# MI Audio Crunch Box Power Supply



## Grant2023 (Jun 26, 2008)

I recently purchased a MI Audio Crunch box (of member JMann, great to deal with btw), and have been running it at 9V. I'm happy with it, but am interested in taking it up to 18V, if it can take it. I don't want to try and end up frying the pedal if it isn't built for higher voltage.

Anyone on here able to comment on it's capacity, and any sonic differences between them? Thanks in advance.


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## mhammer (Nov 30, 2007)

Look at the capacitors used.

Caops tend to come in 16v, 25v and 35v ratings (among others). Standard ptractice is you use the next rating up from the anticipated supply voltage. So, if the Crunch anticipates a 9v supply it would probably use caps rated at least 16v. That's not good enough for use of an 18v supply, so the question is what the *actual* cap voltage ratings are. If it was 1969, the price and size differential would likely mean the caps weren't rated any higher than they absolutely needed to be. But since it's 2009, caps with very high ratings can be found that are physically smaller than their 6v-rated counterparts from 1969, and the price differential between a 16v-rated and 50v-rated is mere pennies. So there is a good chance the caps are okay. Check first, though.

More important, though, is whether the semiconductors can take it. I don't know that unit intimately, but some units use CMOS chips. E.G., DOD uses a 4007 CMOS chip for its switching in darn near 90% of all their pedals. CMOS chips will emit magic blue smoke above 15v, so use of 18v supplies is contra-indicated.

What is it exactly that you hope to achieve with use of an 18v supply? It might even be contrary to your sonic objectives.


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## Grant2023 (Jun 26, 2008)

I'm looking to 'open it up', and give it more of a coloured, full sound. At 9v, it sounds good, but somewhat cold and crunchy. I have read articles of people saying high voltage will achieve this, but it appears to be with the latest edition of the pedal, which is rated for 9-25V. I'm just not sure if the older version is rated the same.


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## mhammer (Nov 30, 2007)

Is it possible to direct me to a schematic of the pedal? If sop, I could tell you whether the possibility truly exists. Sometimes, you know, people hear stuff that isn't there, and with the magic ingredient of Web 2.0, rumours get started.

Some things DO sound better at higher supply voltages, but not everything does.


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## Grant2023 (Jun 26, 2008)

Here's the schematic

http://www.ultimate-guitar.com/forum/showthread.php?t=1044588

I know people hear things that sometimes aren't there. I ran an OCD at 9v for a long time, and recently tried running it at 18v, and while I heard slight difference, it was nothing like the difference Mike Fuller and many forum members hyped it up to be. I'd be interested to try though, just to experiment, if nothing else.


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## erikm5150 (Mar 3, 2006)

mhammer,

i don't have a link to the schem, but i know that the crunchbox is a modded marshall guvnor, with more gain and a simplified tonestack.
the opamp is a LM833
the clippers to ground are 5mm red LEDs

to the OP, 
the pedal can take a higher supply voltage with no problems. I use mine with 18v, but it works fine with 24v also. 
I've been using mine for about 2 years.


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## mhammer (Nov 30, 2007)

Okey dokey. Looking at it now. 18v should be no problem at all. The chip can take it. You can use an LM833 or NE5532. They're both in the same ballpark. Just make sure the caps are all spec'd for a 25v supply voltage or better.

I would recommend a couple of things, though.

1) A 1M to 2M2 resistor to ground from C1 on the input to avoid popping when switching.

2) Rewire the Tone control so that the two outside lugs go to R7 and R8, and the middle lug goes to C8. I dubbed this the Stupidly Wonderful Tone Control a couple of years ago, and the name sort of stuck in the community. When you see a reference to SWTC, that's what it is. The reason for making this change is explained here: http://hammer.ampage.org/files/SWTC.gif


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## Grant2023 (Jun 26, 2008)

Cool, thank you both for your help. I ran it at 18v last night, and I did hear a subtle difference.


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## forum_crawler (Sep 25, 2008)

The website says you can run it from "9V to 25V of headroom"


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## mhammer (Nov 30, 2007)

You can get more headroom IF you run it at higher supply voltages, but the voltage swing of the chip is rarely rail to rail. The datasheet for the LM833 indicates that it can swing from +/-13.5V with a +/-15v supply, so we'll say that it can swing to within 1.5v of each rail. In other words, with a 9v supply, it has enough headroom to swing as wide as +/-3v (that's 9v minus 3v). 

That is, of course, why the change from a 9v to 18v supply produces audible change. With 18v, the swing is +/-7.5v, which is a damn sight better. If you figure the typical input signal for a note plucked on an unwound string is unlikely to be much more than 50mv, that means you could theoretically amplify it by a factor of 15000/50=300 before you ran out of clean headroom. Given that the maximum gain of the unit is roughly 10000x (stage 1 is 101x and stage 2 is 100x) you run out of clean headroom very quickly. But then the LEDs only let you run the signal clean until +/-1.5v anyways (use red LEDs).


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## Mr. G. (Oct 16, 2009)

I've got a Crunchbox v2 and run it at 18v. There is a slight audible difference, but not enough to make me choose one over the other. The only reason I use it at 18v is to free up an extra 9v out on my DC Brick. It's a great sounding pedal either way.


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