Thanks George.
Quite. That's rather what I thought. Your second para describes exactly the issue I am highlighting.
Quite. That's rather what I thought. Your second para describes exactly the issue I am highlighting.
Krell Class A/B power amps, do 'anticipator circuits' work?
Douglas Self stated that increasing bias often leads to increased distortions. He also measured it. The reason for that is changing transconductance. Transconductance is output current to driving voltage gain, that is rapidly changing when two transistor current (biased region) changes into one transistor current (outside of biased region). He stated: It is not generally appreciated that moving into Class-AB, by increasing the quiescent current, does NOT simply trade efficiency for linearity. If the output power is above the level at which Class-A operation can be sustained, THD increases as the bias advances into AB operation. This is due to so-called "gm-doubling" (ie the voltage-gain increase caused by both devices conducting simultaneously in the centre of the output-voltage range, in the Class-A region) putting edges into the distortion residual that generate high-order harmonics much as under-biasing does. This vital fact is little known, presumably because gm-doubling distortion is at a relatively low level and is obscured in most amplifiers by other distortions.You can find it here (5.3): http://www.douglas-self.com/ampins/dipa/dipa.htm The main reason of going into class A is to reduce feedback. Once feedback is set damage is already done (TIM distortions) and playing with bias won’t change it. Perhaps there is a way of compensating for the change in transconductance (gm doubling) or adjusting feedback dynamically but it is very complicated. My Benchmark AHB2, class AB amplifier, uses separate "error output stage" to avoid recursive feedback and TIM distortions (AAA patent). |
Here is a link to Dan D'Agostino's patent for plateau bias. This describes, in general terms, how his circuit works. https://patents.google.com/patent/US5331291 |
Thank you Jaytor, that is extremely helpful. I had not read it previously. It may be seen to settle the question I put. I have read the patent. It relates to a methodology of adjusting the bias current in response to measured changes in current flowing in the load. At no point does the patent claim the bias adjustment can be triggered so fast that when a sudden high current event occurs, that event can be presented in either pure Class A or employing a higher bias current than that being employed immediately prior to the occurrence of the event. The system can adjust the bias in quite complex ways but it cannot provide a raised bias current in time to catch sudden dynamic increases in the programme. Such effect is advantageous to SQ and can only be achieved by running full-time in Class A. I am satisfied the natural order is restored. The application should not have been described by some as 'anticipator circuits'. That would entail time travel. |
Yeah, I think it has more to do with setting the bias based on the current requirements (as in amps) of the current situation (e.g. listening level and speaker load requirements). For many speakers and most listening conditions, a fully biased class A amp is just wasting power, but it's nice to have it in reserve. I owned a pair of Krell MDA-300 monoblocks for a while that were room heaters. I think these were made in the days when Krell class A amps were fully biased. I replaced these with an FPB-300 (which used the plateau biasing technology) which I owned for more than 20 years. There was a couple years separating my ownership of these amps, as well as different speakers, so I never really got a chance to directly compare them, but they were both excellent amps. I don't think I would hesitate (at least from an SQ perepective) to buy one of the newer D'Agostino designed Krells (not sure about the ones after he left). I finally got rid of mine after it failed for a second time with bad caps and it was more expensive to recap it then it was worth. In hind-sight, I wish I had kept it and done the work myself since I've recently gotten into doing a lot of DIY audio electronics. |