Big Muff e-cap question

sinner · Post by **sinner** » 06 Apr 2012, 22:06

Dudes...

In BMP polarized electrolytic capacitors are only in signal patch. Does it matters how they are orientated? AFAIK e-caps blocks only DC current, audio signal is AC, so should be no issues - am I right?

deltafred · Post by **deltafred** » 06 Apr 2012, 23:00

Polarised electrolytic capacitors work best if the the +ve side has a higher voltage on it than the -ve side.

They will tolerate a small reverse polarity (about 10% rated value IIRC - could be wrong). If you reverse bias them too much they start to act as resistors and conduct, very noisy resistors I seem to remember.

The trick is to asses (or measure) which side has the highest voltage on it and make that the +ve side. If one side goes to ground via a resistor or pot then that will (usually) be at 0v as far as DC is concerned so that side would be -ve.

Hope I'm making sense here!

bato001 · Post by **bato001** » 07 Apr 2012, 03:49

Absolutely Deltafred. The positive side is placed on the side of the connection on which you want to block the most DC current.

If you need to block the DC from both sides, place two elecltrolytics with the - sides back to back and with the + sides facing opposite sides, like the electrolytics between Q1 and Q2 on the Old Style Swollen pickle or Rams Head BMP.

The Cap between Q2 and Q3 has the + side pointing at the circuit portion with the highest voltage rating.

sinner · Post by **sinner** » 07 Apr 2012, 05:24

Thanks guys, great info. This whole "back to back" thing sets me off - that's why I was asking

Kitrae · Post by **Kitrae** » 08 Apr 2012, 04:51

EHX often put them in the "wrong" way around in many of the 70's Big Muffs. I have switched them back and forth to hear what changes and I hear very little, if any change either way. I would still put them in the way they are supposed to be, but apparently the EHX guys did not think it mattered too much.

sinner · Post by **sinner** » 08 Apr 2012, 16:55

Here's most common e-cap map, does it looks right?

I wasn't sure C5 orientation, so I just took it from bato001's schemo

Kitrae · Post by **Kitrae** » 08 Apr 2012, 20:51

Going by the most common way EHX did it in the 70's (if that can actually be said about EHX in the 70's), and the most common way they did it from 2000 and on, the + side should always point to the nearest transistor in the path. So your input cap, diode clipping caps, and the coupling cap beside the tone pot should be reversed. If I remember right, that's the layout on one of the original EHX schematics from 1975 too.

sinner · Post by **sinner** » 08 Apr 2012, 21:11

Ok, but that doesn't match deltafred's recommendation

What do you think Richard?

bato001 · Post by **bato001** » 08 Apr 2012, 21:54

I actually think it looks fine Sinner, but I think the best way to know if it is OK is to try it.

sinner · Post by **sinner** » 08 Apr 2012, 21:55

bato001 wrote:I actually think it looks fine Sinner, but I think the best way to know if it is OK is to try it.

It will work ether way around anyway

deltafred · Post by **deltafred** » 08 Apr 2012, 22:03

sinner wrote:Ok, but that doesn't match deltafred's recommendation

Yea, a couple seem the wrong way round but will no doubt work ok due to the low voltages involved.

If in doubt breadboard or build it the circuit and measure the voltages.

bato001 · Post by **bato001** » 09 Apr 2012, 03:10

The only sticking point is finding 470 pF to 560pF electrolytics to put over Q2 and Q3 : )

Kitrae · Post by **Kitrae** » 09 Apr 2012, 05:04

sinner wrote:Ok, but that doesn't match deltafred's recommendation

No, I agree that they should probably point to the higher voltage side, but you said "average" so I was just pointing out what EHX average seemed to be for the electros. As I said, EXH was not consistent, and I really don't think it makes any difference to the sound. I have not bothered switching them in all places of the circuit to verify, just a the ones in the gain stages that seemed wrong in a couple of Ram's Head BMP's.