Vox Distortion Booster Mods

[Willthebold]

Just picked up the V830 Distortion Booster and noticed that the breakup was kind of harsh.  I opened it up and saw the RC4558p chip.  Just wondering if anyone was familiar with this circuit and knew how much swapping in something like a JRC would help the sound.  I assume quite a bit.  Any ideas?

Will

[MartyMart]

Hi, i have one, but did'nt look at a mod just yet!
If there's one or two pairs of Diodes in the clipping section, you could try the old "Diode trick", swop one pair for a single 1N4002 or change one from each "pair" for a 1N4001 for a more "asymetrical" distortion sound, think "tube screamer" or Boss SD-1 etc.
You could even use a 3mm LED, which is a Diode of course, this is a little harsh sounding for me though.
Let me know whats in the box?
Cheers, Martin.

[hairyneil]

I just got one of these...and I'm finding it a bit(read "for too")trebbly.
Is there something that can be done with caps to make it bassier?

[RandomRedLetters]

I just got one of these...and I'm finding it a bit(read "for too")trebbly.
Is there something that can be done with caps to make it bassier?

I havent seen the pedal schematics, but Im guessing if it lacks a tone control then it just uses the input and output cap to filter out the lows. If the input and output caps are less that 0.1uf, (most likely 0.047uf or smaller) than go ahead and try 0.1uf or 0.22uf as input and output caps.

[AL]

It actually does have a tone control although I can't figure out why - it really doesn't work that well. If someone does find a useful mod for this doorstop please let me know. As for right now my best use for it would be to gut it and use the really cool enclosure.

Hmmm? Methinks I sound a bit negative.

I'm not sure if changing the chip would do much. I would go with the previous suggestions of swapping caps and diodes to see what you get.

AL

[Mark Hammer]

You MAY get some audible benefit from a chip change, but don't let the hype put stuff in your ears that isn't there.  I would imagine only modest returns from a chip change at best.  These may change the "character" of the clipping produced in useful ways, but they certainly won't cause any wholesale changes in fizz-vs-grind.

More importantly, there is likely insufficient lowpass filtering for your tastes.  The same pedal that sounds "dull" to someone playing an HB-equipped guitar  with lower-value pots through a lesser quality guitar cord, going into an amp with limited bandwidth and speakers that shave off everything above 5khz, will sound like a screeching banshee to someone playing through single coils and a 1meg volume pot through a high end *short* cable, going into an amp with no way to defeat the "brite" function, and 10" speakers that go well past 8khz.  In short, if there is not much rolling off of high end by any of a number of factors on the way to the speakers, the same pedal can seem to possess unreasonable amounts of excess treble.  The effects of chip changes ARE real, but they will NOT remedy this sort of problem.

A far more likely source of "improving" the tone of the pedal probably lies in upping the value of the feedback cap in the critical gain stage.  That is, the one with the diodes in its feedback loop.

Note that such simple lowpass filtering is 1-pole in nature; 6db reduction in treble content per each additional octave you go up.  Most tone controls in such pedals have a similar slope.  The end result is that, once you factor in the tremendous boost in harmonic content that distortion pedals create (that IS, after all what they do), a great deal of undesirable harmonic content can still remain.  Even IF it is suppressed by 12 or 18db below content on the "good" side of the filtering, it can still be way too obvious and leave the tone "harsh" sounding.

Though I don't have the schematic handy, my guess is that the cap in question will be in the vicinity of 22-470pf.  Increase its value by 30% (or closest common value) by temporarily tacking on another  cap in parallel on the copper side and let us know what you think.

Incidentally, a similar change to the critical cap/s in the tone controls might also move things more in your preferred direction.

[Yun]

maybe try this mod eh?

[DDD]

Maybe there are couple of clipping diodes connected to the ground? In that case there is a capacitor in parallel with the diodes. So to make your sound darker you have to change the capacitor with another one with double capacitance.

[hairyneil]

Wehey!:D
This seems to have worked DDD!

The rest of the suggestions prolly would too...but I'm too dumb to work out exactly what to do...and too scared to guess!

(for the benefit of other folk looking to mod this pedal)
If you open up the bottom, the first board you see, the one without the pots attached.
Where the chip is, there is a small green cap (C5, with a box around it) in parallel with two diodes (D2 and D4) I attached to the back of the cap a "473" (47nF?) cap. This seems to have given me a lot more bass, and reduced the harsh sound!

Thanks for all the help folks!