There is a network containing 0.47 uF, 0.047 uF, 1M, 220k
which acts to reduce LF gain a maximum of 15 dB, starting at 15 Hz.
By 1.5 Hz, at which many amps would oscillate badly without this network,
the LF gain has become about 12 dB less, and as stability is affected by the
amount
of open loop gain and the amount of FB applied, the amp will have far less NFB
applied
at 1.5 Hz than at say 100Hz, so the amp will be rock stable at *all* LF.
The amount of FB that *can* be applied safely without oscillations
occuring is also dependant on the phase shift of an amp, and stablity
is threatened as phase shift approaches 180 degrees betaween input and output.

LF Phase/Gain networks such as you refer too were not very secret at all in the
50's. For example, they were well understood & used in both the McIntosh MC40 &
MC75.

I was being slightly facetious about secrets, but a large number of home diyers
have a rather limited knowledge of the finer points involved about amps they try to
make.

Patrick Turner.





Cheers, John Stewart