Ach, shame, I will be moving to Perth 2-3 years from now (my missus is from Armadale, but we are currently living in Norway). It's a pity that small businesses like yours are folding up one after the other. Hope you'll end up finding someone to take the torch, mate

Geez, sorry, been away for a while.
@Miyagi, maximum signal amplitude is determined by the feedback resistor of the opamp - bigger resistor -> more amplitude. The resistors at drain and source will determine the symmetry and, to some degree, the gain of the differential stage. I would have to check my sims to give you a more complete answer, but I am currently traveling and with no access to my computer. Will get back to you with more info when I'm back
@J M Fahey, thank you for the compliment means a lot to me, esp. coming from you.

If you are talking about the last bjt, emitter follower indeed, I wouldn't go above 10k unless I wanted to purposefully introduce "cathode follower"-ish distortion.
FWIW I breadboarded the 3-stage version of this circuit, not the 4-stage one, and there was plenty of gain and still some wiggle room to push it with a TS without turning into white noise. YMMV
Oh, and ref. biasing, nothing wrong with using 2 resistors in series to get to the right bias either.

Yes, bipolar supply. Look for Gallien-Krueger 200RG model (and similar very early 80s models) schematics to see where that particular gain stage originates : ) they use J113, but for these particular applications, depletion mosfets behave like jfets.
That stage CAN be built to single supply, but then we'd have to tinker with an additional Vref, and since I am running the whole preamp on bipolar, there's absolutely no need to go down that rabbit hole.
GK schematics are an extremely useful resource on how to bias and design jFet gain stages, btw. Most if not all of their guitar amps from the late 70s on (and a lot of their bass amps) feature jfets in one form or another.

Yes, I did use 2N5089. I tried 2N3904 but it did not work as well
BC550 should give you a similar result to 89s though.
Bear in mind that I tinkered with it for less than 1h, so it was a bit of a summary test.
As for the voicing, I'd say marshall-ish with a raw, unfiltered edge to it. Into the right amp it can sound quite nice (think Black Sabbath, Pentagram, etc), but I prefer a bit more filtering and tone-shaping. While it sounds quite full and "amp-like" (not a term that I like, but...), and it has a nicer feel than your standard boss disto-box, I would still be partial to other multi-stage pedals like the AMT Legend ones, for example (check on FSB for schematics).
Oh, I tested it on the clean channel of my Peavey Bandit silver stripe, low input and bright switch off, which is an amazing pedal platform IMO.
Yesterday I was fiddling with a distortion circuit of mine which is quite similar in its topology and voicing, but using LND150 depletion mosfets. To my ears, they sound much more pleasant than BJTs in this kind of cascading gain stage circuits. I will probably post it at some point in the coming days, as I am pleased enough with it to share it 
Again, breadboard and see for yourself! lotsa fun can be had with this circuit, whether you tinker with the values or not.

If R9 is not connected to ground and/or Q2 souce, you will probably get 24V at Q2 drain. also Q3 could be fried and sending its drain voltage back into Q2's drain.

Well I breadboarded it early this week out of curiosity. A few observations:
- 18V instead of 9 does not make much difference.
- bias is finicky, get as close to 4.5V as possible (using 2 resistors in series or a trimmer).
- while the voicing of this circuit is not exactly to my liking, it is undeniably well designed: tried other coupling caps, bypass caps and so on, and it quicky went south. Only tried stuff for about 20min, though.
- snubber caps collector-to-base -> Nope. A 2.2n cap across the last collector bias resistor worked MUCH better to tame the highs.
- somebody at DIYStompboxes suggested using other diodes. I tried BAT85 and it sounded pretty good.

I will have to revisit it at some point during summer, as it definitely has a nice vibe. Hope this helps!

Wish I could, but all my recording gear is on the other side of Europe now and I won't be able to access it until christmas at earliest    phone recordings don't do it justice, unfortunately. I might build one in a stompbox and send it to a mate to test it and record some clips during summer, we'll see...

I'd try 10pF to 22pF, base to collector, similar to the trick employed to smooth down silicon transistors in fuzzes, but that's a matter of taste
I had a bit of downtime this arvo and threw the Vulcan on LtSpice: the frequency response is VERY similar to the Randall RG100Es, so you'll probably hot rodded marshall vibes. That doesnt mean that it will sound like a Randall, though: clipping thresholds and gain increments are very different.
I was also surprised to see that the circuit, as is, is already shaving off some dBs above 6kHz.
I'd be tempted to try it at higher voltages, with low hFE transistors, just for the sake of it.
Oh, and the diodes do mimic grid clipping: they chop almost half of the waveform before the gain stage. Very clever design with just a few parts

Hi there!
I have this one on my to-breadboard along the line.
Seems to be marshall-voiced, judging by the filtering, and pretty high gain.
Might be wrong, but perhaps the diodes mimic grid saturation as in a valve amp.

Indeed breadboard first, especially if using other trannies, and check the voltages. Personally I'd try snubber caps on the last two gain stages.

Good luck!

Thx G1!

Attached some screenshots just after the push-pull mimicker:

Attached schematics:

Hi all,

First post here, long time lurker though - I figured I'd start with a little something that I have been working on in the past weeks.

I wanted a fairly neutral preamplifier circuit to use with dirtboxes and various AIABs, but with the ability to have some flavour and zing of its own. After several iterations and massive overhaulings to an otherwise unremarkable starting point, I arrived to this.
For the sake of simplicity, it can be split in two parts:

First schem shows the input stage, tonestack and limiter/soft clipper.
- Input stage is somewhat ripped from the Ampeg B2. I did try jfets, transconductance stages and whatnot, but in the end I wanted transparency, so buffer + variable gain stage it is. There is a "bright" switch too, because we still want to have a nice and chimey clean sound on its own.
- TB tonestack is a Voigt, just for the sake of it, and because I wanted a) passive, and b) non-interactive. I tweaked the values to get a flat response at the middle of the pot rotation. Fender have a pretty nice passive and non-interactive tonestack in some of their models (Frontman, etc) but I was looking for a frequency response more in line to what I am used to (FMV). Completely subjective choice, though. The drawback of a Voigt tonestack is its high sensitivity to load and very high gain loss, so it is followed by a makeup gain stage.
- Mids control is your bog-standard mids filter: just a scaled-down FMV with treble and bass removed. The switch changes the frequency response of the mid-dip from JC120 to JCM800-ish, which is convenient to have depending on what dirt flavours one is using upstream of the preamp.
- From there we go into a GK G.I.V.E-styled soft clipper. Depletion mosfets gave me a slightly mellower sound than the prescribed J113s from mr. Gallien, so I went with that. The advantage is also their regularity in specs (no massive differences from one device to another), so it allows for the trimpot on the drain to be removed altogether. The resistor in the feedback loop of the transimpedance stage determines the amplitude of the wave, and I kept it fairly low because of what comes after. This stage is the first to clip, and does so in a very gradual and symmetrical way, not too far removed from a LTPI.

The second schematic shows the push-pull power stage mimicking and presence/resonance circuits.
- I had this crazy idea last year when tinkering with the Peavey T-Dynamics circuit. I wanted to have a reasonably simple approximation of some squish and dynamic, without having to resort to feedback from poweramp or other complicated contraptions. The Traynor Dyna series have a pretty cool way of emulating that (sans the crossover distortion), but I hated the idea of using only one half of a LM13700. The Ampeg SVT-8Pro has another very creative way of doing that, which I breadboarded and tried, but no luck: sounded like crap with a clean preamp, and only good if some distortion was added upstream. In the end, I took two of Ampeg's flexwave stages in parallel and made them "push" in opposite directions, while clipping half of the wave: the result is that of a semi-slow compressor that gradually introduces crossover distortion. It is followed by back-to-back 1V Zeners, on account of, y'know, power amp clipping and such, and also to make 100% sure that the following stages do not hit the rails.
- Then comes volume control, followed by a Presence and Resonance circuits VERY similar to those of Rocktron power amps from the late 80s-early 90s. The main difference is that I added a pot for each, to be able to tweak them individually, as opposed to a simple mix pot from the original circuit.

Other considerations:
- I am using LF353 opamps because right now I have more of those than TL072s, and because they sound fairly nice here. Feel free to try other opamps if you are breadboarding or building it, but should make no difference.
- For the last clipping stage, other clipping configurations can be used, provided it is not above 4V ptp.
- Other Jfets or DMOS should work well on the soft clipping stage. Experiment, etc.
- The tone stack is tweaked to the frequencies of standard FMV tonestacks, minus the interaction, but can yield good results with the original Voigt values and, say, the contour from Marshall's Valvestate, or the Focus from several crate amps.
- Resonance is tuned at 120Hz (closed back 412 cab), but other freqs can be used by changing C26 and C27.

Disclaimer:
- NO CLAIM is being made on the superiority of this circuit above any other circuit, quite the opposite. I simply followed some random ideas I had and put all this together; it works and it sounds how I wanted it to sound, and does what I wanted it to do, and that's all I need.
- Yes, I am aware that simpler means can be used to the same goal, but again, my point was to get an idea to reach a concretion (and have fun and learn as I do so). I am only a hobbyist, not an engineer (I wish)

I will work on some modular veroboard layouts in the coming weeks, but as of now, circuit is still on the breadboard, so please do let me know if you have comments, suggestions, observations or criticisms, or if something is terribly wrong somewhere in the circuit.

Cheers and enjoy!

/Alex