Eemu PMOS Bass Preamp

[Fancy Lime]

Greetings fellow tone afficionados!

After years of chasing the "perfect" bass overdrive tone, I have something that I really like and I think others might to. So I thought I'd post it here. I call it the Eemu PMOS Bass Preamp. It illustrates how to use p-channel MOSFETS in a negative ground circuit, which may be educational for some people here since I am unaware of any designs doing that for reasons I really do not understand. It's not difficult just somewhat unusual. Jack Orman wrote an article about this called "Positive Power for the PNP Fuzzface". The BS250 sounds fantastic (although variation between devices seem to be huge from what I've read; I personally had no problems) and is MUCH less noisy than the n-channel equivalents (more or less) 2N7000 and BS170. Thanks to Joe Davisson and his Obsidian Overdrive, where I learned this.



The main design goal was to get that "not clean but not too obviously overdriven" or "hot and dirty" sound (if that makes sense to anyone) that you get from hard working tube power amps. The secondary goal was to make this as quiet (meaning not noisy or hissy) as possible. I experimented with Mu-Amp (aka Minibooster) based designs a la Runoffgroove Ginger (and tons of commercially available pedals) for a while but that did not suit me too well. The overdrive from a 2N5457 or J201 equipped SRPP or Mu-Amp stage sound great at "obviously overdriven" sounds but not so much in the clean-dirty transition region. It reminded me too much of the typical smooth triode preamp distortion and did not provide the rough dynamic raunchiness of a hard working pentode power amp. Not that the the Eemu Preamp sounds exactly like a hot and hard pushed tube power amp but to my ears it nicely emulates the most important characteristics of this very special sound. If I were tasked with designing a two channel guitar or bass preamp, this design would be my starting point for the clean-crunch channel and something based on two Mu-Amp stages would probably become the overdrive channel. I originally built one of the early prototypes with BS170 n-channel stages but that was just way too noisy and basically unusable at higher gains as a studio preamp without a noise gate.

Let me quickly walk you through the design (mostly interesting for those who have never designed a preamp, others may be bored by this paragraph):
Q1 is a simple gain stage with feedback biasing and rather high source and drain resistors, which helps mitigate some of the large differences between individual BS250 transistors. Note that the gain pot is a reverse logarithmic 50k pot. A linear one will also work but with much worse control characteristics. If using a linear pot, 25k gives you better control at higher gains but you loose some of the low gain range. The bright switch could also be replaced my an on-off-on switch to allow two different bright capacitors.
Next comes the mid control. I decided to take the tone controls apart instead of having them all in the same place as in Fender Bassman style tone controls. The reason for that is that I wanted the mid control to strongly influence the overdrive, the bass control to weakly influence the overdrive and the treble to to influence it at all. This allows to dial in a sound where the bass is quite strongly compressed but the mids are almost clean and very dynamic. This contributes a lot to the impression of a "fat but clear" sound. This is quite similar to what the "ultra lo" switch on Ampeg SVTs does only tunable. It is also highly interactive with the gain control.
After the mid control comes the first drive stage which also acts as the second gain stage. The boost switch changes the gain range from mostly clean to mostly crunchy.
The bass control allows only relatively modest bass cuts because radical bass cuts make no sense to me in a bass preamp. You can ommit R15 and replace the bass pot with a log 500k one to make it more radical.
The third BS250 is wired as a buffer but is also pushed into overdrive.
The treble control with the 2N5457 buffer at the end is a hybrid between a Rat Filter and Mark Hammers "Stupidly Wonderful Tone Control". This type of circuit cuts all highs with 6bd per octave and lets you select the corner frequency. Therefore this is actually more like a tunable cabinet simulator than a traditional treble control, where you would select how steeply everything above a fixed frequency is cut. The advantage is that it filters out noise at higher gains a bit more effectively. Also I just like the sound better but thats a matter of taste of course. It works best with a rev log pot imho.

If anyone is interested in building one of these, I'd be thrilled to hear your opinions. Let me know if you find errors in the schematic.
I hope you have fun with it.
Cheers,
Andy

[Hatredman]

I dig your logo!

[Fancy Lime]

Thanks!

[Hatredman]

But...


You posted on the ˜I want de schematic for that commercial pedal that I want to clone" subforum. Hardly anyone comes here, so you will have few responses.

You would have much more visitors on your circuit if you`d posted on the ˜Build your own stompbox" subforum. Why did you choose to post here?

[Fancy Lime]

@Hatredman

Hi there,
I am new to the forum and I was under the impression that a subforum named "Schematics & Layouts. Find schematics and layouts here." would be better suited than one named "Building your own stompbox. Post your questions about building your own stompboxes." since, you know, I want people to find the schematic and not post a question. But I am aware that thou shalt not judge a forum by its name, so thanks for the hint.

I could not find anything on forum policy about moving posts, does anyone here know? Should I post it again on the other subforum? Should I then delete the old post or not?

Thanks and cheers,
Andy

[duck_arse]

fancy, push the button marked "report to moderator", and tell them what you'd like done. (some of them seem like OK people.)

[KarenColumbo]

Watch it, Eemu, here I come:

[duck_arse]

Watch it, Eemu, here I come:

[hey Mrs. Columbo - those are mosfets, static sensitive, don't like being rattled around in plastic boxes so much. you got any alfoil, or conductive foam, or an anti-stat bag to put them in?]

[KarenColumbo]

  Ooh! Thanks for pointing this out. Yeah, I got an antistatic bag - will put them in right away, Sir!

[R.G.]

It illustrates how to use p-channel MOSFETS in a negative ground circuit, which may be educational for some people here since I am unaware of any designs doing that for reasons I really do not understand.

Maybe I can help with this small point.

There is actually a long history of people attempting to improve older PNP designs that use a positive-side-grounded power supply by just moving "ground" to what was the -9V rail and making the previously "ground" positive side of the power supply be +9V. This is quite often tried to "fix" the PNP germanium Fuzz Face so it can be used with the more common negative-side-grounded power supply.

This often works. But not always, and for reasons that are subtle. In theory, it's fine, no problem just do it. In practice, there is a stream of people who wind up posting here about "my reversed-ground pedal doesn't work - please help".  In all the instances where the attempt has been to fix a PNP fuzz face (and not some other grosser error), operation is solidly restored by putting the power supply back the way it was.

That's because, as Yogi Berra said, in theory, theory and practice are the same. In practice, they're not. Theory says that it does not matter whether you use the + or - side of the power supply in a single power supply circuit as ground. Theory also quietly doesn't mention that theory also says that power supplies are true zero-impedance voltage sources. In practice, power supplies have some impedance, and doing the simple reverse-the-ground mod may or may not work depending on how much decoupling the power supply has to make both sides be a low impedance.

I notice that you've done a fairly good job in your schematic of decoupling the power supply. That's good practice in general, and may well be one reason you've run into no problems with inverting the ground so far. The other part of having these things not have excessive noise and oscillation seems to be lead dress and wiring, especially on the ground wires. Contrary to the presumption in schematic theory, real wires are not zero ohm devices, they are very low value resistors. This realization is what makes the whole topic of grounding and ground path wiring something that needs studied.

The higher the gain of the circuit, the more critical power supply decoupling and ground wiring get. And immediately changing some circuit where the positive side of the power supply wiring becomes the ground may unintentionally ( and un-theoretically)  introduce just enough and just the wrong phase feedback through those wires, and bango, it oscillates. This sometimes takes the form of sheer excessive noise as the RF oscillations are heterodyned back down into audio as random RF hiss.

That's one reason you don't see more of these here. Most of the reversed-ground versions work. Even more work if you do very, very good power supply bypassing. However, some don't, and usually the cure for that is to put the circuit  back to the grounding scheme the circuit started with. Your circuit was born cross grounded, but with good bypassing. So wiring layout will be the key in getting them working. But then wiring is one key to getting all high-gain distortion pedals working, too.

[Fancy Lime]

Hi R.G.,

thanks for the explanations! You are right, old PNP fuzz faces being modded to n-ground is pretty much the only widespread use of PNP or p-channel devices in n-gound circuits that I am aware of. I always dismissed the fact that that seems to only work sometimes and attributed it to two things: Fist of all, for me at least, modding often goes bad. If I have a perfectly working circuit and change anything, even the tiniest little thing like changing a tone capacitor, it stops working for reasons I can never figure out. Of course that cannot be the case if converting the FF mack to its original state makes it work again. The other thing that I always suspected to be a problem with modded PNP FFs was biasing. Transistor production in the 60s was not exactly super consistent and even though the FF circuit is not too bad at stabilizing bias for different transistor characteristics, there are limits. So I would expect that changing the power supply wiring might upset the bias enough for it to stop working because, like you said, wires (especially corroded, old, thin, poor quality ones that have been showered in beer and sitting in damp rehearsal basements since Woodstock) and jacks have resistances. But that's just me talking out of my chair warmer, I have no hands-on experience with that. Am I completely off on this?

But all of these things do not really apply to designing a new PMOS or PNP effect, which needs its own new bias anyway, do they? So what I am wondering is: Why no new designs? Is that just a tradition thing? I mean sure, for modern silicon BJTs it does not make a huge difference in sound (if any), so we might as well stick with what we have lying around our workshops. And P-JFETS have their own set of problems. But for PMOS or Ge-BJTs there is some benefit to be had, imho.

Cheers,
Andy

[R.G.]

Am I completely off on this?

No, not completely off. Those things do contribute to the fatalities, of course.

I have to admit, as I do in my postings on this subject when it comes up yet again, that I do not really understand 100% of the problem. If I did understand it, I could do a lot better than saying "here be dragons."

My experience with reversed-ground pedals started from doing a newly-commissioned custom build of some two-transistor pedal that originally had a positive-side-ground. No problem, I'll just do a reversed power/ground and be done with it. Needless to say, it didn't work. Some days of debugging later, I had found that when it had reversed ground, it oscillated in the FM radio band. The only change needed to make it work perfectly again was to reverse what was considered signal ground compared to the power supply, with concomitant changes to the grounds on input, output, and controls.  Note that the DC conditions inside the active circuit did not change, so bias was untouched. Neither were there beer soaked and corroded contacts and components.

Well, OK, I did try drowning it in beer in frustration at one point, but then I had to rebuild it, and the re-build did much the same.

I said to myself, surely this is a power supply bypassing problem or one of those RF black magic things that needs small ceramic capacitors sprinkled in at odd places to suppress/enhance/extend RF magnitude/phase response and/or suppress spurious feedback. Another few days convinced me that capacitor-sprinkling was far inferior to ground changes.

Ahah! (I said to myself) all I need to do it better bypass the power supply and this will go away, as it is one of Maxwell's Demons camped inside the electron paths and those are banished by good power supply decoupling.

No good. Make it positive grounded, it works. Make it negative grounded, it sings but refuses to work. I rewired it so often, including two new and different versions of hanging-garden star grounding, that I wore some leads out. Still, all that works is putting the ground back where it said to in the schematic. I ran out of tricks, both of mine and my grizzled ham-radio friends who also happened to be degreed and practicing EEs. Their collective wisdom was "don't do that, most especially if it works with the intended power supply wiring".

I still do not understand the phenomena completely. It gets better with good power supply bypassing and immaculate wiring, as those have fixed some other things I have done with reversed ground power wiring. But I have run into a few others that can't be fixed with any combination of tricks I've collected over the past 40 years.

I'm forced to conclude that indeed, sometimes there are dragons there. Maybe not often, but sometimes. The incidence is not zero.

But all of these things do not really apply to designing a new PMOS or PNP effect, which needs its own new bias anyway, do they?

No, your comments don't apply to new PMOS or PNP effects. But Nyquist and Mother Nature do. I don't think that it's always bias. Every hard case I actually had in my hands was RF oscillation, including oscillation at frequencies that a 20MHz scope thought didn't exist. PMOS, with its very high frequency response, has the potential to make that worse, which is why I was on the point of suggesting that you put 100R to 1K resistors in series with those MOS gates as close to the FET as you can get them, as some situations can have MOS devices oscillating from lead inductance leading into the gate, capacitive load on the source, or both. But I digress. I don't think that just getting the bias right and not changing is a slam-dunk answer. It's an answer to some degenerative cases, but then...

So what I am wondering is: Why no new designs?

I suspect that it's just become a positive-power world. Silicon's leakage superiority had it simply eat up the bipolar world from germanium, hole and electron mobility be d@mned. Silicon is more naturally an N-type, although advanced processing has recently (last decade or so) made PNPs as good as NPNs, mostly. I think it's more that you have to have a positive reason to pick a harder to find and more expensive part to go P-type.

But for PMOS or Ge-BJTs there is some benefit to be had, imho.

And that leaves us with opinion. Well and good. Many if not the vast majority of effects are set up using the side effects of devices and circuits. If you have discovered that you like some characteristic of PMOS, great. Go with it. There's no reason to do any of this at all unless we are discovering new shiny things that please us for some reason.

Gemanium BJTs - that's another matter. The Fuzz Face has had an  - um, well, effect, I guess - all out of proportion to what it really does. It is a good idea to remember that simply rubbing germanium on an effect does not make it sound better. The purveyors of effects which are simply the 27,386th clone of the Fuzz Face with secret-sauce "mods" to make it sound like flying saucers and lasers would have you believe that germanium is magic, even if it's just used for an emitter follower in front of everything. The Fuzz Face itself was not universally good, and many of the real, no-fooling Fuzz Faces sound simply terrible compared to today's clones, as we have learned to measure the gains of the transistors, sort out the obviously bad ones, then compensate bias for the remainder. But simply using germanium isn't magic. Maybe someone can design a new circuit using a side effect of germanium if there are enough germaniums in the world left to use.

So don't take this as any kind of "don't do that" message. Go do what sounds good. But when and if you run into one of the funny situations, perhaps after increasing the gain a bit, or doing a new layout, remember the caveats. You'll almost certainly be able to work around the issues, having been forewarned.

[Fancy Lime]

Hi R.G.,

thanks for sharing your insights and experiences with this. But I'm young and surely the caveats of previous generations don't apply to me. I'm not afraid of dragons... and that was the last thing they ever heard of him.
Kidding, of course. Seriously, I really appreciate the warnings but will venture deeper into that territory regardless. I'll write home to you when I meet the dragons. It sure helps knowing that others have struggled with this too.

About Ge PNPs: Well, maybe I should have been more specific. I did not mean they sound better. In fact, I have never heard a compelling argument why they should (other than "thats what Jimi used" and even that...). In my limited experience with fuzzes, there are soooooo many factors influencing the sound, that the transistor variations apart from hfe pretty much drown completely to my ears. The material of the transistors should not really matter at all. So, no I'm not buying the whole "Ge sound better" thing. It may sound great when used right but o does Si. What I meant with benefit was that you can sell a Ge-fuzz for twice the price if you do the marketing right and leave your soul on the coat hanger by the entrance. Not my type of thing but boy, oh boy are there many people milking that imaginary cow. I don't mean to hate on those who build Ge fuzzes, they can be great. But at least for commercial pedals I consider it mostly a marketing gimmick. For self made ones its often the "I found a bunch of old Ge tin cans and want to do something fun with it", which I think is a great reason.

For the PMOSs: There seems to be a fundamental reason why certain (not all) PMOSFETs are less noisy than NMOSFETS. I read in some post somewhere (don't remember where) that this has to do with the dopant and doping process of the source and drain regions. But that was only a half sentence in a post about something else, so I don't really know how reliable that is but it kind of makes sense to me. The same post claimed that only "large" (whatever that means) PMOSs are less noisy because smaller ones use a different doping process. I did not really make a effort into finding out why the BS250 is quieter than the BS170 and was just happy that it is, at least the batches I have. But with typical MOSFET variations that may mean nothing in terms of a general rule.

Cheers,
Andy

[R.G.]

I should probably explain the "dragons" remark. Back when the Europeans started exploring the world again after the dark ages, they started drawing maps. The maps were necessarily limited in scope as there were large regions where no European had both gone and returned from.  The map makers, not wanting to appear to not know what would out there, would often mark the places just outside their known places with the inscription "Here Be Dragons".

It's kind of a late-Dark-Ages/early-Enlightenment in-joke.


Of course any dragons that were there, when they saw Europeans coming in quantities too large to kill and immediately eat, ran screaming away.

Smart, them dragons.   

[Fancy Lime]

Hi R.G.,

yes, I got that reference and thought it was very fitting. That's why I spun it further. Is that a strange one? I assumed that the "here be dragons" marker was common knowledge. Then again my work tends to involve maps a lot, so maybe I'm not representative. Also, I'm a huge Terry Pratchett fan and he makes this sort of reference a lot. Not sure about that one in particular but the kind of thing certainly.

But, yeah, thats one of the problems of internet forums. You can't read peoples faces for reactions so you don't know if they understand subtle references, let alone irony and sarcasm. Then again, its still much better than never communicating with all these people at all.

Cheers,
Andy

[Fancy Lime]

Here some pics of my build. Enclosure was etched with ferric chloride plus a bit of hydrochloric acid. Ferric chloride alone etched well but left so much FeO residue that that stopped the etching. Box was then spray painted back and sanded again but not evenly so it got this rough-around-the-edges feel. Finally three matte clear coats. I'm rather pleased with the result, I don't mind saying. Anyone know how to post audio here?





Cheers,
Andy

[swever]

http://picosong.com/ works great if you have an mp3 file. You can also create a soundcloud to keep all your sound samples in one place and organized.

The box looks great!

[Fancy Lime]

Hi,

I uploaded a sample to the gallery:

http://www.aronnelson.com/DIYFiles/up/EemuPreamp.mp3

Bass is a Cort B4 fretless with the active electronics removed. Best 400€ I ever pent on an instrument. Sound mediocre with the on-board preamp but if you take that out and use it passive it is absolutely fantastic. Signal chain: Bass -> Eemu Preamp -> focusrite scarlett 2i4 -> Ardour4. No effects, compression or eq-ing in the software.

First riff is dry, then I go through some different setting that I like. Please excuse the playing quality.

Cheers,
Andy

[Fancy Lime]

Re-upload of the schematic by popular demand:

https://postimg.cc/tYrdvyfF

Cheers,
Andy