Bascom King Interview PS Audio BHK Signature amps

Got it, seven (now eight) days in a row at work have made me slow, sorry.

I still have a DH-200, somewhere. I built a number of them, and modified many more, over the years. It was not real happy with low-Z loads and I thought the bass a bit anemic but overall it was/is a nice amp. My Counterpoint SA-220 was much better, but still not up to the better BJT amps in the bass. The limited listening I have done in the past few years indicates much better performance from MOSFET output stages these days. And most certainly high-power MOSFET technology has improved a great deal over the years.

DonH50 said:

The problem is to define "excessive". Like most things, just a flat statement of how many dB is insufficient. Loop order, open-loop parameters, bandwidth, circuit topology and type of feedback, a myriad of factors determine how feedback impacts the output.

Click to expand...

That's right - audiophiles read reviews and marketing literature and "feedback is bad" becomes ingrained.

When I re-started Genesis, I hired Arnie Nudell and Bascom King as the designer-in-chief and consultant. In the early days, Bascom gave me a very valuable lesson in feedback - he told me that too much is bad, and too little is bad. Even with many "zero feedback" amplifiers, local feedback is necessary to keep components operating linearly. Zero feedback usually means zero global feedback. He encouraged me to listen to what feedback sounds like, and open my mind and listen for myself.

In those days, I was retired in Singapore and bringing up babies and a young family. Hence, I had time to play. On a challenge, I designed a pair of monoblock tube amplifiers and Bascom helped me tweak them.

(Review and a very long-winded story here: http://www.6moons.com/audioreviews/genesis/m60.html - they are long obsolete, so this is not a plug.)

After we had the "Feedback discussion" Bascom sent me a circuit to put into the prototype so that I could dial in the amount of feedback - from zero to too little to too way much. He had intentionally put into the circuit enough bandwidth so that I could dial the amp from zero feedback (which resulted in IIRC damping <1) to positive feedback which resulted in a negative output impedance.

Little changes in twisting that dial gave me a huge insight into the sonic and musical workings of feedback that is still one of the most valued pieces of my design education. From that lesson, I started working on the design of the servo-control circuit for the Genesis woofers, and that is when the breakthrough came after nearly 3 years to eliminate the "dry boring slow bass" that used to be the bug-bear of servo-controlled bass.