Transformer coupled / Capacitor coupled

Clueless,
I believe per the article that the transformer application he is attempting is in a passive device. I think that is why the active transformer coupled pre's don't have that issue. They are 600 ohm instead of the 50k or 100k many actives use. We drive not only a variety of tube amps but the Gamut solid state which has 10k ohm active balanced curcuit with unbelievable authority and resolution. This amp is in stark contrast to the tube amps we drive with input impedance numbers as high as 100-150k. Wide range for a single pre do be able to handle such extremes with no loss of any musical content. His article points out the very reasons passive devices cannot compete in most real world situations. Thanks again for the heads up. I like to stay current on all this cool stuff. Wish you were local. You could attend one of our shootouts:)

Jc: I'm about as "local" as you can get. I just don't live by you.

I'd love to hear one of your shootouts.

Sincerely
I remain,

The Audio Note, EAR (E11??), the big Jadis and Atma-Sphere (P-2) preamps are the only high-end audio transformer-coupled units out there to my knowledge.

Transformer-coupling is common in professional tube gear, particularly from the 50s and 60s. This is because the equipment had to match to the 600 ohm balanced line standard (which is still very much around today). Tubes coupled by output coupling capacitors would never be able to play any bass driving a 600 ohm load!

We built the P-2 (discontinued when it was replaced by the MP-3) so it could drive the 600 ohm standard as well. The nice thing about the 600 ohm standard is that it ameleorates the role that interconnect cables play in the sound of the system, which is why the standard has been used for the last 5 decades by the professional recording and broadcast arts. Its always been a puzzle to my why audiophiles are so slow to embrace the same standards, despite having the same concern about cable qualities. This has spawned the high-end audio multi-million dollar/year cable industry.

We're an OTL manufacturer, but we've been a major supporter of balanced line technology for the last 14 years (mostly due to prolonged exposure to the recording studio); our preamps support the standard using direct-coupled ouptuts, which is the only other way to do it as capacitors won't work (since no-one would take a tube preamp with an electrolytic output coupling cap seriously...). IMO, its a shame that more manufacturer's aren't wise to what's happening here (sorry for the hype blast).

No reason to apologize for recommending what appears to be sound engineering practice.

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