red LEDs for clipping, one already has a pretty wide dynamic range.   If one wants less clipping and more apparent dynamics, you merely turn the gain down.  Note that the Guv'nor, on which the circuit is based, aims for less total gain.  The Guv'nor has a max gain of 46x in stage 1 and 68x in stage 2, compared to 101x and 100x in the Crunch box.  Of course 46 x 68 still gets you a total cumulative gain of 3159x, which vastly exceeds the capability of  the circuit on a measly 9v supply., so 101 x 100 doesn't really get you anything appreciably more, although it changes where in the range of the Gain pot that point of maximum saturation sets in.

My advice would be to reduce the value of the feedback resistance in stage 2 from 1M to 470k, or even 390k, and revert back to the GUv'nor's value of 2k2 for the ground-leg resistor in stage 1.  That will still net you a total maximum gain of 46x * 39x =  1794x, which is more than enough gain to consistently hit the limits of what the chip can deliver and exceed the forward voltage of the LEDs.

I would recommend changing the 100pf feedback cap in stage 1 and replacing it with 330pf.  At lower gain settings, it will have no perceptible impact on tone.  At maximum gain it will roll off treble starting around 4.8khz, which will help to remove the fizzies, and also reduce hiss a bit.

[ct_anthony]

Thx for the information, I think I'll try it with a 470k fixed resistor + 500k pot for the second feedback loop, this way I can get everything from "reduced" to stock... I'm currently not sure about the 1k vs 2.2k in the "first ground connection"... It will give less gain but also change the frequency away from the "stock" TS value (~720hz) too the full drive (?) value (~320hz)... basically I see this circuit as a "TS light" into opamp clipper into LED clipper.

Your recommendation about the cabs is also interesting... my first intuition would have been to increase the second one... I'll give this a try

Thx!

[rthryhorysak]

Almost positive the @ uses a clipping diode switch. PCB looks like SMD LED and a pair of silicon diodes. As far as the arrangement of the switch, I'm not sure, but it's not more than a change in diodes.

[Ben Lyman]

Almost positive the @ uses a clipping diode switch... it's not more than a change in diodes.

Correct. The 3 way switch selects Red LEDs, Bat diode, or mosfet clipping. The different clipping thresholds provide 3 different levels of distortion. This must be what JHS is referring to when they say it has a "less compressed" option. I breadboarded the @ and was underwhelmed, the stock red LED's are my favorite. Actually my new favorite for Crunchbox and Rat is a single red LED and 2x series 1n914's, crunchy and open sounding, definitely less compressed but still very heavy.

[Mark Hammer]

It's not any sort of "best practice" or gospel, but I find it best to spool out the top end bit by bit, and where there are multiple stages I like to lop off the treble under 5khz in the first stage so that one isn't generating too many harmonics of harmonics via the clipping.  The "Presence" control is actually there to accomplish that same thing.  Use of that control, in tandem with controlled top end, will yield a sound with more than adequate bite and sizzle, but with the sharp edges rounded off.

The suggested use of a basic 470k with an additional 500k *can* be a useful idea.  Personally, I don't think it requires a pot.  Keep in mind that even a 470k feedback resistance in stage 2 will yield a max gain of over 2000x.  Adding another 500k will yield a max gain over 4000x, which won't likely provide any detectable difference, since it will hit the maximum voltage swing of the chip when you even breathe on it.  Most of the clipping you hear will be from the chip and not the diodes.  So adding 50k, 100k, 250k, or 500k to the basic 470k as you adjust that knob won't be all that audible.  All that will happen is a change in the dialability of the sounds you want with the gain pot.  T'wer I, I'd stick a 680k resistor in that position, and use a toggle to add a 390k in parallel with it to provide 248k.  That will give you an "intense" and moderately-gritty range.  But if intense and super-intense is your desire, stick a 1M in the feedback loop, and use a toggle to put  an 820k in parallel.