What is stereo?

There are now two system of high fidelity, monophonic (monaural) and stereophonic. Monophonic is a system that starts from one microphone and is fed through a single high fidelity set. Stereophonic is a double system. Two separate microphones are placed at different sides of the orchestra and two different systems are used to keep the two signals or channels separated. Two separate speakers are used, placed on different sides of you room. Stereo is much like 3-D photography, two slightly different sound reach your ears giving you a new dimension in sound.

H.H. Scott '59

Spectral Audio DMC-20

The spectral DMC-20 is a true dual monaural reference preamplifier which incorporates a long list of technical innovations and enhancements to achieve new heights of signal linearity and speed of signal response. The result is stunning musical coherence and transparency. A tour through the DMC-20's outstanding features will serve to highlight its remarkable capabilities.

Volume. The DMC-20's volume control is a custom made, conductive plastic, precision stepped attenuator built to Spectral's design. Our extensive comparative testing confirms that this device, with its satisfying torque feel, offers by far the most sonically transparent operation, precise tracking (channels within .10 decibel) and extended life of any volume attenuator made today.

Balance. Spectral takes the most uncompromising approach to balance control, utilizing a precision stepped attenuator with 14 settings employing metal film resistors. The DMC-20's balance control features a center position bypass - in this position the control is out of the circuit altogether.

Input selector. The input selector is not in the signal path, but controls the relay logic system which switches between source inputs at optimal positions in the circuit. This sealed, mil-spec rotary switch features highest-grade heavy plated gold over coin silver contacts for extended durability and reliability.

Internal Controls:

Input Trim 20 dB range for tuner, disc, aux 1, aux 2, tape

Input Trim Bypass Switches

Phono Input Impedance selector (opt.)

Protection System bypass switch

Balanced Input Level 20 dB range

Tape output level

Balanced High Level operation switch

Rear Panel Connectors:

Signal Ground post (gold)

5 RCA Input connector sets (gold)

Balanced Input Neutrik XLR connectors for aux 3 (gold)

Tape input and output RCA connector set (gold)

Balanced output RCA connectors for non-inverting, inverting

Balanced output Neutrik XLR connectors (gold)

DC Power Hi-Rel input connector

One look at the DMS-20 confirms the boldness and meticulous attention to detail with which Spectral engineered this ground-breaking preamplifier power supply. Using cascaded power supplies, individually regulated for each rail of each channel, the DMS-20 delivers flawlessly clean and well-regulated DC power to the DMC-20, completely isolated from ground currents and decoupled from high frequency interference. With a total output of 80 volts, rare for a solid state device, the DMS-20 has the reserves of a 100-watt-per-channel power amplifier.
It is Spectral's growing conviction that analog circuits, although conceptualized in two dimensions, should be realized so as to take full advantage of all three physical dimensions for ultimate performance. In the DMC-20 the power and ground conductors, and housekeeping functions such as ferrying control signals from the front panel to the relay system, are carried on a slide-out "mother board." The amplifier modules for line stage amplification and optional phono stage and balanced input conversion, are located on their own smaller PC boards which float above the mother board. Power and ground connections are made by "popping up" the relevant conductors at the precise points where they are needed in each circuit. And fully 90% of the signal path is carried on specially designed, phase-aligned, individually terminated MIT/Spectral cabling which acts as the preamplifier's signal bus. With the DMC-20, Spectral for the first time carries the audiophile cable concept which it pioneered with MIT into the active heart of the amplifier circuit.
This three-tiered architecture offers the most innovative and uncompromising response to the classic preamplifier challenge to reduce the contamination of sensitive, low level music signals by extraneous noise to an absolute minimum. Power, ground and signal conductors no longer snake around one another on two dimensional printed circuit boards with their inherently imperfect dielectrics.
Spectral's polysulfone (Teflon derived) printed circuit boards, custom made for us by Tektronix, have no rival in the audio field. But the low level music signal's extreme sensitivity to radiant energy fields makes it imperative, we believe, to remove it from the crowded domain of the PC board at every opportunity.
The signal bus is routed in bundles to prevent interference from radiated fields, and interconnections are made at a 90 degree angle to eliminate crosstalk between channels. Intricate and costly, this approach is a challenge to engineers and craftspeople alike. But we think the resulting sonic purity and impeccable transparency make any other approach unthinkable for a state-of-the-art instrument like the DMC-20.

Specifications DMC-20

Input Compliment: 7 Inputs - tuner, disc, aux 1, aux 2, aux 3, (balanced, phono) optional

Output Compliment: 4 ouputs - normal, inverted, balanced, tape

Control Compliment

Output Attenuation: 32 step conductive plastic, modified log taper

Output Balance: 15 step attenuator switch with center position bypass

Mute Switch: 0 - 20 dB attenuation

High Level Trim: 0 to 10 dB attenuation for each channel all high level inputs, with attenuation bypass switches.

Mode Switches: Stereo/mono, absolute phase

Input Selections: 6 plus tape monitor

Recording Amplifiers

Protection Circuit

Protection Mode: DC Offset, oscillation

Method: Crowbar output relays

Sensitivity: 20 mV continuous DC

Recovery: Approx. 10 seconds

Chassis Temperature: Internal 48° C @ 25° room temp.

Model 201 Balanced Output Module

Fully Balanced Operation: All D.C. coupled

Frequency Response (any level to 36 V RMS):

DC to 4 MHz, -3 dB

DC to 2.5 MHz, -0.10 dB

Slew Rate: 1000 V/us or greater

Rise Time: 70 nanoseconds

Distortion: Less than 0.01% THD and IM

Crosstalk: Greater than 90 dB

Noise: 105 dB A Weighted, ref 100 mV @ 1 KHz

Output Voltage: 1.6 VRMS, 100V P-P max.

Output Current: 1 Amp max. per channel, balanced out

Gain: 40 dB unbalanced, 34 dB balanced

DMS-20 Power Supply Technical Data

Output Voltages:

1) Dual ± 40 VDC @ 0.6A

2) Dual ± 40 VDC @ 0.6A

3) Single ± 5 VDC @ 1A

4) Single ± 10 VDC @ 1A

Noise and Ripple: Less than 200 mV peak to peak

Reserve Capacity: 180 Joules per supply (±40 VAC)

Input Mains Voltage: 100, 120, 220 V AC ±10% @ 47.5 Hz to 440 Hz (factory set)

Power Consumption: 80 watts normal, 175 watts max.

Protection: Short circuit, over voltage, line transient protected

DMC-20 Preamplifier Mechanical

Weight: 5,25 kg (12 lbs)

Dimensions (W x H x D): 483 x 635 x 317 mm (19" x 2½" x 12½")

DMS-20 Power Supply Mechanical

Weight: 49 kg (22 lbs)

Dimensions (W x H x D): 483 x 635 x 318 mm (19" x 2½" x 12½")

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