[Amps] Solid State Amps (or rather the NCL-2000...)

[Manfred Mornhinweg]

Jim, Gary,

> I too use my Bose QC-15s in the shack for contesting. With 2 amplifiers 
> running, it really helps! 

About 15 years ago, when I was seriously into powered paraglider flying, I glued 
headphones to my helmet, and I built a homebrew noise cancelling system into 
them. I used a small electret mic in each ear cavity, as close to the ears as 
possible, and brew up several different kinds of loops to cancel out any noise. 
After all, having a screaming 2-stroke engine right behind my head was _really_ 
a problem!

My crude homebrew noise cancelling system could only be made to work at pretty 
low frequencies, though. Whenever I tried to extend it into the midrange, I got 
mighty oscillators on assorted frequencies, rather than any cancellation effect. 
There was simply too much delay in the headphone-mic path. And probably 
reflections too. Even so, it was somewhat useful, because the headphones were a 
closed design and rejected mid and high frequency noise reasonably well, while 
lower frequency noise got through. And my system at least reduced it somewhat.

The technology used was limited to LM386's, an LM324, and a bunch of passive 
parts. In addition to producing the proper anti-noise in teh low frequency 
range, the headphones carried the audio of my TH-28 radio. Those were the 
glorious days of XQ2FOD air mobile!

Now I'm too arthritic to keep flying.

But in the shack, I prefer not using headphones at all. I always feel confined, 
enclosed, limited, and "attached", when wearing phones. I much prefer a good 
speaker, and a quiet environment.

> There is a variable wirewound resistor in the circuit of my amplifier
> fan. What I'd really like to do is; lower the fan's speed to minimum,
> insert a thermal probe into the airstream from my amp's exhaust, have
> it trigger a relay when the exhaust temperature rises to 100 degrees
> & then raise the fan speed to maximum as it is currently set to. That
> would save the fan from early failure, keep it quiet most of the time
> and properly cool the tubes when it needs to.

The same could be done electronically. A simple thermistor can be used as 
sensor. A Schmitt trigger should be used to get clean high/low switching. 100°C 
seems fine to switch to high, and it might switch back to low when the air is 
coming out at no more than 60°C or so, to avoid constant speed changes. This 
would have to be tested. The power element can be a MOSFET if the fan has a DC 
motor, or either a TRIAC or two back-to-back MOSFETs if it's an AC fan. If you 
use a TRIAC to switch an AC motor, it normally needs a snubber across it to 
handle the inductive load. But with that resistor you describe, probably no 
additional snubber would be needed.

> I need to find a small unit that will fit into the amp to do this
> cleanly, haven't researched finding those components yet.

It would be a nice little project. But make sure you get enough air flow to cool 
the tube base even after the fins are cool.

With my amp I tried different blower speeds, but even at pretty low speed this 
particular blower makes lots of noise. It's a tingling, ringing noise, 
apparently caused by the blower fins resonating. In addition to that is the 
rushing noise of the air, complety with irregular turbulence noise. That air 
noise goes down at lower speed, but the ringing doesn't.

Manfred

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