The AM Forum

Every so often I find another handbook .. this time it is a 1967 Bill Orr where he discusses Heising modulation and a way to make it more efficient by moving the bias toward cutoff between words ... only takes a few low power parts ... has enybody give dis a go?

I think it produces a controlled-carrier effect, which can be very annoying at the receiving end, with certain receiver agc characteristics.

thanks for answering, Don.  I believe controlled carrier would throttle down power outpoot between words... I don't think the bias shift Heising method affects carrier power but only the biasing of the modulator tube ... It would be very interesting to hear on-the-air to see if the attacks were distorted ... 73 ..John

John
Don

In its simplest form... the subjective effect (putting aside the efficiency savings) would be very much like a noise gate. If the background noise were almost imperceptible, then the listener at the other end might not notice it.

One could imagine a much more complex implementation in which modulating background noise (from a low power, efficient modulator) is injected during the periods when the main modulator bias is adjusted to turn off the main modulator... or, perhaps, injected all the time, to mask the effect of the varying background noise from the main modulator. That would be a lot a complexity, but it would make for some interesting experiments.

Stu

yes Stu ... or if using a compressor to use the sidebar signal to shift the bias in the modulator ... could be pretty slick ... morebeanz

I think there is a fellow here in town who did something like this, he is using a single 833 as a modulator, and it is Heising configured. In order to keep the plate power down, it goes into and out of class A as he talks. I gave him some kW modulation iron so I know he was using a reactor or autotransformer.

No, the carrier doesn't change., the bias to the Modulator tube changes. The average plate voltage on the RF stages says the same as with standard heising modulation. At low audio levels the current thru the modulator tube is low, but as audio level increases the bias to the modulator is reduced to increase the current thru the modulator tube so the voltage across the tube can swing further.
The bias voltage is changed at a sylabalic(?) rate, the idea being that most of the time the modulator tube current can be low, thus reducing dissapation.

The only thing a listener might hear is that as the bias on the mod tube changes, there will be some low frequency  'thump' as the bias goes up and down. However this will probably be well below the audio response of most receivers.
                                                                                                     Ian VK3KRI   

Ian

Thanks... now I understand the approach:

a. Class A modulator in true Heising configuration
b. Adjust resting plate current slowly by changing the bias... mostly to reduce dissipation/power consumption during silent intervals and, possibly, between syllables.


The rate of bias change must be slow enough to produce only sub-audible frequencies... but fast enough to accommodate voice peaks... which seems to me to imply that saving power during silent intervals will be a lot easier than saving power between syllables... unless you are willing to tolerate a noticeable degradation in the audio quality. The challenge (and the trade off) is somewhat (but not exactly) like designing an AGC circuit in a receiver.

A version of this, employing modern technology, might store (delay) the audio in a buffer for a few milliseconds... so that the bias can be adjusted in anticipation of audio peaks.

It would seem that this would be pretty easy to try out with something like a Globe Scout.

Stu

I vaguely recall a similar scheme for rf linear amplifiers, I believe in late 50's or early 60's CQ magazine.  The idea is to keep the idling plate dissipation down between syllables.