- 24 step AVC Copper EI Core - $469 /pair
- 24 step AVC Copper C Core - $579 /pair
- 24 step AVC Silver EI Core - $579 /pair
- 24 step AVC Silver C Core - $694 /pair
|Transformer Volume Control|
Why do people use Autoformer Volume Control or Transformer Volume Control? Before we go there, let's analyze resistive volume control electrical characteristics on an amplifier,
Using the most common 100K A*ps Blue pot as example, measured with just simple multimeter, it was found that the resistance at the center tap of the pot between left and right channels are not the same. Sometime the difference can be huge! Just do yourself a favor. Go measure it at home, regardless of what brush type of pots you use.
Left and right channel is not matched and the loudness between left and right will be different. This is still manageable if you use a stepped resistive attenuator.
What about the characteristics at different loads? Say we fix the VR and we use 2 different loads, 47K and 10K Ohm. We found that at different loads, the output gain changes. What happens if your next stage is an input transformer that has variable impedance at different frequency?
Different loads (input impedance of next stage) give different volume!
If one measures the output voltage at different frequency driving a tube preamp with input capacitor or SS preamp with input capacitor, he may find that the output voltage actually changes with respect to frequency! The dB of change can be up to 1 or 2dB that is audible to human ear. This may happen on preamplifier without input capacitor too due to Miller Capacitance at the grid of the tube.
The higher the frequency, the higher the attenuation/loss. Why? Because the VR forms a LPF (low pass filter) with the input capacitor or the Miller Capacitance.Conclusion: resistive attenuation scheme is flawed!
All these would not happen if you use autoformer volume control or transformer volume control. With the input stage to the AVC / TVC, above problems can be avoided. That is why so many people preferred AVC / TVC compared to traditional resistive volume control!
It's time to order you own unit of Autoformer Volume Control / Transformer Volume Control!
J&K Audio Design
8 June 2014