NBS UNIVERSAL II PREAMPLIFIER
DESCRIPTION OF DESIGN
DESCRIPTION OF THE UNIT
The Universal II Preamplifier is a
comprehensive control unit which incorporates the functions which often require
three separate pieces of equipment:
A phono preamplifier
A line-stage preamplifier
An equalizer, or as we prefer to call it a serious tone control
The phono section has separate
inputs (and input circuits) for moving-coil pickups and
moving-magnet/moving-iron pickups. Front-panel adjustment of the moving-coil
loading is provided.
The line-stage section has 2 sets
of single-ended inputs on RCA jacks, as well as 1 set of balanced inputs on XL
female jacks. The outputs are available single-ended on RCA jacks as well as
balanced on XL male jacks. The volume control has an LED indication of status
and can also be remote controlled.
The tone control section is
modelled after the NBS Universal Tone Control. There is one low-frequency band
centered at 100 Hz and three high frequency bands centered at 2.5 kHz, 5 kHz
and 10 kHz. Unlike a conventional tone control, this gives enough flexibility
to be truly useful in adjusting for poor recordings or those not to one's
The Universal II Preamplifier will
operate correctly from any mains voltage without adjustment.
No signal path passes through any
mechanical switch contacts. All the mechanical switches are simply input
devices to a microprocesser control system which operates the internal
electronic switches for signal routing and control. The signal path itself is
entirely analog. No age-related or use-related deterioration will occur.
FRONT PANEL CONTROLS
The volume control operates in 0.5
dB steps over a range of 80 dB. The vertical row of LEDs gives a coarse
indication of the setting of the control. Each LED represents 14-16 steps or
about an 8dB interval. When the volume is adjusted in the range between LEDs,
the next LED up wil blink briefly to give a visual indication of the
adjustment. Small adjustments can be made 0.5dB at a time. A quick turn of the
knob will give much faster response. The volume can also be adjusted via the
supplied remote control.
1: Input 1 is single-ended line level
2: Input 2 is single-ended line level
3: Input 3 is balanced line level
MC: Moving-coil phono input -stereo
Mono Moving-coil phono input -
L&R summed to mono
MM: Moving-magnet/moving-iron phono input - stereo
phono input - L&R summed to mono
50 50 Ohm load applied to moving-coil inputs
100 100 Ohm load applied to moving-coil inputs
250 250 Ohm load applied to moving-coil inputs
500 500 Ohm load applied to moving-coil inputs
1000 1000 Ohm load applied to
The audible effect of loading on a
MC pickup can only be determined while playing a record. That is why it is a
front-panel control. There is no "correct" setting. You need to
determine which setting you prefer. It may not be the same for all recordings.
TONE CONTROL 100 Hz
A band of frequencies centered at
100 Hz can be increased or decreased in loudness as follows:
+1 1 dB boost (increase)
+2 2 dB boost
+4 4 dB boost
+7 7 dB boost
+10 10 dB boost
-1 1 dB cut (decrease)
-2 2 dB cut
-4 4 dB cut
-7 7 dB cut
-10 10 dB cut
TONE CONTROL 2.5KHz
The operation is as described above
for 100 Hz, but the center frequency is 2500 Hz
TONE CONTROL 5KHz
The operation is as described above
for 100 Hz, but the center frequency is 5000 Hz
TONE CONTROL 10KHz
The operation is as described above
for 100 Hz, but the center frequency is 10000 Hz
The musical effect of the 4 bands
of the tone control section can be described as follows:
solidity, mid-bass 100 Hz control
upper midrange 2500 Hz control
sibilance, low treble 5 kHz control
high treble 10 kHz control
Small adjustments (+/- 1 or 2 dB)
work better than big ones. You would likely only need
10dB in an extreme case.
BACK PANEL CONNECTIONS AND CONTROLS
This is the large black switch. It
is the only control on the back of the unit.
It is also the circuit breaker
which protects the unit against internal electrical failure.
When the handle is toward the side
of the unit, this is the "ON" position.
Leave the power switch off until
all connections are made.
Use ONLY the supplied AC mains
cable. It is an extremely high performance cable which is part of the overall
performance of the preamp.
Connect the unit to any properly
installed domestic single-phase supply in the nominal range of 100V to 240V, 50
Hz or 60 Hz. No further adjustments are necessary.
Do not defeat the safety ground. If
you have hum in the system which requires lifting the safety ground, identify
the source of the problem and correct it. As a general practice, never defeat
the safety ground.
There is one pair of single-ended
(unbalanced) outputs on RCA connectors.
There is one set of balanced
outputs on XL male connectors.
The outputs may be used
simultaneously. The balanced and unbalanced outputs have no interaction with
INPUT JACKS 1 & 2:
These are conventional single-ended
line-level input jacks which can be fed from any common line-level source.
These stereo pairs correspond to "1" and "2" on the front
panel input selector
INPUT JACKS 3:
These are balanced line-level
inputs which can be fed from any balanced or unbalanced line-level source. The
full benefit will only be realized with a balanced source. This input pair
corresponds to "3" on the front panel input selector.
INPUT JACKS MM PHONO:
These input jacks are used for
moving-magnet phono pickups. The gain in the MM preamp is 40dB.
INPUT JACKS MC PHONO:
These input jacks are used for
moving-coil phono pickups. The gain in the MC preamp is 60dB.
Single-ended line input buffers:
These are gain of 1 (0dB) amplifiers whose performance is completely
independent of the impedance of the source. The single-ended inputs are AC
coupled with a corner frequency of 3.6 Hz.
Balanced line input buffers: These
are gain of 1 (0dB) amplifiers whose performance is independent of source
impedance. Further, the common-mode input impedance is extremely high which
prevents shield-current induced noise. The balanced inputs are flat to DC.
Moving-coil (MC) phono inputs:
These are differential-input instrumentation amplifiers. The inputs are
presented on the back panel as RCA connectors, but the connectors are floating
and the signal on the phono leads is received differentially. Because the
cartridge only sees the input offset current of the amplifier (I.e. the
difference between the two bias currents) the use of input coupling capacitors
can be avoided. This eliminates a high-pass pole in the response, which is
always desireable. A small restriction which is imposed by this arrangement is
that the cartridge signals must be free of ground, which is usually the case.
Because the high gain of the input stage extends to DC, there is a
servomechanism in the second stage to eliminate any offset. The resulting
high-pass frequency is 0.5 Hz.
The resistive load placed on the MC
pickup can strongly affect the sound with some cartridges. There is no
“correct” loading and it can only be determined by listening. For this reason
the MC loading is a front-panel control.
Moving-magnet (MM) phono inputs:
These can be described the same as the MC inputs, but with different
optimization for lower gain and higher input impedance. The input loading for
the MM inputs is fixed at 47K Ohms and 100 pF, the industry standard. The DC
servo high-pass frequency in the MM preamp is 1.9 Hz.
Phono equalization: This is
accomplished by a proprietary dual integrator pole-zero intercept topology. The
curve provided is true RIAA. It does not incorporate the later IEC 20 Hz
high-pass, nor does it incorporate the so-called “Neumann” stop in the high
frequency rolloff. Both of these modifications degrade the sound quality and in
any event are unnecessary with a high-quality turntable setup.
Line-level control functions: All
input and mode selection is performed electronically as is the volume control.
The signal range at all points in the circuit is greater than 7V rms which
represents a headroom of 27 dB above the nominal signal level of 316 mV rms.
Tone control: The tone control
section is derived from the NBS Universal Tone Control which we introduced in
2011. That unit has 6 frequency bands. The present unit has 4 bands, chosen on
the basis of user experience. The general description above specifies the
center frequencies and available amplitude adjustments. The purpose of the tone
control section is to be easily and quickly adjustable so that you can spend
your time listening to the music. While you can certainly use the tone control
to compensate your system or your room, the real intention is to use it to fix
recordings that were either not very well made or do not correspond to your
preferences. The circuit is a so-called parametric equalizer (parameters are F,
Q and dB boost/cut). We have predetermined the center frequencies (F) and the
Q. The boost/cut is what you control from the front panel. The
frequency-dependent circuits are LCR networks which absorb input or feedback
energy depending on whether cut or boost (respectively) is called for. As with
the preamplifier section, the front-panel switches are not handling any audio.
The switches provide control inputs to a microprocessor. The microprocessor
then commands the signal processing circuits.
Output line amplifiers: The
balanced and single-ended outputs are fed by completely separate amplifiers.
All the output may be used simultaneously and there will be no interaction. The
balanced outputs may forced into unbalance by grounding one side and they will
still operate correctly. All outputs operate correctly into any load greater
than 500 Ohms. The output impedance is uniform to DC.
General circuitry: There are no
electrolytic capacitors anywhere in the signal path. There are no mechanical
switches or potentiometers anywhere in the signal path. All operating voltages
are regulated at least twice. The signal handling circuit boards are 4-layers,
one of which is an uninterrupted ground-plane. The circuit is completely
analog. Even though there is digital control, the audio signal itself is never
Single-ended inputs: 2 stereo pairs, Zin = 20KΩ
Balanced inputs: 1 stereo pair, Zin = 48KΩ
MC Phono inputs: 1 stereo pair, balanced on floating RCA jacks
Zin = 50,
100, 250, 500, 1KΩ via front-panel switch
MM Phono inputs: 1 stereo pair, balanced on floating RCA jacks
Single-ended outputs: 1 stereo pair, Zgen = 220 Ω, Load Z = >500 Ω
Balanced outputs: 1 stereo pair, Zgen = 50 Ω, Load Z > 500 Ω
Max. Gain (Vol CW): Line input to line output = 10 dB unloaded
input to line output = 48 dB
input to line output = 71 dB
Frequency response (Tone flat): Single-ended in: +0/-1dB 1Hz - >100 kHz
in: +0/-1dB DC - >100 kHz
RIAA +/- 0.1dB 20Hz – 20 kHz
Signal/noise ratio re 1V rms, Line
inputs > 100dB, 30 kHz BW, A wtd
> 78dB, 30 kHz BW, A wtd
input > 75dB, 30 kHz BW, A wtd
Total Harmonic Distortion: Line in
to line out, 4Vrms <.005%
to line out, 4V rms <.02%
Mains requirements: Properly installed, grounded service
100VAC - 240VAC 48-62 Hz