Hi to u all ,
a long time ago peter walker and mike albiston patented the current dumping circuit in which a powerful class b amp took over from a little linear class amp to provide the muscle drive needed by hungry lodspeakers through a wheatstone bridge.I would like to know if that topology has been used by other manufacturers and if so,what are its advantages and disadvantages compared to other conventional circuits?thanks.
a long time ago peter walker and mike albiston patented the current dumping circuit in which a powerful class b amp took over from a little linear class amp to provide the muscle drive needed by hungry lodspeakers through a wheatstone bridge.I would like to know if that topology has been used by other manufacturers and if so,what are its advantages and disadvantages compared to other conventional circuits?thanks.
One of the main disadvantages which have hindered widespread acceptance is that the bridge has to be carefully balanced to get the maximum distortion null.
The 405 had a bridge which was reasonably accurate, but that meant that the distortion performance was not much different from other good amps.
So from a practical point of view, you get an amp that is comparable to other good amps, so there is no real advantage other than for marketing.
Jan
Avoiding setting the output bias in the factory without having more distorsion than standard topologies was the Walker and Albinson's aim when they invented the current dumping scheme.
Technical advantages:
Getting rid of the issue of output bias current depending on temperature changes of the output transistors and of the resulting effect of programme dynamics on distortion.
Getting rid of factory trimming.
Other advantages for QUAD:
Marketing advantage: being able to claim that the output is controlled by a class A amplifier.
Lots of free publicity in audio magazines and the Journal of the Audio Engineering Society.
Even more free publicity by getting the Queen's Award for Technological Achievement.
Intellectual satisfaction of coming up with something else than the same old Lynn configuration everyone else was using.
In the 1980's a Japanese company, I think it was Technics, also made amplifiers that consisted of a combination of a low-powered class-A amplifier with a higher-powered amplifier, but they added voltages rather than currents and needed inconvenient floating supplies.
Getting rid of the issue of output bias current depending on temperature changes of the output transistors and of the resulting effect of programme dynamics on distortion.
Getting rid of factory trimming.
Other advantages for QUAD:
Marketing advantage: being able to claim that the output is controlled by a class A amplifier.
Lots of free publicity in audio magazines and the Journal of the Audio Engineering Society.
Even more free publicity by getting the Queen's Award for Technological Achievement.
Intellectual satisfaction of coming up with something else than the same old Lynn configuration everyone else was using.
In the 1980's a Japanese company, I think it was Technics, also made amplifiers that consisted of a combination of a low-powered class-A amplifier with a higher-powered amplifier, but they added voltages rather than currents and needed inconvenient floating supplies.
True enough. In the later versions of the 405 (405-2 and beyond) they used a single diode bias for the output stage to get it closer to some bias current, but still without factory adjustment. This improved matters a bit, especially high-order harmonics.
Jan
The main advantage was the absence of the Vbe multiplier and the associated potentiometer as well as the annoying idle current adjustment in the cold phase so as again after the warm up phase.
This is possible by the use of a small pure class-A single ended stage with approximately 50mA idle cuttent (very stable) together with a class-B power stage.
A better approach - at least from my view is the follow:
Yamaha's Hyperbolic Conversion Amplification (HCA) Circuit
and
http://www.diyaudio.com/forums/solid-state/142233-yamaha-hca-circuit.html#post5534305
How works actually the adjustment-free bias control from Linn's LK-2 ?
go to PDF file 8-10 at post #51 under
tweaking classic Linn LK1 & LK2 / LK280
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This is itself has been tried many times. Elektor for instance had several designs like this in the past.
The problem with the concept is that at crossover, the driving amp has to traverse the zero bias region very fast, from pos bias for the N output device through zero to neg bias for the P output device.
So you need to do this extremely fast to avoid crossover distortion and in practice there is always some xover distortion, and it is more than in a properly biased class B output stage where the handover from N output device to P output device is gradual due to the bias current.
To get more in detail: even if you go very fast from pos to neg, you go through a regimen where the output stage is completely off which means the feedback varies enormously in that small area, leading to wild changes in (closed) loop gain again leading to xover distortion.
The genius of Peter Walker was to use a balanced bridge between the driving amp and the output stage in such a way that when the output stage comes on, the feedback factor changes to keep the closed loop gain constant. So that part of the problem was taken care of.
The requirement for a very fast transition through crossover still remains an issue, and he realized that and helped it partly later on by using a single diode bias, so the output stage never turned hard off while still no need for adjustment.
The current dumping concept as developed by Peter is very ingenious and a lot has been written about it at the time. It took the community some time to really understand what was going on with it, much, much more than a simple low power class A driving a class B. The analysis by Vanderkooy and Lipshitz is a classic.
Jan
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The Quad circuit was a victim of new technology, higher speed BJTs and Mosfets became available in the 80s. These allowed 100W and above amplifiers using conventional circuits to have decent performance at moderate prices. Back when 2N3773 with 200kHz Ft was the standard, getting decent THD was hard.
It is a practical way to avoid output power transistor bias setting.
I'm having trouble making out the numbers on the schematic but R38 is the resistor between the transconductance amp and the speaker and I think it is 4.7 ohms, implying the transconductance amp supplies the first +/-100mA or so before the power transistors conduct at which point the transistors provide a massive, non-linear transconductance boost. Obviously, this causes gross open-loop non-linearity.
But not a problem just chuck some feedback around it...
I'm having trouble making out the numbers on the schematic but R38 is the resistor between the transconductance amp and the speaker and I think it is 4.7 ohms, implying the transconductance amp supplies the first +/-100mA or so before the power transistors conduct at which point the transistors provide a massive, non-linear transconductance boost. Obviously, this causes gross open-loop non-linearity.
But not a problem just chuck some feedback around it...
Attachments
Did you read the 1976 paper than Jan linked, The beauty of the bridge is that the distortion is not just being reduced by negative feedback, which is why the distortion was dominated by the low order non-linearities of the Class A section.But not a problem just chuck some feedback around it...
I've skimmed over the Vanderkooy and Lipshitz article. The bridge arrangement intends to eliminate Vbe of the power transistors from appearing at the op-amp input, which they found it did fairly well at least at low frequencies. I'm saying this is not the only source of distortion in the system by a long shot.
I don't know about you but I find the article quite turgid and I think it would be easier to analyze using a Spice model. Has anyone got one?
I don't know about you but I find the article quite turgid and I think it would be easier to analyze using a Spice model. Has anyone got one?
True. Also, there are generally a multitude of factors that determine whether a specific design 'makes it' in the marketplace or not, most having nothing to do with the technical merit.
Jan
Right. Quad has been around for decades. I met Peter Walker in the early 1980s. He was demo'ing his electrostatic speakers.
Before "current dumping" Quad was famous for its valve amplifiers.
The "current dumping" idea is a great marketing hook. It sounds clever and sexy. A good story has always been used to market audio products. The reality is that it makes manufacturing a lot easier and perhaps field reliability (I'm not sure) but didn't break any new sound quality ground. It was easily exceeded by valve amps at the time and by conventionally biased BJT amplifiers, such as Naim's.
Peter Walker was a great businessman and won Queen's awards for it.
Before "current dumping" Quad was famous for its valve amplifiers.
The "current dumping" idea is a great marketing hook. It sounds clever and sexy. A good story has always been used to market audio products. The reality is that it makes manufacturing a lot easier and perhaps field reliability (I'm not sure) but didn't break any new sound quality ground. It was easily exceeded by valve amps at the time and by conventionally biased BJT amplifiers, such as Naim's.
Peter Walker was a great businessman and won Queen's awards for it.
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The cuning of the dumping circuit is often looked as how the output impedance stays constant, through the feedback network, between its two states : when only the class A stage is delivering signal and when the power transistors conduct to assist it.
Inspired by it, with Mosfets at the output,
the Quasar amplifier (2008) : QUASAR a QUAD405 reborn design
the Renardson's MJR9 : MJR9
Inspired by it, with Mosfets at the output,
the Quasar amplifier (2008) : QUASAR a QUAD405 reborn design
the Renardson's MJR9 : MJR9