Hattor Audio Autoformer Reference Preamplifier

@atmasphere - You do understand the autoformers (auto-transformers) being discussed here are being used as volume controls, right?

I certainly respect your work in designing balanced tubed electronics, and your adherence to the balanced line standard in your products, but I am not the person to convince since I have no dog in the fight, except as the owner of a preamp that uses four autoformers to adjust volume while providing balanced inputs and outputs.

I am not clear whether you are suggesting an alternative, or simply questioning why the designers are not using a single autoformer per channel.  Either way, the main commercial designers of these products (EM/IA-Jackson/Slagle, icon-Nagy, and Bent-Chapman, and now Hattor-Kallas) all use/used four autoformers to adjust volume when configuring their stereo preamp products for balanced operation. That would be surprising if they were aware that it was degrading CMRR by 20-30 dB.  Maybe you could figure out why they are not using just one per channel.

@atmasphere - Ok, fair enough. If you have more than a passing curiousity, I suggest reaching out to one of the designers that I listed.  I would suggest Dave Slagle at Intact Audio and EM/IA, since many seem to use his autoformers.  If you find out anything, I too would be curious to hear about it just because I like to know how things work.

@atmasphere - Back to your question of why 4 autoformers, this (link) will take you to the following Q & A, which was posted on the Intact Audio website:

Q - Will they work for balanced  attenuation.?

A - They have been used with good results in balanced systems but it requires two autoformers per channel. If both the source and load adhere to the broadcast standard, a single autoformer will work.

I believe the last sentence makes your point. If you have further questions, you could maybe contact Dave Slagle at Intact Audio or one of the other designers who use four autoformers in their balanced offerings.

Hey Teajay, thanks for reaching out. I purchased the copper version and it is currently being built.  I didn’t choose copper particularly because of the price difference but because I have not read overwhelming consensus that silver is better wrt AVCs.

Arek knows me as a long-time customer so, if you are game, maybe we could ask him if we could swap units after your review and then we could both directly compare the silver and copper versions.  We could even schedule the hand-offs so that each of us would have both units in our houses at the same time for a week.  It would require him to treat both units as demo models and I would commit to either staying with the copper version that I already purchased, or sending it back and upgrading to a silver version, either the demo unit if you don’t buy it, or  a new unit. Of course, I would do Audiogon write-ups on both and the comparison.  Let me know if that sounds interesting to you and, if yes, we can reach out to Arek.

I have had AVC preamps here from Acoustic Imagery (Jay-Sho) and more recently, Pal Nagy’s icOn 4PRO, with four Slagle autoformers.  I owned the PRO4 and had it for a more extended time but in both cases, I noticed a bit of sound that I found somewhat “unnatural” compared to the resistor based preamps I was used to. It was sort of a very slight sheen or slickness (hard to describe) that just seemed a little off.  However, it may have simply been that I was used to resistor volume controls.  In any event, I did not find it to be a deal breaker and in both cases, the AVC sound was transparent, organic, and displayed a touch of warmth or body. They were inviting to listen to and, in the absence of other really good sounding choices, I could have been happy with the PRO4 in my system.  Because of the impedance matching benefit of the autoformers, the AVC preamps are one choice that can be run passively and not sound lightweight or tonally thin. In fact, running the output of the icOn PRO4 through my SMc Audio unity-gain buffer did not seem to noticeably improve the sound.

I look forward to trying the Hattor Autoformer Reference Preamp (balanced model with 4 autoformers) both as a stand-alone device directly into my amplifiers and as a passive volume control with the output routed through my unity-gain buffer. In the case of Hattor’s resistor based preamps, the buffer improves on the tone and body over simply connecting the output to my amps in passive mode. Their own internal op-amp based active stage (gain adjustable by the remote - very cool) improves on the passive output in some ways but the op-amps don’t match the sound quality of my SMc buffer. I have also owned the Hattor Tube Active Stage and it was a very nice sounding step up from the op-amp active stage, but was also not quite to the level of the SMc buffer, IMO. In general, it is my experience that to improving on Hattor preamplification options requires spending multiples of the price.

I believe Slagle at EM/IA (link), Nagy at icOn (link), Bent Audio AVC-1 (link), and now Arek Kallas at Hattor Audio (link) all use two AVCs for single ended preamps and four for their true balanced preamps.  The picture on the Hattor website link shows four and the verbiage in the Bent and icOn links discuss that four are required in their balanced versions.  As to why, I never questioned it but assumed each autoformer has an input and an associated output tap depending on the voltage reduction (i.e., volume setting), and that one AVC was required for each audio signal line so, one per channel single-ended and two per channel for balanced.  Whether it could be done differently I don't know, but they all seem to use two for single ended and four for balanced.  Maybe somebody who has built one can answer your question.