MDP-1 Vacuum Tube Mic/DI Preamp- Operating
· All tube, short signal path design
· Pure class A, high voltage circuitry
· Transformerless tube output stage capable of delivering
· Low noise, wide bandwidth and superior transient response
· Choice of mic input transformer for 'full' or 'focused'
· 30dB gain range in 3dB steps
· Fully transformerless DI with 100k, 1M and 10M input
· Ten position mic lo-cut filter (20-180Hz)
· Large illuminated VU meters fully buffered from the
signal path for low distortion.
· Soft-start warm-up with output muting
· Gold-plated switch contacts, I/O connectors and tube
· Fully regulated high voltage (300V) and filament dc
· Custom toroidal power transformer with shield for minimum
· Polypropylene capacitors and metal film resistors (pairs
matched to 0.1%)
· Single-sided pc boards to minimize capacitive interaction
Thank you for purchasing the Pendulum MDP-1, a modern two
channel vacuum tube Mic/DI preamp designed to be the ultimate
way to get your mic or source directly to tape or hard disk.
Unlike vintage or hybrid designs, the MDP-1 uses a pure tube,
class A high voltage circuit topology with a transformerless
output stage to deliver an open, intimate sound with a level
of detail that meets the requirements of the most demanding recording
applications. Our short signal path design excels in audio performance,
delivering extremely wide bandwidth, low noise, and high headroom.
The MDP-1 has all the things you'd expect in a full-featured
mic preamp, including phantom power, phase reverse, input pad,
a mic lo cut filter with 10 frequencies and large, illuminated
VU meters. In addition, the full-featured DI input on each channel
is has an entirely transformerless signal path, and accepts line
and instrument level signals at a variety of input load impedances.
Since a key component of the sound of any tube mic preamp
is the input transformer, a choice of two low turns-ratio transformers
is available. The MDP-1A is equipped with two Jensen 13K7 transformers
for a 'full', open sound with plenty of sparkle in the high end
and a deep low end - perfect for recording all sources, especially
instrumental music. The MDP-1B is equipped with two custom-wound
transformers for a more 'focused' sound for extra presence and
clarity in the midrange - ideal for a vocal sound that easily
distinguishes itself in a mix. The MDP-1C is configured with
a Jensen 13K7 in channel 1, and a custom transformer in channel
The unit was carefully packed at the factory to protect against
damage in transit. Nevertheless, be sure to inspect the unit
and shipping carton for any signs of damage that may have occurred
during shipment. If there is any damage, notify us immediately
for further instructions. It's also a good idea to save the carton
and packing materials should you ever need to return the unit
for repair. The shipping carton should contain the following
items: the MDP-1 Mic/DI Preamp, an IEC 3 prong power cord, and
this operating manual.
The MDP-1 uses two EIA-standard rack spaces, and can be
mounted in any standard 19 inch (483mm) equipment rack. If the
MDP-1 is mounted in a mobile rack or road case, it is important
that the rear of the chassis is supported to prevent possible
damage from mechanical shock and vibration. Excessive shock and
vibration can cause damage or premature failure of the vacuum
tubes, or cause them to shaken loose from their sockets. Please
avoid rough handling.
For proper operation, it is very important that adequate
ventilation is provided. Vacuum tubes produce a significant amount
of heat that must be removed from inside the chassis. The side
panel vent holes and top panel slit vents should never be blocked
in any way. Never mount the MDP-1 below a rack unit with a depth
greater than 7 inches (178mm). Do not mount the MDP-1 near other
heat-producing equipment such as power amplifiers or other vacuum
tube products. If possible, leave open at least one rack space
above the unit, and use a rack spacer with a ventilation grille.
Never operate the MDP-1 inside a road case where the side panels
are cushioned in foam.
Preventing Ground Loop Hum
One of the reasons the MDP-1 sounds so good is that unlike
many other vacuum tube products, it operates single-ended, Class
A without an output transformer. However, it does not benefit
from the galvanic isolation provided by an output transformer.
For this reason, a few precautions are necessary to insure hum-free
· Isolate the front panel from the rack rails. Use plastic
shoulder washers to prevent electrical contact between the rack
ears of the MDP-1 and the metal rails of the equipment rack.
· Isolate the MDP-1 from units mounted above or below
it in the rack. Make sure the front panels are not in electrical
contact and that the top or bottom cover screws of the MDP-1
are not touching those of any other units.
· Connect the 3 prong IEC power cord to the single-point
star grounded electrical source for your
The idea here is to make sure the MDP-1 seeks ground at only
one point. For safety reasons, do not lift the ground at the
IEC power cord. Keep in mind that in a properly grounded hookup,
the MDP-1 does not hum. Please, take the time to do this right,
and you will be rewarded with hum-free operation. Call us if
you have any questions. Please note that pin 2 is hot.
The MDP-1 is equipped with a 3-prong IEC power connector and
detachable cord. Never operate the MDP-1 with the ground on the
power cord defeated. Unless otherwise stated, this unit operates
from 120V/60 Hz at 30W. The ac fuse is accessible from the rear
panel and is rated at 1A/250V (3AG) SLO-BLO. To check or replace
the fuse, make sure the unit is unplugged. Operation at 240V/50Hz
is available as an option. Please contact us for more information.
Other than changing the tubes, the user should not attempt to
service the MDP-1 beyond that described in this manual. Never
remove the covers or attempt to replace the tubes until the unit
has been disconnected from the ac power source, and all circuits
inside have been allowed to discharge for a period of at least
30 minutes. The vacuum tubes become very hot once the unit has
been turned on, and they should not be touched until they have
cooled to room temperature. To reduce the risk of fire or electrical
shock, do not expose to rain or moisture, or operate it where
it is exposed to water. Since potentially lethal voltages are
present inside the unit, it should only be opened by qualified
service personnel. Refer all servicing, or any questions about
servicing, to Pendulum Audio, Inc.
While the operation of the MDP-1 may appear to be rather straightforward,
there are a few features which may differ from what you're accustomed
to seeing on other stand-alone mic preamps. You may find it useful
to refer to the Condensed Operating
Instructions to quickly identify the operation of the
front panel controls. However, we suggest you read through this
section to take advantage of all its features, and to make sure
you are operating the MDP-1 in the way most appropriate for the
type of recording you're doing.
Please refer to the rear panel layout (see the Condensed Operating
Instructions) for the location of the inputs, outputs, ac power
inlet and power switch. Make all connections to the MDP-1 before
· On the right hand side of the rear panel are the input
jacks for channels 1 and 2. The female XLR connectors on top
are the transformer-balanced mic inputs. Connect microphones
to these inputs using standard balanced XLR mic cables. Pin 1
= ground. Pin 2 = + (positive phase), pin 3 = - (negative phase).
Note: these inputs may be +48v phantom powered, as selected by
the '+48' switches on the front panel. Do not use phantom power
on a microphone that does not require it! (e.g. dynamic, ribbon,
or tube microphones). Make all mic connections before applying
· The two 1/4" jacks below the XLR connectors
are the unbalanced DI input connectors for channels 1 and 2.
The input impedance of these jacks is 1M when the DI input switch
on the front panel set to Instrument, or 100K with a 20dB pad
when the DI input switch set to Line. These inputs can be used
for connecting an instrument directly to the rear panel, or for
connecting the MDP-1 to an unbalanced patch bay. The two 1/4"
inputs on the front panel can be used for either instrument (10M)
or line level (100k/-20dB) signals.
· To the left of the input jacks are the output jacks
for channels 1 and 2. The male XLR connectors on top are 3 pin
unbalanced outputs, with Pin 1 + Pin 3 = ground, and Pin 2 =
output. Connect these outputs to 3 pin balanced console, converter,
or tape inputs. If you encounter ground loop hum when connecting
to active-balanced inputs, lift the shield from Pin 1 on the
input end of the cable. When connecting to a balanced patch bay,
be sure that Pin 2 = Tip. For connection to transformer-balanced
inputs, do not lift the ground at either end of the cable.
· The two 1/4" jacks below the XLR connectors
are unbalanced output connectors, wired in parallel to the XLR
outputs, with Tip = Pin 2, Ring = Pin 1. Use them for connecting
the MDP-1 to unbalanced inputs.
On the left side of the rear panel is the IEC input socket. Connect
to a 120V/60Hz receptacle with the 3 prong IEC power cable supplied
with the MDP-1. For safety reasons, do not lift the ground on
the power plug by using a 3-to-2 ground lift adapter.
· Turn on the power to the unit using the ac power
switch located above the power inlet socket. Although it might
seem a bit awkward to put the ac power switch on the rear panel
of the MDP-1, there is a very good reason for it. By keeping
the ac away from the front panel, hum induced by the ac line
into the front panel circuitry can be virtually eliminated. This
is especially important with the MDP-1, since the DI instrument
input jacks on the front panel operate at 10M, and are very susceptible
to ac hum.
· If necessary, replace the 1A/250v 3AG SLO-BLO fuse
only with the same type and rating.
To prolong tube life, the MDP-1 goes through a soft-start sequence
for gently applying power to the tubes and stabilizing the circuit
before engaging the outputs. When the power switch is turned
on, the outputs are relay-muted to ground and the dc voltage
on the tube filaments is ramped up to 12.6Vdc. Next, the high
voltage supply is slowly increased to 300V and the circuit is
allowed to stabilize for about 2 minutes. Finally, the relays
lift the outputs from ground and the blue 'on' led on the front
panel is illuminated. For best results, please allow the MDP-1
to warm up for at 10 minutes or longer before using it.
Using the MDP-1 as a Mic Preamp
Please refer to the front panel layout (see the Condensed
Operating Instructions) for the location of all switches and
controls discussed below. The operation of both channels is identical.
All switch contacts that pass audio are gold-plated for high
Input Mode: MIC
With the input switch in the 'MIC' position, the MDP-1 is configured
as a mic preamp, with the XLR input connectors enabled and the
DI inputs on both the front and rear panels disabled. In this
mode the signal path consists of the input transformer followed
by a Class A tube gain stage with a transformerless output. The
nominal source impedance is 150. When the MDP-1 is used with
a transformerless microphone with a very low output impedance
(50 or less), an external impedance-matching network (available
from us) may be necessary to improve transient response.
Phantom Power: +48V/0
In the +48V position, 48 volts is supplied to pins 2 and 3 of
the XLR input. The phantom voltage is applied via two 6.81k metal-film
resistors that are hand-matched to better than 0.1%.
Mic Input Pad: 0/-20
In the -20 position, a 20dB impedance-matched resistive pad is
inserted in front of the input transformer to prevent overload.
To optimize common-mode rejection, the metal-film resistors are
hand-matched to better than 0.1%. Use this position when recording
with mics placed on sources with high sound pressure levels such
as drums, guitar amps, or when close-micing a singer with a high
output mic. Use if distortion is heard, or if the output level
of the preamp is too high with the GAIN selector switch at its
lowest setting (+33dB). For the best noise performance, use only
In the - position, the phase of the mic signal is inverted at
the secondary of the input transformer. Configured this way,
the phase of the mic can be switched without interrupting the
dc voltage to a mic requiring phantom power. Positive phase is
Pin 2 hot.
Mic Lo Cut: Off to 180Hz
The MIC LO CUT is an 11 position rotary switch that provides
a 6dB per octave rolloff with 10 user-selectable turnover frequencies:
20, 30, 40, 50, 60, 80, 100, 120, 150 and 180Hz. It offers a
greater selection of frequencies and gentler low frequency contouring
than the low cut filters on most mics or mixers. In the 'Off'
position, the filter circuit is removed from the signal path.
Note that the Mic Lo Cut filter affects only the mic input, and
not the DI input.
GAIN: +33 to +66dB
The GAIN Control is an 11 position rotary switch that adjusts
the gain of the tube stage from +33dB to +66dB in precise 3dB
increments. The 1% metal-film resistors that determine gain are
selected for precise matching between the two channels. At low
gain settings, there is more global feedback in the tube circuit,
which offers a more 'accurate' and 'controlled' sound. At higher
gain settings, the sound is a little more 'open' and 'harmonically
rich'. Used in conjunction with the OUTPUT control (discussed
below), subtle variations in the character of sound can be achieved.
OUTPUT: 0 to 10
The OUTPUT Control is a passive attenuator positioned between
the final tube stage and the output connectors. It can be used
in conjunction with the GAIN control to adjust the level that
is sent to a tape machine or hard disk recorder. For the cleanest
sound, set the OUTPUT control fully clockwise (completely out
of the signal path) and use the GAIN control to set the overall
signal level. To add more 'harmonic content' by driving the tubes
at a higher signal level, set the GAIN control to a higher gain
setting and use the OUTPUT attenuator to bring the output level
down to a more useable range. Or, use the OUTPUT control to make
fine adjustments (within the 3dB range of the GAIN switch settings)
to the level sent to a tape machine. Use the VU meter GAIN setting
(discussed below) to monitor how hard the tubes are being driven.
Used sparingly, this gentle tube 'overdrive' can create subtle
changes in the harmonic balance of the source. Used to excess,
it will cause audible distortion. Proceed with caution. With
the OUTPUT attenuator set to mid-range, the level of attenuation
is 20dB for load impedances greater than 10k.
Note: Our use of a resistive output attenuator after
a low impedance tube output stage is sure to raise an eyebrow.
First, the output impedance is increased from 600 up to as much
as 5k at the 3:00 position of the output potentiometer. Second,
a higher output impedance may in principle cause problems when
attempting to drive a long run of high-capacitance cable.
If this moderate increase in output impedance troubles you,
keep the output control turned all the way up, use the GAIN control
exclusively and the MDP-1 will deliver a true 600 output impedance
with high output drive capability and good stability. In practice
however, when the preamp is use in a typical control room and
connected by a reasonable length of cable to a tape or A to D
converter input, the OUTPUT control offers a very convenient
way to make minor level adjustments, or to create subtle variations
in the character of sound. Try it, and you may like it.
The illuminated ANSI VU meters are electronically isolated
from the signal path, and can be switched to measure levels at
two points in the signal path. Keep in mind that a VU meter is
a mechanical device, designed in accordance with a with a well-accepted
ballistic standard, to indicate an average loudness level. On
the other hand, the led meters on your digital recorder are reading
a peak program level, and faithfully register all those short
transient spikes that add little to the perceived loudness of
the program material. The ratio of the peak to average levels
can be 20dB or greater depending on the source (e.g. drums).
So, if you're wondering why the led meters on your recorder are
dancing around zero, but the VU meters on the MDP-1 are hovering
at or below -10, you're simply seeing the difference between
the peak and average program level.
When the METER switch is in the OUTPUT position, the VU meter
indicates the signal level at the XLR and 1/4" output connectors
(post - OUTPUT attenuator). The meter is calibrated to 0VU =
+4dBu (1.23vrms). Use this setting to monitor the average program
level sent to a tape machine or console input.
When the METER switch is in the GAIN position, the VU meter indicates
the signal of the tube stage (pre - OUTPUT attenuator). The meter
is calibrated to 0VU = +20dBu (7.75vrms). Use this setting to
monitor how hard the tube stage is being driven, and to gauge
the onset of audible distortion.
The center position of the METER switch turns off the VU meter.
Using the Mic Preamp Without the Input
Here's another tip that may raise an eyebrow. The MDP-1 can
be used with high output tube microphones as a fully transformerless
mic preamp. Simply connect the mic to the rear panel DI input
using an XLR to 1/4" adapter, and switch the input mode
to DI. Increase the GAIN about 20dB to compensate for the lack
of transformer gain and the 6dB loss when running the mic unbalanced.
The noise performance suffers a bit, but when used for recording
vocals with a high output microphone (e.g. Neumann M149), the
results are superb. Keep the mic cable as short as possible.
Using the MDP-1 as a DI Preamp
The MDP-1 features a full-function DI with a signal path that
is entirely transformerless. Please refer to the front panel
layout (see the Condensed Operating Instructions) for the location
of the switches and inputs discussed below. The operation of
both channels is identical. All switch contacts that pass audio
are gold-plated for high reliability.
Input Mode: DI
With the input switch in the 'DI' position, the MDP-1 is
configured as a line-level DI preamp, with the DI inputs on both
the front and rear panels enabled and the XLR mic input connectors
disabled. In this mode the signal path consists of a Class A
tube line stage with a transformerless output. The input impedance
is 100k for line level inputs, or 1M and 10M for instrument inputs.
The front and rear panel jacks are wired so that the rear jack
is disabled when a 1/4" plug is inserted into the front
DI Input: Instrument
With the DI INPUT switch on the front panel in the INSTRUMENT
position, the 1/4" input jacks on the front and rear panels
are configured to accept unbalanced high impedance sources. The
GAIN range is +20 to +50dB in 3dB increments.
· The front panel jack has an input impedance of 10M,
which ideal for very high impedance sources such as piezo transducers.
Use this input when minimum loading of the source is desired.
· The rear panel jack has an input impedance of 1M,
which is ideal for moderate impedance sources such as passive
magnetic guitar or bass pickups. Use this input when light loading
of the source is desired. Loading the magnetic coil of a bass
guitar pickup with 1M can sometimes tighten up the low end, adding
definition. Since this effect depends on the inductance any given
pickup, try it both ways and see which sounds the best.
DI Input: Line
With the DI INPUT switch on the front panel in the LINE position,
the 1/4" input jacks on the front and rear panels are configured
to accept unbalanced high level sources.
· The input impedance is 100k for both front and rear
input jacks, with a -20dB resistive pad inserted between the
input and the tube line stage.
· The range of the GAIN control is 0 to +30dB, which
is ideal for line-level sources such as -10dBV unbalanced consumer
audio equipment or keyboard outputs.
· Use the rear panel jacks to connect the DI inputs
to an unbalanced patch bay.
Gain and Output Controls
See 'Using the MDP-1 as a Mic Preamp' for a discussion of how
to use the GAIN and OUTPUT controls to achieve subtle changes
in the character of the sound of the DI. Note that while the
MDP-1 can in principle be softly 'overdriven' like an guitar
preamp, it doesn't have the same tone-shaping EQ. However, the
MDP-1 can be used to overdrive the front end of a tube guitar
Replacing the Tubes
All vacuum tubes have a limited life due to reduced electron
emission from the oxide coating on the cathode and/or a buildup
of impurity gases is the bulb. The life of the preamp tubes in
the MDP-1 is estimated to be several years. If you notice the
sound quality deteriorating - higher distortion, muddiness, or
microphonic behavior - it's time to change the tubes. We recommend
changing all the tubes at once. If you are uncomfortable with
replacing the tubes yourself, please have it done by qualified
service personnel. Replacement tubes are available directly from
1. Unplug the MDP-1 and wait at least 30 minutes for the high
voltage in the unit to discharge and for the tubes to cool to
2. Remove the top cover by removing the nine #6-32 Phillips-head
DO NOT remove the bottom cover.
3. Note the position of the four tubes (V1-V4) in the porcelain
· The input tubes are ECC83/12AX7A (V1 and V3)
· The output tubes are 6922/6DJ8 (V2 and V4)
4. Remove each tube and replace with the same type removed
from the each socket.
DO NOT install the tubes in the wrong positions!
5. Reinstall the top cover and screws.
There are a large variety of ECC83/12AX7s available. Each
type has slightly different internal structure and design. Consequently,
each type has its own sonic signature. Sometimes the differences
are subtle - sometime not. You are encouraged to sample the different
varieties and pick the one that sounds the best to you.
The 6922 is a rugged, military style 6DJ8. Since it is used
as a high current output driver, we recommend replacing it with
the same type and rating. The 6922 used in this fashion has much
less influence on the sound of the MDP-1 than the 12AX7A input
Circuit Type: Class A vacuum tube signal path
with transformerless output
Input Gain: +33 to +63dB in 3dB steps
Input Impedance: 1500 transformer balanced and floating
Frequency Response: -1.0dB 6Hz and 65kHz with 10K load, +45dB
-1.0dB 20Hz and 65kHz with 600 load, +45dB gain
Noise:EIN less than -125dBu with 150 input load, unweighted
EIN less than -118dBu with 150 input load, unweighted (Custom
Distortion: THD+N less than 0.03% from 100Hz-10kHz,
less than 0.07% at 20Hz, measured with -20dBu input,
+10dBu output, +45dB gain
Input Gain: +20 to +50dB (Instrument), 0 to +20dB
(Line) in 3dB steps
Input Impedance: 10M or 1M (Instrument), 100k with 20dB pad
Frequency Response:-1.0dB 5Hz and 250kHz with 10K load, +20dB
-1.0dB 20Hz and 300kHz with 600 load, +20dB gain
Noise: EIN less than -116dBu with input shorted, unweighted
Distortion: THD+N less than 0.04% from 20Hz-10kHz
measured with +10dBu output, +40dB gain
Output Level: +35dBu into 10k load, Output control
+25dBu into 600 load, Output control at maximum
Meter: selection of output level (0VU = +4dBu)
or tube gain (0VU = +20dBu)
Phantom Power: +48Vdc applied to pins 2 and
Mic Pad: -20dB pad at the primary of the mic input
Mic Phase: inverts the phase at the secondary of the mic
Mic Lo Cut: 6 dB/octave rolloff from 20-180Hz in 10
Polarity: input and output XLR connectors are pin 2 hot
Vacuum Tubes: (2) ECC83/12AX7A, (2) 6922
Power:120Vac, 30W (240Vac optional)
Power Supplies: +300Vdc and +12.6Vdc, fully regulated
with soft-start warm-up and output muting
Dimensions: 2U enclosure, 19" x 3.5" x 12.5"
(48.2 x 8.8 x 31.8 cm)
Weight: 14lbs (6.4kg)
Pendulum Audio, Inc. warrants to the first purchaser of a
new Pendulum MDP-1 Vacuum Tube Mic/DI Preamp that the unit is
free of manufacturing defects in materials and workmanship for
a period of one (1) year from the date of purchase. Pendulum
Audio, Inc.'s sole obligation under this warranty shall be to
provide, without charge, parts and labor necessary to remedy
defects, if any, which appear within one (1) year from the date
of purchase. All warranties expressed or implied made by Pendulum
Audio, Inc., including warranties of merchantability and fitness,
are limited to the period of this warranty. Pendulum Audio, Inc.
is not responsible for indirect, incidental or consequential
damages arising from the use or failure of this product, including
injury to persons or property.
This warranty does not cover damage due to: misuse, abuse,
modification, accident or negligence. The warranty does not apply
if the unit is repaired or altered by persons unauthorized by
Pendulum Audio, Inc. in such a manner as to injure, in Pendulum's
sole judgment, the performance, stability or reliability of the
unit. The warranty does not apply if the unit is connected, installed
or used otherwise than in accordance with the instructions furnished
by Pendulum Audio, Inc. There is no warranty on vacuum tubes.
If the equipment requires warranty repair, return authorization
must be obtained from Pendulum Audio, Inc. prior to shipment.
Equipment should not be shipped to Pendulum Audio, Inc. until
return authorization and the proper shipping address is obtained
from us. The equipment (with all its components parts and connecting
cables) must be suitably packaged, including a note with the
owner's name, address, telephone number and a description of
the reason for return. The owner pays two-way shipping (we recommend
UPS), and we suggest that the shipment be insured for its full
This limited warranty is in lieu of all other warranties,
expressed or implied, and no representative or person is authorized
to represent or assume for us any liability in connection with
the sale of our products than set forth herein. This limited
warranty gives you specific legal rights, and you may also have
other rights which vary from state to state.
Pendulum Audio, Inc.· P.O. Box 339, Gillette, NJ 07933.·
(908) 665-9333 · www.pendulumaudio.com