Is an autoformer (AVC) always superior to pots and resistors ?

Great question.  I have been looking at the icOn 4PRO passive preamplifier (that uses Slagle autoformers) as a possibility to provide remote volume control and a display in front of my solid state buffer.  I am currently using a Hattor passive preamp in that role but I have been curious about the icOn and how the sound of an autoformer based volume control would compare to what I hear from the AMRG resistors in the Hattor.

I had an Acoustic Imagery Jay-Sho (autoformer) preamp a few years ago but found that using it alone (without a buffer) left me wishing for more weight behind the dynamics.  To me, it sounded a little too smooth compared to what I was used to with resistor based units.  However, using autoformers to control volume in front of an active buffer or low-gain active stage might be a design approach that would provide the best of both worlds.  I would like to try the Jay-Sho again, or the icOn, in front of my SMc buffer or Hattor's Tube Active Stage.

@sns - Your Coincident Statement MkII pre may use autoformer volume control pots but it is an active gain preamp using DHTs to provide 13dB of gain.  That is a whole different kettle of fish than what those report hearing from “passive” autoformer preamps such as the Bent Tap X, JaySho, or icOn Pro4 where the only gain(if any) comes from the autoformer transformers themselves.  I am not surprised you like what you hear from the active Coincident Pre.

@mrdecibel 

I think the semantics can get tangled when discussing this stuff.  Plus, I do not have that much experience with TVC or autoformer preamps.  My SMc buffer is designed as an active unity gain buffer.  The input voltage is very close to the output voltage but the active stage prevents impedance mismatches and helps control the interconnect cables. 

My buffer does use output transformers to provide balanced operation, and the designer could have set those for up to 6dB of gain instead of the unity gain that I chose.  I believe the older Bent Audio Tap X was set up for +6dB gain, resulting from the transformers.  I assumed it may be possible to set up autoformers the same way, but I am not sure.  I still wouldn't consider that an active stage but others might.  Below is a quote about transformers and gain from a review of the Music First TVC preamp:

The transformers have dual primaries, allowing them to be connected in series as a step-up device offering the +6-dB option, or parallel as a 1:1 transformer.  Bear in mind that selecting the +6-dB option does cut the range of attenuation by an equivalent amount, but it also allows pairing with older components with lower outputs. It can still drive your power amplifier to full output, and sound quality is not compromised in the least by selecting this option.

If your question was about my comment about the Coincident preamp, I would not be surprised if an autoformer volume control in front of a low'ish gain active stage could sound quite good.  That is what I am considering doing with the icOn 4Pro in front of my buffer.

I ended up purchasing the icOn 4PRO balanced unit, which has four Slagle autoformers (copper). It is supposed to arrive tomorrow and I look forward to trying it in my system, both as a stand-alone preamp and also as a volume control in front of my SMc unity-gain buffer.

I have been using a Hattor passive preamp (Amtrans AMRG resistors) in front of the SMc buffer for a pretty good sound, and recently tried swapping the Hattor for a Goldpoint passive (SMD resistors) I still have here. Both sound good with the Goldpoint providing a bit more precision and the Hattor providing a bit more body and smoothness - maybe due to the carbon film vs. SMD resistors?

An equipment manufacturer I know has stuck with resistive preamps and told me, in his opinion, "TVCs sounded excellent with simple material, but started getting a bit irritating and “confused” with complex, dynamic music. Ymmv." I notice he said "TVCs" and not "AVCs" so I don’t know whether there will be a difference between the TVC units he has heard and the AVC icOn unit. I also don’t know whether he used the units alone, or in front of an active buffer, or both, and whether that even makes a difference. I will give it a try.

@j_andrews - Nice work!  I like the use of a stepped switch.  Thanks for sharing.
I wish I had the knowledge, then I would do a similar but balanced unit - very simple with one input and two parallel outputs.  The main complication would be that I have come to like having a remote and display, which would add complexity to the build.

@atmasphere

Hi Ralph, Not sure you noticed but the OP was asking about AVCs and not TVCs. I have read that AVCs have wider bandwidth, with less ringing, coloration, and smearing than TVCs. Here is more info about AVCs, and here is a review about an AVC preamp.

To the OP’s question, the answer would likely depend on the answer to the question, “the best at what?” In my listening experience, AVC volume controls do sound different from discrete resistor volume controls, and not necessarily better in all situations or with all types of music. Both types can be implemented as a stand-alone passive solution, or as part of a preamp with an active buffer or active gain stage, so the implementation will have a significant effect on the sound.

I have been using a discrete resistor type passive volume control with a unity-gain active buffer and so far have not found anything that sounds better to me based on stuff I have tried up to about $10K. However, I recently acquired an AVC preamp to compare with the VC/buffer set-up I have been using and since the trial is early, my only comment is that the quality of sound from each is very good but they do not sound the same.

Like many things in this hobby, there does not seem to be a clear-cut performance hierarchy between TVCs, AVCs, volume pots, and discrete resistor switches/arrays. The reviews I have read of TVC and AVC offerings mostly paint them as nothing short of the crème-de-la-crème of preamplification. Reviews of EM/IA, Townshend, Music First, icOn, and Bespoke are all extremely positive. On the other side of the coin, I have read about their susceptibility to ringing, as discussed earlier in this thread, and possible difficulties processing complex, dynamic music. The many positive reviews left me intrigued enough to look at trying an AVC or TVC preamp for myself.

I have the icOn 4PRO balanced preamp here now. It is a Model 4b, which is fully balanced, including 4 Slagle autoformers, and offers both balanced (XLR) and single-ended (RCA) inputs and outputs. Functionally, it seems to be at the top of the pile of TVC/AVC preamps in that it offers fully balanced processing of the signal, remote control of inputs, outputs, balance, mute, and volume, and a very nice segmental display that is the easiest to read of any I have owned. Sonically, most of the reviews I have read are hesitant to call out significant differences between the better known options such as S&B TVCs, Slagle AVCs, or even the Bespoke TVCs. There does seem to be a clear preference (by many but not all) for silver wired TVCs or AVCs, which cost about twice as much.

I initially thought I would probably like the icOn as a passive volume control in front of my SMc Audio unity-gain buffer (using a Hatter discrete resistor passive preamp in that role now), but surprisingly the icOn seems to sound better as a stand-alone preamp connected directly to my amplifiers. Even though it has RCA inputs and outputs, in my system I seem to like the sound best when using the balanced inputs and outputs. One nice feature is that you can select which output (or both) are live, so I use the balanced outputs for my main amps and the RCA outputs for my two subs. Since I have only had the icOn here about a week, my final subjective assessment compared to the Hattor/SMc combination will have to wait, but I can say that the icOn sounds good. However, based on what I have heard so far, I suspect it will be a close call on which I ultimately like better in my system and for the music I listen to.

That initial statement was intended to address the different volume attenuator options, as discussed in the OP.  My current rig includes a passive resistive unit into an active buffer.  The icOn 4PRO is a passive preamp based on Slagle autoformers.  It so far seems to sound better alone than it does run into the buffer.

I doubt the guide on grounding and shielding connectors in active equipment presented in the form of the AES Standard 48-xxxx (2019 Draft Revised), applies to the icOn 4PRO, since it is a passive unit. Fortunately, I cannot detect any noise issues when using the icOn.

My concerns are functionality and sound quality, and right now both options offer similar functionality and pretty good sound quality, which is up there with the many good preamps I have owned (including a tricked out MP-3). 

@mrdecibel 

The only "test" is for me to compare the icOn AVC preamp against my current Hattor/SMc buffer set-up, for the sole purpose of determining which of those two options sounds better to me.  If the icOn sounded better through the buffer then that would be ok too, but that doesn't seem to be the case - it sounds better as a stand-alone preamp.  I haven't yet decided which will remain in my main system.