Dear Clueless,
There is a world of difference between the transformer volume control that Thorsten is talking about in this article and the use of transformers in the output of a pre-amplifier.
Take the M3 for example, it uses an output transformer driven by a single 5687 with the two halves wired in series, it has a 33:1 step down ratio, an output impedance of about 4 Ohms and a bandwidth from about 8 Hz to over 150kHz minus 1 dB, for all intents and purposes that is at least 3dB above and below any audio signal it is likely to be presented with.
Due to the enormous gain (hence the high stepdown ratio) the line stage does not limit neither the dynamic nor the frequency envelope of the signal it is presented with, which is why a good transformer coupled pre-amplifier sounds so free, easy and uncluttered.
Transformer volume controls have many great advantages when correctly designed and wound (which I might add is not a simple matter), however there are several key problems implementing them into a pre-amplifier circuit,
1.) The input impedance is typically too low to be driven directly from a normal no-feedback high gain pre-amplifier stage.
a.) This means that you either have to apply feedback to the gain stage which defeats the sonic advantage the transformer volume control provides.
b.) Or you have to drive the input winding on the transformer volume control with a driver transformer from a no-feedback tube gain stage to get the full benefit.
This is not an inexpensive solution!
2.) The frequency/phase behaviour of the secondary windings MUST be very similar, otherwise each step on the volume register will sound different.
This requires enormously accurate winding and interleaving, also not inexpensive.
3.) The overload/saturation behaviour of the mumetal core is very critical, again you require a large mumetal core to get the best results and they are cheap either.
There are other issues.
All in all, transformer volume controls are unquestionably the way forward to achieve the greatest sonic performance, but they come at a price and none of the "passive" implementations I have seen or read about sofar achieve this aim.
Hope this helps.
Sincerely,
Peter Qvortrup
There is a world of difference between the transformer volume control that Thorsten is talking about in this article and the use of transformers in the output of a pre-amplifier.
Take the M3 for example, it uses an output transformer driven by a single 5687 with the two halves wired in series, it has a 33:1 step down ratio, an output impedance of about 4 Ohms and a bandwidth from about 8 Hz to over 150kHz minus 1 dB, for all intents and purposes that is at least 3dB above and below any audio signal it is likely to be presented with.
Due to the enormous gain (hence the high stepdown ratio) the line stage does not limit neither the dynamic nor the frequency envelope of the signal it is presented with, which is why a good transformer coupled pre-amplifier sounds so free, easy and uncluttered.
Transformer volume controls have many great advantages when correctly designed and wound (which I might add is not a simple matter), however there are several key problems implementing them into a pre-amplifier circuit,
1.) The input impedance is typically too low to be driven directly from a normal no-feedback high gain pre-amplifier stage.
a.) This means that you either have to apply feedback to the gain stage which defeats the sonic advantage the transformer volume control provides.
b.) Or you have to drive the input winding on the transformer volume control with a driver transformer from a no-feedback tube gain stage to get the full benefit.
This is not an inexpensive solution!
2.) The frequency/phase behaviour of the secondary windings MUST be very similar, otherwise each step on the volume register will sound different.
This requires enormously accurate winding and interleaving, also not inexpensive.
3.) The overload/saturation behaviour of the mumetal core is very critical, again you require a large mumetal core to get the best results and they are cheap either.
There are other issues.
All in all, transformer volume controls are unquestionably the way forward to achieve the greatest sonic performance, but they come at a price and none of the "passive" implementations I have seen or read about sofar achieve this aim.
Hope this helps.
Sincerely,
Peter Qvortrup