Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 8)

Perhaps it is because those near opposite characters of
the two circuits, that when combined or blended the
audible benefits and strengths of each prevail, while
the weaknesses of each are minimized. Of course, the
better each circuit is optimized, the closer they tend to
meet in the middle (transparency). Perhaps it is because
the approach of respecting both voltage and current; it
results in a form of optimal power transfer. Optimal
power transfer is a very old topic in electronics and
it relates to the old 600 ohm impedances pro audio
inherited from the telephone industry and the 75 ohm
terminations we need to be concerned with for word
clock and video lines. Maybe with complex sources,
there is valuable information carried both in voltage
and in current and maybe most mic pre inputs are
not as simple as a basic resistor. In other words, to
some degree a typical microphone is a complex source
(especially dynamic, ribbon and transformer coupled
condenser mics) and a typical mic preamp input may
also be a complex load (especially if it is transformer
coupled) and the cable and connections between
the two might also be viewed as a combination of
resistance, capacitance, inductance and distance. So
maybe it is all too complex to grasp without some
serious computer modelling, but maybe it can be easy
enough just to hear in some situations - and we’ll leave
that up to you.
We would like to speculate that one of the complexities
that you might experience will be cable length. While
we were designing the TNT we noticed excess high
frequency sibilance coming in at the highest impedance
settings. We finally traced it to the mic cables that we
were using. If we doubled the length, the problem
doubled and if we used a very short cable the problem
disappeared. And the problem wasn’t apparent at low
impedance settings. OK but why? Here is where we
have to speculate.
At low Z settings, it may be akin to our old 600
ohm terminated lines that pro audio inherited from
the telephone industry. And that standard was set up
to reduce echoes in early long distance lines. It also
resembles the 50 ohm or 75 ohm terminated lines
used for video and word clock where cable reflections
impact high frequencies. Normally we don’t consider
audio frequency cable reflections to be a concern
because they don’t seem to affect the 20 kHz
frequency response or square waves on our ‘scopes.
So our speculation might be a question. How far do
these cable reflections need to decay (in dBs given
that -60 dB is 1/1000) before they do not intersect
with our abilities to perceive transients? Or how many
microseconds of reflections and down to what dB?
Just use a shorter cable.
Impedance Issues and Microphones
OK, the above might be a bit of technical mumbo-
jumbo and what you really want to know is what to
expect and listen for when you change the impedance
switch in your session. Simply, at very high impedance
settings, there are usually a little more highs. At low
impedance settings, with dynamic and ribbon mics
the bottom often tightens up. In the middle impedance
settings, the preamp may sound closest to what you
have grown to expect with that mic because most mic
preamps are medium impedance and typically 1000 –
3000 Ohms.
With high impedance settings, one may be affecting
the mic and cable in a few ways. First, with
transformer coupled mics like most dynamic, ribbon,
and tube condenser mics, one might be setting up a
high frequency peak in the mic’s transformer that may
have been intended for 1 kOhm to 3 kOhm preamp
impedances. The opposite is also common, where very
low impedances may cause the transformer to roll-off
highs earlier than the designer intended (yeah, but its
your mic and your session, so choose the setting with
your ears). The other effect goes back to that rambling
about cables and time domain effects. Listen carefully
for excessive sibilance and what might be described
as an artificial harshness, and what perhaps the most
finely honed ears will hear as time-smearing in the top
octaves. This effect is directly related to cable type
and length, and once you lock into it, you can prove it
by doubling (or halving) the cable length. Even better
is moving the TNT into the studio once you have your
settings, and using a 4 to 8 foot cable from the mic to
the preamp. It seems puny low level mic signals are
more fragile to these effects than hotter line level (and
robust line driver driven) signals but we don’t know
why. Give it a try. If this seems a bit inconvenient for
level tweaks, add a simple passive variable attenuator
(fader or pot) in the control room near your converter
(if it has inconvenient input level adjustments) if you
are a purist, or use the compressor or EQ gain controls,
if you’re not. The TNT has quite a bit of headroom
(except in 60’s / 70’s mode) so there won’t be much
chance of overloading it and the real thing to keep an
eagle eye on is the analog to digital converter at the
end of the chain.
Away from the extremes, the 2K (2000 Ohms) setting
represents the standard impedance of most mic
preamps and what most microphones are designed to
drive. In other words, its safe, and maybe a bit ‘vanilla’
and this isn’t a bad thing. The 600 ohm setting is also
pretty safe and may have some advantages because
it gets closer to a 50/50 blend of voltage and current
mode preamps.
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