Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 7)

The TNT was getting interesting. We now had a tube
preamp from the SLAM! on one side plus two solid
state preamps working together on the other side, and
each of those 3 circuits had a unique sound or subtle
flavor yet there was some common theme or style.
The tube channel is based on a JFET / Vacuum Tube
Triode cascode circuit that is quite unusual in that it
is a blend of old and new components. The cascoded
combination allows for high gain, low noise, and low
distortion without using negative feedback.
On the solid state channel, one of the preamps also
uses JFETs in a cascode topology. The high impedance
circuit operates in the voltage mode. The ideal voltage
amplifier would have infinite input impedance and
the current flow would be zero. We use a paralleled
cascaded ultra-low-noise FET / op-amp circuit and the
TNT requires a pair of those hybrids for balancing (8
discrete matched FETs). There are a few interesting
twists here too. When phantom power is engaged,
it uses the conventional 6.8 kOhm phantom power
resistors plus the usual DC blocking capacitors. So
even if the impedance switch is set to 2 Meg Ohms,
the phantom circuit limits the impedance down to
about 14 kOhms. But if phantom is turned off, TNT
removes those resistors (rather than switching them
to ground, as is standard procedure) and bypasses
the DC blocking caps, so that you truly have 2 meg
input impedance and DC coupling restored. And no
free lunch here either. Compared to the current mode
amplifier, the voltage mode amplifier typically tends to
have opposite characteristics in terms of its strengths
and weaknesses.
The low impedance circuit is based on a special
Lundahl transformer designed to operate in the current
mode. This allows the transformer to work down to
near DC yet be very small physically. This was paired
with an ultra-low noise, ultra-low distortion op-amp
that won in our listening tests.
By combining the JFET and Current mode preamps we
were able to create a variable impedance that uniquely
sensed and amplified both voltage and current. And
this in turn provided much better gain consistency
while usually sounding a bit better than either
approach alone. In other words the volume didn’t
jump or shift as the impedance control was changed
unlike most (or all) other approaches. Some switch
transformer taps, but transformer frequency response
is very source dependent. And some preamps had 20
dB gain changes as the impedance knob was adjusted.
Now that users can accurately hear and compare
the effect of variable impedance without huge gain
changes and without significant frequency response
changes, it becomes truly interesting to hear what
effects there are. In most cases these effects are not
life changing. Even where low impedance settings
seem to affect the damping or tightness a dynamic or
ribbon mic might exhibit, one then may be faced with
an unfamiliar sound from a familiar mic. This may be
a good thing sometimes, but often one picks a familiar
mic for its familiar sound. And one may hear similarly
questionable highs in the high impedance settings.
One might hear excess sibilance or harshness that may
be due more to cables and cable distance than anything
else. In the end, one may be most comfortable with the
two middle settings of 600 and 2400 as the idea of
extreme impedances gradually lose appeal.
Maybe the TNT might be viewed as a bit of a myth-
buster in regards to “variable impedance”. Yes, it is
sometimes useful, but can often be subtle - not exactly
the most important feature for a mic preamp.
And then the basic premise of a tube preamp and
solid state preamp being very different sounding
animals might be a myth too. At one time we had both
preamps tuned and adjusted to be extremely clean
and transparent. One might guess that two transparent
gain stages regardless of the technologies might sound
the same, which was essentially inaudible. We had to
go backward and re-introduce some of the “flaws”
to recreate some of the creative differences that we
all expected. We added some internal jumpers that
essentially un-trim the tube bias trimmer pots that are
tweaked to set up minimum distortion.
Same thing with the IRON control..... So much has
been said about the sound of transformers lately that
many people expect that one component to almost be
responsible for a product’s signature sound - wrong!
Sorry another myth. Most modern transformers are
pretty transparent when used properly.
In fact, we had to use several techniques just to make
the IRON control ‘audible’ including designing
the transformer with unusually low permeability
laminations, driving it with a non-optimum source,
and forcing DC into a tertiary winding to create more
distortions. In other words it was a bit of a fight to make
it audible enough to wrap a control circuit around it.
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