[Heathkit] Broadcast quality audio???

[Robert & Linda McGraw K4TAX]

One of the neat things that Heath did with the TX-1 was to provide 
non-symmetrical diode clipping so as to increase positive modulation 
percentage in excess of 100% without incurring negative modulation exceeding 
100%.  The result of any diode clipping action is the production of lots of 
harmonics related to the fundamental signal thus the inclusion of the band 
pass filter after the clipper.  Also, realizing the need for audio punch, 
the low end was eliminated via the use of small interstage coupling 
capacitors and the small value of the cathode bypass cap on the audio 
driver.  All of this was done for the benefit of busting DX pile-ups and 
cutting through QRM and making for a "loud" AM signal.

Well today, there's few if any AM DX pileups around and as to cutting 
through QRM there are much better methods of processing that allow for 
higher positive modulation peaks with out the concerns of exceeding 100% 
negative modulation and the resultant band spatter generated.  External or 
outboard dynamic processing, band width limiting to remain legal, and the 
ability to concentrate the modulation energy in an effective spectrum so as 
not to soak up excessive modulator power is quite easy with some of the new 
solid state microprocessor controlled devices.  Yet all of this is done 
without concerns for splatter and adjacent interference.  With these devices 
the average modulation can be raised far more significantly and efficiently 
than with any diode clipper and filter network.

My efforts were to conclude that the TX-1 in it's basic form, its driver 
transformer and modulation transformer and associated choice of tubes and 
power supply can produce a very nice smooth frequency response, provide very 
low modulated distortion, maintain a good signal to noise ratio with just a 
few basic changes of components.  Major circuit changes and modifications 
are not required.

As to external processing, well that is up to the user as the how, what, why 
and how much.  From my viewpoint, what I put into the transmitter I want to 
come out of the transmitter.  That's what I call "broadcast quality".

73
Bob, K4TAX