The original Leak "Vari-Slope" pre-amplifier was designed to enable the high-frequency distortion from inadequate disc reproduction of the day to be filtered out - with minimal effect on the audio signal proper. Like other Leak pre-amps it was designed to be easily mounted in a timber cabinet if required.
Below you will find a transcription taken from a sales brochure for the
then new "Vari-Slope" pre-amplifier, introduced in early 1952.
Leak went on to produce a series of these pre-amplifiers: the Vari-Slope
II and III, and later the "Varislope Stereo". (Ironically the
solid state Leak amplifiers of the 1960's also had a two-slope filter but
implemented with inductors and capacitors!)
The original photos of the Vari-Slope were generously provided by Gary Smith of Australia,
and the sales brochure below was generously supplied by Bruce Scahill of New Zealand.
The new "Vari-Slope" pre-amplifier has a refinement which will doubtless set the pattern for future high-fidelity reproducing amplifiers. This advance consists of variable-slope "electronic" low-pass filters operating on negative voltage feedback principles.
NO INDUCTORS ("CHOKES") ARE USED and all their disadvantages are completely eliminated.
It is common knowledge that owing to defects in radio transmissions, and (more particularly) in records, the most satisfying results are almost always obtained when the high-frequency response of the reproducing amplifier is restricted. The simplicity of this simple statement of fact hides the very real difficulties of making equipment to meet the need. These difficulties arise from two factors: first, the nature and magnitude of the distortions inherent in records varies greatly from record to record. Second, with any particular record, the magnitude of the high frequency restriction which is deemed to be required, and the manner is which the restriction is distributed over the frequency band, will depend on three things: the taste of the individual listener, the characteristics of his pickup and loudspeaker, and the acoustic characteristics of his listening room.
From the above it follows that the requirement is for a very flexible system which will vary not only the frequency at which the response begins to fall but also the rate at which the response falls off. Creditable attempts have been made to meet theses requirements by means of conventional "low-pass" choke-capacitance filters, bet even when properly designed and constructed these filters introduce considerable distortions on transient, which is only to be expected from the oscillatory nature of a circuit containing inductance and capacitance. This transient distortion is very real: it can be predicted from theory, demonstrated by oscilloscope, and most importantly, it is audible, particularly with steep slopes of attenuation. This is the main reason why we have always expressed dissatisfaction with conventional filters of this type, even though we make and market them.
THE SOLUTION to the the problem is the elimination of all choke-coils, and this has been achieved by using modified twin-T resistor-capacitor networks in negative feedback loops. We have evolved an ideal form of continuously variable slope control which increases the feedback as the slope is made steeper. The obvious advantages of our electronic feedback method over conventional choke-coil filters are: -
(a) Improved transient response characteristics and the consequent reduction
(b) Extremely low harmonic and intermodulation distortion due to the negative voltage feedback action.
(c) No discontinuities in the rates of slope when the slope control is operated, and no change in signal level at frequncies below cut-off. (Both these faults occur in variable-slope choke-capacitor filters due to the slope control altering the terminating impedance and the insertion loss).
(d) There are no chokes to give rise to magnetic hum.
(e) Smaller size, lighter weight, greater uniformity in production
Extended listener-research established that the cut-off frequency should be between 5kc/s and 9kc/s and that the slope of attenuation should be continuously variable between 5dB and 50dB over the octave immediately following the cut-off frequency.
Response curves obtained on
"TREBLE -3" position (turnover 5kc/s).
The "-2" and "-1" positions give
similar continuously variable ranges of
slopes, turning over at 7kc/s and
The equalisation for the recording characteristic of records is obtained in all three positions for "RECORDS" by resistor-capacitor feedback networks.
The "78B" positions is very accurately the inverse of the E.M.I. characteristic. The Decca 78 characteristic is the same except that the treble cut is called for above 4kc/s, and the inverse can be accurately obtained by using the slope control.
The "78A" position is for American records, including LP's for it very accurately follows the standard playback curve stipulated by The Audio Engineering Society, whose membership includes the foremost American recording engineers.
The "LP" position is for
Decca ("London" in the U.S.), the new
E.M.I. 33-1/3 and 45 records
(to be released in Britain later in 1952)
and for Columbia (U.S.) LP's. The solid line
shows the built-in equalisation, and is compared
with Decca and E.M.I. characteristic.
The pre-amplifier will operate from any pickup generally available in the world. The sensitivity is 15mV at 1,000c/s for full power from the associated TL/12 amplifier. The input impedance is 100,00 Ohms.
The pre-amplifier will operate from any radio unit generally available in the world, the sensitivity being 60mV. Flat response.
The sensitivity is 4mV, and the input impedance is 200,00 Ohms. Flat response.
The effects obtainable with the
feedback tone control system.
| Dimensions as on RC/PA/U:|
Fornt panel, 10-3/4" x 3-1/2" (27.3 x 8.9 cms).
Chassis, 10" x 3" x 2-3/4" (25.4 x 7.6 x 7.0 cms).
Weight, 4lbs. 8ozs. (2.04kgs).
Valve, ECC81 (12AT7).
BRITISH USERS OF THE RC/PA/U PRE-AMPLIFIER. Your old model can be re-built as a standard "Vari-Slope" at a charge of 8 gns. Orders will be booked in rotation. Please do not send your pre-amp to us until we advise you of a date.
OVERSEAS USERS. Please contact yout national agent regarding the availability of this re-building service, ahich is dependent on freight and re-import duties being resonably low.
|Phone: SHEperherds Bush 1173/4|| Telegrams: Sinusoidal, Ealux, London
Printed in Gt. Britain
|Foreign: Sinusoidal, London|