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RC/PA RC/PA/U Pre-Amplifiers

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The Leak "RC/PA/U" Pre-Amplifier

Leak "RC/PA" (Dec 1948)

With the release of the "TL/12" power amplifier in December 1948, Leak also released an associated pre-amplifier called the "RC/PA" (an abbreviation of "Remote Control Pre-Amplifier"). The list price was 6 Pounds-15 Shillings (the TL/12 alone was 25 Pounds-15 Shillings). The following details were features in sales information:

• An original distortionless feedback tone-control circuit which will become a standard.
• Switching for Pick-Up, Microphone and Radio with automatic alteration of tone control characteristics (presumably this is a reference to the equalisation required for discs).
• High sensitivities - will operated from any moving coil, moving iron or crystal pick-up; from any moving coil microphone ; from any radio unit.
• Controls: Input selector; Bass gain and loss; Treble gain and loss; Volume.
• The unit will mount on motor-board through a cut out of 10-1/8" x 3-1/8 ", or it can be bolted to the power amplifier, when, with a top cover the whole assembly becomes portable.

(Circuit diagram not available for this unit.)

RC/PA Description

The preamplifier contains a single miniature pentode valve (an EF40) arranged to give a voltage gain of 100 times, or 40dB, before feedback is applied. As the valve has to provide an output voltage of only 160mV to load the TL/12 power amplifier the distortion is extremely small. When feedback is applied, the gain falls from 100 times to 3 times, and distortion is reduced to an immeasurable figure.

The feedback reduces the input impedance from grid to cathode, and the output impedance from anode to cathode, to values of a few thousand ohms. These low output impedances bring two major advantages: an exceptional freedom from hum pickup within the valve, and an extremely flat frequency response. The low input impedance does not appear across the signal sources, these being padded out by a resistor accross which condensers are switched to provide 3 steps of treble rise.

Bass rise is achieved by releasing some of the total feedback in such a manner that the rise is asymptotic to 6dB per octave over the 3 octaves between 400Hz (cps) and 50Hz (cps) when the bass control is set at +3. This setting is, of course, only for use on record reproduction. By deliberate engineering, the rise is retarded below 35Hz and the response is then made to fall, thus obviating a major disadvantage of the usual bass rise circuits which often rise continuously to 20Hz or below. This disadvantage, together with the fact that many pickups have a resonance of considerable magnitude between 35Hz and 20Hz, results in excessive "motor rumble".

Treble loss is provided by increasing the feedback at high frequencies, and by additional capacity in the input circuits when switched to the -2 and -3 positions. These methods decrease harmonic distortion, wheras the more usual method of shunting a valve by a capacitor tends to increase distortion. Bass loss is provided by a resistance-capacitance network in the output circuit, feedback being fully maintained.

RC/PA Specifications:

Controls:

-Input control to select radio tuner, microphone or pickup.

-Bass control. A seven position switch.

-Treble control, as above.
-Volume control (100K log pot)

Sensitivities:

- pickup: 50mVrms, impedance is fixed (unlike the RC/PA/U) and is 60Kohms.

"will load from any moving iron or crystal pickup".

- microphone: 5mVrms impedance is 250K

- radio: 350mVrms, impedance is 60K

Frequency response: with controls in level position, 30Hz to 20Hz, +/- 1dB.

Distortion: less than 0.05%

Valve: EF40

Power Supplies: from TL/12 or TL/25 0.2A at 6.3V AC rms 0.6mA at 420V.

Reference and Acknowledgement:

RC/PA Description and Specification data sourced from the 1948 Leak "Point One" booklet, and kindly supplied by Patrick Cheffins of the United Kingdom.

Leak "RC/PA/U" (Nov 1950)

During November 1950 Leak released a revised pre-amplifier - the "RC/PA/U" (the "U" standing for Universal). A photo of this model is shown above. Note that the large terminal block on the top left hand side is where signal sources are connected to the unit (RCA input connectors were evidently not ubiquitous at this time).

This pre-amplifier featured user-selectable equalisation for disc reproduction. At the time, the recording industry was shifting from 78 rpm records to the new LP micro-groove types, and so there was a requirement for the pre-amp to have various equalisation depending on the source of the signal. * The cost of this pre-amp was higher than the previous model, being 8 Pound - 15 Shillings.

Leak stated that this pre-amp was a complete re-design of the RC/PA. The pre-amplifier contained a miniature twin-triode (the ECC40) with negative feedback on each stage. A full description is given in the 1951 booklet "Leak Point One Amplifiers" at this web site.

*This situation was to continue for some years as the USA, UK and various companies adopted slightly differing equalisation. The Leak "Varislope II" pre amplifier incorporated six different equalisation curves!