Cambridge Audio 640P

[jrdmedford]

The copper cover over the "Discrete Input Stage(s)" serves two purposes:
RFI shielding and Temperature Stabilization of the PNP transistors underneath.

The stage itself is simply a Red LED biased CCS (constant current source) driving a
differential amplifier using two sets of three matched PNP epitaxial transistors; in tube-parlance - a long tailed pair. One side of the diff amp is grounded, the other side
receives its signal from the MC or MM cartridge through a coupling capacitor. The
design principle here is pretty simple:

1. Parallel together three transistors, already closely matched by the OEM to further
reduce noise factor.
2. Use a CCS driving the differential amplifier to achieve very high PSRR/CMRR.

There is little you could do to improve what Cambridge has done there... It's an excellent
input stage for the price point. The only way to possible improve this would be to
entirely gut the Discrete Stage and replace it with a similar circuit based on the
Linear Systems LSK389 and cascode current source. The only advantage of resorting
that that option would be to permit direct coupling from the cartridge to the JFET gate
and eliminating the input capacitors and furthermore, that treatment would only really
benefit someone using MC cartridges and not high-level MM that most people have.

You might make marginal improvements of the tail resistors in the existing ckt...
These are R54,R71 and R62, R79. You could replace these with 0.1% MF resistors. The more closely matched the tail resistors - the better the CMRR of that diff pair.



-- Jim