Manley TNT MICROPHONE PREAMPLIFIER User Manual
TNT
Manley Laboratories, Inc.
13880 Magnolia Ave.
Chino, California 91710
Tel: +1 (909) 627-4256
Fax: +1 (909) 628-2482
www.manley.com
CONTENTS
SECTION PAGE
INTRODUCTION 3
BACK PANEL & CONNECTING 4
FRONT PANEL 5
BEGINNINGS 6
IMPEDANCE 8
TUBE CHANNEL 10
COOL CHANNEL 13
TROUBLESHOOTING 19
CURVES 20
MAINS CONNECTIONS 24
SPECIFICATIONS 25
APPENDIX 1 - TEMPLATE FOR STORING SETTINGS 26
3/15/11revCD
INTRODUCTION
THANK YOU !
For choosing the Manley TNT Microphone Preamplier. The name of this mic preamplier “TNT” is
a reference to the two different channels, one that is Tube and the other No Tube. We see people today
hungry for a variety of preamps as a means of getting a variety of sounds and tones from the start. Of
course, mic selection and mic position can produce a wider variety of colors, but once that is locked
in, the preamp choice can be a signicant factor in some sessions and that subtle difference in others.
The designer considers the reference to TNT (the explosive) might be a bit apropos too. Both share
a reputation as a relatively powerful and particularly useful tool for the ‘difcult’ tasks while being
relatively safe (well, much safer and more stable than Nitro-Glycerine, its main component). Manley
would like to thank Steve Pogact for suggesting the name.
Most mic preamps are stereo, which is ne when one needs to record true stereo with matched mics
(few are), but the most common situations where one can compare and choose a particular preamp,
are the often the more relaxed overdubs. These are typically single miked or multi-miked where each
mic might be a different type and different distance from the source. So the TNT might be the ideal
cost-effective solution for these situations – maximum choices & minimum cost and rack space and
where you need to know you are using the one of the best.
Some people use the TNT simply because it offers two different sounds and is different from their
other preamps. Some people choose it because each channel may be a contender for the highest
caliber preamp in either the “tube category” or “solid-state category” and they demand the best. And
some users just like the unique controls and features. Of course, given a variety of sounds and features, different people will gravitate to one side or the other and one feature or another. This is to be
expected and probably encouraged, however let us emphasize that we encourage you to dig in, use it
to its fullest and form you own opinions and methods that work for you.
GENERAL NOTES
LOCATION & VENTILATION
The Manley TNT must be installed in a stable location with ample ventilation. It is recommended, if
this unit is rack mounted, that you allow enough clearance on the top of the unit such that a constant
ow of air can move through the ventilation holes. Airow is primarily through the back panel vents
and out through the top.
You should also not mount the TNT where there is likely to be strong magnetic elds such as directly
over or under power ampliers or large power consuming devices. The other gear’s fuse values tend
to give a hint of whether it draws major power and is likely to create a bigger magnetic eld. Magnetic elds might cause a hum in the preamp and occasionally you may need to experiment with placement in the rack to eliminate the hum. In most situations it should be quiet and trouble free.
WATER & MOISTURE
As with any electrical equipment, this equipment should not be used near water or moisture.
SERVICING
The user should not attempt to service this unit beyond that described in the owner’s manual. Refer all
servicing to your dealer or Manley Laboratories. The factory technicians are available for questions
by phone (909) 627-4256 ext. 325 or via our website at www.manley.com. Fill in your warranty card!
Check the manual - your question is probably anticipated within these pages......
3
THE BACK PANEL
SERIALNUMBER
+4UNBALANCED
+4 BAL
-10UNBALANCED
OUTPUT MODE
MANLEY LABORATORIES, INC
www.manleylabs.com
+4UNBALANCED
CAUTION -RISK OF ELECTRIC SHOCK. DO
TRANSFORMER & XLR
DESIGNEDBY HUTCH
TNT
DOES NOT BYPASS
CHANNEL 2
OUTPUT
TRANSFORMERLESS
OUTPUT
N9512422
1
N9512423
FUSE 1A @ 117V
FUSE .5A @ 220V
CAUTION: RISK OF ELECTRIC SHOCK. DO
NOT OPEN. REFER SERVICING TO
QUALIFIEDPERSONNEL ONLY
OUTPUT
TRANSFORMER
ISOLATED
TRANSFORMERLESS
OUTPUT
DOES NOT BYPASS
TRANSFORMER & XLR
OUTPUT MODE
+4 UNBALANCED
-10 UNBALANCED
+4 BAL
MIC
+4 UNBALANCED
TRANSFORMER & XLR
OUTPUT
BYPASS
OUTPUT
DESIGNED BY HUTCH
MIC
TNT
SERIAL NUMBER
MANLEY LABORATORIES INC.
13880 MAGNOLIA AVE., CHINO, CA 91710
PHONE (909) 627-4256 FAX(909) 628-2482
www.manleylabs.com
1 3 8 9 10 7 6 5 4 2
First connect all the cables, then turn on the power, wait 30 seconds, then have fun, as if we had to tell you....
1) POWER CONNECTOR. First verify the POWER SWITCH on the front panel is off (CCW). Use the power cable supplied with
your TNT. One end goes here and the other end goes to the wall outlet. You know all this.
2) VOLTAGE LABEL (ON SERIAL STICKER). Just check that it indicates the same voltage as is normal in your country. It should
be. If it says 120V and your country is 220V, then call your dealer up. If it says 120V and you expect 110 it should work ne.
3) FUSE. Unplug the power cable rst. The Fuse Cap needs a push then turn a quarter twist CCW to pull off. Fuses are meant to
“blow” when an electrical problem occurs and is essentially a safety device to prevent res, shocks and big repair bills. Only replace
it if it has “blown” and only with the same value and type (500mA slow-blow for 120V, 250mA slow-blow for 220V). A blown fuse
either looks blackened internally or the little wire inside looks broken. A blown fuse will prevent all the LEDS from lighting and will
prevent any power and audio.
4) TUBE CHANNEL XLR JACK INPUT. ( Left) Accepts balanced or unbalanced sources. The pin out is as follows: PIN 2 = Positive = Hot, PIN 3 = Negative = Low or ground, PIN 1 = Chassis Ground. Be sure that the PIN 3 is connected to the negative or ground
and not “open” or a 6dB loss or loss of signal will happen. This input is transformer coupled. Switching PHANTOM POWER on, puts
approximately 48 volts via 6.8k resistors (double regulated 46-47 vdc typical) on pins 2&3.
5) TUBE CHANNEL XLR JACK OUTPUT. (Channel One or Left) Transformer Balanced and Floating. Only for +4dBu pro levels.
The pin out is as above. NOTE: Inserting a 1/4” plug into the “UNBALANCED OUTPUT” jack disables this XLR output !
6) TUBE CHANNEL PHONE JACK OUTPUT. (Channel One or Left) Unbalanced output only. Factory set-up for +4dBu pro levels and mono or 2 conductor plugs. NOTE: Inserting a 1/4” plug into this jack disables the XLR output because it switches the transformer out of the circuit to prevent any loading. This 1/4” output should be used if the goal is to record as clean and tight a low end as
possible from tubes. This jack also should be considered if the destination or cables present a difcult load for the XLR transformer
output (high capacitance from long cables, or low resistance from 600 ohm vintage gear or more transformers).
7) COOL CHANNEL XLR JACK INPUT. ( Left) Accepts balanced or unbalanced sources. The pin out is as follows: PIN 2 = Posi-
tive = Hot, PIN 3 = Negative = Low or ground, PIN 1 = Chassis Ground. Be sure that the PIN 3 is connected to the negative or ground
and not “open” or a 6dB loss or loss of signal will happen. This input is transformer coupled. Switching PHANTOM POWER on, puts
approximately 48 volts via 6.8k resistors (double regulated 46-47 vdc typical) on pins 2&3.
8) COOL CHANNEL XLR JACK OUTPUT. (Channel One or Left) Transformer Balanced and Floating. The pin out is as above.
The 1/4” jack has no effect on this XLR (both can be used at once) but the OUTPUT MODE switch should be properly set (generally
+4 Balanced for maximum headroom) however it is worth mentioning that a transformer oating output is equally happy driving balanced or unbalanced inputs. But we will remind you that most unbalanced inputs are designed for -10 levels so in order for TNT’s levels and LEDs to work as designed, if the destination is -10 consumer level the switch should be set for “-10 unbalanced” (usually ;>)
9) COOL CHANNEL PHONE JACK OUTPUT. (Channel One or Left) Balanced or Unbalanced output so a 2way or 3way(stereo)
plug can be used. Factory set-up for +4dBu pro levels. This 1/4” output should be used if the goal is to record as clean and tight a low
end as possible (transformerless). This jack also should be considered if the destination or cables present a difcult load for the XLR
transformer output (high capacitance from long cables, or low resistance from 600 ohm vintage gear or more transformers). The OUTPUT MODE switch should be properly set (depending on the destination and wires) because these outputs do not use cross-coupled
op-amps and are designed to properly drive either balanced or unbalanced systems providing the switch is properly set.
10) OUTPUT MODE SWITCH. Sets both the output level and whether both the XLR and 1/4” jack are balanced or unbalanced.
Normally this switch should be in the “+4 BAL” or Center position. Some PRO gear may prefer the “+4 unbalanced” position but this
style of input is becoming increasingly rare, however because the TNT loses 6 dB of headroom in this mode, it can be used for creative “drive” cleverness. Similarly the “-10 UNBALANCED” mode loses another 6 dB of headroom when ‘mismatched’ in typical +4
balanced systems. However, this mode is more likely to nd service because a fair amount of semi-pro and consumer gear with RCA
jack inputs are compatible with this level and 1/4” plug to RCA plug adapters are very common.
4
THE FRONT PANEL
1 3 8 10 15
TRIMGAIN
COLOR
60s
0
50
55
-4
-6
60
65
-8
70
70s
-2
+2
CLEAN
+4
IRON
+6
+8
P
-1
+10-10
B
+22
TNT
OUTPUT
+10
INPUT
POWER
0L
ON
OFF
SIG
230
BY MANLEY LABS
PHANTOM
ON
OFF
PHASE
180
0
FILTER
80 Hz
FLAT
IMPEDANCE
10K
2400
600
Ω
3MEG
1MEG
300K
TUBE
CHANNEL
INSTRUMENT
TRIMGAIN
ON
45
50
40
35
30
25
P
20
70
B
0
-2
+2
55
-4
60
-6
65
-8
OFF
+4
PHASE
+6
180
+8
0
P
+10-10
B
300c
60 Hz
120 Hz
FLAT
IMPEDANCE
2M 10M
2K
600
300
COOL
CHANNEL
INSTRUMENT
Ω
3M
1M
300K
100K
45
40
35
30
25
P
20
B
FILTER
PHANTOM
2 4 5 6 7 9 11 12 13 14 16 17 18
TUBE CHANNEL
1) PHANTOM POWER: Toggle up turns 48 volt phantom power on (and the red LED) which is needed for most FET condenser
microphones. Turn your monitors down because it may make a big ‘POP’. Avoid patching the mic lines if phantom is turned on. Some
ribbon mics can be damaged if phantom is on and cables or patches are changed, so don’t use phantom with ribbons - or be careful.
2) PHASE SWITCH: Reverses the polarity (180 degrees) of the microphone signal. Sometimes needed in situations where two mics
are used and sometimes useful for vocals when headphones are used. Amber LED is ON when PHASE switch is engaged.
3) HIGH PASS FILTER: Toggle up engages a basic 80 Hz lter. Used to remove excess lows, rumble and some air conditioning
noise.
4) IMPEDANCE SWITCH: 3 position toggle changes the loading characteristics presented to the microphone or instrument. This
can subtly change the sound of the mic. “2400” ohms is considered normal or typical. “10K” is a lighter load and may be appropriate
for some sounds and often for ribbon mics. “600” ohms may tighten up the lows on some mics. Unlike many Impedance switches, the
volume will not change by 6 dB, and will stay relatively constant making comparisons easier.
indicate the impedance given on the 1/4” Instrument Input, where 1 Meg simulates a typical amp. Higher usually means brighter.
5) INSTRUMENT INPUT: Plug your guitar or bass in here. A plug inserted in this jack will disable the XLR Microphone Input.
6) GAIN SWITCH: This rotary switch sets the gain for the rst amplication stage. The steps range from +20 dB to +70 dB and
when used with the TRIM (below) provides a range of +10 to +80 dB of gain. The bottom two LEDs (see #17) indicate “signal present” and “overload” of this rst stage. If internal jumpers are properly set, one can turn this switch higher for overdrive and turn down
the GAIN TRIM to optimise the level again. If the jumpers are set for “clean” then it may be difcult to overdrive this stage.
7) GAIN TRIM POT: This pot is typically used to nely adjust the gain as needed for the recording device or converter.
LEDs are associated with this knob, and are intended to help set optimal levels, which are well (about 10dB) below when the TNT
clips.
The numbers to the right of the switch
The two top
COOL CHANNEL
8) PHANTOM POWER: Toggle up turns 48 volt phantom power on (and the red LED) which is needed for most FET condenser
microphones. Turn your monitors down because it may make a big ‘POP’. Avoid patching the mic lines if phantom is turned on. Some
ribbon mics can be damaged if phantom is on and cables or patches are changed, so don’t use phantom with ribbons - or be careful.
9) PHASE SWITCH: Reverses the polarity (180 degrees) of the output signal.
10) HIGH PASS FILTER: Toggle middle position is a 120 Hz HP lter, Toggle up engages a less drastic 60 Hz lter.
11) IMPEDANCE SWITCH: This is a 5 position rotary switch that both controls the loading on a connected microphone and internally directs and shares the signal between two different preamp circuits or topologies. The least amount of loading is the 2 MEG
setting and the next setting marked 2K (2000 ohms) might be considered traditional or typical (most preamps were designed for 2000
to 3000 ohms). Both of these two only use the cascode FET preamp. The next two settings share both the cascode FET preamp and the
current mode preamp, 600 and 300. The nal setting, 300C, only uses the current mode preamp and relays bypass the sub-circuit used
for mixing the preamps, so it has some purist function.
12) INSTRUMENT INPUT: Plug your guitar or bass in here. A plug inserted in this jack will disable the XLR Microphone Input.
13) GAIN SWITCH: This rotary switch sets the gain for the rst amplication stage. The steps range from +20 dB to +70 dB and
when used with the TRIM (below) provides a range of +10 to +80 dB of gain. The bottom two LEDs (see #17) indicate “signal present” and “overload” of this rst stage. Use this switch in conjunction with the 60’s-70’
color or distortion. In this situation the GAIN SWITCH becomes a “drive” control too.
14) GAIN TRIM POT: This pot is typically used to nely adjust the gain as needed for the recording device or converter.
top LEDs are associated with this knob, and are intended to help set optimal levels, which are well (about 10dB) below when the TNT
clips.
15) COLOR SWITCH: With the switch in 60’s or 70’s, a circuit is added that is designed to clip in an interesting way that somewhat
simulates the way magnetic tape, and guitar amps clip. Use the GAIN SWITCH to control the depth of distortion. In general best
results are obtained when the desired effect is subtle and this circuit is just lightly driven and obvious distortion is minimal. Of course,
sometimes more drastic effects are desired and the GAIN SWITCH can be turned up. Thicker distortion may take several processors.
The 60’s - 70’s switch can alter the highs depending on drive levels.
16) IRON EFFECT: This knob adjusts how audible the output transformer may be from exaggerated at +3 to near zero at 0 and even
becoming inverse or the opposite of a transformer at -1. This control is essentially “out-of-circuit” with the knob straight up (12:00)
(like an EQ). The knob controls several subtle effects including low frequency level, low frequency distortion, high frequency level
and high frequency dynamics. This control is often subtle and somewhat signal dependent.
s switch and the LEDs to control the amount of
The two
17) LED LEVEL INDICATORS: Simple indicators to show signal presence, rst stage clipping, and more or less appropriate levels
for the next device. See page 000 for more details.
18) POWER SWITCH: With this switch UP, LEDs should come on and maybe sounds might come out the back XLRs......
5
BEGINNINGS
The TNT project began due to requests from fans of
the SLAM! and in particular Lynn Fuston of 3D Audio
Inc. The request was simply “Can you bring out just
the SLAM! Mic preamps without the Limiters and
other features. Then came the 3D Audio bulletin board
“Dream Pre-Your ideas wanted” thread which had a lot
of great ideas and diverse opinions. Perhaps the most
obvious theme was that engineers were now using
several mic preamps at their disposal for a variety of
colors like they had always done with mics. This was
actually a pretty new trend in ‘92. How about a box
with a few different preamp topologies for different
sounds?
At the 3D Preamp Summit, the topic of “vintagestyle” electronics came up, not because of huge
desire from the engineers but more as a designer topic
relating the headaches of recreating transformers and
obsolete parts accurate enough to be comparable to
the original. However where there was interest in old
school style was when the recording engineers began
to talk about how they did those classic 60’s sessions,
and the focus was on production technique and war
stories rather than components and gear. Maybe the
engineers were saturated with recreations of old gear
and were craving both the magic of old sessions and
yet new exciting toys to do their job with tomorrow.
A bit of both.
We began to experiment with some simple discrete
preamp design concepts and breadboarded a few
approaches. Hutch also had developed the “Rapture”
gain stage intended for a proposed digital converter
that was impressively un-colored and which became
the standard against which other ‘experiments’ were
compared.
Then the 3D mic preamp summit “Preamps in
Paradise” happened January 2004. This was a
historic event in Tennessee with a panel of 10 notable
preamp designers, and 7-10 famous engineers known
with reputations as preamp connoisseurs. Amongst
fascinating stories and a sharing of approaches, this
designer was hearing a chorus of requests, a short list
that included “variable impedance – but minus the
typical gain changes” and “some new useful control
or knob”. Up to then, the solid state preamp we were
working on was envisioned as a typical minimalist ‘2
knob’ discrete channel in some ways similar to the
tube side borrowed from SLAM!. But the engineers
were asking for more control, and were describing
approaches based on sonic characteristics such as
clean / not so clean, bright / not so bright, transformer
or transformerless and this resonated with the
designer’s session experience. Designers talk about
discrete topologies, tube types and transformer details
but recording engineers talk about sounds and controls
and session techniques.
The initial concept of the TNT was to put two very
different mic preamp technologies in the same box,
and that each were to be as simple as “plug in a mic
and it sounds ne”, without a lot of controls to get in
the way. In the end, the TNT did get new features and
controls such as the IRON knob, 60’s - 70’s switch
and its unique impedance control. And these were
largely due to engineer’s requests from the “Preamps
in Paradise” event.
So we went back to the ol’ drawing board and back
to the lab bench and experimented with a variety of
circuits and topologies, but this time holding truer to
the end result the user would appreciate rather than the
internal workings that might have buzz word appeal.
Already the SLAM! preamp was not “all-tube” but
more of a hybrid FET-Tube cascode, so why stick to
“all-discrete” or “vintage-clone” when the engineers
seemed to be just concerned with sound or tone and
occasionally hoped they could get a few new controls
if possible.
The TNT solid state side evolved into a mix of discrete
and op-amps, plus it ended up with the Rapture Amp
for the line driver. Why this mix? Just our decision
based on listening comparisons where our choices
generally favored the cleaner or most true to the source
as a base. The users could always add stuff that gave
some color or texture onto that base and we provided
a few too.
We did come up with a few controls that gave some
possibilities for “tints” or “avors” that could be dialed
in. For the most part these were designed to be subtle
rather than drastic because the TNT is a mic preamp
and not an EQ or a typical processor. In fact even the
IMPEDANCE switches were designed to not mangle
the sound in unintended ways - they should be “just”
impedance controls without baggage. Some users may
expect bigger sonic changes from huge impedance
changes or major audible effects from varying the
IRON content, but the folklore surrounding those
ideas is maybe more dramatic than reality. These
controls were designed to reect reality which seemed
appropriate on a high end Mic Pre and tend to be more
like tweaks and trims. And this was more in line with
the original concept of a basic good plug-in-a-micand-go preamp.
6
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
avor 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
amplier would have innite input impedance and
the current ow 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
amplier, the voltage mode amplier 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 amplied 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 signicant 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 mythbuster 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 “aws”
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 ght to make
it audible enough to wrap a control circuit around it.
7
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 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 nally 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 reections
impact high frequencies. Normally we don’t consider
audio frequency 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) before 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 mumbojumbo 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 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 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.
8
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