Passive Preamp with Transformer Volume Control

[Anthony]

Originally Posted by cma29

My transformer volume control preamp has been built and tested and is on its way to Columbia.

I should have it this week. Can't wait to plug it into my system.

Carlos, I'm very keen to hear your considered views once you get a handle on its virtues.

Anthony
Australia

[PlanarSpeakerFan]

Originally Posted by cma29

My transformer volume control preamp has been built and tested and is on its way to Columbia.

I should have it this week. Can't wait to plug it into my system.

That's terrific, Carlos. I wish you the best of luck with it. Please give us a full review once it's broken in.

Best,
Ken

[PlanarSpeakerFan]

Hi Carlos,

Did you take delivery on that Passive Preamp yet?

Best,
Ken

[cma29]

Thanks for asking, Ken.

I did receive the preamp on Saturday. I hooked it up immediately and what I've heard so far is impressive particularly at lower listening levels. I think there is something to the transformer-based volume control design principle. I'll write more as the unit breaks in.

So far it's a

[PlanarSpeakerFan]

Originally Posted by cma29

Thanks for asking, Ken.

I did receive the preamp on Saturday. I hooked it up immediately and what I've heard so far is impressive particularly at lower listening levels. I think there is something to the transformer-based volume control design principle. I'll write more as the unit breaks in.

So far it's a

Hi Carlos,

Wow, that is terrific, congratulations!!! I'll be interested in hearing more details about the sound of the unit as it breaks in. I'm also interested in learning more about what led you on your passive preamp journey in the first place. I am impressed by your carefully planned "out of the box" approach you've taken to building your system. I am sure it is leading you to a unique and beautiful sounding system.

Best,
Ken

[cma29]

Ken - thanks for your comments, but wait a minute. I see the XA100.8 in your signature now. Please tell us more!

[jdandy]

Originally Posted by cma29

Thanks for asking, Ken.

I did receive the preamp on Saturday. I hooked it up immediately and what I've heard so far is impressive particularly at lower listening levels. I think there is something to the transformer-based volume control design principle. I'll write more as the unit breaks in.

So far it's a

Carlos.......Don't forget how much we like photos.

[cma29]

Originally Posted by jdandy

Carlos.......Don't forget how much we like photos.

Definitely, Dan.

I'll take a few pictures, say a few words about the unit, why it works well for me and why it caught my eye. Nothing as fancy as your reviews but I'll do my best.

I'll work on it after I get back from my business trip and have a change to listen to it more.

[cma29]

Originally Posted by Anthony

Carlos, I'm very keen to hear your considered views once you get a handle on its virtues.

Anthony
Australia

Originally Posted by PlanarSpeakerFan

Hi Carlos,

Wow, that is terrific, congratulations!!! I'll be interested in hearing more details about the sound of the unit as it breaks in. I'm also interested in learning more about what led you on your passive preamp journey in the first place. I am impressed by your carefully planned "out of the box" approach you've taken to building your system. I am sure it is leading you to a unique and beautiful sounding system.

Best,
Ken

Now that I’ve had to time to enjoy my Bent Audio AVC-1 Passive Line Stage, I wanted to provide a follow up for those who might be interested and to highlight this excellent unit to those who might be looking for a passive preamplifier.

Some Background
A while back I had an Audio Research line stage between my K-03 and my amps. The preamp used ten (!) tubes and it yielded nice results, but the sheer number of tubes added some noise to the signal and I became frustrated with tube matching and chasing down 10 quality tubes that would work well together. Out of curiosity I tried the K-03’s DAC connected directly to the XA100.5s and the results were impressive: very clean, low noise sound that sounded robust and lush. The fact that my amps only need 1.41 volts to reach full power while the K-03 can put out almost 5 volts and that the input-to-output impedance ratio was 300 certainly had something to do with the great performance. I decided to go the direct route and very much enjoyed the minimalist approach.

After a while using the direct set up, I learned more about how the K-03 attenuates the signal that drives the amps: by dropping bits as attenuation increases at the rate of 1 bit for every 6dB of attenuation. Most of the time I listen to my system in a highly attenuated mode so the volume control was dropping a lot of bits to reach the desired sound pressure levels. Why was I paying to top dollar for the K-03 if I was going to be dropping bits?

This bothered me, so I figured the solution might be to add a passive preamp to maintain the minimalist simplicity I was enjoying, but with a better way to attenuate the signal to the amplifiers. I did some research and found the...

Bent Audio AVC-1 Modular Remote Control Autoformer Preamplifier
The unique design principle behind this preamp is the use of transformers (100% passive and sonically transparent - the entire signal path includes only pure magnet wire and high quality switching elements made by Pickering in England) that keep signal power (from the DAC) constant as the gain to the amp is attenuated. So as the voltage to drive the amps goes down, the current increases and this works to effectively lower the output impedance of the preamp to maintain the integrity of the low-level signal that drives the amplifiers. Most volume controls are resistive; when you turn down the volume a proportion of the voltage and current is thrown away along with dynamic contrasts. Not the Bent Audio preamplifier.

This unit is not available from a dealer. I had to contact the builder, John Chapman, in Vancouver, British Columbia directly. He sells the unit as a kit, but I’m not into DIY when it comes to my stereo equipment so I asked him to build and test the preamp for me using the kit he sells to DIYers. John is the nicest guy to deal with and we worked together over a few weeks to design the unit I needed. To maximize simplicity I went with a single XLR input unit and two sets of amplifier XLR outputs. Since the unit is fully balanced I needed 4 autoformers (the special transformer at the heart of this unit). As part of the kit each autoformer costs $450. Ultimately I received the preamp packaged in a high-quality aluminum enclosure along with a very nice remote. Fit and finish are excellent. My cost was $2700 for the unit shipped to Missouri from Canada. This unit looks right at home on my rack next to the Esoteric K-03 and the Pass Labs XA100.5 monoblocks. The remote weighs half a pound and would be suitable for a $25,000 preamp. It feels nice in your hand and it allows for volume control for each channel (e.g., like a Balance control), mute and display shut off.

Volume range is -54dB to +7dB in 1dB increments. Power for the unit’s display and other minor functions is via a 9-volt DC power supply.

Autoformers:


Finished Unit


Inside with four autoformers (not my unit)


Remote Control


Sound
In my system and for the kind of music I listen to (orchestral classical), the Bent Audio AVC-1 has done a superb job helping to bring out the secondary harmonic nuances that make acoustic music come alive. Brass, woodwinds and strings timbers just sound real. The brass has that special “blatt” rough sound that is so authentic. Transparency is superb allowing me to follow complex melodies and counterpoint easily. Listening fatigue is non-existent and dynamics are tremendous.

To conclude I’ll quote a professional reviewer who described the Bent Audio AVC-1 thus: “Oodles of resolution, excellent imaging and spatial effects, silent noise floor, subtlety and almost visible texture.” I could not agree more. For $2700, this passive preamplifier is a great bargain that allows my Esoteric K-03 and the rest of my system reach their full potential.

[PlanarSpeakerFan]

Great review Carlos and a beautiful preamp. I'm so glad it worked out for you.

Ken