On 2015-11-09 05:14:30 +0000, Phil Allison said:

Jim Lesurf wrote:

Arthur Quinn
+

I like like idea of the secondary feedback loop to make the
capacitor-coupled output impedance less reactive at low frequencies.

It wasn't a new idea entirely as IIRC some valve amps used the same trick
to reduce the effects of output transformer imperfections. The challenge is
to apply 'just the right amount'.


** Taking some or even all NFB from the speaker side of the output
capacitor was pretty common with amplifiers using one DC supply. Doing
so however raises the issue of low frequency instability.

I recall seeing a Kenwood solid state receiver of late 60s vintage that
when connected to a Philips 8 inch low resonance woofer ( AD8065)
slowly went into oscillation at about 3Hz.

The trick was to use only a judicious amount OR have a capacitor or two
internally rolling off the gain of the power stage at low frequencies
- as is done with most valve amps.


... Phil

Yes, the review says of the damping factor that "from about 2Hz to 50Hz
it tends towards slightly negative and infinity", implying that the
resistive component of the amplifier output impedance passes through
zero to negative within that frequency range. Oviously the negative
resistance will have been made much smaller than the expected
loudspeaker resistance to ensure stability.

Arthur

--
real email arthur at bellacat dot com