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Old 26th June 2004, 03:32 AM   #1
mjarve is offline mjarve  United States
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Question Non-switching amp same as Class A?

I recently acquired a Pioneer SX-3900 receiver (circa 1980) and had a question about part of its design. The amplifier section is stated as being a Non-Switching amp, and the sales literature goes in to how standard class AB amps are more efficient, but are subject to crossover distortion and so on, but this receiver, with its non-switching amp produces only 0.005% THD at it's full rated power. The power rating is unimportant (it is rated at 120 WPC cont. RMS 20-20,000Hz), but if I understand the design correctly, this would be a class A amp.

Or is it? Is a non-switching amp by default a class A amp, or is there another design that does not use a switching scheme, but is still not class A. Another point that comes to mind is that class A amps, as a rule, are very inefficient, and produce high amounts of heat regardless of power level. The amp section of this receiver does get very hot, and has relatively large heat sinks for a 120 watt amp, but that again does not necessarily mean much.

Thanks for your input.
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Old 26th June 2004, 12:41 PM   #2
sreten is offline sreten  United Kingdom
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Non-switching does not mean it is class A, though class A
is non-switching within the current limits of class A operation.

Without the exact details of what is meant by non-switching
its hard to quantify what exactly is going on, or the amplifier
class, but they are usually regarded as being a combination
of classes, e.g. class A with B, or class A with C.

sreten.
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Old 26th June 2004, 07:43 PM   #3
Steven is offline Steven  Netherlands
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In a non-switching class AB amplifier none of the output transistors is switched off completely at any time.

In a class A amplifier normally a current increase in one output transistor goes together with an (almost) equal decrease of current in the other.
In a class AB amplifier normally an increase in current through one output transistor (with approx the bias current) makes the other transistor non-conducting.
In a non-switching cass AB amplifier an increase in current through one output transistor lowers the current in the other transistor in a non-linear way and such that it will not go to zero. The advantage is that this transistor will not be reversed biased (as will happen in normal class AB) and this makes switching on again much faster.

Steven
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