 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
69.118.107.235
In Reply to: RE: my feeling is different posted by AbeCollins on November 22, 2024 at 08:11:54
Unless you can predict music, otherwise you always fall behind.
Follow Ups:
Unless you have a 20 second or longer gap in the music or between tracks it doesn't 'fall behind'. I never experienced it. I believe Krell had a patent on that clever design. But all of that aside, the KSA-50S was a very robust and sweet sounding amp that like many Krell (and BMW's) output significantly more power than shown on the manufacturer's spec sheet.But there are different ranges of Krell products. I also had the KAV-2250 for a short while. That was a powerful but awful sounding Krell as was the case with many of their "KAV" products including one of the preamps I had.
The 'baby Krell' KSA-50S which was their smallest "Sustained Plateau Bias" amp of that era was released in the early 1990's but I wouldn't consider buying any 30 year old amp today.
I personally wouldn't consider a true Class A amp for a few reasons including my listening habits. I want to power up the equipment and start listening immediately. Class A amps can take a good hour to "warm up" all that thermal mass to sound their best. The other option would be to leave the Class A amp ON 24/7. Not advisable with that constant huge AC power consumption and space heater heat.
Edits: 11/22/24 11/22/24 11/22/24 11/22/24
1. KSA200. heatsink too small, had field issue.
2. KSA250. greatly reduced bias. about 30W in class A.
3. KSA200S. variable bias.
Edits: 11/22/24
Krell KSA-200S Class A bias resides at 5 discrete steps, not continuously variable. Regardless, I'm pretty much done with huge power amps while I enjoy the efficiency and excellent sonics from modern Class D.Sustained Plateau Biasing
How is a modern, high-performance, "class-A" amplifier designed to have a low, "green," standby power consumption?Obviously, the class-A designation requires that the dynamic music envelope fits within a static, class-A current and voltage combination for a rated load impedance. Many "dynamic" or pseudo-class-A designs do not feature true class-A operation for their output stages. Generally, the "dynamic," "sliding-bias," and "Stasis" amplifier designs feature signal-controlled biasing of the output stages. The bias level is wound up according to the signal's current demand, with its minimum value carefully controlled to avoid classic class-B crossover distortion. Complex circuitry is common, with unwanted, signal-related modulation effects sometimes apparent in the output waveform.
In the case of the current Krells, several factors working in combination appear to redefine the topic of dynamic biasing. The key to the design was the use of an ultra-fast bias-control circuit. The technology was borrowed from the high-speed, current-feedback amplifiers used for I/V conversion in digital systems.
Using current-feedback technology, this bias circuit looks ahead at the music envelope, and predicts and then sets the output stage's equivalent class-A bias levels before that same music's leading edge has actually passed through the amplifier's circuit. When it finally arrives at the end of the relatively slower main amplifier chain, the required bias level has been anticipated; the output stage is therefore in a relatively stable, static condition of class-A bias current.
Krell has avoided the worst music-related modulation effects by learning from the psychoacoustic aspects of the design of compressor/limiters. Such limiters have a fast attack and a slow decay. Similarly, the slow decay of the Krell's bias level after a music transient has passed allows the whole decay envelope of the music to reach completion long before any bias shifts occur. A very generous delay of 20 to 30 seconds is provided before any downshifting of bias occurs.
Another factor reducing the potential audibility of such program-related "gear changing" is the concept of "plateau biasing" at one of five appropriate levels. This means that the bias level is not constantly dynamic, but varies with the program power envelope only over a few preset levels.
The bias in this Krell series of designs settles at one of the five levels for an extended period, and is not switched up or down unless the longer-term power demand needs it. This means that the bias level is not being switched very often, and then only for an instant of transition. Thus, signal-induced distortions, if present, can only occur for a tiny fraction of the time. The prolonged plateau bias levels do, in fact, imply a very high proportion of true, non-varying class-A operation.
Concerning the relative "speeds" of the power amplifier and the bias anticipator, Krell correctly reminds us that the maximum slew rate found in high-level recorded music is a modest 10V/µs. The "S"-amplifiers have an intrinsic slew rate well above that at 100V/µs, ensuring a generous safety margin. On the other hand, the anticipator circuit operates at a "speed" in excess of 1000V/µs, well in advance of the main amplifier.
Edits: 11/22/24
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
