"Jim Lesurf" wrote in message
...

Can only speculate at this point as I have never tried using these
circuits
for heaters in audio amps. However...

With 'direct' heating the heater psu is - I presume - connected to the
same physical place as the cathode signal. Having a 'constant current' PSU
means the PSU looks like a high impedance connection, so will be less
likely to have a loading effect on signal drive to the cathode if the PSU
common mode isolation is poor. i.e. the problem is that the cathode signal
has to drive any loading it sees due to the PSU, so a high impedance might
be preferred.


No, it's nothing to do with loading. What is actually happening is that the
resistance of the filament is partially in series with the cathode current.
So it acts as an unbypassed cathode resistor creating a little bit of
negative feedback. Effectively shorting that out with a low impedence
filament supply reduces this negative feedback.

David.