Perhaps it is because those near opposite characters of
the two circuits, that when combined or blended the
audible benets 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 electron-
ics 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
condensor 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 resist-
ance, 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 complexi-
ties that you might experience will be cable length.
While we were designing the TNT we noticed excess
high frequency sibilence coming in at the highest im-
pedance settings. We nally traced it to the mic ca-
bles 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 appar-
ent 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 tele-
phone 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 reections impact high
frequencies. Normally we don’t consider audio fre-
quency cable reections 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 reections
need to decay (in dBs given that -60 dB is 1/1000) be-
fore they do not intersect with our abilities to perceive
transients? Or how many microseconds of reections
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 trans-
former 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 imped-
ances. 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 articial harshness, and what perhaps
the most nely 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 hot-
ter 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) set-
ting 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 ‘va-
nilla’ and this isn’t a bad thing. The 600 ohm setting
is also pretty safe and may have some advantages be-
cause it gets closer to a 50/50 blend of voltage and
current mode preamps.
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