passive preamp with buffer

audio-kraut · 12th February 2006, 06:29 PM

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 · 12th February 2006, 06:34 PM

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

audio-kraut · 12th February 2006, 07:15 PM

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 · 12th February 2006, 07:44 PM

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 · 12th February 2006, 08:04 PM

Quote:

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 · 12th February 2006, 08:06 PM

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 · 12th February 2006, 08:11 PM

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 · 12th February 2006, 10:56 PM

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

12th February 2006, 06:29 PM	#1
audio-kraut diyAudio Member Join Date: Apr 2004 Location: 12km off the alaska highway in northern BC	passive preamp with buffer 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

12th February 2006, 06:34 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Well, for a start, it's hardly a passive preamp! __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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12th February 2006, 07:15 PM	#3
audio-kraut diyAudio Member Join Date: Apr 2004 Location: 12km off the alaska highway in northern BC	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.

12th February 2006, 07:44 PM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

12th February 2006, 08:06 PM	#6
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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

12th February 2006, 08:11 PM	#7
audio-kraut diyAudio Member Join Date: Apr 2004 Location: 12km off the alaska highway in northern BC	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.

12th February 2006, 10:56 PM	#8
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	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