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Keelhauler MKII - a reasonably nice sounding preamp

Started by Umlaut, May 12, 2024, 11:39:24 AM

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Umlaut

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





Umlaut


g1

Nice work and great name for your project.   :)

Umlaut

Thx G1! :)

Attached some screenshots just after the push-pull mimicker:

joecool85

Looks interesting. I'd love to hear some clips!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Umlaut

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...

Miyagi_83

That's an interesting idea you've got there. Say, are you running those LND150's at +/- 15V? Am I reading the circuit diagram correctly?

Umlaut

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.

Miyagi_83

Quote from: Umlaut on May 21, 2024, 03:18:58 AMGK 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.

So much to learn, so little time...  :)

J M Fahey

Looks VERY good in general.
Specially the third screen capture which shows a waveform with a kink near zero crossing plus rounded tops, quite close to a Pentode push pull clipping, the expected "tube power amp clipping". 👍🏻

Miyagi_83

@Umlaut
Hi, in practice, what's the maximum signal amplitude coming from that GIVE stage? Is there a limit?
Best regards,
M.