Review: Threshold SL-10 Preamplifier

Category: Preamps

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.

Below is further information from Threshold circa 1980 in regards to the SL 10.


Model SL 10:

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.

Biasing preamplifier transistors to the range realized in the SL 10, however, creates undesirable voltage offset characteristics which impede the application of such levels in a dc coupled system. In the conventional approach to dc coupled design, input circuitry is optimized to the overriding requirement of low offset voltage. This avoids the cascading dc amplification of voltage and bias currents which result in severe driver offset or even damaged woofer assemblies. Unfortunately, to optimize for this single parameter in the high speed circuitry of the SL 10 would have compromised the system's performance as an audio processor of maximum resolution. In order to realize the wideband phase integrity of a dc coupled design with the linearity of ultra-high bias levels, it was necessary to develop a unique offset nulling circiut that would assure absolutely stable dc performance under all conditions. This peripheral control system, completely outside the signal processing path, watches for the presence of dc offset through the signal carrying circuits and nulls it to "0" at the SL 10 output over a time span far outside the region of subsonic information. The action of this null circuit even includes effective cancellation of offset originating in associated
equipment connected to an input of the SL 10. As a result, the SL 10 exhibits the dc stability of a capacitively coupled feedback loop but with the only capacitors appearing in the signal path being the individually calibrated polystyrene and tantalum devices used to obtain RIAA equalization.

The phono stage circuitry of the SL 10 employs an advanced approach to the handling of RIAA equalization in the feedback loop. Threshold's investigations into transient distortion phenomena have shown the desirability of maintaining a constant amount of feedback across the audio band. Most commonly this is achieved, in power amplifiers, through very low, and thus constant, open loop gain. The governing parameter is consistency and greater open loop feedback is permissible as long as the level of feedback remains constant versus frequency. Unfortunately, in an RIAA phono stage, a flat open loop characteristic results in a feedback variation of 100 to 1 across the audio band resulting in high feedback at high frequencies and low feedback at low frequencies. This causes lack of control for bass information and stability problems at high frequencies.

One solution that has found recent favor has been to split the phono stage into two flat gain stages which are separated by passive RIAA equalization. This allows constant feedback for each stage. The technique has two disadvantages however: the additional degradation caused by two circuits in series, instead of one, and greater susceptibility to high frequency input overload from the pre-emphasized disc.

Threshold uses an original approach to the problem whereby the open loop curve of the phono stage is shaped to compliment the RIAA characteristic, resulting in a virtually constant amount of feedback across the audio band. The SL 10 exhibits high open loop gain when 60 dB of closed loop gain is required at low frequencies and low open loop gain at high frequencies where only 20 dB of closed loop gain is called for. This technique allows constant feedback to be achieved without the use of two circuits, and with a phono overload point which increases with frequency to accommodate disc pre-emphasis.

The Threshold SL 10 is designed to accept all velocity characteristic phono cartridges, providing equalization to the exact reverse of the RIAA recording characteristic, and includes a highly advanced pre-preamp section. Not a transformer, this preliminary phono gain circuitry circumvents the performance limitations inherent in these devices to attain very low noise, extraordinary definition, and high rejection of rf interference.

Threshold model SL 10 technical data:

The extraordinary speed of the SL 10 basic circuit is revealed in the input vs output pulse burst display above. Shown is the SL 10 basic circuit responding to a 200 nanosecond pulse burst. The upper trace is the input pulse of a 100 millivolt peak burst. The lower trace is the output of the SL 10 basic circuit, driven from a 50 ohm source, reproducing the burst at a 2 volt peak output. The dispiay is calibrated to 100 nanoseconds per division (1 microsecond across the screen). Note the extreme simiiarity of the two traces at this extended performance levei.


Two channel, low-level signal processing unit having switch selected capacitance or impedance load characteristics for matching all velocity characteristic cartridges. Built-in, preliminary gain phono stage, operated without feedback, for moving coil or ribbon cartridges. Three high level amplitude characteristic (flat) inputs. Record/Monitor facilities for a single tape recorder. Front panel control functions consist of program selection, instantaneous source or recorder monitoring, channel balance, and audio level. Audio processing circuits are powered from a separate power supply module and operate with a total of 20.000 microfarads capacitive regulation throughout the supply circuitry.

Audio circuits incorporate ultra-fast, cascode/class A, direct coupled operation. High gain. wide bandwidth (200,000,000 Hz) semiconductors are selected through noise and curve-trace linearity analysis. Auto-null assures dc offset of no more than ± 10 mv maximum at the output, even when injected at the input by associated equipment, without affecting audio response. Superior power supply rejection through quadruple decoupling.

Active Circuit Data:

performance of basic gain stages. (measured to IHF standards A202 for distortion specifications).

preliminary phono gain stage:

(all measurements made at highest input impedance) 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.


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.




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.

Available Accessories:

Rack mount kit for S 1 power supply consisting of 19 inch wide faceplate matching that of the SL 10 audio module, hardware, and instructions.

The superiority of DC response capability is clearly illustrated in the oscilloscope display above. Shown is the performance of the SL 10 basic circuit reproducing a one-half Hertz square wave. Note the total absence of sag and, thereby, the capability for unencumbered low frequency performance.

The spectral noise density for the SL 10 preliminary phono gain stage is shown above and indicates the extremely low, broadband, stochastic noise levels that can be obtained with an advanced, feedback-free design. The less than 50 nanovolts per '/Hz noise density establishes a midband noise floor greater than 125dB below the 100 millivolt rated output. The spectrum display is calibrated to 50 nanovolts per vertical division. Frequencies shown are 20 Hz through 20,000 Hz with a filter bandwidth of 10 Hz. The input to the SL 10 is shorted.

Because Threshold is constantly researching new technology and materials the option is reserved to incorporate design refinements and/or modifications into existing product lines without notice or obligation.

Threshold Corporation

Only the mind of Nelson Pass could have come up with this over the top, state of the art preamp, and to think this came out in 1979 just boggles the mind. One just has to hear this in ones system to fully appreciate the prowess of the Threshold SL 10, although some 30 years since new, it will mesmerize you with it sonics. Find one if you can and have it serviced by Jon Soderberg at Vintage Amp Repair. He worked with Nelson Pass at Threshold, and has the knowledge and skill to maintain any Threshold product. he has done several Threshold pieces for me and I have never been dissappointed with his work and service.

Keep in mind if you do find a SL 10 it will need service as the caps are near end of service life. Once service is completed your good for another 20 to 30 years.

Of all the preamps that have passed through here over the years the SL 10 has very few peers indeed. And by chance if your like me and still spin a lot of vinyl, (I play about 30 hours per week of vinyl) you have found a true preamp that will allow your vinyl library to shine as never before.

In todays preamp market to find a current preamp with a phono section on board will cost you dearly and I seriously doubt if the newer ones can come close to the phono section of the SL 10. This came on the market at $1,295.00 in 1979, in todays dollars that relates to $4,013.66. So you can see this preamp even by todays dollars is in very lofty company.

Of course theres not many of us left that still embrace vinyl playback, so some of you just do not know the excellence of this phono section, when you have had to deal with outboard units or the newer poorly designed units. No expense was spared in the phono section of the SL 10 and that goes for the line stages as well.

Of course it has just stunning cosmetics, the sculptered control knobs are a true work of art, seldom found at any price, past or present. Audiophiles are just drawn to it once they see its understated elegance that defies time.

Listed Below is a review from "The Audio Journal" December 1979 issue.

The Threshold SL-10 (straight line-ten) as it name indicates is a preamp constructed with the purist in mind. The slim rack mountable look of the front panel is dressed with only four controls: audio level, channel balance, tape monitor and input selector. The selector control provides positions for one turntable input and three high level inputs. A separate power supply box is provided that houses the power transformer and filter capacitors. The SL-10 has a clean, uncluttered appearance, but as simple as it looks there is more here than meets the eye.

Even though the user can only access one turntable input from the front panel, The SL-10 sports two sets of phono jacks, one labeled "low impedance and the other "high impednace". The former is for moving coil cartridges and the latter for moving iron and moving magnet types. A rear panle switch selects between the two inputs. Two other rear panel switched are also provided to tailor the preamp to specific cartridges. For moving coils, the impedance termination switch has three impedance ranges: 1-15 ohms, 15-30 ohms and 30 to 50 ohms. For magnetic types, tha capactive loading switch provides positions for 100, 200, and 400 picofarad loads. So the stock SL-10 will handle all velocity type cartridges without any kind of add-ons.

A closer inspection of the SL-10 reveals the utmost quality that we have come to expect from Threshold. The Chassis is constructed from sheet steel instead of aluminum ( the face plate is the only aluminum part). The controls operate smoothly and precisely without any pops or clicks. The input and output jacks are all gold plated. All of the active parts are mounted on a single circuit board. The foil patterns of the circuit board appear on both sides and are gold plated. The bottom of the chassis has a removal panel to access the bottom of the circuit board in the event that service is ever required.

The nomenclature for the rear panel connections appear on the top cover along with a block diagram of the SL-10 circuits. According to the owners manual, the SL-10 has individually curve-traced transistors that have a bandwidth of 200,000,000 hertz!

As we shall see, the Threshold SL-10 is the most important step forward in transistor preamps since the introduction of the Levinson JC-2 several years ago. At that time the JC-2 set new standards for the definition. extended frequency extremes, low distortion and low background noise. However the JC-2 had a compressive effect on sound. Spatial qualities of recordings, most notably the midrange depth, collapsed through the JC-2. Audio enthusiast, especially, vacumn tube afficionados, were quick to point an accusing finger at the JC-2.

Perhaps this concept requires a bit more explanation. The easiest way we know to hear spatial qualities of a preamp is to listen to a good recording of some type of vocals, preferably a solist. Spatial qualities can be heard with all kinds of music, but for sake of argument, we think vocals are the easiest to apply here since the human voice is very difficult to reproduce properly.

When reproduced properly, the voice should maintain a separate idendity from the accompanying instruments. The vocal sound should have a roundness and a fullness so that it has a "glow" about it, so to speak. In other words, the voice should have a three dimesnional character so that the listener can almost imagine that the vocalist is standing in his listening room.

The age old gripe with the transistor units, including the JC-2 is that such a vocal sound would sound flat instead of round. The voice would tend to flatten out against the accompanying instruments so that much of its individual character would be lost. Vacumn tupe preamps that have the "classic tube sound", whilw much better in this respect tend to over glorify the threedimensional effect and add a sort of "fuzziness" around the edge of the vocals.

Well, the SL-10 has all the good qualities of the JC-2 and none of the bad ones. The spatial qualities are also there, but without the "fuzziness" or over-glorification of depth. With onlt the possible exception of the Theta preamp,( also reviewed in this issue ), the Sl 1o has the most musically accurate midrange we have ever heard out of any preamp tube or solid state.

To go along with that superlative midrange, are equally impressive frequency extremes. the treble region hasn't a trace of grain. The perfectly seamless texture of the high end allows cymbals, triangles,etc to have their natural ring and decay. The treble is very extended but isn't irritating even after hours of listening.

Bass reproduction has always been a strong point of transistor preamps and the SL-10 is certainly no exception. Bass from the SL-10 has the control and tightness of live music. There doesn't seem to be a limit to how deep it can go. In fact, the specifications for the SL-10 say that the bass is flat to DC! Need we say more ?

We shouldn't go any further without saying that the majority of our listening with the SL-10 has been with moving coil cartridges, primarily the Ortofon MC-30, the Denon 103D and the GAS Sleeping Beauty. We did try several magnetics. The performance with these is superlative as well. The moving coil cartridge definitely has some sonic advantages, but every head amp or transformer/preamp combination we tried has left something to be desired. It wasn't until we began using the SL-10 that we realized just how good these cartridges were.

Part of the secret here, we think must lie in the fact that the head amp is built in and is especially designed to mate with the phono section. Even with the lowest output cartridges the Sl-10 is super quiet. The music stands out against a deal silent background. With every separate head amp we have tried we encountered problems with radio frequency interference (RFI) as well as hum, hiss and a list of other noises too long to menetion. With the SL-10 we haven't heard any RFI at all. We are happy to say goodbye to our old friend ,RFI, foreever. Of course, there have been other preamps with built in head amps that didn't work as well. So, obviously there is more to this story than we know how to explain. However, we do know that moving coil fans will not be able to live without the SL-10 once they hear it.

Now that we've had the SL-10 for awhile, we don't think we could live without either. We must admit we like the approach of the SL-10 excellent sound, simple yet complete. Of course there are other good preamps available and we even use a few of them ourselves. Of all the preamps we've used though, we can't think of another that does so much well.

Yes it looks like Threshold has done it again!!

Manufactured by: Threshold Corp., 1832 Tribute Road, Suite E, Sacramento, CA 95815

Serial Number: L7908275

The Audio Journal - December 1979 issue.

So there you have it one of the great preamps from the golden age of high end audio made in USA.

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