TVC preamps: good or bad?

What was the system setting when you tried the TVC?

TVCs can be considerably more neutral than many active line stages. But not **all** line stages!

A lot depends on how careful the design of the TVC actually is. The issues of TVCs are: bandwidth, hysteresis loss (introducing a loss of low level detail) and ringing (introducing brightness).

The advantage of a TVC is of course no noise and circuit simplicity. If designed properly, ringing attributes can be reduced, but one should note: in order to do this, the TVC has to have a loading resistance that is different for every volume control position. The reason for this is that the TVC must present an amenable load to the sources (DAC, phono preamp etc.) while at the same time being able to change the turns ratio. This is usually accomplished by having the multiple taps of the transformer on the secondary side, selected by the switch. Without an appropriate loading resistor for each position, the TVC will express the inter-winding capacitances rather than the actual turns ratios. Most designers set up an approximate loading resistance, but this ignores the ringing issue, which can only be addressed by a different loading resistor for each tap. With only one loading resistance, the bandwidth and ringing issues will be different with each volume control position. It is likely that many have found that there is a particular volume position that seems to sound the 'best' as a result.

To date I've seen no TVCs that pay attention to this issue. Anyone who has worked with audio transformers will realize how important this is.

Even with proper loading, hysteresis losses inside the core of the device will rob the signal of low level detail in much the same way that push-pull output transformers in a tube amp will (part of the reason SETs have made such a comeback in the last 20 years).

It is a testament to how bad some line stages are that TVCs often outperform them. Circuit simplicity is a good thing in audio (less places for the signal to get messed up) and TVCs are not only simple, but can provide gain and impedance control of the output if designed properly.

Despite this a **properly** designed line section can easily trump the best TVCs. This is a good example of getting what you pay for.

Atmasphere, while opinions vary on whether a passive preamp can surpass a top-notch active preamp, there seem to be a number of posters who find TVC passives preferable to resistive passives. I don't think I've ever read anyone comment that they prefer a resistive passive to a TVC. Which leads me to wonder, how would a TVC or autoformer volume control sound if used in a top quality active preamp in place of a traditional resistive attenuator volume control? Have you ever tried it? Apart from the issue of sonics, there could be a problem with increased cost & available real estate within the preamp chassis. But maybe the ability of a TVC to increase current drive at high voltage attenuation might off-set somewhat its other theoretical compromises.

I have tried three transformer volume controls and they were considered some of the best. The first was Dave Slagle's Autotransformer Volume Control (dubbed AVC), the next was the S&B TVC, and the third was my own design AVC. I find the AVC has better sound despite the fact that there can be no ground isolation.

The AVC is superior technically to me mostly because it has far greater bandwidth (mine has about 1 Hz - 300 KHz bandwidth, +0, -3 dB) driven by a 50-ohm output impedance source. Notice I didn't say +/- 2 dB because the FR is virtually ruler flat. There is no measureable ringing that I can detect on a scope. The square wave at 10 KHz is almost perfect, even with high load changes, or even with a near open on the secondary.

The reason the autotransformer does this is because it is a built-in power divider even if it loses much of its mutual inductance. It's all on one coil.

It's also a step-down device, and so what comes out is pretty close to a straight division of what went in, a technique used in precision ratio transformers for metrology. And if you drive it with a very low output impedance device, the distortion drops dramatically, overcoming hysteresis distortion and low initial permeability.

To make it perform better under imperfect conditions, I chose a slightly oversized (not gonna saturate or bend much) "SuperPerm 80" nickel based laminations. This material also has the highest initial permeability, which means it takes the least signal level to start moving the magnetic domains. That maintains the best low level detail retrieval. The material is also designed for low hysteresis, much better than any steel based lamination.

Also, my AVC was voiced by using two thicknesses of these SuperPerm 80 laminations: about 5 thin ones (0.006") to one thicker one (0.014"). This actually smoothed down the sound a little and cut down ringing to that "almost immeasurable" point.

S&B uses silver plated copper wire which I don't care for. I used high purity thin gauge copper wire with no splices on the taps, similar technique to Dave Slagle's. Mine is a one-off, and is not for sale. It remains the best preamp I have used, and I'm never looking for another line stage.

Kurt

Ktstrain, you obviously know something about the subject. Do you think an AVC or TVC could be designed to perform as well with a higher impedance source, such as a tube phono stage with an output impedance up to 5K?