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carlmart 30th April 2002 04:28 PM

Preamp & headphone amp designs
 
Hi!

I am willing to try some preamp and headphone amp quality designs.

The first option I am considering are two variations designed by Walt Jung, one published in Audio Electronis (when it was The Audio Amateur) and the other on a three-part article on Electronic Design.

The first used an AD744 on its first stage and an AD811 on the second. The other uses an AD823 and an AD812 respectively.

On the second design he applied more local feedback for error correction and exgtend bandwidth. The chips are also dual, probably to make a more compact stereo arrangement.

A variation of this circuits can be implemented as a headphone amp.

Has anyone built these circuits?

If you're not familiar with these circuits I can attach them.



Carlos E. Martinez

mlloyd1 30th April 2002 05:52 PM

Hi:
I've built both. I still use the first one (AD744 + AD811) built almost 10 years ago. Currently, the original serves as my primary preamp and another unit built later serves as a headphone amp for my Beyer DT990 phones. Pics on page 2 of my web site.

Sounds great: very low distortion, wide bandwidth, very stable (I've had no oscillation problems at all in spite of the speed of the AD811), and low noise.

The Analog Devices web site probably has more info. Look for Analog Dialogue, volume 27, number 1, 1993" Op Amps in Line-Driver and Receiver: Part 2 Audio Applications" as one example.

If you're looking for PCBs, Audio Express had a project a few years ago using this scheme in a preamp. The name escapes me at the moment (Ryan, maybe? Some else will fill it in). I know they still had boards as recent as last year.

What do you want to know exactly? :)
mlloyd1

carlmart 30th April 2002 06:15 PM

Preamp quest
 
Thanks for your comments: it's exactly what I was looking for.

My quest is a preamp that is transparent enough as no preamp at all. And I am interested in knowing how good different designs sound, if possible compared to the same reference.

Until now it has been difficult to convince myself into using active preamp, as I have been quite satisfied with the passive control I use: just selection switch and volume pot in the signal path.

But I know I maybe wrong in that approach, so I'm willing to try a different way, be it solid state (chip or discrete), valve or even transformers for low level signals.

For power amps I'm more into solid state, as I'm not too convinced a transformer can so good interfacing with a real speaker.


Carlos

mlloyd1 30th April 2002 09:58 PM

I remember now; :D the project was the Valkyrie Preamp by Grayson King.
mlloyd1
Quote:

Originally posted by mlloyd1
.....Audio Express had a project a few years ago using this scheme in a preamp. The name escapes me at the moment ......
mlloyd1

I also just remembered to point out - VERY IMPORTANT -
If you use the AD811 with +/- 15V rails, USE A HEATSINK!!!
It's a high quiescent current device.
Good Luck!

carlmart 1st May 2002 12:49 AM

Yes, the Valkyrie was an adaptation of that Jung arrangement.

The supply was not too sophisticated, just using LT1085/1033 regulators, but it probably was a fine preamp.

Later on I got to know that you shouldn't use large capacitors at the output of these regulators, and they did on that project.

A version of the Jung lineamp using Jung's latest low noise regulators seemed to be the reference used by Gary Galo on his Audio Electronics/ Audio Express tests. I don't know if he still does.

Yes, you had to put heatsinks on the AD811 if you went over +/-12V on the supply. I wonder why they didn't release it in a heatsinkable version as they do on buffers. Or did they?

You certainly have to heatsink if you use that preamp as headphone amplifier, which Galo also did. It seemed to beat Ben Duncan's headphone amp, which is powered by gel batteries. That was on another test.

One thing I am curious on is using the AD812 (which is a dual 811 if I am not wrong) and parallel the chips to handle more current on a headphone amp version.

You said you also tried the second Jung version for this lineamp?


Carlos


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