Museum of Communication object page


Acoustical QUAD II Control Unit

Manufacturer Acoustical
DescriptionQuad control unit
FunctionAmplifier - Pre-Amp unit
TypeUnit Audio
TechnologyValve
ClassAudioApplicationvolAC
Object No545Accession1680LocationA55/2




Acoustical manufactured the QUAD QC II control unit (mono preamplifier) from 1953 until 1959. During that time approximately 20,000 were sold. They were designed to be used with the QUAD II power amplifier. The front panel employs a similar layout and styling to other early QUAD control units with a large volume control to the left beside smaller rotary controls for bass, treble, a filter slope/level control, and a switch to choose the turnover point of the lowpass filter. The filter offers choice between three turnover frequencies – 5kHz, 7kHz, and 10kHz. The ‘cancel’ option removes theses filters and is often taken to be a ‘bypass’. But in practice it leaves in circuit a filter that cuts the response above 20 kHz to reduce any ‘ultrasonic’ signals. The volume control also acted as the on/off switch for the QC II and the associated power amplifier.


The row of buttons below the rotary controls perform two functions. The two buttons at the left-hand of the row allow the user to select input from various types of source. In particular, a choice of two tuner inputs to cater for using separate AM and FM tuners. The remaining four buttons were all for disc replay, selecting various equalisation characteristics. This was because at the time the various recording companies were using a range of different equalisations. These equalisation settings varied from one version of the QC II to another. In some cases depressing pairs of buttons could select further equalisations.


In the example shown here, these buttons could select between two kinds of LP equalisation – Columbia LP, and AES – or two kinds of 78 rpm disc equalisation – FFRR 78, and Standard 78. During the 1950’s the choice of various equalisation curves by recording companies kept changing until it gradually settled on them all adopting what we now call the ‘RIAA’ standard for LPs and 45s. The popularity of the 78 rpm disc also dwindled as it was replaced by the 45 rpm single. Acoustical adapted to keep up with these rapid developments. As a result, the most obvious difference between the later versions of the QC II are changes to the choices of equalisation offered by the front-panel buttons.





The sockets and leads at the side of the unit are for power connections, e.g. for the tuners. At the time Acoustical made both AM and FM tuners and the outputs were designed for use with these, feeding appropriate H.T. levels, etc. Both switched and unswitched power was provided. The pair of male split-pins are for mains a.c. input. The arrangements allowed the QC II to be able to control the power to the tuners and power amplifier. Turning the QC II’s volume control fully anti-clockwise allowed it to switch off the power to all the units.





A number of different versions of the QC II control unit were made. The earliest examples had fewer input sockets on the rear than this example. So this is an example of one of the later versions.


The radio/tape inputs have a nominal sensitivity of 100 mV into an impedance of 100 kOhms. The sensitivity and input impedance of the disc input is determined by the choice of ‘adaptor unit’. This was very useful given the range of disc replay pick-ups / cartridges available at the time. As a result, QUAD provided a range of these adaptors. The appropriate unit was connected via the round multipin socket shown on the rear panel. The standard adaptors for magnetic pick-ups offered sensitivities from 8 mV up to 100 mV and impedances ranging from 25 kOhms up to 100 kOhms. There were also other adaptors – e.g. one suitable for ceramic pick-ups providing a sensitivity of 750mV and an input impedance of 2·2 MegOhms in series with 100 pF. (N.B. The ceramic pickup adaptor is described as above in a QUAD II amplifier User Manual. However I suspect it is a misprint and the 100 pF is actually in parallel with the resistance.)