Combining single- and dual-rail devices. How to?

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Combining single- and dual-rail devices. How to?

Postby meffcio » 19 Jan 2013, 23:44

I want to merge an old Marshall Lead 12 (3005, schematic) preamp with Taylor's (from DIYSB) Tiny Giant amplifier (schematic and info). What disturbs me is that TG is a single-rail amplifier, and the Lead 12 preamp is designed for dual-supply use. I'll be using this setup with a regulated laptop power supply, because I don't want to mess with transformers and stuff like that. How should I deal with the supply rails difference?
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Re: Combining single- and dual-rail devices. How to?

Postby meffcio » 22 Jan 2013, 23:20

I've come up with that kind of idea:
schematic.png

I suppose it is going to work, but I'm not sure about value of the input cap. However, I'm pretty sure there's impedance mismatch between pre- and power amp. How to deal with it? Should I restore the buffer? Or a simple tone stack scaling would do the trick?
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Re: Combining single- and dual-rail devices. How to?

Postby Dirk_Hendrik » 25 Jan 2013, 13:35

Must!
- Make R4 something like 470k and move it to the + input of the opamp instead of the - input. You're now adding a DC component to the output signal of 5.6*Vb. While it's not clear how high Vb is it suggests that the DC level on the output will be at Va level (unable to go higher) all the time.

can do (but it should work in your current schem though since Vb is a "virtual ground")
- C2 can go to ground just as well.
- The Tone stack components (C18 and the mid control) can go to ground provided you add a cap in between the 2nd opamp output and the tone stack. This would have my personal preference. Bring the master CCW connection to ground in that case as well.
Sorry. Plain out of planes.

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Re: Combining single- and dual-rail devices. How to?

Postby meffcio » 25 Jan 2013, 16:47

Well, certainly an opportunity to learn some stuff.

Dirk_Hendrik wrote:Must!
- Make R4 something like 470k and move it to the + input of the opamp instead of the - input. You're now adding a DC component to the output signal of 5.6*Vb. While it's not clear how high Vb is it suggests that the DC level on the output will be at Va level (unable to go higher) all the time.

It's funny, because the schematic is straight-forward copy of Marshall's amp. I suppose the problem occured because of what I'm trying to achieve here about powering this thing, am I right? Or the original design is faulty?

Dirk_Hendrik wrote:C2 can go to ground just as well.

What about lug 2 of the gain pot then?

Dirk_Hendrik wrote:The Tone stack components (C18 and the mid control) can go to ground provided you add a cap in between the 2nd opamp output and the tone stack. This would have my personal preference. Bring the master CCW connection to ground in that case as well.

Thought so, but wasn't sure. What cap value would you recommend? Or is it really a matter of trial and error?

This is what I understood:
schem2.png
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