Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 17)

The IRON knob may disappoint some who expect a
radical and obvious effect. It can be subtle, especially
for mid-range dominant sounds like vocals and guitars
because, for the most part, a transformer affects the
extreme lows, extreme highs, and to some degree
the dynamics. This control is build around the actual
output transformer and, lets face it, a good audio
transformer shouldn’t be particularly colored and
messy. On the other hand, it has always been the
transformers in vintage gear that helped them sound
warm, smooth, and round, yet were quite OK at passing
audio pleasantly and without major damage. The
IRON control allows you to adjust the “Transformer
Contribution” from near zero audible effect, through
to “typical” at 12:00, and further to where the effect is
exaggerated and practically tripled which, by the way,
is still not always obvious.
At very low frequencies a transformer can be said to
have 3 significant effects: 1) it rolls off the subsonic
frequencies starting around 10 Hz, which is audible
due to the phase shift that happens mostly below 50
Hz, which acts to slightly delay the extreme lows. 2)
Hysteresis shapes the waveform for low level, low
frequency signals, putting a couple of bends around
the zero-crossing, which simply adds harmonics. 3)
Hot low frequency signals can cause the transformer
to saturate or overload, again causing harmonics and
may cause earlier roll-off and more phase shift. It is
in the transformer design details that set the amount
and balance between those effects, and is mostly
a function of the core lamination material and size
(weight or bulk), but also a function of the number of
turns of wire and how the transformer is driven, and
the expected load. We design and manufacture these
transformers in house at Manley Labs.
At very high frequencies, the biggest effect is an
ultra-sonic roll-off. This can be as gentle as 6 dB per
octave for several octaves, but typically becomes
18 dB per octave. In fact, the way that capacitance
between windings and leakage inductance interact,
it usually creates a significant bump in the frequency
response somewhere between 30 kHz and 100 kHz,
which is then tamed, as standard procedure, by adding
a resistor and capacitor at the output or secondary,
leaving a smooth predictable roll-off usually around
80k – 120 kHz. This still leaves some phase shift
in the 5k - 20kHz range which is audible and gives
one the impression that the highs are softer and don’t
extent to infinity.
This may surprise some people, but some designers
and researchers, including yours truly, have
experimented and determined for themselves, at
least, that we might need a frequency response out
to 500kHz or 1 megaHz to completely avoid any
audible phase shift in analog audio circuits and where
absolute transparency is the goal. Then again, our
ears adjust rapidly to rather drastic roll-offs even
as low as 10kHz and we often choose a rolled-off
high frequency response as being more pleasant and
comfortable. And some might say, “no microphone
or speaker can reproduce those frequencies, so why
bother?” but these effects are relative and additive,
so every little bit may be audible for those that can
compare. And considering the best audio systems only
approximate full range live performances, then maybe
one of the significant reasons is these infra-sonic and
ultra-sonic roll-offs happen too early, causing phase
shift in the audible spectrum and audible time smear.
Dynamic performance is another reason, but lets not
dwell on that now. Given the capabilities of modern
audio and the demands of the audience, much of the
goal is creating illusions and emotional impact. This is
how we should approach the controls on the TNT, and
maybe what should become the criteria for choosing it
against other preamps.
By now, you have probably correctly guessed that
turning the IRON control counter-clockwise results
in a cleaner, tighter bottom and extension in the
highs. Turning the IRON control clockwise results
in a rounder, smoother, warmer, and softer tone. The
mids won’t be affected much, and mostly only in a
relative way compared to the lows and highs, but if
your ears are great, you might notice a subtle effect
with IRON maxed where the mids arrive slightly
before the lows and highs and that this is a big part
of the vintage sound. And if you have been reading
carefully, the transformer is biased in “70’s” mode so
the IRON control will have a slightly different (and
greater) effect. You may have also picked up that the
IRON knob acts like an EQ knob, where it has near
zero effect as a circuit when the knob is straight up at
12:00.
We also mentioned that the ¼” output does not use
the transformer but is affected by the IRON knob. As
before, the IRON circuit is essentially bypassed when
the knob is straight up. Turning it counter-clockwise
introduces the transformer correction circuit, and with
this output creates an Anti-IRON effect. Extreme
highs and lows are boosted slightly. As the Iron knob
is turned up, you introduce the effects of a transformer
into a transformerless output.
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