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Old 14th August 2002, 11:51 AM   #21
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Dear Mr. Pass

The enclosed schematic is a simplified example of how a BCA output stage might look like. At least how I understand it. Otherwise I might just have made a cool invention !

If the input signal is zero then both "switches" are on at the same time and off at the same time with a duty cycle of 50%.
If the input voltage rises then the upper switches on-time increases, the lower switches on time decreases according to the input voltage. For negative signals it is vice-versa.

As you can see, the topology is single-ended, i.e. the load is referenced to ground (O.K. one can also reference one leg of a bridge to ground but I assume you know what I want to say).

The three main advantages of this toplology are:

1.) No need to suppress shoot-through current by dead-time and the like (nothing to adjust !! )

2.) No problems with reverse diode turn-off times.

3.) Good supression (theoreticall a 100% cancellation) of the switching residual for low signal levels.

Having done quite a lot of thinking regarding switching amplifiers I must enviously admit that it is one of the coolest ideas anybody had, regarding switching amplifiers, so far.

Regards

Charles
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Old 14th August 2002, 09:53 PM   #22
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Default Nelson Pass Is Right...

....nothing impressive about class-d (pwm), amplification...at least with respect to hi-fidelity reproduction.

The singular failing of the pwm approach is the fact that the mandatory output RF filter permits a flat frequency response into one...and only one design load....which is probably why a class D device may produce low distortion, (~500ppm thd+noise, ref 1Khz, 80khz bandwidth, smpte ), into a dummy laboratory load, and deliver an absolutely horrendous performance when terminated with a loudspeaker system, whose impedance characteristics vary with frequency.....

This is the single greatest limitation of the new breed of so-called digital power processing amps. by grundig, texas instruments...etc....

Do not believe the hype.....linear power processing, (aka good old fashioned analogue amps), still remains the only way to produce true hi-fidelity....for the moment....i do'nt myself see the status quo changing in the immediate future...
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Old 14th August 2002, 11:04 PM   #23
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.....worse still, for anything approaching passable hi-fi, the switching frequency would have to be of the order of 100 times the highest nominal audible audio frequency....which means a 2MHz oscillator is mandated....this requirement can be watered down (20Khz switching freq.), if the design load is to be a single subwoofer driver...... operating at say 2 Kilowatts+....., and a 200hZ low-pass cut-off freq...
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Old 15th August 2002, 03:05 AM   #24
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Charles, you've got the basic output stage concept correct as well as the key features of BCA. Somewhere on one of my mag optical disks I've got some papers Gerald sent me back in 1999/2000 when he was evaluating our CoolMOS transistors for the switching amp (these are charge compensation high voltage MOSFETs designed for much lower Ron and faster switching speed than standard vertical MOSFETS- Ron is about 1/6 of a standard device of the same chip area).

This is pretty humorous... from the reaction to my post above, it seems I'm perceived as an advocate of Class D amps...


My only point in posting was to explain why Gerald Stanley's invention has some sigificant advantages compared with a conventional half bridge or full bridge PWM switching amplifier. These advantages extend to robustness and freedom from the most common failure mode plaguing these designs, as well as extensibility which makes some incremental improvements in fidelity possible.

On the whole, for someone looking for a sound reinforcement amplifier, or perhaps a high power sub amp, the K series are decent well engineered products, for their intended market and applications.

Techron was producing amplifiers for the MRI market using this concept, with some significant success, as I understand.

Though I've had an on again, off again fascination with the class D amps since the seventies, and had a few papers published on the topic, including an AES paper, I'll readily admit you won't find one in my audio system...

my own tinkerings are non-loop feedback designs, and my current favorite endeavor is rebuilding Aragon amplifiers into something more listenable. You can pick up used ones pretty inexpensively, and the chasis, heatsink, and power transformers are an acceptable starting point, without the hassle of doing your own metal working-


Best regards,

Jon
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Old 15th August 2002, 05:35 AM   #25
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Default Trouble with tribles

Anyone know how Linn gets decent sound out of amps with switching power supplies?

They do sound decent- right? right?
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Old 15th August 2002, 05:54 AM   #26
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Default Re: Trouble with tribles

Quote:
Originally posted by Variac
Anyone know how Linn gets decent sound out of amps with switching power supplies?

They do sound decent- right? right?
They do sound decent, but are kinda excessively priced. There are a couple companies getting good results with analog amps and switching supplies. Chord?

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Old 15th August 2002, 06:13 AM   #27
Variac is offline Variac  United States
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Ok, but how do they address the switched supply issue?
-usually a recipe for evil sound I've heard, yet they seem to get good sound?
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Old 15th August 2002, 06:22 AM   #28
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Quote:
Originally posted by Variac
Ok, but how do they address the switched supply issue?
-usually a recipe for evil sound I've heard, yet they seem to get good sound?
Linn starts by putting each half of the amp into its own very isolated solid aluminum well. And they must have spent 10 years & a lot of R&D dollars to get it to work. They aren't perfect, perhaps a little smoothed over sounding. Certainly not worth the asking price, but show that if you throw enuff time, money, and smarts at it you can get something decent.

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Old 15th August 2002, 07:14 AM   #29
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Hi All

Regarding switching PSUs and amplifiers there was already a discussion within another thread. I can only repeat that it IS possible to build a good sounding amplifier with a switching PSU but then you can't optimize your PSU for cost as it is done with your PC's and TV's PSU. They can even be built in a way that they send less garbage back into the mains than the best conventional PSUs (for DIY I would of course prefer the latter ones) !!
Just have a look at those CHORD amplifiers.

The lowpass filter issue is in fact one of the biggest problems within Class-D amplifiers.

But the statement that it has to be designed for a given load would be the least problem for me: I am into active speakers anyway
Apart from that, all the other amplifiers are also grateful if they can drive a resistive load (at least it doesn't hurt) !
The load related peaks and drops generated by the output filter are usually harmless compared to the response anomalities of some speakers (metal domes for instance).

The real culprit is the bandwidth LIMITATION and phase distortion which I don't like. There is of course a possibility to get around this by paralleling of a conventional and a switching amplifier but this is 1.) not an easy task and 2.) there is only about 85 % of efficiency left.

The class-D amp I developed back then was a lab power source and it's bandwidth was from DC to 45 kHz. It's THD was 0.44% and mainly k2. Carrier suppresion was 80dB. It could deliver up to 250 Watts.
We saw how THD could be improved but left it as it was because we already achieved the goal of THD < 1%. Apart from some faint treble noise it was quite pleasing to listen to it, although not intended for this purpose.

Because I have some demand I will once make another try to develop a better one but for domestic purposes I do also prefer to listen to conventional amplifiers.

But I am still convinced that the majority of applications could easily be covered by switching amplifiers because most people simply don't care about sound quality that much.

Regards

Charles
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Old 15th August 2002, 09:24 PM   #30
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Alright, I relent:

The Crown circuit is actually clever, but the promotional
bullsh*t is not.

Personally, I agree that one of these days switching amps
will rule the world, and old fogeys like me will be sitting
around pining for the good old analog days. If there's
anything left to sit on.

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