And the project is complete in case anyone's interested:

woohoo! finally! I was missing the coupling capacitors. Sorry about the monologue

ORIGINAL CIRCUIT



BIASED CIRCUIT

...and of course, once the opamp inputs are corrected, now I don't get the same frequency response anymore or anywhere near the gain of the original. What gives?

ORIGINAL CIRCUIT



BIASED CIRCUIT

Jesus christ, am I dumb or what... I had the opamp inputs reversed. 

Damn it, I had another mistake in the biased circuit. The input resistor R4 was set to 680 instead of 680K. What I don't understand is, when I set it to 680K, the simulation output does this... minus 200dB? wtf? shouldn't the gain increase when increasing the input impedance? It seems to work fine up to 150K or thereabouts, but if I increase it further, I start losing tons of gain. Any pointers please?

Quote from: Enzo on July 09, 2018, 08:36:41 AM
You have a dead short wired across C4 and R2 in one of them.

Oh, that's just the way I wired the 22K gain potentiometer. But there was a wiring error anyway. R9 goes in between R2 & C4 instead of ground.

Actually now that's corrected both look more similar

OK, I've been doing some reading on the subject over the weekend and managed to simulate the circuit with LTSpice... and I think I've got it. Here's the original circuit:



And here's the biased one I came up with



The response curves are not quite the same, I take it the added capacitors bleed out some frequencies (acting as high pass filters?) but they're close enough I guess. Maybe I can tweak C2, C3, C4 & C7 to get as close to the original as possible.

One thing I don't quite understand is whether C4 is really necessary or not. I put it there because I believe I need to DC decouple the inverting input of the second opamp, but the simulation works almost the same without it.

So should I leave it there?

I have breadboarded the first stage of the preamp and it seems to work fine, but haven't got around trying the whole thing yet, so I'm relying on these simulations so far

This is where I am right now.

I'll be breadboarding it this weekend and will also try modeling it in autodesk.

Quote from: Enzo on July 05, 2018, 10:16:18 PM
Not the original, the original did not have the op amps offset from ground.  Pin 7 on the original would be at zero DC.  Now it will be sitting at +7.  Thus now ther will be DC on the controls where it wasn't before.

ouch  I see, thanks

Quote from: Enzo on July 05, 2018, 06:21:18 PM
Grab a dual op amp from your drawer and dummy up that exact circuit on your bench...find out.

Thanks Enzo, will try that

Quote from: Enzo on July 05, 2018, 06:21:18 PM

Pin 7 has a DC path to ground through the bass and mids pots.  They will be scratchy

Weird, that's the original schematics. I'll add a coupling cap at the output of U1B

Hi guys. I'm trying to modify the circuit of a  Marshall Lead 12 preamp to use it with a single +15V power supply, so I need to bias the opamps to Vdd/2.

The first attachment is the original circuit, and the second is my attempt at biasing those opamps. I'm not too sure about the second one, does it look about right?

Thanks

Quote from: phatt on June 27, 2018, 06:51:49 AM
Well it sounds like the power amp is unstable and oscillating.
As we have no idea of how you wired it up it's anyone's guess as to what has gone wrong.
Maybe post a schematic of the power amp you built and also a pic of the actual circuit.
Phil.

I used one of these pre-built TDA7297 boards for the power amp (pic bellow)
The thing I tested a couple other small amps I have (the 1W tin can TDA67A based one and an aftermarket headphone amp) and those also do that annoying 5.5Khz intermittent squeal. I gets attenuated quite a lot if I touch the lead casing, so I'm guessing it's picking up some mains interference and maybe I need to double check everything is properly grounded. I'll see if I can test it somewhere else later today

I have an issue with the amp, not even sure if it just started happening or it always did it and I only noticed it now. It's making an intermittent, high pitched hissing/squealing noise at a frequency of 5.5KHz or thereabouts. I've ruled out power supply ripple by powering it with a pack of lithium batteries. I've been googling it and ordered some ferrite beads just in case, but I believe that won't help since it's supposed to filter radio frequencies well above those 5.5KHz. I've tried bypassing each of the 3 opamp stages in the preamp and managed to rule out the second one as the source of the problem, but when I bypass the other two, the whole thing goes nuts and starts buzzing real bad (oscillation perhaps?). I'm just about to try bypassing the entire preamp to make sure the problem is there.

I should mention I added gain pots for opamps #1 & #3 with switchable clipping diodes (asymmetrical) in all three opamps, not sure that's relevant but anyway.
Also the noise is there with no guitar, just the cable plugged in the input, but goes away if I unplug the cable.

Any ideas on how to troubleshoot it please?

ETA: I just connected the output of an overdrive pedal to the power amp's input and it's still doing the same thing, so I'm guessing it's not the preamp? But then again I also built that OD pedal so it might have the same problem (both layouts and overall build quality are atrocious TBH). I'll try a couple other pedals, I'm afraid also builds of mine...

ETA2: It's still doing it with the the other two pedals as preamps, so that seems to rule out the preamp entirely.

Un-friggin-believable, I figured it out! finally!
Turns out the volume pot was bad, but it seemed to be doing what it's supposed to do: control the volume  :lmao:
By just by sheer luck I accidentally poked it "the right way" as I was testing the amp for the emptienth time and bam! it worked for a split second.

Man it was driving me crazy. haha

How can a pot fail in such a misleading way? Do they have no decency? Won't somebody please think of the children?   :grr

Quote from: phatt on March 18, 2018, 07:15:16 AM
A bad move cause it's a Hi Z tone circuit and can't work into a 10k load.
If you want to hear how sad the outcome will be then just temporarily bridge R12 (1Meg) with a 10k resistor. Your tone controls will be rendered close to useless. :duh
Phil.

Damn, I see. Thanks Phil.
So if I try wiring the tone stack like that, with those caps and resistors at the pots, the ones I put in the board (R11, C10, C11, C12) will be left floating. We'll see how that goes