it may help to think of it this way. A Duo does not have ALC, nor does it have a slow-start heater supply, nor a thermionic valve output stage. The traditional reasons for needing ALC in a transceiver no longer apply. Nor do the reasons which required back-coupling of a so-called ALC voltage from an external amplifier to a transceiver.
A digital transceiver, such as the Duo, which generates RF directly from a D/A converter at the output frequency can, in essence, run open-loop, because the gain of the PA stage(s) post D/A is relatively small, in dB terms, and relatively unvarying. Thus any needed control may be applied in the DSP, rather than the analogue, domain. A similar argument applies if the transceiver drives an external amplifier of modern, solid-state, design.
So why did we ever use ALC? It was a bodge which worked using '60's technology. It allowed for both a degree of speech compression and avoidance of overdriving PAs (or external amplifiers). But note that speech compression and avoidance of overdriving are incompatible given a common mechanism. How do we know that it was a bodge? Just try finding a specification for what a given ALC voltage means, in terms of gain reduction or power output or even IMD. It's not there, merely a phono socket labelled ALC, but more accurately identified by Neil earlier!
The 2018 ARRL handbook puts it more prosaically in section 14.1.2:
"…Since we have absolute control over the sig-73,
On 04/11/2019 12:30, Tony EI8JK wrote:
Am I right in thinking that it's not possible to access the Duo's ALC?