TVC

A magnetic volume control

Some time ago I constructed an experimental preamplifier using FETs. I decided not to use the usual ALPS potentiometer for volume control, but to try something new. I ordered a 2-deck, 24-way ELMA switch and started to calculate resistance values.

The result was a log-tapered, switched resistor attenuator. The resolution and detail brought to the music was a real joy. The difference was substantial. I have since then used stepped attenuators only. Well, that was not entirely true. My ULBO preamp uses a T-network for volume control, but it still uses switched resistors.

Playing music on my modified CD players and Leonardo power amplifier, with the stepped attenuator in between, is very nice and enjoyable. After a few years, however, the feeling of something missing has become obvious. Both the CD players and the Leonardo have been tweaked for some time, but now was the time to upgrade the volume control.

A few articles and posts had made me interested in transformer volume controls (TVC). A few e-mails to people using them confirmed that the TVC is worth trying. Said and done, two Steven & Billington TX-102 were ordered. During the weeks of waiting that followed I planned and prepared a suitable box. The completed TVC is shown on the pictures.

The sonic performance of the TX-102 is really stunning! I thought the shift from high-end pots to stepped attenuators was huge, but the TVC brings volume control to new heights.

The first and most obvious impression is the transparency. It conveys a visible clarity, some kind of analogue continuity to the soundstage. I would like to say that there is a new kind of visibility on stage. The level of precision and detail has improved, the perceived size of the individual instruments is more realistic, and (I really love this!) the quiet space between the instruments are really quiet. Dead quiet.

I should make the remark that these statements obviously only apply to the recordings good enough to carry this information.

I was previously quite convinced I had good (well sort of) dynamic range. The TX-102s have set new standards in my listening room. I have never before listened to such a dynamic range. Excellent!

One effect I had not expected, is an improved bass range. The bass is given not only an extension to the low end, but also a softer, cleaner and more realistic presentation. Further, it seems better integrated with the low mids. This may seem to be a funny statement, but I suspect this phenomenon is all about impedance matching and its associated roll-offs.

I mounted the transformers on top of a nice aluminum box, using gold-plated clamps (actually made for capacitors). For switching I used an ELMA 2 deck, 24 position gold-plated switch. Good circuit diagrams are provided by Bent Audio in Canada, and various implementations can be found on the web.

For convenience I also included three pairs of input terminals and a signal source switch (again a gold-plated ELMA). As is also shown on the pictures, the outputs are doubled to enable a tap to the bass amplifiers.

Construction is straightforward. A bit of planning may be recommended, because there a many leads to manage and control. I strongly recommend that the grounds of the two channels be thoroughly separated throughout the TVC.

I also recommend that primary-side grounds be separated from secondary-side grounds. Although not strictly required, this offers the opportunity to separate the input and output devices galvanically, which sometimes may be essential. I added a small switch on the back of the box, to connect or disconnect primary- and secondary-side grounds.

Depending on the capabilities of your audio gear, a TVC can make a big difference. It certainly did in my case. It is not a cheap project, but my experience is that it gives good value for the dollars.

[ previous ] [ back to top ] [ next ]