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Old March 4th 19, 06:11 PM posted to
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Default More than 30W per chanel Class A transistor amps

On Monday, March 4, 2019 at 11:32:18 AM UTC-5, Howard Stone wrote:
> On Monday, 4 March 2019 16:06:40 UTC, Peter Wieck wrote:
> > On Monday, March 4, 2019 at 9:59:57 AM UTC-5, Howard Stone wrote:
> >
> > > Aha, this is an argument for active pre-amps. Passive pre-amps presumably all have no gain.

> >
> > Correct. Passive pre-amps are no more than switches.
> >
> > Peter Wieck
> > Melrose Park.

> So I guess that, given that my project is about exploring Class A
> with my speakers, that IF I buy a pre-amp, it too needs to be Class A.

The chances you're going to come across a preamp whose active
'stages are NOT biased class-A is pretty small. I can think of none
of the top of my head. (Some earlier IC-based preamp may have used
op-amps that had a push-pull output stage that may have run class
AB, but, again, I can think of none).

> And I suppose I find out whether there are any Class A DACs,
> because I think their analogue end must involve some gain.

Same goes here as well: you're not likely to find a DAC output
stage that is NOT class-A biased.

NOTE: A "DAC" really consists of two fundamental parts: the
digital-to-analog stage, which, these days, will also
include the bulk of the anti-imaging filtering, done
in the digital domain: this part feeds an analog signal
to the line driver stage, which, again, is very unlikely
to be anything but class-A.

Now, the issue of whether an audio power amp is biased class-A
vs class-AB or even class-B (and, yes, even various switch-mode
implementations) is a design issue forced on the designer byu
considerations of efficiency and thus weight (less efficiency
means more copper and iron in the power supply) heat dissipation
(larger heat sinks, bigger chassis area for heat sinks), cost
and therefore success in the chosen market segment. When you're
talking about even, say, a modest output power of 50 watts/channel
continuous, there's a very large difference in whether you
implement it as a class-A or class AB, and even withing those,
a large range of demands on power, heat and the subsequent costs.

However, in a preamp, where the total power output requirements
are orders of magnitude less, these considerations become far
less of a design driver and thus do not force decisions between
biasing classes that are either engineering or economically

Putting it more simply, it's MUCH easier to design, manufacture
and support class A amplifiers dealing with low(er) signal levels
and substantially lower powers than it is to to the same where you
need LOTS of power.

Case in point: consider a sophisticated solid state preamp with,
maybe, 4 gain stages (RIAA, gain/balance, tone and output), using
the output stage as one instance, say it's rated at 2VRMS into
10 kOhm load: that's, lessee 0.02 uW (or, to make it even more
impressive sounding: 20 billionths of a watt). And let's say each
stage has roughly the same power output requirement. The difference
in heat dissipation and power supply requirements from running all
those stages at class A vs all of them at some class-AB is probably
on the order of the amount of power required to run the front panel

Plus the fact that, in the context of audio amplifiers, the choices
over whether to run things class-A vs class-AB is ONLY applicable
to push-pull topologies. To flip this around, if you wanted to have
the choice of running things class-A vs class-AB in a preamp, you
would have had, first, to make the design choice that you're designing
a push-pull preamp from input to output.

Don't spend a lot of time worrying about whither the preamp you
might buy is class A or class AB: because you'd be spending a HUGE
amount of time searching for the class-AB preamp that you don't want
to buy.

| Dick Pierce |
| Professional Audio Development |
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