Threshold SL-10

The SL 10 was the second preamp released from Threshold following the tradition of the now legendary NS 10 preamp that was the first from Nelson Pass.

The SL 10 is a fully cascode Class A preamp of exemplary build and construction. The SL 10 set new standards for the time and remains to this day a leading edge preamplifier by any standard one would care to judge.

The interior of the Threshold SL 10 reveals its premium components. The basic circuit board is military grade, double sided, gold plated, plated-through construction. All connectors and switch contacts are gold plated. Rotating controls are environmentally sealed. The separate power supply employs Mallory computer grade capacitors and carries an immense reserve capability with quadruple decoupling from the audio circuits.

Eight transistors per channel, directly coupled to the input and operated without feedback, the SL 10 circuit forms the active synthesis of a transformer while avoiding the reactance effects that compromise transformer performance. The circuit is a system of such accuracy that it does not require frequency compensation or corrective feedback and therefore maintains extremely high levels of speed and phase integrity.

Designed as a "modular" system, the Threshold SL 10 consists of two interconnected components. In order to eliminate any possibility for the noise pickup that occurs when audio circuits are in proximity to ac power components, the SL 10 utilizes an independent power supply module that may be located at a distance from the audio module. The reserve capability of the supply lies far beyond the demands of the audio module.

Its construction features computer grade Mallory capacitors and there is a total of 20,000 microfarads regulation exhibited within the supply circuitry. Quadruple decoupling, consisting of 14 electrolytics and monolythics in parallel, provides isolation equivalent to a separate power supply for each preamplifier gain stage. Absolutely no interaction between stages or channels is allowed to occur through this elaborate power source system.

The Threshold SL 10 is a discrete design employing components of the highest grade. Dale and Corning metal film resistors are used throughout. All connectors, circuit paths, and switch contacts are gold plated. Audio level is controlled through a Waters, conductive plastic, dual potentiometer which is environmentally sealed, as are all rotating switches.

For maximum noise shielding, the chassis and cover of the SL 10 and its power supply are fabricated of sheet steel. All transistors used in the SL 10 are individually curve traced on Tektronix equipment to determine gain and linearity characteristics, while those transistors actually in the signal path are further selected for an ultra-low noise component.

As a component evolving from the most advanced engineering concepts, the Threshold SL 10 will provide uncompromised performance when coupled with associated equipment of the highest calibre. The audio path is so simple, so distortion free, and so extraordinarily fast that within the broadest definition of "audio range" transient and phase anomalies simply do not exist. No expense has been spared in engineering or construction to assure that the SL 10 fully justifies the Threshold claim to design concepts that constitute the leading edge of audio technology.

The Threshold model SL 10 preamplifier is the vanguard for a new generation of low-level signal processing units whose performance margins lie substantially beyond those required by the emerging, high technology, recording systems. As an exercise in highly advanced engineering, the Threshold SL 10 represents fresh design concepts and empirical performance that establish new accuracy domains in the transfer of signal from source to amplifier. Several unique operating configurations are employed in the comprehensive SL 10 system, allowing it to process at extremely high levels of information integrity.

Input transistors of the phono and high level circuits are operated in the cascode mode, which increases bandwidth through a reduction of "Miller" capacitances and provides additional isolation between the source, the power supply, and the gain circuitry. In addition, the active input devices are biased to current levels an order of magnitude beyond those normally applied to solid-state preamplifiers, particularly those reaching for the lowest possible "noise" figures.

These very large bias currents substantially reduce the distortions in the gain transistors, yielding a "super class A" operating mode where the idling currents are many times larger than the current called for in actual operation. Not only does the high bias assure extreme linearity but also increases the phono input transistor overload to approximately 2 volts, measured at 20,000 Hz, for the high level magnetic cartridge input, and greater than 70 millivolts peak, at any frequency, for the low level, "moving coil," cartridge input.

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Preliminary phono gain stage

FREQUENCY RESPONSE: .3 Hz through 500,000 Hz, +0, -3dB.
SQUARE WAVE RISE TIME: .5 microseconds.
TOTAL HARMONIC DISTORTION: .015% second harmonic @ 50 millivolts out .. 03% second harmonic @ 100 millivolts out, NOISE: (preliminary phono gain stage + RIAA gain stage): -80dB A weighted referenced to 1 volt out. - 72dB unweighted referenced to 1 volt out.
CROSSTALK: (left minus right) > -7OdB @ 1,000 Hz.
INPUT OVERLOAD: 70 millivolts peak for any frequency.
INPUT IMPEDANCE: selectable 1 through 15 ohms, 15 through 30 ohms, 30 through 50 ohms.
RIAA phono gain stage:
RIAA EQUALIZATION: no greater than .5dB deviation at frequency extremes.
TOTAL HARMONIC DISTORTION: 012%. 20Hz through 20,000 Hz @ 2 volts out. typically: .008% @ 1.000 Hz.
SMPTE INTERMODULATION DISTORTION: .006% @ 1 volt out.
NOISE: -90dB A weighted referenced to 1 volt out. -75 dB unweighted referenced to 1· volt out.
CROSSTALK: (left minus right) -70 dB @ 1.000 Hz.
INPUT IMPEDANCE: 47.000 ohm and selectable 100 pI, 200 pf. or 400 pf.
INPUT OVERLOAD: 320 millivolt peak @ 1.000 Hz.
OUTPUT IMPEDANCE (tape out): 1,000 ohms.
GAIN FACTOR: +33dB @ 1,000 Hz.

High Level Gain Stage

(performance of the basic gain stage outside the system)
FREQUENCY RESPONSE: dc through 500,000 Hz. +0. -3dB.
SQUARE WAVE RISE TIME: <.6 microseconds.
PROPAGATION DELAY: input to output transit time: <.02 microseconds.
PHASE SHIFT: <10 degrees at 1,000,000 Hz. SLEW RATE: 150 volts/microsecond.
TOTAL HARMONIC DISTORTION: .003%, 20 Hz through 5,000 Hz .. 006%, 5,000 Hz through 20,000 Hz @ 5 volts out into 10,000 ohm, 1,000 pf load.
SMPTE INTERMODULATION DISTORTION: .008%, 10.000 Hz @ 5 volts out into 10,000 ohm, 1,000 pf load.
CROSSTALK: left channel to right channel; -6OdB @ 1,000 Hz. tuner to auxiliary; -65dB @ 1,000 Hz.
GAIN FACTOR: +2OdB.
MAXIMUM OUTPUT BEFORE CLIPPING: ::12 volts peak.

Dimensions:
Faceplate: 19 inches wide, 2.62 inches high.
Chassis: 17 inches wide, 2.215 inches high, 8 inches deep.
Power supply: 8 inches wide. 3.375 inches high, 5 inches deep.