There appear to be misconceptions about this fine piece of equipment! I'll try to shed some light on the subject.
1. What is the CD4 Demodulator?
It is a 4-Channel RIAA amplifier, and IMHO, one of the best in the world, even compared with all the fancy new digital op-amps available today. Its transparency and clarity is surpassed by only the best Tube amps. B&O pulled out all the stops when they came up with this one.
The CD4 Demodulator provides full line-level output to all 4 channels, all the time (even if only 2 channels are recorded on your LP).
The choice of your BeoMaster (or other) Receiver/Amplifier will determine whether you hear all 4 channels (Quadraphonic) or only 2 channels (Stereo).
2. What the CD4 switch does:
The switch changes only the Upper Limit of the Frequency Response (Low-Pass Filter) of the output signal from the BeoGram Turntable.
With the CD4 switch OFF, the Upper Limit of the frequency response of the output signal is limited to about 20kHz.
With the CD4 switch ON, the Upper Limit of the frequency response of the output signal is extended to about 50kHz.
That's the only function of this CD4 switch.
The switch position makes NO difference to the output volume level, EVER!
The switch does not switch the RIAA amplifier ON or OFF, EVER! The RIAA amplifier is permanently ON (as long as you have AC power to the BeoGram).
3. Why is the CD4 switch required?
The switch is required because the Quadraphonic CD4 signal is encoded in a special track, recorded at approximately 44kHz.
A regular stylus is incapable of picking up this signal, and the BG4002/6000 needs a MMC5000, MMC6000, or MMC20CL to pick up this high-frequency 44kHz signal.
If you play a regular stereo LP (non-Quadraphonic) on this turntable, you may hear high-frequency distortion. Switching the CD4 switch OFF, lowers the Low-Pass Filter's upper limit to 20kHz, thus eliminating this high-level distortion.
4. Rebuilding this CD4 demodulator.
I have now rebuilt 4 of these, and I can state with confidence that its sound quality as an RIAA amplifier is absolutely superb. If anyone is not satisfied with their CD4 Demodulator's performance as an RIAA amplifier, feel free to contact me, and I will rebuild it for you. It is easily disconnected from the Chassis, and sent by mail. Absolutely, do not throw it away - Please!!!!
Regards
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!