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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