Thread: New Preamp

I posted last year about a DIY preamp I build using a Khozmo attenuator. I was really pleased with the SQ of that preamp which was a nice improvement over the Parasound JC-2 I had been using. I've really been enjoying building my own gear over the past couple years, so I thought I'd see if I could build another preamp pulling out all the stops (at least to the extent of my ability) to see if I could make a meaningful improvement.

It's taken me most of the year to complete this, partly because I've had to wait for boards and parts, and also because I had a couple of other projects I started in the meantime. But as of today, it's in my main system and sounding pretty sweet. Since it has no break-in and barely any warmup, I can't really assess how much of an improvement it is, but it doesn't seem to be a step back in any way.



The new preamp, shown in the upper right, is a two chassis design with a separate power supply chassis (middle left side). I gave DIYAudio.com contributor "Salas" top billing since I'm using output boards and shunt regulators that he designed.



The power supply has a custom Toroidy 400VA transformer with nine secondaries. These feed four independent +/- linear supplies with CRC filtering to provide independent DC supplies for each channel and for each of the two voltages required (+/- 12V and +/- 17V). A separate 5V regulator supply provides power for the Khozmo attenuator. This is connected to the main chassis with a 14 conductor umbilical cable.



The main chassis uses two 2-channel Khozmo relay-switched attenuator boards to support fully balanced input selection and attenuation. I'm using Takman REY shunt resistors and Audio Note silver tantalums for the series resistors. I set this up like my previous preamp with three balanced inputs and three single ended. After switching, these feed a unity-gain input buffer so that my preamp has an input impedance of 500K. With my previous preamp, I did not use an input buffer, so I had to use a much higher impedance attenuator, and I think this still caused some issues with my DAC which has a high output impedance. This input buffer also converts the single ended inputs to balanced before the attenuator.

The attenuator feeds two separate output stages. One is the same AMB.Org A24 buffer I used in my previous preamp, built with Z-foil resistors. This provides both a single ended and balanced output. I'm using the single-ended outputs to drive my subs.

The second output stage is a fully-balanced implementation of the Salas DCG-3 single-ended class A design. This is a very low distortion discrete high-current line stage (and headphone) amp that has predominantly second-order distortion. This is built with hand-match transistors and premium components (Z-foil and AN tantalum resistors, Nichicon and Wima decoupling caps, etc.).

All audio wiring is done with solid-core silver with PTFE insulation. Connectors are Viborg and Furutech.

The DC supplies from the power supply chassis are regulated using Salas UltraBib regulators. These are class A shunt regulators which provide low output impedance and very low noise, but require a fair amount of heat sinking.



I built the chassis primarily using custom designed powder coated panels from Front Panel Express. I used 6mm panels for the top and bottom. The thicker panels from PFE aren't perfectly flat, but overall I'm pretty pleased with the way it came out.

Great job Jay, I look forward to your future posts on how it sounds.

Originally Posted by nc42acc

Great job Jay, I look forward to your future posts on how it sounds.

+1

Well done! That’s a great job.

Thanks for all the kind words. I really enjoy the process of the design and build, and it's very satisfying when it actually sounds good. That hasn't been the case with all my projects, but my hit rate is improving over time. With about five hours of operation so far, this preamp is sounding very nice. I can't wait to hear what it sounds like after a week or two of burn-in.

Super nice job.
The layout and implementation can be quite a challenge getting everything to fit and align properly without looking like you threw spaghetti in there.
Yes it is very rewarding to have a project come together and actually perform as expected.
Keep us posted.

amazing! congrats! ...i really with is had the knowledge to be able to build something that technically sophisticated. there is nothing more satisfying than a well executed diy project.

Very nice! I also love that you are using Arek's attenuators. Arek Kallas is the owner of both Hattor and Khozmo which is his line mainly designed for DIYers.

Cool !

Really really nice!

Looks great!