Here’s your chance to spend time with Bascom H. King, designer of the new BHK Signature power and preamplifier, scheduled for release this summer. Bascom shares with us the secrets of its design, we get a lesson in MOSFET, tube and transistors. It is a rare glimpse into the the mind of a true master of the arts.
+1. That was great! Old (I mean “experienced”) guys rule.
Here’s what I’m not clear on…There seems to be two schools of thought within the audiophile community, in that 1) using no feedback in a amp designs makes for ‘better’ sounding music, and 2) the use of feedback is a good thing because it lowers distortion (among other things). Because of this contradiction, I did a little ‘online research.’ I learned that adding feedback increases higher harmonics (e.g.: 4th, 5th 6th 7th, etc… order harmonics). Music, comprised of the simple harmonics, is said to ‘sound better’ within the 2nd and 3rd harmonics… BHK stated that he uses 40 dB of feedback in his design. Is this considered a ‘high feedback’ design, and how does this impact the higher harmonics/lower harmonics?
Feedback’s a whole subject books can be written on. Using it takes years of trial and error to know where it’s best. 40dB, in general, isn’t a lot - but it’s where the feedback is placed that makes the difference in sound. For example, in the BHK amp there’s no feedback around the tube, feedback around only the output stage where it belongs. Those distinctions are important. In the case of the input stage, the use of no feedback allows the music to be amplified by the tubes in a way that preserves everything, maintaining the delicious open sound the BHK has. Once the signal’s been captured and amplifier by the zero feedback tube input stage, it is passed along to the MOSFET output stage and only there, where it’s important, is loop feedback applied.
It is a cerful blend of design that gets one where they want to go, musically speaking.
Thanks Paul. This is absolutely fascinating. There’s so much more to designing a high end amplifier than ‘meets the eye!’ The entire video, with Annie, Bascom and you is outstanding…Highly informative and entertaining.
Very interesting and informative reply - Thanks Paul. This goes a long way to addressing similar queries made a while back.
Would be interesting to see a summary of the best links on the topic of feedback. I did post some links to Nelson Pass when I was trying to understand feedback a while back.
So many types and methods of implementing feedback - but, it would still be a very good thing to understand the relative pluses of BHK’s global feedback vs other types of feedback (single stage global or negative in solid state amps).
There’s also an interesting Leaving Class A article from Pass Labs:
As always, all pointers to better information than links I’ve nominated would be keenly read & appreciated.
@Green Machine…Nice article by Pass.
According to Bruno Putzeys, there’s no such thing as too much feedback in an amplifier design: http://www.edn.com/design/consumer/4418798/Negative-feedback-in-audio-amplifiers--Why-there-is-no-such-thing-as-too-much
That seems to ‘fly in the face’ of the philosophies of some designers (i.e.: in that there may be a limit to how much feedback ‘should’ be used).
I think it has been proven that both design concepts - low to ‘no’ feedback designs vs applied negative feedback designs - can produce excellent results (i.e.: in that if the design is executed effectively, one concept is not necessarily superior to the other).
I think Bruno (correctly) rails against blanket rules of thumb and oft quoted truisms which have no apparent basis in reality.
I like this slide deck from him: http://www.hypex.nl/docs/papers/AES123BP.pdf He shows that some feedback can give more distortion than either less feedback or more feedback in a (particular) otherwise identical circuit. In my words (not necessarily Bruno’s) you can’t really jump to any conclusions about feedback in a circuit without understanding the whole circuit in question and in particular until you know your phase margins, etc. you don’t have enough information to say anything.