Passive Preamp with Transformer Volume Control

[cma29]

Hi, guys.

Does anyone have experience with passive line stages that use a transformer for volume control?

I'm considering one to drive my Pass Labs XA100.5 (1.41 volts required for full output) from the K-03 DAC (2.45 volt output at 0 dB with optional +6 dB gain available).

What I like about these transformer volume control-based line stages is that no power is lost as volume is attenuated and actually the output impedance drops as the gain decreases so the sound does not deteriorate at low levels.

Here is what I found in a Stereophile review about this:

I've also tried various so-called passive preamps—including those that use transformers. These are usually called transformer volume controls (TVC), or magnetic passive preamplifiers. Generally speaking, they use an auto-transformer designed so that the user can tap in to different sections of its windings via a switch. As each section is engaged, the voltage level swings up or down, as does the output impedance.

There are several advantages to this. A transformer acts to isolate your power amp from whatever radio-frequency interference (RFI) your source components are trying to send it. Also, compared to a simple, conventional, less-expensive potentiometer, a transformer passes along more of the incoming voltage from those source components. Less power is lost.

A key point is that a passive preamp's output impedance drops the more you attenuate the signal, which means that the sound actually improves at lower volume settings. This has a very practical benefit: You don't have to crank up the volume to get more detail and better dynamics.

[cma29]

Bump

Any thoughts, anyone, before I pull the trigger on this design? Thanks.

[AudioNut]

Today is your lucky day Carlos. I was able to find the transformer you need and it is being delivered to my house this afternoon.

As soon as I'm able to add the passive circuitry, I'll ship it to you in Missouri.

Cost was a little more than I anticipated, but I don't think that will bother a man of your means.

Best wishes,

Julian

[cma29]

Originally Posted by AudioNut

Today is your lucky day Carlos. I was able to find the transformer you need and it is being delivered to my house this afternoon.

As soon as I'm able to add the passive circuitry, I'll ship it to you in Missouri.

Cost was a little more than I anticipated, but I don't think that will bother a man of your means.

Best wishes,

Julian

Thanks, Julian. I was hoping for something a little heftier, but this will do for now.

[PlanarSpeakerFan]

Hi Carlos,

How are you liking the Bent Audio Line Stage?

Best,
Ken

[cma29]

Originally Posted by PlanarSpeakerFan

Hi Carlos,

How are you liking the Bent Audio Line Stage?

Best,
Ken

Hi, Ken. Thanks for asking, but I don't have it yet.

John Chapman, the guy behind Bent Audio is building the preamp for me right now. To keep it as simple as possible, mine will have one XLR input and 2 XLR outputs.

What I like about this unit is its simplicity and its ability to increase current delivery to the amps as voltage (gain) is turned down, so the lower the gain the lower its output impedance. It makes sense to me, but the final verdict will have to wait until I actually install the unit into my system!

Here is a link to a review for one of these units: Bent Audio AVC-1 Modular Remote Control Autoformer Preamplifier Oodles of resolution, excellent imaging and spatial effects! Review By Clive Meakins

My version is a little more involved because of the XLR balanced operation.

[Masterlu]

Brings new meaning to "Get Bent"

[cma29]

Originally Posted by Masterlu

Brings new meaning to "Get Bent"

Yeah. It's not the best brand name in the world.

[PlanarSpeakerFan]

Originally Posted by cma29

Hi, Ken. Thanks for asking, but I don't have it yet.

John Chapman, the guy behind Bent Audio is building the preamp for me right now. To keep it as simple as possible, mine will have one XLR input and 2 XLR outputs.

What I like about this unit is its simplicity and its ability to increase current delivery to the amps as voltage (gain) is turned down, so the lower the gain the lower its output impedance. It makes sense to me, but the final verdict will have to wait until I actually install the unit into my system!

Here is a link to a review for one of these units: Bent Audio AVC-1 Modular Remote Control Autoformer Preamplifier Oodles of resolution, excellent imaging and spatial effects! Review By Clive Meakins

My version is a little more involved because of the XLR balanced operation.

Hi Carlos,

That was an interesting read. I can see the potential advantages of this design. I wish you a lot of luck with it. Please let us know how you like it once you take delivery.

Best,
Ken

[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.