Hattor Audio Autoformer Reference Preamplifier

My experience of using a Autoformer as a Standalone Volume Control is one where I am able to detect a colouration injected by the Autoformer.

The same has been detected when it has been used along side a TVC, where the TVC has increased the noticeability of the colouration that is able to be detected.

My AVC is not my first pre amp option of late and it is out on loan, it is the VC for a DHT Pre Amp Design a friend has been fine tuning using Type 26 Valves.

As a very very lay description, the AVC is used at the back end of the Circuit when the Signal is sent to the Power Amp. In a system I know very well with a Pre Amp Design I am familiarising myself with, I can't detect the Colouration that the AVC / TVC has added when used as a more conventional device within a system. 

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.

and offers four autoformers for the balanced version.

@mitch2 Any transformer is capable of balanced operation. I'd be interested to know why they think they need four...

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.