Autoformer volume control in an active tube preamp?

[tommylion]

Does anyone make something like this? Has anyone successfully replaced the standard volume control in an active tube pre with an autoformer, like the Elma 47 position from Dave Slagle/Intact Audio?

intact audio

 

[Golum]

Yes I have it in my Tobian SC12 DHT preamp...I had pictures of it somewhere - if I find them will post...
But I believe he makes them in house...

 

[Glide3]

Does anyone make something like this? Has anyone successfully replaced the standard volume control in an active tube pre with an autoformer, like the Elma 47 position from Dave Slagle/Intact Audio?

intact audio

I have an Emotive audio Epifinia preamplifier that has the option to route the signal to a Slagle TVC.

 

[sjs]

Most Thomas Mayer preamps use an autoformer volume control, typically from Intact Audio, occasionally from Tribute.

 

[Salectric]

Fred Volz told me, maybe 10 years ago, the best sounding preamp he ever made had the Epifania circuitry with Slagle autoformers on the output. @Glide3 maybe that was yours? My custom Epifania predates that one.

I also have a standalone Slagle AVC box and I have compared it to the Epifania many times. My Epifania has a number of mods one of which was to replace Fred’s stepped attenuators with an Alps Black Beauty pot. For a long time after getting the Slagle AVC, I felt the Epifania and AVC were very similar but the AVC was slightly better. However, after changing the output caps in the Epifania, the two are still very similar but I now prefer the Epifania.

I realize that doesn’t address the question posed by @tommylion about using an autoformer after the tube circuitry, similar to what Fred Volz described and Thomas Mayer uses on most of his preamps. My Epifania doesn’t have enough space inside for the 47-step AVC or else I would have tried it. I did try hooking up the outboard AVC box to the output of the Epifania, and that was a mixed bag. It was too long ago for me to recall the details but I do remember sound quality took a step backwards compared to using either one alone. That doesn’t mean much since the Black Beauty was still in the circuit and I had another interconnect in the chain as well. The AVC on the output does have the advantage of reducing any inherent noise level of the tube circuitry in the linestage.

I do have one other comment that may be relevant. Most tube lineages have too much gain in my opinion. The Epifania is on the low side(12 or 15 db as I recall) but even it has much more than I want given my other equipment and high-efficiency speakers. A pot on the input lets me reduce the volume enough for my high-output DAC. A stepped control, whether an AVC or stepped resistor control, doesn’t have the same flexibility at the low end of its range.

 

[sjs]

AVC and TVC tend to work best when driven with a low impedance source, hence the layout used by Thomas Mayer where the AVC follows the step down output transformer is a very good solution, and helps to reduce any valve noise as pointed out by @Salectric.
I have both Slagle and Tribute AVCs and find they work best when used in this way.
I have tried them on the input of a tube line stage, but due to the non zero output impedance of my phono stage it is not my preferred approach.