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

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.

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.

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).