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Problem with CA 640A V2


Active Member
I recently purchased a Cambridge Audio 640A V2 amplifier for my Mordaunt Short 914s. I do however have a nagging problem.

When the speakers are wired to either output "A" or output "B", all is fine. The problem however occurs when I try to biwire and use both of the outputs together.

When turning the amplifier on, the protection circuits engage, causing the protection LED to flash in single bursts (the manual says this is because of a DC tection).

If I unplug one of the Low Frequency cables for either channel, it turns on fine.
Also if I disable the B output (currently connected to the High Frequency inputs on the speaker) all is fine.

Finally, if I turn the amplifier on with the B output disabled, then turn the B output back on whilst the amp is on, all works fine (currently what I have been doing).

Any ideas as to what would cause this?

Thanks, Andrew.

Ed Selley

AVF Reviewer
The startup impedance is dipping below the lower level that the amp is set to. This is (it seems) different to that across a single set of terminals. You can either biwire from A terminals only (it is taken off the same output anyway), engage speaker b each time or not biwire- the latter being worth a punt as I can't say I have heard much (any?) of a difference when compared blind.


Active Member
Cheers for the advice.

I must admit, I have noticed an improved sound now it is biwired, but it's new kit, so it might be that its just wearing in. I will get someone to help me do a blind test at some stage to see if I really do notice any difference.

Bi-wiring from one terminal could be a problem. I'm using 4mm cable on 4-5mm banana plugs, so fitting two cables into one terminal could be difficult :(

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