Increasing the power-supply voltage to 16V makes the pre-amp more stable. The reason why Quad used an 11 Volt power-supply was the size of the capacitors at the time the 33 was designed. We will also remove the 5V line on pin 4 of the output that was intended for switching on the power-amplifier but that was never used with the Quad power-amps.
And... That's it!
Quad 33 Control Unit Mono /Stereo switching facilities. And the optimum way to connect a CD player.
Mono Stereo facilities
There are a lot of questions about these facilities, in most cases these questions arise after the upgrade of a 33 Control Unit. As pointed out below, the Disc, Radio1, Radio2 and Tape replay have different mono/stereo functionalities. The only common thing is an input combined with the Stereo button. This document is based on the circuit diagram above serial number 11.500. Below serial number 7500 some functionality is missing, in the last part of this paragraph you will find the functional differences.
No pushbuttons activated
If no pushbutton is activated, all the input signals are shortened to pin 2, earth. Quad assumed in those days that all the sources could cope with this situation. Check if all your source equipment can deal with short circuit outputs!
Disc
Disc combined with «M : L+R to L output
Disc combined with M» : L+R to R output
Disc combined with «M, M» : L+R to L and R output, full mono
Disc combined with Stereo and «M : L to both outputs
Disc combined with Stereo and M» : R to both outputs
Disc combined with Stereo and «M, M» : L+R to both outputs, full mono
Radio1
This is a special situation because the Quad FM2 and FM3 tuners carry the mono signal on pin 1 of the Din connector. So if a device is connected, without a signal on pin 1, there will be no output in the first three situations described below!
Radio1 combined with «M : Signal pin 1 to L output
Radio1 combined with M» : Signal pin 1 to R output
Radio1 combined with «M, M» : Signal pin 1 to L and R output, full mono
Radio1 combined with Stereo and «M : L to both outputs
Radio1 combined with Stereo and M» : R to both outputs
Radio1 combined with Stereo and «M, M» : L+R to both outputs, full mono of pin 3 and 5
Radio2 and Tape replay
Radio2 combined with «M : L to L output
Radio2 combined with M» : R to R output
Radio2 combined with «M, M» : L to L output, R to R output
Radio2 combined with Stereo and «M : L to both outputs
Radio2 combined with Stereo and M» : R to both outputs
Radio2 combined with Stereo and «M, M» : L+R to both outputs, full mono
On units below serial number 7500 there was no possibility to fed one channel, Left or Right, to both outputs.
Connecting a CD player to a 33 Control Unit.
Quad advised in a previous factory document three ways to connect a CD player
A modified Disc Adapter board
Via Radio 1 or 2
Via the Tape input
From a technical point of view the Tape input is the best option, the signal from that input is fed through a resistive network on the tape adapter board and than connected via the switch to the volume pot meter. Also the input sensitivity can be set to 1V.
The second best solution is the usage of one of the two radio inputs. The sensitivity of those inputs is 100mV and the signal is fed trough the tape adapter board, one transistor stage extra.
The worst option is the usage of the modified Disc Adapter board. The signal is fed through an extra two transistors, compared with the first option a total of three transistors extra, and associated components. The only advantage is the sensitivity setting can be adjusted by changing some components.
If you want to use one of the radio inputs for a CD player, buy a special cable with at least a 10dB attenuation, otherwise, in most cases the CD player will be too loud with normal volume settings.
Joost Plugge
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