Biasing opamps in a Marshall Lead 12

[dazz]

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

[Enzo]

Grab a dual op amp from your drawer and dummy up that exact circuit on your bench...find out.


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

[dazz]

Grab a dual op amp from your drawer and dummy up that exact circuit on your bench...find out.

Thanks Enzo, will try that

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

[Enzo]

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.

[dazz]

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

[dazz]

This is where I am right now.

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

[dazz]

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

[Enzo]

You have a dead short wired across C4 and R2 in one of them.

[dazz]

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

[dazz]

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?

[dazz]

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

[dazz]

...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

[dazz]

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

ORIGINAL CIRCUIT



BIASED CIRCUIT

[dazz]

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

[phatt]

Err??
It may pass signal but it still not right,
Pin 5 has no bias 
R9 needs to go to pin5
There are other problems as well which are a bit hard to explain.
Some circuits are hard to convert to single supply and you will have to alter more than R9.
I'll see if I can redraw it for you in the next day or two.

Opamp Rule of thumb; The positive input has to have a DC path to a reference voltage for single supply or ground if it's split supply.
Phil.