I'm working on pedal recreation of the preamp section of a solid state guitar amp. I want to stay true to the original and use the same or similarly spec'd op amps throughout. Due to the circuit design, I could either use two separate single op amps for the input and output sections, or use one dual op amp for both – one side each for input and output. This is a cascaded high-gain preamp and the output could potentially swing up to ~24V peak-to-peak, while the input just provides a modest boost to the raw guitar signal. Is this a recipe for oscillations due to the proximity of both signals in the same op amp? Or could it be enough to keep the actual traces well away from each other to avoid any issues?
Using the same dual op amp for both input and output stage - bad idea?
- Thread starter lowpitch
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Blackboarcult
Member
Not the best practice IIRC, but depending on the circuit you might get away with it. Would that be the Randall X2-G3 or similar?
I suspected as much. Some of those older Randalls look intriguing for sure, but I'm adapting the boost channel of the Marshall Valvestate 8100 with (probably) FETs for tubes. Uses TL072 and obsolete M5201 switching op amps throughout, which seem to be very similar in specs to plain old 4558s other than the switching capabilities.
Blackboarcult
Member
Funny, I breadboarded that channel two weeks ago, with LND150 in lieu of tubes, and the (heavily modified) diode section of the Ethos twe-1 instead of leds. Quite nice sounding actually, I can post a schem if interested. I used TL062 for opamps (a tad less harsh than 72s or 4558s), ran the whole thing at 18v because I couldnt be bothered to set up a bipolar charge pump, and used the other half of the last opamp as buffered Vbias for the whole thing. Generally, there's little difference in sound between 18v and 24v IMO.
Cool idea. I'd like to check out that schematic.
Currently planning to build it stock with a DC-DC converter for +/-15V, then see what I do and don't like about the circuit and go on from there. Simulations in LTSpice seem to suggest a bit of a lack in bass content, but it's hard to hear with your eyes only.
Would be curious to see the schem too. I recently went down the LND150 rabbit hole. I'm quite keen to experiment with them some times soon.
Blackboarcult
Member
There goes.
I find that biasing LND150 with a diode gives a bit more snarl than resistor+cap, but ever so slightly lower gain.
I didn't emulate the feedback around the valve stage because it would have defeated the mosfet grind I was getting
"cathode follower" can be anything you fancy, bjt, mosfet, jfet... not much difference.
The whole section after it could be replaced by the 8100 tonestack verbatim, but I was aiming for a pedal to put in front of a clean preamp, not a full-blown preamp.
Opamps can also be whatever, I preferred TL062, but TL072, LM1458, RC4558 all work. Difference is minimal.
C51 can be increased to 1n, 2.2n or 3.3n if needed, depending on how sensitive one is to hi-freq nasties.
R27 is big in order to make the response of contour stay only in the mid frequencies, a lower value resistor might decrease lows along with contour pot rotation. Best to use a jfet opamp for the buffer that comes after.
R49 and R53 can be lowered for more sag, or increased for a stiffer response.
I used a W005 bridge rectifier for the diode clipping, as Marshall used them in a few amps and I had them lying around, but any diode array will work the same (1N914, 1N4001, etc). I just like the 1 part vs 4 on the breadboard factor
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This topology is super interesting. http://milas.spb.ru/~kmg/jcm800fetLnd150_en.html