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Here's some more clarification.

Posted by kurt s on October 22, 2006 at 17:32:47

In Reply to: Re: Need some clarification... posted by Janos on October 21, 2006 at 21:07:59:

The problem about the thinning out with higher volume control settings is real for a TVC or AVC. Neither of these should necessarily be better than the other except in your specific cases you tested. Another AVC can outperform your TVC. I received an AVC that I thought might have a bit too few windings on too small a core. So I designed my own with a larger core and more primary windings. The result is less of the problems you describe when you crank up the volume setting.

Now the idea of using high level signals with another gain stage in front of it is a good one in this case, or in other words, it's now better to place the AVC after the gain stage rather than in front as in a traditional preamp just because of the nature of tranformers compared to resistors. With the transformer after the gain stage you have a high signal level from which to further attenuate. This means that the turns ratio of the transformer will be higher to drop the signal back down where you want it compared to not having the gain stage there. This reduces distortion in the core by having plenty of voltage swing up front with a high turns ratio - and this means the B field in the core is actually lowered than if the transformer has to "work" harder by using a smaller turns ratio. So for the same volume levels the extra gain stage can improve the situation, at the cost of the distortion and signal degradation of that gain stage.

It's a balancing act. If your AVC is not working well with a line level DAC output, usually an op amp but sometimes a tubed cathode follower, then that might mean the AVC is not designed as well as it could be if a TVC proves not to exhibit as much of this. The AVC might need to be bigger in core size and in number of primary turns. That was the solution I came up with. I would not use another gain stage for my system with my new AVC at this point because overall the results would be worse.

Yes it is true that inductance falls with increasing frequency with all magnetic-cored inductors and transformers, but that doesn't mean a low impedance op amp is not good for driving the AVC. The opposite is true. An op amp's low output impedance is especially good for driving the AVC for this reason. But it might not be swinging enough volts to get it operating in the AVC's sweet spot. It's not the impedance, it's usually the voltage swing that's a problem in these TVC/AVC devices. I believe that the physics for that is pretty solid.

A TVC will likely have less bandwidth than an AVC. That's just because of the close intimacy of the turns in an AVC compared to a TVC. This could be good or bad in a digital system, I don't know.

I like all the observations taken down here, and the solutions generated to overcome them in an appropriate way. I wanted to point out that it is possible to design these autotransformer volume controls with too small a core and too few primary turns. This will lead to such artifacts in the sound. Now it is true that you cannot make it completely go away. But a good AVC still sounds better to me than by using anything active added in the chain. So it is to me the ultimate "line stage", one AVC designed properly to operate well at line level signal levels over the range that best fits the gain structure of the entire system. Do it right and it's damn tough to beat.

Kurt

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