Big Muff (op-amp version) cap and pot questions

[j_flanders]

Hi,

Looking at a lot of schematics for the op-amp Big Muff it seems like all of them have one capacitor incorrectly.
I have found 3 pictures showing the pcb of an original Big Muff. I mean that yellow ceramic capacitor marked 160J or 150:




I'm new to this but as far as I can tell 150 here means 15 x 10pow0 or 15 x 1, so I think this value should be 15pF or 16pF.
But all online schematics use 150pF or 160pF and some even suggest 220pF or 330pF.
I guess I could simply stick to the original and/or try the other values as well, but does anyone have an explanation why every schematic out there uses such a large cap?

Next question is about the volume, tone and sustain pots in an op-amp big muff.
There seem to be as many combinations as there are schematics. Pots being used in the schematics I have found:

V  25k   ?   T 1Ok   ?    S 10k   ?
V  50k   ?   T 10k   ?    S 10k   ?
V  50k log   T 10k lin    S 10k lin
V  50k lin   T 10k lin    S 10k lin (150k resistor on Vol)
V 100k lin   T 10k lin    S 10k lin (150k resistor on Vol)
V 100k   ?   T 1Ok   ?    S 10k   ?
V 100k   ?   T 10k lin    S 10k lin
V 100k lin   T 10k lin    S 10k lin
V 100k log   T 10k lin    S 10k lin
V 100k lin   T 10k log    S 10k lin

Normally for this kind of info I go to :
http://www.kitrae.net/music/big_muff_op_amp_history.html
http://www.kitrae.net/music/big_muff_guts.html#ReplacementParts

But there I find entirely different values:

V4 and V5 op-amp Big Muffs were made with 130k linear taper Sustain and Tone pots. The Volume pot was 150k linear.
Later V4 and V5 Muffs had all 150k linear pots.

I haven't been able to find any confirmation on that info nor have I seen any of those values being used in any schematic.
Some say that pots (log, lin, value) don't matter that much (same sound just in different positions) while others explain that it actually does make a difference in a(n opamp) Big Muff. Explained here:
http://www.thegearpage.net/board/showpost.php?p=16922677&postcount=7
and here:
http://www.madbeanpedals.com/forum/index.php?topic=9163.msg80710#msg80710

Does anyone know (for sure) what the actual values were in the original pedal?

Thanks.

John

[duck_arse]

there is (apparently) a quirk in the way some parts, notably small value caps are marked. this was discussed here a little while ago, and I came across the problem this very day in a build. if a cap is marked "151", it will be 150pF, and is the rightest way to mark it using the three digit code. however, if the part is marked "150", it is not 15pF, but 150pF, again. if it is marked "15", you can be sure it is 15pF.

in the bigmuff opamp, it is across a 470k in the feedback loop. the 150pF//470k combo produces a cutoff frequency of 2250-ish Hertz. with a 15pF cap, this becomes 22kHz, so is really not very usefull as a tone shaping/controlling value. as for being 150 or 160, it matters little. it can be 220pF or 120pF if that's all you have.

as for the original value pots, I can't comment. I'd just restrain myself to looking at one circuit, and use the values shown on it. or the values currently readilly available, or the values I had on hand. if they don't sound right, try higher or lower. an opamp will drive a 10k vol pot as easily as a 25k vol pot.

[edit:] I hope this helps. oh, and welcome to the forum. that's a very big picture ^.

[PRR]

Older ceramic caps just wrote the pFd. "150" is 150pFd.

Newer ones do use the xxd notation, where "d" is a decimal multiplier. So "150" would be 15 times 10^0 or 15pFd.

You can also look at the circuit and ask "does this make sense?". As a general thing, 15pFd is "too small to do much" in audio systems, 150pFd will tame treble in networks with ~~100K impedance.

[duck_arse]

and Philips (and Siemens) mark ceramics 15p or n15 for 15pF or 150pF. no confusion.

[j_flanders]

Thanks for the replies. That pretty much clears it up as far as that cap goes. Also good to know why it should be that value (wrt cutoff freq).