Transforming volume

December 19, 2016
 by Paul McGowan

Ok, the last post on fringe methods of controlling preamp volume before we get to the meat of this subject, the Gain Cell.

We’ve learned the job of a volume control is to divide the music signal into smaller levels with as little harm as possible. Typically we use resistors: fixed pairs for stepped attenuators, a metal contact sliding over resistive elements to make a pot, changing levels of light stimulating a cadmium photoresistor in an LDR.

All the above-mentioned techniques rely upon using resistors to change levels.

There is another means that does not pass a signal through resistance. A transformer. As its name implies a transformer transforms electrical signals from one form to another. In this case by converting electricity into magnetism and back again. Place a voltage into a coil of wire and you make a magnet. Place another coil in close proximity to the first and the process reverses if the electricity is moving between plus and minus (called AC – Alternating Current). Music is AC. What comes out of your wall socket is AC.

In a transformer, electricity in, equals electricity out, without physical contact. Instead, the transfer of energy happens through a magnetic field.

What’s cool about transformers is their ability to change ratios. If the input and output coils of wire are identical, then 1 volt AC in equals 1 volt AC out. Change that ratio and what comes in is no longer what comes out. Half the length of wire coiled up on the output cuts the input voltage by that same ratio. The pattern is pretty easy to understand.

Now imagine the output coil with detours. Instead of using the two ends of the coil of wire for your output, the designer attaches another wire somewhere along the coil as a second output. Where along the coil that wire is attached will determine what percentage of the signal is captured. This extra wire is called a “tap”.

Start with two identical coils of wire. Signal in equals signal out. To the output coil, add taps. The number of these taps determines the number of steps in our level control. Where along the output coil you place each wire determines what portion of the signal you get: one-tenth the length of the output coil gives one tenth the possible output signal, and so on.

Each of these taps typically connects to our friend the mechanical selector switch, (though it could also connect to relays or a CMOS switch if remote control were desired) which we understand has a rather limited number of positions. 20, 30, 40, maybe even 50 steps require a big switch and lots of taps.

The upside with this type of attenuator is the sound of a transformer is added, something many people like. The downsides are twofold: limited number of steps with correspondingly big jumps, restricted frequency response at the extremes.

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13 comments on “Transforming volume”

  1. Can we learn, Paul, if your partiality concerning this type of passive attenuators is based on theories or empirical listening results or other findings? There are even companies judging output transformers as mandatory audiophile core components.

  2. I don’t have a position on this. But the multi tapped transformer seems to be the only way to make a passive line stage that can drive difficult long cables without compromising dynamics.

        1. Yes a friend just built a passive preamp using two autoformers, one for each channel. He claims wonderful sounds from this and it is dead silence from any hum or noise.

  3. I can witness that under one set of conditions, a transformer based volume control surpassed all competing preamp technologies for absolute sound qualities by unanimous decision of an expert panel of listeners. Note that we gauged by visceral reactions as well as conscious attributes – polygraph parameters and the toe tapping test are more important than the “visualization” test since most “imaging” is fake and learned. At least two of the participants attended acoustic concerts regularly (I went to over 100 that year) so the “HiFi” bias of ears broken in to speakers was considered.

    Removing the veils is essential for this level of discernment. It starts with a pristine recording of acoustic music in a real music venue to provide an absolute reference. Any studio manipulations bias the test in favor of the artificial music reproduction ecology. Music mixed and mastered for mediocre speakers sounds worse on more accurate systems. (IMNSHO any mixing and mastering sounds inferior to straight through mic pair signal. I stopped listening to mixed and mastered recordings years ago.)

    The speakers were refurbished Quad ESL63 which have exceptionally flat high frequency extension and coherence. They were driven by a custom amp built on a Dynaco Stereo 70 chassis. It had a differential input and cascode driver feeding a more or less stock push-pull output stage with the Dyna Williamson tap transformers. The circuit configuration eliminated all but one coupling cap, which was a military grade Pentacap oil filled, skived Teflon, anti-inductive unit chosen by ear. This amplifier later surpassed all comers in an amplifier shootout driving a pair of Meadowlark speakers.

    I can’t explain why the transformer sounded better than a pot – it has narrower bandpass and slightly higher distortion measured by a SoundTech analyzer. Perhaps it is impedance matching.

    We were curious about RFI effect, and tried putting a cell phone next to the amplifier and calling it. This was not audible, which speaks to careful layout, linearity of tubes, good cables and the monopoint star grounding of the system.

  4. Quads are not known for their bass or highs Acuvox. Tube amps require the highest bandwidth output transformers to achieve good bass response also. This is one reason OTL tube amps are still around even with the expense they entail.

    I believe a transformer based preamp could be a great thing and not very expensive if was implemented correctly.

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