Hattor Audio Autoformer Reference Preamplifier

Hey mitch2,

It’s interesting that you made this post. I just recently inquired about setting up a review for Stereo Times on the Hattor silver version of this preamplifier. If they are interested and set up the review with me I will have the information you are seeking

Teajay Terry London

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.

@mitch2 I also run the Hattor The Big Preamplifier and am looking forward to your comparison with the new autoformer pre. Also would like to hear your experience with how The Big Pre has compared to other brands.

Many moons ago I acquired his Ultimate Passive preamp and loved everything

about it when I had a Marantz HD-CD player, but the lazar died. I got a great deal on an Audio Alchemy DDP-1 + PS-5 and sold the Hattor, though I miss it to this day. It was jewelry, Decades ago I had an autoformer pre from Antique Sound IMO way ahead of its' time. IMHO, get them cryoed 

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.

Having been both a happy customer of Arek's resistor based passive and a former AVC/TVC user, I look forward to your opinion on this new preamp. 

@mitch2 stated " the AVC sound was transparent, organic, and displayed a touch of warmth or body. "

This is not a criticism of the above description but a alternative description on how detecting such traits effected a different individual.

I agree there is Warmth or Body, In a variety of systems having heard a AVC / TVC in use, to my sensitivity and perception,  Warmth or Body, equated to a Weighty Underpinning of a Note > Vocal, to the the point it was a distraction for being the trait being noticeable.

For myself who has Transparent Devices, by Transparent I mean as good as invisible as a influence on a End Sound, Invisible to the Point, the Sources Effect on the Power Amp is all what is being produced as an End Sound.   

My experience of a AVC / TVC as a conventional place in a circuit as a VC does not leave me being able to suggest the encounter can be described as being  Transparent, the influence of the AVC / TVC on the End Sound produced is easy to be discerned.  

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.

@mitch2 FWIW Dept.:

I've been designing balanced audio equipment for the high end market for 40 years; we pretty much introduced balanced line operation to high end audio.  All you need to do to make a transformer operate balanced is not connect either side to ground (the connections going to pins 2 and 3 of the XLR connector) and only ground the case of the transformer (which ties to pin 1 of the XLR). That's how its done in the recording studio and broadcast. If what you're saying is correct, my surmise is they don't understand this aspect of the balanced line system (codified by AES48, one of the balanced line standards). But I've run into that in high end audio a lot. 

With two transformers per channel (one for each phase) its pretty obvious that one side of each transformer is grounded. When you do that you lose one of the primary aspects of balanced line operation, which is interconnect cable immunity; to prevent the cable having a 'sound'. Put another way if you hear differences between cables you've got a problem. Another reason you run a single transformer you maximize Common Mode Rejection Ratio, which is to say you improve the system's ability to reject that which is not the signal, such as hum and noise. Two transformers for one channel would require the transformers and all the components associated be matched; even if this were done to tolerances less than 1% you'd still have a dramatic loss of CMRR by a good 20-40dB. 

A single transformer driving a balanced line or receiving a balanced line has a nearly ideal CMRR (which is to say as high as it can get, as much as 120dB) by comparison. 

@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.

@mitch2 Atma-Sphere has been doing balanced line longer than anyone else in high end audio to the best of my knowledge- nearly 50 years. 

The 'broadcast standard' to which he refers is probably the balanced line standard. The reason for its use is to minimize noise and to also minimize colorations from the cable itself. For the latter, if you've ever auditioned cables and heard a difference you know what I'm talking about. 

The use of one transformer without a center tap in a balance application is used to minimize noise pickup, which is done through something called the 'Common Mode Rejection Ratio'. The CMRR is essentially when a signal is common to both inputs (non-inverting and inverting), it does not get amplified (its rejected). When you use a center tap or when dual transformers are used, the CMRR is significantly degraded, by well more than 20-30dB!

It is on that account that whenever transformers are used in a balanced application, there is no center tap- the outputs are simply tied to pin 2 and pin 3 of the XLR, and pin 1 (ground) is not tied to the transformer at all. There is never a need for dual transformers. 

A transformer is inherently able to be run single-ended or balanced since it does not care if one side is tied to ground or floated- it works the same in terms of passing a signal. Even if the output of the transformer is loaded in some fashion to prevent ringing, it would still work the same- there's literally no reason to use dual transformers- that simply degrades performance. 

@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.

@mitch2 To your question, yes. 

I don't have a dog in the fight either. I'm simply presenting the facts. 

I'm still curious about this practice as you can see in my first post. 

@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.