Hi Tom and group,
I've had the chance to do some analysis of transformers over the years.
Let me preface this by stating I am in NO way an expert on them! So, to the
real experts out there, please don't beat me up too bad; these are
generalizations only. This will be a very short treatment, devoid of actual
measurements (for now). Audio transformers are a subject that I have a
great interest in and have done some work in measurement in an attempt to
quantify the effect they have.
If the intention of installing transformers is for isolation, breaking up
ground loops, ease of interface, etc. then you want to find a high quality
unit that meets the band width specification AT the rated AC voltage that
will be applied to it. Keep an eye out for how the ratings are stated. For
example, a band width of 20Hz-20KHz at 100mV would not necessarily
correlate to the same at 2V! The frequency extension at both ends is
limited by core magnetizations/saturation and the distortion products
generated by the increased applied voltage. The other way to look at that
is that the band width narrows as the input voltage is increased beyond the
rating of the device.
Now the interesting way to look at that is backwards...............say
you want to use the transformers for a coloration effect. You can then take
advantage of the band width at lower specified input voltage by driving
the transformer beyond that. This is what results in a "transformer sound".
Meaning, either the band width becomes limited and/or the distortion
products become audible. So, in this case you may not want to use a very
high quality unit, but either something kind of funky or voltage under
rated or band limited.
When most (competently designed) transformers are used within the
ratings, we weren't able to measure much of anything in regard to band
limiting or distortion products. However if you push the input level to
where the core saturates, some interesting and audible artifacts were noticed.
Some years ago a client came to use with the idea of building a dedicated
"transformer box", to be inserted anywhere in a line level signal path with
the intent of adding some unique coloration. The specific units he wanted
to use were a pair Western Electric 111C "repeat coils". These were line
level 600:600R (or could be strapped for a couple different ratios). So we
were tasked with the design of a circuit to bring maintain input : output
at unity gain and package the whole thing up in a useable fashion.
http://www.analogbros.com/pictures/coil%20box/
At the time we were working with a colleague who was investigating a very
interesting amplifier topology as published by Dr N Thaggard. The initial
design was as a low level phono stage, which could be easily adapted for
say a mic level. We manipulated this for line level. In short, the active
circuit measured essentially flat within the intended band width, exceeding
it by a quite a bit on the high frequency end.
Anyway, to cut to the chase. To be honest, when the project was complete,
I really couldn't hear any significant difference between input and output
when the levels were carefully calibrated.
However...........................there is a kind of built in "adjustment"
that transformers have to allow you to skew the response...........the
secondary termination. There is likely a manufactures suggested
termination; depending on where in a given circuit and type of transformer,
this could be a simple R or an RC combination. Manipulation of this can
indeed result in audible differences. We ended up installing a variable R
termination on the device to allow for interjectional mis-adjustment of the
termination.
Given the time, there is plenty more I could write and if I can locate
the data from the experiments I'll be happy to post it. Got to run now,
work to do.
Regards, Jim
At 12:34 PM 3/14/2011, you wrote:
Hello,
I have been thinking about adding some output transformers to my Series
One. I am thinking about building a rack chassis with two (for now)
transformers and having them available via patchbay... so obviously they
are not technically in the Neotek. I could then patch them in on a direct
output or on the master outs before hitting tape or my convertors.
Should be easy enough, but my question is if anyone has some suggestions
as to what transformer is a good match sonically with a Series One. I am
considering this only for more options in terms of adding some different
sonic charactor and color. The design would be just a passive circuit.
Tom
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