> distortion in common base preamp

The distortion generated with the small signals encountered will be
negligible wrt the distortion generated by the speaker proper. There are
few small speakers that get under 1% distortion and they are unlikely to
be used as microphones in such a circuit.

The amplitude on the collector will most likely be 100mVpkpk at most while
speaking directly into the speaker at 10 cm or so. This is a small
percentage of the output capability and distortion will be low. You can
model it in SPICE if you like.

3% distortion is acceptable for speech (some say 10%). A cheap small
speaker will give 5% distortion at least. Remember that the emf in the
voice coil depends linearly on the magnetic field in the gap and the field
depends on the (inverse) square of the apparent gap (wrt voice coil
position). 1% distortion in this context means gap accuracy of 0.01^2 =
0.0001. A 1mm gap would require machining and geometrical tolerance to 100
nanometers for that. Oops, not your average far eastern tiny speaker. Oh,
by the way, this assumes everything else is perfect, like suspension,
damping, centering, coil geometry, and acoustics.

The 0.01% distortion implied by some HiFi amps implies speakers with a
field that accurate and the gap parts should be machined to 1e-8 which is
20nm for a 2mm gap. Re-oops. Better don't look at your expensive speakers.
20nm surface quality and geometry is mirror grade (good enough for laser
mirror I think). And there are no shortcuts I know of here.

One of the less published reasons for which good speakers go for huge
magnets is this. Big magnets afford a larger gap and that puts precision
within machinability limits, besides increasing efficiency and
electrical coupling/damping. However big magnets are expensive. If the
manufacturers could get to these results otherwise, they would. The
ferro-fluid gap fillers etc play a role here but I think that an air gap
is cleaner.

Peter

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