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

@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!

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).

@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.