passive preamp with buffer

[audio-kraut]

Hi,
I'm planning to build a passive preamp with the following feature:

three phono inputs, switched to a phono pre amp (studio type, with balanced out, the board I have already and it is very quiet)
one balanced input
two single ended inputs

all inputs will have level control through some noble pots
all single ended inputs get convertet to balanced via OPA 633 and then fed to a buffer amp build with the OPA 627 (all those I have in my kit)
Output is balanced only and will feed a digital chain.

For the converter circuit I will use the one in the opa 633 manual.

Any suggestion or thoughts?

Thanks for any deedback in advance

 

[pinkmouse]

Well, for a start, it's hardly a passive preamp!

 

[audio-kraut]

Passive in the sense that the buffer does not supply any gain, but I think I need? it because of the level controls.
The only gain in this amp will be from the phono pre.

I have chosen the opa 633 conversion to balanced as the transformer solution is rather expensive.

 

[anatech]

Hi audio-kraut,
Yes, you should use buffers to drive the output. They should give much better performance than transformers if you build them right. I question the balanced thing, but okay.

TI / Burr Brown have a new single ended to balanced converter chip you can try. It's the DRV-134.

-Chris

 

[audio-kraut]

I question the balanced thing

thanks for the hint.
I like to stay balanced as I feed into a balanced digital processing chain which right now consistst of the scr/deq/dcx 2496.

I thought it therefore cleanest to run balanced right through the preamp, especially as the phono preamp board outputs balanced signal.

Switching will be done by multilevel five pole switches.

As to the buffer circuit including the opa 627 - the only one I could find is on the burr brown data sheet. Any others out there?

This is my first time trying to design something from scratch w/o the benefit of a well funded electronic education, so I need some help sometimes, even in an apparent simple matter as this project.

 

[anatech]

Hi audio-kraut,
Ah, the number I gave you was a convertor. Look for a Diamond buffer circuit. You may find it in RF circuits.

-Chris

 

[audio-kraut]

Just checked again - I am using the DRV134 (free samples) as the converter, not the 633..too early on a sunday afternoon I guess.

Wanted to use the 627 as a buffer because I have some left from previous "upgrading" projects on some phono preamps.

 

[anatech]

Hi audio-kraut,
The DRV134 doesn't need a driver. It will do all that by itself. Better than many op amp driver type circuits. It does "want" to see a low impedance source, so a buffer before the DRV134 is a good idea.

So all you have to do now is focus on a quiet regulator circuit and wiring. Esp. the grounds.

-Chris