EH Soul Preacher Comp opinions

[markm]

Hi y'all,
I've been working on a layout off and on for the EH Soul Preacher but I would like to have some opinions on the circuit itself.
Have any of you fellas built the Soul Preacher and if so, what did you think?
I have searched this subject in the forum and I came up with some posts about noise etc.
This circuit is actually a "glorified" O/S for the most part so, I expect it would sound similar however, if any of you
have some first hand experience with this circuit, I'd like to know your thoughts and opionions of it.
Thanks!! 

[Meanderthal]

 I never built one, but used to own one of the old ones. It was an overbearing squasher of a thing that was more like the Walco Sustainer than an OS, but not nearly as noisy... Might be cool for those who want an EXTREME compressor! I do see prople show an interest in building the Walco occasionally, this is a much more usable circuit than THAT!(I have a Walco... ugh, the noise, the NOISE!!!)

[Mark Hammer]

I haven't built it or even used/heard it, but the summation as "glorified OS" isn't far off the mark.

I have two different schematic files for this.  One seems like a "factory original", given its stylistic similarity to a great many hand drawn E-H schematics I have seen, and the other is a cleaner, more photocopy-friendly re-draw by RG Keen, sometimes in the early 90's or earlier.  I suspect there may be an error in both, but more on that in a moment.

How, one might legitimately ask, is it an improved Orange Squeezer?  In several ways.  First there is a bipolar input stage, which the Armstrong OS lacks.  Second, there is variable gain in the op-amp gain stage, so as to adjust the amount of envelope signal amplitude and the amount of compression.  The OS has fixed gain.  Third, the Soul Preacher uses a full-wave, rather than half-wave rectified circuit, which should reduce envelope ripple in the decay portion of sustained notes.  These three features turn it into a hopped-up version of what the OS is.  You will note that all of these things have been discussed, at one time or another, as add-ons for the basic OS circuit, and responded to favourably by those who have tried them out.

Is the posted-around SP circuit in error?  This may reflect my naiveté about some things, but I suspect yes.  The OS uses an 82k fixed resistor and a FET drain-source path as two legs of a voltage-controlled potentiometer.  Instead of the overall "pot" having a fixed resistance, and the dividing point between the two legs shifting around, one leg (the 82k resistor) stays fixed, and the other leg (the FET) goes higher and lower in resistance.  In the SP, the same fundamental "attenuation engine" exists, but the order of the DC-blocking cap and series resistor are flipped around.  In the OS the input signal sees a 47nf cap at the input, then an 82k resistor and the combined resistance of the FET and trimpot going to ground.  In the posted SP diagram, the input signal sees (after the input buffer stage), a 220k resistor, a 100nf cap, and then the FET to foating ground (4.5v).  My gut sense is that the 220k and .1uf components need to be flipped in order for the virtual pot to work, but I may be wrong about that.

Some comments floating around described the Soul Preacher as noisy.  But you know, just about EVERY compressor is noisy when used improperly, and not a whole heckuva lot of players seem to know how to use them properly.  So, the reported experience of it being noisy does not surprise me at all.  But that is more a reflection on the typical user (and the form and function of "legends" on the net), than on the circuit itself.  It IS a notably improved Orange Squeezer in many respects, and if attention is paid to the way it is used, and the components used, I imagine it would sound as good as a great many compressors out there.

As for "extremeness", that can be easily improved by: a) turning the gain down or adjusting the minimum gain on the op-amp, and b) reducing the 220k resistor that serves as one leg of the virtual pot.  Keep in mind it is the ratio of the one leg to the other on a pot which determines how much attenuation there is.  If the 220k resistor is reduced, then large variations in the FET resistance will have less of an impact.  Works the same way on the Orange Squeezer.

[markm]

Thanks for the explaination Mark, I greatly appreciate it.
Trying to get the circuit together in a Layout that makes sense has been a bit of a hassle and then, I have to overcome the hurdle of whether my parts substitutions are going to be correct.
I was going to try using 1N34a for the diodes and 2N3904/5088's for the trannies along with an MPF102 for the JFET.
Just guesses at this point but, I'm always game for trying out compression circuits.
I like comps I guess.....don't know why I just do! 
Thanks again Mark and Meanderthal for the help!!

[aron]

I had it and mine must have been defective. If I wasn't playing it would start seriouly compressing until .....wooooooh it's loud, then if I played a single note; as in READY TO TAKE A SOLO, it would clamp down and I wouldn't hear my guitar for about 5-10 seconds. It would slowly fade back in.

So I would mute and strum my guitar before the solo and then let it rip.

[markm]

 
Something's wrong there huh?!
Not sure if I wanna build this now 

[aron]

I really hated it but that's all I could afford at the time. I liked the Orange Squeezer a lot more.

[Mark Hammer]

I think the issue is that you knew less about compressors at that time than you do now.  ALL side-chain operated circuits make certain assumptions about the incoming signal, and if the signal you feed it does not conform to those expectations, generally the pedal will seem to underperform.  The dreaded "undersignalitis" in auto-wahs is a perfect example of that, as is the erratic behaviour of some noise gates.  As has been noted in many places, designers often predicated their designs on assumptions about weak pickups.  Given that E-H started out as a budget-conscious company (and seems to have retained this ethic to some extent), chances were pretty good their anticipated client would have low output pickups.

[Meanderthal]

 Actually, aron's description was pretty accurate, There was no way to use it "properly", it did the same thing on bass, Tiesco, Strat, and Les Paul(copy). It seems to have been intended to be as much a swell pedal as a crazy compressor. Once ya got used to it it was kinda cool in it's own way, but this wasn't what you would want to make your tele twang jump out and sparkle, or keep your chords even with your single notes or anything, it was more for long winded bluesy solos with notes lasting like 2 measures or something.
Yes, it would clamp down on the note and suck it in slowly... like a Walco.
The fact that the circuit resembles a hot-rod OS is cool though, the mods that Mark Hammer suggested might be well worth looking into... there's no reason it MUST do that overblown stuff, it's just what it did stock.

[markm]

The reports and opinions I'm reading about this circuit aren't exactly "favorable" it seems. 
Makes me think I may just abandon this project, at least for now.
It certainly looked interesting to me but, it could explain why there are no layouts around for it.

[RickL]

I built one from one of the posted schematics and it worked. I don't remember what it sounded like and it's buried in one of 12 or 14 milk cartons filled with effects so it might take a while to find it in order to make a report. I'll see if I can find it and report back.

[markm]

I built one from one of the posted schematics and it worked. I don't remember what it sounded like and it's buried in one of 12 or 14 milk cartons filled with effects so it might take a while to find it in order to make a report. I'll see if I can find it and report back.

I'm interested in your findings.
Please do post a report.

[aron]

Yes, it would clamp down on the note and suck it in slowly... like a Walco.

WOW, that undid years of thinking I had a crappy one! Amazing! I really did dislike it...... I used it with an old ES-225 and it worked ok as long as you strummed the strings to get ready for your solo!

[Meanderthal]

 It's possible that we both had defective ones... I bought mine NOS sometime around 1983 or so(still had an ancient stock battery with it), it was really cheap, and may in fact have been a factory reject or something. Also, there's a possibility that the schematic comes out different when built- someone may have addressed the clamp/suck issue at some point... I'm gonna stay tuned for the build report(outta curiosity, I already have more compressors than I need).

One thing to add- although I got a kick out of the thing it did, I got rid of it at some point... meaning it  probably was more annoying than useful, or someone wanted to trade something more interesting, I can't remember which.

[Mark Hammer]

Well, whatever it was or wasn't...stock...it is easily "rehabillitated", and likely worth doing.

I suppose it is the same thing with pedals as with many amps and guitars.  You often see products that were 95% solid, then someone went and made a bad design move like poor choice of speaker or pickups or switch location, etc., and the product ends up being relegated to the back of the shelf.  In this case it would seem to be either the degree of sensitivity, the lack of control for compression amount, or both.  I always thought that the reason it disappeared was largely because many people misunderstand compressors and have unrealistic expectations for them (effect not obvious enough).  Turns out the product itself may well have helped in its own demise.

In any event, the FET-based attenuator is something that enjoys good reputation in plenty of studio gear, if everything around it is tweaked just so.  There is no reason to think otherwise of the Soul Preacher, and if something even simpler, like the OS can do the job well, then so can the SP.

[markm]

I think I'm going to try and see this thing through and finish a layout for it just to satisfy myself I suppose.
I did take Mark H's advice and flip-flopped the .1uF cap and 220K resistor......I wonder if this could cause the clamping issue that some have had with this circuit??

[Mark Hammer]

Consider using a 150k fixed resistor and 100k pot or trimpot in series to replace the 220k component.  This will alter the response curve of the unit. As the resistance goes from 220k downward, the changes in the FET's resistance will have less impact, hence less squash. 

A second consideration is the amount of gain in the op-amp.  Stock, the minimum gain is x11, and max gain is x112, which seems a little much, especially given the input stage.  Try changing the 1k resistor in the op-amp stage to 2k2.  This will shift the gain range to 5.5 - 51, which may lend itself to more subtlety.

The switched capacitor on the output is intended to give some advantage to the treble on the output.  I've been suggesting this for the Dynacomp and OS for a while.  Wonder where I got it from? 

I now realize that I have seen 3 schematic drawings of the Soul Preacher, one "original", one by RG, and one here: http://www.freeinfosociety.com/electronics/schematics/audio/soulpreacher.pdf  It's funny how the multiple drawings can convince you that this is what the circuit MUST be.  I don't have any confirmation from anyone yet that those drawings are in error and that my suggested "component flip" is correct (as opposed to me talking through my hat...again).  But....it bears noting that the two cleaner drawings are redraws of the original hand-drawn circuit, and if THAT one had an unintended error, it would be repeatedseveral times over, further entrenching the idea that the drawing is unassailable.

[markm]

Interesting to say the least Mark
The way I have the layout set up, it's easy to flip flop the cap and resistor around.
Out of curiousity, would you want to take a little peak at it if I PM it to you?
Maybe give it the once over for me??
I've been using the free info schem I beleive but not sure as I printed it out about 4 months ago!

[Mark Hammer]

Not likely I'll have the concentration and time this week, but maybe others might have a chance.

[markm]

Not likely I'll have the concentration and time this week, but maybe others might have a chance.

No problem, very understandable my friend but, a Big Thank you for your assistance here, it's been very enlightening for sure!