Manley TNT two channel Mic Pre Owner's Manual
OWNER'S MANUAL
MANLEY
TNT
MICROPHONE
PREAMPLIFIER
MANLEY
LABORATORIES, INC.
MANLEY LABORATORIES, INC.
13880 MAGNOLIA AVE.
CHINO, CA. 91710
TEL: (909) 627-4256
FAX: (909) 628-2482
CONTENTS
SECTION PAGE
INTRODUCTION 3
BACK PANEL & CONNECTING 4
FRONT PANEL 5,6,7,8
CREDITS 8
THE MASSIVE PASSIVE
BEGINNINGS, THE SUPER PULTEC 9
THE PASSIVE PARAMETRIC 10
WHY PASSIVE, WHY PARALLEL 11
PHASE SHIFT, WHY TUBES 12
CURVES 13 to16
TUBE LOCATIONS, ETC 17, 18
EQUALIZING
EQUALIZERS (GENERAL) 19
EQUALIZER TECHNIQUES 20 to 24
TRANSLATIONS 25
TROUBLESHOOTING 26, 27
MAINS CONNECTIONS 28
SPECIFICATIONS 29
WARRANTY 30
WARRANTY REGISTRATION 31
APPENDIX 1 - EXAMPLE SETTINGS 32
APPENDIX 1 - TEMPLATE FOR STORING SETTINGS 33
THANK YOU !
For choosing the Manley TNT Microphone Preamplifier. The name of this mic preamplifier “TNT” is a
reference to the two different channels, one that is
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 significant 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 ‘difficult’ 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 fine 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.
INTRODUCTION
Tube and the other No Tube. We see people today hungry
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 flow of
air can move through the ventilation holes. Airflow 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 fields such as directly over
or under power amplifiers 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 field. Magnetic fields 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, or (928) 425-9333 or by email at <service@manleylabs.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 LABORA TORIES, INC
13880 MAGNOLIA AVE., CHINO, CA 91710
PHONE (909) 627-4256 F AX(909) 628-2482
SERIAL NUMBER
FUSE 1A @ 1 17V
FUSE .5A @ 220V
www.manleylabs.com
+4UNBALANCED
CAUTION -RISK OF ELECTRIC SHOCK. DO
NOT OPEN. REFER SERVICING TO
CAUTION: RISK OF ELECTRIC SHOCK. DO
QUALIFIEDPERSONNEL ONLY
TRANSFORMER & XLR
DESIGNEDBY HUTCH
TNT
DOES NOT BYP ASS
TRANSFORMER
ISOLATED
CHANNEL 2
OUTPUT
TRANSFORMERLESS
OUTPUT
OUTPUT
MIC
MIC
N9512422
1
TRANSFORMERLESS
OUTPUT
+4 UNBALANCED
OUTPUT
DESIGNED BY HUTCH
OUTPUT
TRANSFORMER & XLR
BYPASS
DOES NOT BYP ASS
TRANSFORMER & XLR
OUTPUT MODE
+4 UNBALANCED
+4 BAL
-10 UNBALANCED
MANLEY LABORA TORIES INC
.
TNT
www.manleylabs.com
N9512423
OUTPUT MODE
+4 UNBALANCED
-10 UNBALANCED
10
+4 BAL
+4 UNBALANCED
OUTPUT
TRANSFORMER & XLR
BYPASS
67
DESIGNED BY HUTCH
5
4
SERIAL NUMBER
MANLEY LABORA TORIES INC
13880 MAGNOLIA AVE., CHINO, CA 91710
PHONE (909) 627-4256 F AX(909) 628-2482
23
.
1
FUSE 1A @ 1 17V
FUSE .5A @ 220V
CAUTION: RISK OF ELECTRIC SHOCK. DO
NOT OPEN. REFER SERVICING TO
QUALIFIEDPERSONNEL ONLY
8
TRANSFORMERLESS
OUTPUT
DOES NOT BYP ASS
TRANSFORMER & XLR
9
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 fine.
3) FUSE. Unplug the power cable first. 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 fires, shocks and big repair bills. Only replace it if it has
"blown" and only with the same value and type (2A slow-blow for 120V, 1A 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 difficult 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 = 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.
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 floating 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 towork as designed, if the desination 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 difficult 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 find 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
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TUBE CHANNEL
1) PHANTOM POWER: Toggle up turns 48 volt phantom power on (and the amber LED) which is needed for most FET condensor
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. Red LED is ON when PHASE switch is engaged.
3) HIGH PASS FILTER: Toggle up engages a basic 80 Hz filter, 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. The numbers to the right of the switch indicate the
impedance given on the 1/4" Instrument Input, where 1 Meg simulateds 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 first amplification 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 first 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 difficult to overdrive this stage.
7) GAIN TRIM POT: This pot is typically used to finely adjust the gain as needed for the recording device or converter. The two 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.
COOL CHANNEL
8) PHANTOM POWER: Toggle up turns 48 volt phantom power on (and the amber LED) which is needed for most FET condensor
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 filter, Toggle up engages a less drastic 60 Hz filter.
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 final 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 first amplification 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 first stage. Use this switch in conjunction with the 60's-70's switch and the LEDs to control the amount of color or
distortion. In this situation the GAIN SWITCH becomes a "drive" control too.
14) GAIN TRIM POT: This pot is typically used to finely adjust the gain as needed for the recording device or converter. The two 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 exagerated 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.
17) LED LEVEL INDICATORS: Simple indicators to show signal presence, first stage clipping, and more or less appropriate levels
for the next device. See page 000 for more deatails.
18) POWER SWITCH: With this switch UP, LEDs should come on and maybe sounds might come out the back XLRs......
5
6
7
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 "vintage-style"
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 had 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 fine", 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 choice 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 "flavors" 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 reflect 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 mic and go preamp.
8
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 circuits 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 k-ohm 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 consistancy 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 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 "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 every products 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.
9
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 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 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
sibilence 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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At very low impedance settings with dynamic and
ribbon mics another effect can come into play. Damping
is a term normally associated with speakers that refers
to the fact that a dynamic speaker comes to rest faster
and is better controlled when connected to a very low
impedance amp. (It was one of the biggest selling-point
features when early solid-state amps with tons of negative
feedback first came out.) Damping often has a dramatic
effect on a speaker’s frequency response and is one
reason why some speakers work better with solid state
and some better with tube amps (they were designed to,
or what they were designed with). A similar effect often
happens with dynamic mics and the bottom tightens up
at low impedances. Whether this is desirable tends to
depend on whether you are aiming for a tight and
probably ‘truer-to-the-source’ sound or the sound of the
mic that you and the world is more familiar with and
might be viewed as more authentic or traditional. You
might even use the tight 300 ohm settings for tubby
instruments and the 2K setting for drier sources and
mics.
To complicate matters further, it depends on the mic.
Modern FET condenser direct coupled (transformerless)
mics are mostly immune to whatever setting impedance
you select, (though you might isolate some of those cable
and preamp circuit effects described above) so expect
generally very subtle or negligible differences. At worst,
with loud sources and lowest impedances you might
introduce early clipping with the occasional FET
condenser mic. Tube condenser mics are fairly immune
but the impedance may affect the frequency response of
the transformer. And with ribbon mics, one concern
might be getting enough highs to start with, so you might
want to especially watch out for losing highs while you
focus on the tighter bottom with low impedances. But
the good news is that ribbon mics tend to be famously
forgiving when boosting the highs with a good EQ so it
may be easy to “get the cake and eat it” this time.
And for those who just don’t want to be bogged down by
any technical issues and complications: you are in luck
again – just turn the knob and pick the setting that
sounds best for this track. In fact, this is the best advice
for those that love all the technical explanations too, and
when you get down to the session, the most important
thing is to listen and choose based on the heart and the
tapping foot rather than the intellect and some words in
a manual or web-site. Remember the music, remember
to listen. The old adage remains valid “If it sounds good,
it is good”.
We touched on using the impedance switch by ear and
how the low impedance settings may be tighter in the
lows, the medium impedance settings might be the ticket
for the advantages of blended preamp those settings
represent typical mic pre impedances and how the
highest impedance may be useful for squeezing the last
drop of highs (but not necessarily the most accurate
highs).
One way (not the only way) to approach the
IMPEDANCE switch is to begin at the middle or “600”
setting. Listen to what you get. If the sound strikes you
as OK but already a bit bright, then try the lower
impedance settings. If your first impression is that this
instrument/mic sounds good but a little dark, then try the
higher settings. Quite likely, your first impression is that
it won’t sound exactly OK or good enough, so the best
advice, is that you should be really be out in the studio
adjusting the mic position and you are not at a point
where the subtle effects of adjusting preamp impedance
will help enough. Maybe you started off on the wrong
foot, or wrong mic in this case. You might try approaching
mics the same way as the IMPEDANCE switch. If the
instrument is hard sounding, try a softer sounding mic,
and vice versa. When you have a bright stinging
instrument, maybe you don’t want to use the brightest
mic in the brightest position and coupled to a bright
preamp in its brightest setting, followed with EQ in
maximum “air” settings. The real trick to getting “air”
is letting that track and the others to “breathe”, give it
some room to move, rather than add some electronic
artifacts. It doesn’t take Einstein to suggest sounds or
tones in a song are “relative” (and so is volume).
A proven approach, is first to listen to the instrument in
the studio, walking around, getting a handle on where it
sounds best and how the tone changes around the
instrument. We do that because instrumental projection
isn’t necessarily obvious and because it gives you a
starting point and the information needed to tweak mic
positions. Then one might choose a microphone or 3 and
maybe based on complementary characteristics. We
might also suggest experimenting with mic positions by
ear rather than by the eyes, or ego. One might say the
first task of an engineer is re-creating the sound that the
musician is hearing and intended. The second task might
be understanding the musician intended it to sound
better than what he got in the room and that maybe
something larger than life (as opposed to squashed and
smaller) might be what the sound becomes in the mix.
Some of you laugh and say "Not my clients, not my
mixes!" One can hope.
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