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

@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.  

Mitch, I think using an additional set of interconnects, vs the icOn going direct into amps, might make this " test " invalid to the discussion at hand ( apples to apples / apples to oranges ), although, what ever sounds best to you. My best....

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). 

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.

You left out active preamps.

Its possible to direct couple using tubes to create a balanced output. IME when you do this you have transparency and bandwidth on tap that's very hard to get any other way.

When running balanced I feel its important to support the balanced standard (AES48) since it prevents ground loops and minimizes cable interactions.

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.

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.

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.

@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.

This is an argument some of my friends made to me. AVC is always the best volume control, better than anything else such as rk50 or resistor based volume controls.

@smodtactical The correct answer is 'sometimes- maybe??'.

TVCs can generate distortion which PVCs cannot. This is because if the transformer isn't loaded correctly (and being an inductor) it can 'ring', which is to say it will generate harmonics.

All coupling transformers have something called 'critical damping' wherein if you pass a square wave through it, with the correct load the square wave will be intact with minimal overshoot at the output. Loaded too heavily (output load impedance is too low) and the square wave will be rolled off (rounded corners on the square wave). Loaded too high and the square wave will overshoot and may not look like a square wave at all!

The source impedance (such as that of a CD player), the output load (the amplifier) and the turns ratio (the volume control position) all play a role.

Because of this its nearly impossible for the designer of the TVC to have the device loaded correctly at all volume control positions. You can see that it might be advantageous to have a different resistor loading the output at each position on the control, but that value will be different depending on the load that the amplifier presents to the device.

But sometimes you luck out. I wouldn't count on that though. IMO/IME you are far better off with a buffered volume control at the very least. A buffered control minimizes variables you always encounter when working with passive devices (TVC or PVC).

@j_andrews

I say the same thing to anybody who will listen, get one of Dave Slagle’s autoformers with a balance control and be done with typical resistor type volume control. If it has a sound - sign me up. I love it!

@jmolsberg That's great, how do you like it?  What is your amp?  Do you feel the autoformers have a sound or not?  Can't wait to wrap this thing up..just need to set aside some time to inhale some soldering fumes.

@mitch2 , Yes all the Slagle autoformers would require a stepped switch as you're actually connecting to different taps on the autofomer,  As I mentioned, I purchased the most basic units which are only 16 steps...so not a lot of resolution between volume steps but I think I'll be OK with that as I usually set it and leave it alone.

At one point, he did offer some sort of collaboration with Bent where there was a remote control unit but I don't think that's available anymore.

@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.

@j_andrews I use slagle passive in front of oliver sayes with omega speakers! Your build looks nice 😊 

I’m in the process of building a simple avc attenuator with the budget autoformers from Slagle. To be used with my Sayes SET and Omega single drivers.

2 inputs with a switch and 2 outputs

vintage 16 step rotary knob

progress pic HERE  and FRONT

Really looking forward to finally being done with it.

Still need to put in the rca connector and wire it up.

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.

low powered set amp, an inductive volume control, and high efficient speakers = audio nirvana 😃

The Statement sounds like it does everything well. However, I have eliminated tubes from my system. Enjoy ! My best, MrD.

I did understand the autoformers different than TVC in my Statement, which  by the way is dual TVC, one for each channel in fully balanced active pre. I just threw it in there since I thought we were discussing different volume controls in general.

As for passives, I do find @mrdecibel  comments to be accurate as to what I've experienced with passives with pots or resistor based VC. Not heard the autoformers, although was considering at one point, saw usual complaints about passives in general. Not to say passives may work in some setups

@mitch2 I apologize for my misunderstanding. I commented based on your statement " where the only gain ( if any ) comes from the autoformer transformers themselves ". If designed with +6 db gain, yes, but we are talking unity gain, so the source creates the gain. My best ! Always, MrD.

@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.

@sns, Mitch is correct, it is not the same. I am speaking unity gain, and my experience is exactly as stated. @mitch2, with a unity gain passive device, the gain is provided by the source, your last statement is unclear to me. sorry....... 

@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.

no softened transients here with my slagle autoformers. wonderfully impactful at low level listening, which is my preference.

Its all about implementation, no way, no how my system softens transients! In fact transient performance, especially of the micro type are quite incredible. Coincident Statement has likely the most robust power supply I've ever seen in a pre, much more like what you see in power hungry amps.

TVCs, ime, rounds things out and softens transients, but like everything else, it is what your ears like. TVCs are supposed to be better with impedance matching between source and amp.

I can only say I love my Coincident Statement MkII pre with dual transformer volume control, superior to any pot or resistor based volume pre I've owned. Absolutely most stable imaging imaginable, no tracking issues whatsoever.

Dunno.

I prefer optical comparator, crossing at zero (silent)

The PS Audio gain cell design is superior also.

I've tried an AVC incorporating Slagle's parts. It sounded dead quiet but my system needs a bit more boost and I like the coloration added by my 6SN7 tube.

My experience has been to get one of Dave Slagle’s autoformers and be done with typical resistor type volume control. 

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.