Soul Preacher distorting

[rankot]

I've built this one long ago, but never succeeded to make it sound as I expected it should - it seems that it is compressing when sustain is up to 1/10th of range, everything above that produces overdriven sound.



I used standard schematic checked it many times from different sources and it's OK. All the parts are used as they should be, although I didn't have tantalum caps and hand, so I used electrolytic. I have also used 1N4148 instead of 1N3666 (can't buy them or anything similar here). Transistors used: 2N3904 and 2N3906. JFET 2N5457.

[rankot]

It seems that posting this actually helped me to find problem - maybe I just collected all my thoughts... After I posted this, I first checked all the joints and resoldered all that looked suspicious, but it didn't help. So I concluded that the problem is most probably in JFET, removed it and placed socket instead. Then I tried another JFET and - voila! - it started to work.

So I tested this with different JFETs and I like J202 the most of all, sure it's a winner. And this little 2N5457 that was there first - it was faulty at all.

However, I must ask people knowing more than me to help me understand if those tantalum capacitors are really that important, or I can leave electrolytic instead.

[rankot]

For those who want to know, I have measured different JFETs for Soul Preacher. I like J202 and 2N5459 the most:

type           I_dss            V_gs(off)
J309          2.0mA                          1.40V
J202          1.0                               0.73
PN4393      3.9                               2.80
2N5459      2.0                               1.40
J310          4.4                               3.00

[Mark Hammer]

If you built it from the schematic you show, then no wonder it doesn't work properly.  I looked at it and thought "Why would there be two 1N3666 diodes in the same orientation connected to the same place, and why would pin 7 of the chip connect back to the 10k resistor feeding pin 5?"  That simply made no sense.  So I looked in my hard drive collection of schematics, and found this:



Break the connection between that lower diode, and pin 7 of the op-amp, and things should work much better.

[rankot]

Mark, you are completely right, I have a few of those schematics' and I managed to send a wrong one - actually I have used the one you sent later 

So to conclude, it works like it should right now, the real troublemaker was a bogus JFET. But question about tantalum vs electrolytic remains

[iainpunk]

Heyy,

Tantalums seem to work better in RF circuits due to a lower parasitic high frequency inductance and a (slightly) lower ESR (equivalent series resistance), but they are worse in most other ways, most notably the price point. Also, the temperature dependence seems to be (far) higher in my experience, but i couldn't cite any scources (except the measurements i made at school).

I personally use foil caps for everything smaller than 10uf, but im in a luxury position of having those.

So, yeah, elcos are, in pedals anyway, much better and cheaper.

Have fun, Iain

[rankot]

Thanks! I usually use metal film for everything from 1u and less, mostly because of the size - 10u at 63V is really huge But I see a note about tantalums on this schematic, so I was curious.

[Lordrahulcool]

I saw a layout i  effectslayout blog  of soulprecher which has 1n34a  can i replace it with 1n4148?

[antonis]

It should be more interesting to replace only one diode (e.g. one Ge and one Si..)
"Asymmetrical" octave, perhaps..??