In article , Wally
wrote:
Given that my speaker rebuild fantasy is gradually taking shape, I'm
starting to think about what to do about crossovers. The plan is to
bi-amp, one amp to the B139 drivers, the other to the B110/T27. I'd
rather ditch the orginal KEF DN12 units and make new ones for the
mid/top, probably keeping to the spec crossover frequency. So, I have
some questions about components...

I don't know the details of the crossover networks in question. However if
you are going to bi-amp the system it may make sense to remove some/all the
networks between power amps and speakers and, instead, use networks at the
inputs to the power amps. Done correctly, this may well give much better
results that networks that have to work at high powers in the complex
loads.

Am I right in thinking that air-cored inductors are better than those
with some sort of ferrite(?) core?

Depends upon your definition of 'best' and the circumstances.

Ferrite cores mean you can reduce the length of wire required. This can
mean a physically smaller coil and/or a lower coil resistance and/or lower
resistance due to internal impedance, etc. Hence the ferrite may improve
things on the conductor side. However it may, itself, increase magnetic
'singing' of the coil and/or introduce some non-linearity. (That said, it
may well be that the speaker units introduce more distortion than the use
of a suitable ferrite.) Sizes and shapes also affect the amount of coupling
from/to a coil and surrounding components. Ferrite non-linearity or
saturation may not matter much in some circumstances, but make ferrites a
poor choice in other circumstances.

So, "It depends"... :-)

For the same value, would an air-cored version be physically larger?

Probably.

If so, is that because it requires more turns?

and/or a larger cross-sectional area.

What is the best type of capacitor to use wthin sensible cost limits?

Answer similar to the above. :-)

Are you wishing to compare bipolar electrolytics with other types, or are
you excluding them?

The advantage of using crossover/equalising networks between pre and power
amps is that you can avoid a lot of the above problems as the required
voltage and current levels in the components will be much lower. This means
you can use smaller, cheaper, and better perfoming coils/caps.

The disadvantage is that producing the correct network may involve a fair
bit of work as it won't be a simple 'copy' of the speaker arrangement.

Slainte,

Jim

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