|NBS UNIVERSAL II PRE AMPLIFIER|
THE NBS UNIVERSAL II PRE AMPLIFIER
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:
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 taste.
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 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:
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:
Small adjustments (+/- 1 or 2 dB) work better than big ones. You would likely only need
BACK PANEL CONNECTIONS AND CONTROLS
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.
The outputs may be used simultaneously. The balanced and unbalanced outputs have no interaction with each other.
INPUT JACKS 1 & 2:
INPUT JACKS 3:
INPUT JACKS MM PHONO:
INPUT JACKS MC PHONO:
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 digitized.
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