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

Audio Research SP-6 (1978)

 Audio Research SP-6 High Definition music  stereo preamplifier features:
Close tolerance segmented gain control; 2 dB steps (guaranteed, 1½ dB tracking, ½ dB typical)
Front panel "mute" switch for repeat settings, interruptions, etc.
Rear panel gain "range" switch (+10 dB) for high efficiency speakers, high output cartridges, etc.
Special "off-chassis" construction, with all inputs/outputs isolated form chassis, plus circuit filters to eliminate or minimize "RFI", "TVI", and "CB" interference.
Rear panel provision to disconnect "common" from chassis so that rack mount installations with power amplifiers, etc., may be accomplished without ground-loop induced hum
Separate on-off switch and indicator for amplifier (etc.) power receptacles (3), relay controlled, 1600 watt capacity.
Front and rear panels are of two-color anodized aluminum construction for permanent finish
Industrial grade components and construction for long service life. Rated for continuous commercial service.
 With the introduction of power amplifiers such as the D-52, D-110 and D-350, and the availability of a number of brands of high performance speaker systems, the need for a new and higher level of preamplifier performance for the best systems has occurred.
After a significant amount of research and study , we have determined that from a cost-effectiveness standpoint, once again, the vacuum tube must be used.
We then felt that if such a project were to be undertaken, it would be desirable to execute it to the highest standards reasonable, recognizing that in all probability it would be the last significant vacuum tube product offered to the music world.
One of the expensive (and little recognized) requirements for such a concept would be a power supply capable of providing isolated and regulated voltages to each circuit section. This has been accomplished in the SP-6 by utilizing an all solid state triple section regulator with massive "filtering" at the output of each section.
With such a power supply available, it now becomes possible to increase the bandwidth of an AC coupled vacuum tube design without compromising the all important circuit stability. We have done this, drawing upon the Sp-3 basic circuit design. Increased interstage time constans, larger output coupling capacitors, and "DC concept" feedback circuit innovations, all combine with the new power supply to provide a new standard of sound reproduction.
Only high quality turntables with stable pickup arms and cartridges should be used with this preamplifier.
We suggest that this preamplifier will provide the state-of-the-art. Reference well into the '80s. We further suggest that inclusion of this unit in any otherwise good music reproducing system will effect more improvement than any other single component change.
Frequency Response: 
High level Section : ± .25 dB, 10 Hz to 30 kHz, -3 dB points approx. 1hz and 250 kHz into hi-Z load. (-3 dB point approx. 1 Hz into 100 k Ω load)
Magnetic Phono : ±1 dB of RIAA from 30 Hz to 15 kHz
Harmonic Distortion:  less than .03% at 5 volts RMS output, IHF load, 20 Hz to 20 kHz (typically about .003% in midband)
Intermodulation Distortion:  less than .008% at 5 volts RMS output, IHF load, SMPTE method
Magnetic Phono Input to Tape Output : 34 dB
Magnetic Phono Input to Main Output : 60 dB
High Level Input to Tape Output : 0 dB
High Level Inputs to Main Output : 26 dB
Input Impedance:  50 k Ω , all inputs (Magnetic phono may have any value from 10 Ω to 100k Ω subsituted. Also has provision to add input capacitance for matching certain magnetic cartridges)
Output Impedance:  500 Ω at 1 kHz, rising to 2,1k Ω at 20 Hz , all outputs (recommended minimum load for maximum audio quality, 20 k Ω)
Maximum Inputs:
Magnetic Phono : 700 mV at 1 kHz (.2 volts RMS, 100 kHz)
High Level Inputs : esentially overload proof.
Rated Outputs:  5 volts RMS 20 Hz to 30 kHz, all outputs, IHF load.  (Main output capability is 60 volts RMS output at < 1% THD at 1 kHz into 100 k Ω load with 3 volts RMS high level input)
High Level Noise:
250 uV RMS max. Residual unweighted wide-band noise at main output with gain control minimum (86 dB below 5 volts RMS output)
More than 90 dB below 1 volt RMS input (less than 20 uV equivalent input noise)
Magnetic Phono Noise:  5 uV equivalent input noise, wide-band RMS (-66 dB reference 10 mV input)
Tube Complement:  6 premium grade ECC 83/12AX7
Power Requirements:  105 -120 / 210 - 240 V AC, 50/60 Hz,
Power Consumption:  35 watts
Dimensions (W x H x D):  480 x 134 x 260 mm (19" x 5¼" x 10¼")
Handles extend : 41 mm (1-5/8") forward of front panel
Rear Chassis fittings extend : 23 mm (7/8")

Weight:  10 kg (22 lbs) net

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