Marshall Lead 12 - But It's a Pedal

[Box_Stuffer]

I want to make a pedal out of the Marshall Lead 12 pre-amp section. I have seen it done before. I found this schematic for the amp. The part I am interested in is the upper portion. It is basically an over-driven LM1458 with a 3 band EQ.

It does not show any power in or a voltage divider on the input signals. So I guess I need to know if the voltage divider would come just before the input resistors shown (R2 R3)? Would I need a separate divider for each input or just one and connect it to both inputs?

[stallik]

It shows +19v to IC1 pin 8

[Ripthorn]

I did this same thing for the 5210. It's a great overdrive. Just scale it for single supply voltage. It takes some careful checking, but not hard.

[Elijah-Baley]

Maybe this could be useful?

http://tagboardeffects.blogspot.com/2018/10/marshall-3005-preamp.html

[duck_arse]

It shows +19v to IC1 pin 8

..... and - -16V IC1 pin 4. bottom left corner.

[Box_Stuffer]

So if you have a +V in and a -V out, then you DON"T have to make a voltage divider for the input?!

What would happen if I run it with 9 volts - mainly for convenience? I have seen how to make +V / -V using 2 9volt batteries hooked together, but where can I find how to do it using a 9volt wall wart power supply?

[Box_Stuffer]

I did this same thing for the 5210. It's a great overdrive. Just scale it for single supply voltage. It takes some careful checking, but not hard.

Can you tell me more about scaling it down? Power is what I was wondering about. Can I power it with 9 volts to make it simpler? I'm a noob and I have only made a very basic single stage op-amp circuit so far. It used a simple voltage divider and I have not worked with +V / -V yet.

[GibsonGM]

Opamps using bipolar supplies (both plus and minus) don't need a reference voltage - their reference is ground, zero volts!  The power supply (generally) swings positive and negative around this point so no 'special' reference needs to be set.  Using a unipolar supply was kind of a 'hack' done long ago so that the consumer market could access the wonderful world of opamps.

You CAN slap a +/- together from a wallwart, but proper design/build practices inform us that we should make a 'real' power supply.   

One way is using 2 voltage regulators (plus and minus, of course), and another common method is to use an IC and small circuit to do this, taken from the link Elijah posted above:

http://tagboardeffects.blogspot.com/2014/01/bipolar-voltage-converter.html

I'd be partial to making the IC version...leaving the LM78XX regulators for something larger like an actual amp, since they take up more space.   

I'd suggest learning more about how and why we set up power supplies this way, to be either unipolar or bipolar.  The preamp you want to work on isn't complicated and should be 'hackable' to run on say, a 12V uni supply and still sound good.

There are also TONS of dirtboxes you could do first that run on 9V to become familiar with how this all works Best thing about this hobby is that you'll never run out of projects!!

[Ripthorn]

Read up on single supply versus bipolar supply opamp stages. It has to do with how each opamp stage is biased. Here is my project page that gives some info: https://scientificguitarist.wixsite.com/home/the-beatdown

[kenjib]

Maybe this could be useful?

http://tagboardeffects.blogspot.com/2018/10/marshall-3005-preamp.html

There was more than one version of the Marshall Lead 12 as well. That circuit (and the schematic in the original post) looks like the op amp clipping version, which I think is the newer version. I have one of those and it sounds great. It's too bright but I think that's probably the speaker's rather than the amp's fault. I have to try it through my 2x12 one of these days.

The infamous ZZ Top "My Head's In Mississippi" Lead 12, however, I think was the older version which uses diodes for clipping and gets a heavier nastier tone when you push the gain past a certain point. but I might be remembering wrong. You will need a different circuit for that tone if that's what you want.

I would note, too, that the gain on the op amp version does also sound really great when cranked and also takes gain pedals really well... It's a fun little amp but you won't quite get the ZZ Top tone out of that circuit IMO. Close though.