Anybody own, or is thinking about or working on a Quad amp?
The unique aspect of this amplifier is that its output stages run
pure class B. That is, they're not forward biased at all.
This is a British made amp. first one probably made in the 60's.
It's specs are about as good as the best audio power amps.
Another amp, the Mattes, was I think probably made also in the sixties. Iit too
ran its output stages class B.
I'd love to get any info on this amp too.
Thanks Jessjh
The unique aspect of this amplifier is that its output stages run
pure class B. That is, they're not forward biased at all.
This is a British made amp. first one probably made in the 60's.
It's specs are about as good as the best audio power amps.
Another amp, the Mattes, was I think probably made also in the sixties. Iit too
ran its output stages class B.
I'd love to get any info on this amp too.
Thanks Jessjh
You can find the QUAD 405 schematic here;
http://www.audio-circuit.dk/Schematics/QUAD 405 Schematic.pdf
http://www.audio-circuit.dk/Schematics/QUAD 405 Schematic.pdf
Hi,
http://quad405.com/
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/QUAD/QUAD-405-Modification/QUAD-405-Mods.html
Samples from the wealth of Quad info on the web.
http://quad405.com/
http://www.dc-daylight.ltd.uk/Valve-Audio-Interest/QUAD/QUAD-405-Modification/QUAD-405-Mods.html
Samples from the wealth of Quad info on the web.
forr said:
Actually, the definition of Class C is that each device has a conduction angle of <180 degrees, whereas in Class B each device has a conduction angle of exactly 180 degrees. In that respect, because the output transistors are not perfect devices and need >0.6V to switch them on, their conduction angles must be <180 degrees, so the current dumping part of the 405 should strictly be called Class C. Nevertheless, it is commonly referred to as Class B even though it doesn't quite meet the strict definition.
And yes, the 405 includes a Class A smaller amplifier. There was a very lively discussion in the "Letters" column of Wireless World at the time as to how the entire circuit worked.
I think an explanation sometimes given about the Quad principle
does not give an easy picture to pick up. It says that the classe A stage "assists" or "replaces"the output stage in its non-conduction region. In fact, it is much easier to understand if you imagine that the main amplifier is the class A stage and the output stage only assists it above a demand for a certain quantity of current. Hence the name "current dumping".
Subjectivly, it is a well reviewed amplifier. But I have some difficulties in understanding the use of such a complex low power stage and its many compensations.
does not give an easy picture to pick up. It says that the classe A stage "assists" or "replaces"the output stage in its non-conduction region. In fact, it is much easier to understand if you imagine that the main amplifier is the class A stage and the output stage only assists it above a demand for a certain quantity of current. Hence the name "current dumping".
Subjectivly, it is a well reviewed amplifier. But I have some difficulties in understanding the use of such a complex low power stage and its many compensations.
Jessjh said:
It's "specs" are even better, because crossover distortions happen on higher power than in an ordinary amp, so they are less audible.
Also, BE junctions of output transistors are shunted by low resistance, that smoothens distortions and discharges non-linear capacitances.
I exploited the similar idea of stepped approximation of transfer function many times, it works very well.
Here is the example:
I've found that the main source of audible distortions in such topology is the driver stage that's why in my last amps I use a vacuum tube.
forr said:
Forr,
One main point to remember is that whenever the dumpers come on, it automagically decreases the gain of the class A stage so that the total amp gain does not change. This is done through the bridge type feedback scheme.
Why is this significant? Generally the gain of an output stage in an amp varies through the cycle. For instance, in a class AB output stage, during the 'A' part of the cycle the gain of the stage is higher (both transistors contributing) than in the 'B' part. This is called 'Gm doubling'. It is the feedback loop that irons out the differencesto a large extend so that the gain appears more or less constant in the cycle and therefore there is very little distortion which would be caused by the non-linear gain.
The beauty of the current dumper is that the varying gain of the output stage (the 'dumpers') is compensated by an equal gain change in the class A stage. So, in theory, the gain variation is zero and the amp as such is perfectly linear. This works assuming that the A-stage is linear, of course. The dumper concept essentially buys you class-A linearity with a class AB configuration.
Jan Didden
Re: Re: Re: British Quad amplifiers
They are more linear per given power gain when used in certain output voltage/current range, especially when you assume dynamic non-linearities, also their non-linearity is different and is close to non-linearity of our perceprions. We learnt from the birth how to compensate such distortions....
Any art is about fooling of human perceptions to form certain imaginations, right?
Audio art is not the exception.
janneman said:
They are more linear per given power gain when used in certain output voltage/current range, especially when you assume dynamic non-linearities, also their non-linearity is different and is close to non-linearity of our perceprions. We learnt from the birth how to compensate such distortions....
Any art is about fooling of human perceptions to form certain imaginations, right?
Audio art is not the exception.
Hi Janneman,
Thanks for your complementary explanations which make me remember the subtle role of the bridge.
I think current dumping has an other overlooked feature which might be, in its inventors's mind, even more important than class A performances with class B dissipation : the lack of any preset to adjust in the factory.
Thanks for your complementary explanations which make me remember the subtle role of the bridge.
I think current dumping has an other overlooked feature which might be, in its inventors's mind, even more important than class A performances with class B dissipation : the lack of any preset to adjust in the factory.
forr said:
Class A may be well designed with no need to adjust any preset. The same way, 2 resistors in feedback loopss in my topology (see the pic, in series with capacitors) are also adjustable for better linearity.
Even the traditional amp with abused complementary emitter followers may be designed without the need to adjust in the factory each and every amp...
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