Arthur Quinn wrote:


** 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.



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.


** The get such test results implies the amplifier was being driven at its output with a varying frequency of known current while watching the voltage and phase at the terminals. It's a powerful technique that not many reviewers ever used.

BTW

The output stage of the 626 has very similar topology to the 40watt design published in the Philips "Audio Amplifier Systems" application book of 1970 - minus the regulated PSU.


..... Phil