Idle current adjustment on amplifiers
Jørn-E. Årnes
Does anyone know if there are any "standard procedures" for where to measure the mVDC
when adjusting IDLE CURRENT on an amplifier?
I've got this monster of a class A amplifier with no less then 220000µF power stage (yes
220000) to provide this decent output of 41W stereo over 8 Ohms (300W/1 Ohm)! Oh well...
I now want to make some correct DC-balance and idle current adjustments before the top
cover assembly take place. DC-balance is OK, but as mentioned I'm not quite sure where
to put the probes for idle ajustment.
I _think_ it should be done placing the + measuring probe directly on the stage speaker
output + terminal and then the minus probe on the "last in line" emitter output pin. In
this case the milliVoltage (DC) over the emitter-resistors will be measured. Please
correct me if I'm wrong.
I've already got the Voltage specs. for this adjustment: 286mV.
Thanks in advance!
Greetings, Jørn
Nicholas Bodley
If it's really a class A amplifier, I suspect that there's no established
standard circuit or adjustment procedure; however, my knowledge of class-A
transistor amplifiers is quite limited, because extremely few transistor
amplifiers are class A. (however, see Douglas Self's article in Sept(?)
Electronics World (UK). I'd highly recommend that you do what you can to
find the proper procedure! Don't be surprised if the output stages have
current greater than an ampere with no signal.
Your questions seem to suggest that you're thinking of the more-common
type (class B, or something close; I don't honestly know the proper letter
(not C!)). These amplifiers run with their output transistors "idling"
with no signal; they conduct only a very modest current.
Unless I'm very confused, an audio class A amplifier runs with about half
of the max. permissible current in the output stages with no signal. Its
advantage is that the transistor(s) conduct all the time.
In most transistor audio amplifiers, half of the output transistors are
cut off for one signal polarity, and their opposites carry the signal.
When the signal changes polarity (we're talking milliseconds or less
here), both sides conduct briefly, then the other side takes over. It's
this changeover, or commutation, that in some people's opinion creates
more-or-less subtle degradation of the sound. However, the no-signal power
dissipation/consumption of such an amplifier is quite tolerable. A class A
audio amplifier, by comparison, with 200 W audio output would be scary--
huge, would keep at least one room warm in winter, and be costly to run
and even more costly to build.
Best regards,
|* Nicholas Bodley *|* Electronic Technician {*} Autodidact & Polymath
|* Waltham, Mass. *|* -----------------------------------------------
|* nbo...@tiac.net *|* When the year 2000 begins, we'll celebrate
|* Amateur musician *|* the 2000th anniversary of the year 0.
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