Quote:
Originally Posted by Synchrodyne
I am not sure quite what the effect would have been from including the tone controls within the main feedback loop, but perhaps the initial slope of the curves would have been reduced, with marked steepening at the extremities.
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Good grief!
There are several ways of looking at it
The overall feedback would act to reduce the slopes created by the tone controls, but the reduction factor would depend on the amount of gain reduction the feedback loop was giving... (open loop gain)/(closed loop gain) which depends on the setting of the volume control.
With the volume control at the loud end there will be a scale model of the Baxandall curves available through use of the tone controls. As the volume control is reduced, so the Baxandall curves will be progressively scaled-down further.
This is weird, because the human ear becomes progressively less flat at lower sound pressure levels - the Fletcher-Munson curves - and the listener is most likely to want to add some bass and treble boost at low levels.
It looks like someone at Armstrong had invented the unloudness control
1953 is about 14 years too early to ask what on earth were they smoking. It looks like someone who didn't really understand what they were doing had tried to amalgamate two circuits they'd come across.
Another way of looking at the thing is that the operation of the tone controls will affect the stability margin of the outer loop. Treble boost looking particularly risky at first sight. But treble boost in the tone control produces a phase lead as it produces a rising response. These two effects tend to compensate each other's effects on the outer loop's stability margin.
With the number of active stages enclosed in the outer loop, and no doubt an economy model output transformer, you might expect them not to be using agressive amounts of feedback, but the volume control needs plenty at the low end if it is to have much range.
'Orrible! No wonder they dropped it.
David