Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 14)

The Impedance switch also has markings on the right
hand side that are for the INSTRUMENT input only.
How a magnetic pick-up reacts somewhat depends
on the impedance it is driving. This effect is diluted
because most guitars and basses have those volume
and tone controls that pretty much set the maximum
impedance the pickup will ‘see’. The pickups also
have to drive a length of cable, which often has
appreciable capacitance and may roll off some highs.
A fun trick for a session instrument is to bypass the
volume and tone controls (maybe on a switch??), just
leaving the pickup switch and jack, and then using
a fairly short low capacitance cable into a variable
impedance preamp like the TNT. In that situation the
IMPEDANCE switch becomes very dramatic and
the high impedance settings like “3 meg” and “10
meg” often get bright enough to sound much more
acoustic-like. Typical authentic guitar amp tones are
the 300K and 1 meg settings. The 100K setting may
be similar (thick and un-bright) whether the volume
control bypass mod is there or not. Guitars and basses
with “active pickups” (battery included) and guitars
feeding stomp boxes before the TNT, should be
mostly immune to the IMPEDANCE switch settings
and the 100K setting may have marginally less noise.
After the preamps are selected or are combined in
a very transparent summing amp, the next stage is
the “60’s / 70’s module” that contains a fairly simple
class-A all-discrete circuit, and is inserted when the
60’s or 70’s switch is selected, or relay-bypassed
when that switch is set to “CLEAN”. The module is
meant to simulate some of the qualities of vintage
discrete nonlinearities, tape overload (what some
call tape compression), and changes the frequency
response slightly. It is not meant as a straight
simulation of any particular piece of vintage gear
and is meant to evoke some general characteristics
of those eras and give the user a few more variations
in the ol’ tool-kit. The 60’s/70’s switch also adds DC
bias to the output transformer to further simulate
some old technologies (and which can be further
adjusted with the IRON control). If sufficient interest
is shown, Manley may be able to offer alternative
“modules”, as this block is easily removed and
replaced.
Then the signal hits the GAIN TRIM conductive
plastic pot, and is routed to the RAPTURE AMPS,
which are the final line drivers in this preamp.
It drives the ¼” output directly and the output
transformer (custom designed and manufactured in
house for the TNT) for the XLR output. Two controls
are wrapped around the RAPTURE AMP.
One is the IRON knob, which compares the input
and output of the transformer, derives an error signal,
which is then sent to the IRON pot, that acts much
like an EQ boost/cut pot with zero ‘effect’ at 12:00 or
straight up. This way the user can reduce the effect
of the transformer to near inaudibility or exaggerate
it or just leave it as it is where this additional circuit
is essentially bypassed and the transformer output
is just pure conventional transformer and the ¼”
output is just clean and unaffected. In fact, if you use
the ¼” output and adjust the IRON control counter-
clockwise, you get an effect that might be called
“Anti-Iron” and could practically reduce the effect
of a transformer in the next piece of gear following
TNT, such as a compressor – kind of like “cleaner
than clean”.
The other circuit wrapped around the RAPTURE
AMP is the OUTPUT MODE switch on the back
panel. Rather than use the conventional cross-fed
feedback output that many use to make a balanced
output a bit fool-proof and to simulate a transformer
and how it accommodates balanced and unbalanced
inputs, we have this switch. Why? For one thing, we
have the real transformer, for another, those circuits
are inherently usually within .5 dB of being called
an oscillator, and thirdly, the switch is 3 position,
so we can properly accommodate +4 unbalanced,
+4 balanced, and –10 unbalanced – all driven low
impedance, high current (full headroom down to 50
Ohms) and unconditionally stable. One might also
note that these two outputs are isolated and can serve
to drive two separate distant destinations easily.
The RAPTURE AMP itself was the result of months
of auditioning almost every discrete op-amp and gain
stage and every chip op-amp known to us to ever be
used for an audio product or published DIY project.
And we experimented with most of the tricks used to
further enhance all of these op-amps. In the end, we
found a circuit that was practically unique in its lack
of artifacts and sonic purity. After all those months of
R&D, it was decided that it should go in a block of
epoxy.
A similar routine of comparing the whole TNT
preamp to a lot of known expensive reference
preamps was performed while also continually
comparing the raw source to the preamps attenuated.
Lets just say we are confident that it is a winner and
particularly true to the source. Hope you like it!
14