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

@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.

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.

@mitch2 Whether a transformer or autoformer, the same rules apply in exactly the same way. The main difference might be that since there is only one winding you will have less stray capacitance. But the capacity to ring when improperly loaded is exactly the same.

Autoformers, transformers, optocouplers, potentiometers, resistors I have all of them. My friend the late Roger Modjeski preferred the old Noble 100k or 50k potentiometers which we have plenty of and sell to folks who want to DIY a simple pot-in-a-box. It takes a lot of pieces to come together to get the best out of any of them. Never tried a buffer in my system, but Nelson Pass seems to find them helpful and they would be just to have that control over the output impedance. Like active preamps they have their warts as well, but with active devices its all about the circuit, that will define the component.

In my digital system I use either a Lightspeed attenuator or AVC I built myself using the top level Slagle Autoformers. In my analog system I use an active preamp because I need the gain for LOMC and analog tape. @atmasphere is right about the rules, the only other thing the transformer will give you is galvanic isolation.

Antique Sound Labs used to make a preamp that used either an AVC or TVC as the attenuator (I can't remember which) with a tube output. Way back there was a well regarded active preamp made by Melos that used optocouplers, this before the Lightspeed I use came on the seen. So the technology has been used before in different applications with various levels of success.

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.