[Amps] Blower noise

Tue Aug 11 22:13:14 EDT 2015

Hello Manfred.  I dealt with fan noise a few years ago and got it down to an acceptable level.  Looking at what I think is your amplifier (National NCL-2000?) , the fan motor appears to be what I called a record player motor when I was a kid.  I remember them as being relatively low RPM, maybe 1500-1600 RPM, so I am surprised at the noise level being produced.  My Drake L-4B runs a 1550 RPM motor and is very very quiet.  
I do realize ceramic tubes like your 8122's need a bit more airflow than 3-500z's; hence, more noise.

My noise issue was with an ITT AM-6155 amplifier.  It runs a ceramic Amperex DX-393's which require quite a bit of air.  The fan motors in these beasts run at 400hz generated by an internal DC to AC power supply.  The motor running at 400hz creates noise at that frequency.  The motor speed is something like 3400 rpm, and the motor employs stainless steel instrument bearings-they too are noisy.  At that RPM, the fan blade noise equates to a high-pitched ringing sound.  Add the air noise, and the result is one noisy fan!!!
I replaced the s/s instrument bearings with standard sealed bearings.  That took a significant amount of noise away.  The next step was isolating the fan motor from the chassis using cutout pieces of an old mouse pad--major improvement there.  Playing with the 400hz power supply, the noise emanated by the motor  dropped significantly by *increasing* the fan rpm slightly.  The last thing was isolating the amplifier from the table using a plastic real estate sign--like plastic cardboard.  Locating the amp so the remaining sound was directed away helped as well.  
Obviously, most of these are not applicable to your fan, except mechanically isolating the motor from the chassis.  It would be well worth looking into.  Even if you end up putting the amp in a box or behind a wall as others have suggested, isolating that 50hz vibe will reduce your noise that much more.
A word of caution about using cheapee 12 volt fans in amplifiers......been there and done that.  I found that in the presence of RF, they can shut down.  Definitely not what you want under key.
My 2 cents worth :-)
Best 73,RussKW6T  

     From: Manfred Mornhinweg <manfred at ludens.cl>
 To: amps at contesting.com 
 Sent: Tuesday, August 11, 2015 9:13 AM
 Subject: [Amps] Blower noise

Does anyone have a good idea for reducing blower noise?

I end up rarely using my NCL-2000 amplifier, because the blower noise is so 
disturbing! I measured it now. The intensity is 52dBA, measured at 1 meter 
distance, which happens to be roughly the distance between the amp and my ears 
during operation. I also measured the spectrum of the noise: Averaged over 
several seconds, it's a clean pink noise caused by the airflow, plus strong 50Hz 
line frequency hum, plus a strong peak at 907Hz. What one hears is mostly this 
907Hz whine, which is unstable in amplitude. It's probably caused by air 
turbulence around the impeller blades, as the frequency seems to be the number 
of blades multiplied by the rotation speed.

I removed the blower from the amplifier. Running it outside, while holding it in 
my hand, the noise is quite acceptable. It's more wind noise than anything. The 
whine isn't present, nor is the 50Hz hum. The whine appears when I restrict the 
air flow with my hand over the outlet. At a certain level of restriction, the 
impeller slows down, and the whine appears. If I restrict the air flow further, 
  the whine disappears again.

For all I can see and hear and feel, the whine comes strictly from air 
turbulence, not from any motor vibration due to worn bearings, or anything like 
that. The whine has a somewhat ringing sound. It might be that the impeller has 
a bell resonance at that frequency.

The bearings feel smooth and good, the only play is in the axial sense.

The line-frequency hum becomes audible, and even loud, as soon as I touch the 
blower to the desk. It's obviously motor vibration that gets audible only when 
coupled to a radiator large enough for that frequency.

Since the blower fits tightly in the amp, I don't see how I could install any 
damping to keep the hum from coupling to the amp's structure. Anyway the most 
disturbing sound is that variable whine, and that seems caused by turbulence 
happening when the blower runs with just the amount of back pressure the tubes 
happen to put on it. I don't think I can change that, at least not without 
losing too much air flow!

Does anybody have any good idea? If not, I will buy a modern Chinese plastic 
blower on eBay. There are some in the right size, that can amply provide the 
needed airflow and pressure, and they cost 10 bucks, shipping included. They are 
12V-powered, so it would be necessary to add small power supply. It could go in 
the same are where the original AC motor goes, as the Chinese blowers have their 
motor built into the center area of the impeller.

The main problem is that these modern blowers are all mirror-views of the 
NCL-2000's blower! They have the outlet on the wrong side. That will require 
quite some hacking. And of course I have no idea whether these are free from the 
whining noise! Their noise spec is roughly the same as that of the original 
blower, but that doesn't tell much about actual behavior when installed, or 
about how disturbing a noise it produces.

The good thing with 12V DC blowers is that it's easy to slow them down. I would 
make a switch to run it at high speed during TX, and low speed during RX. With 
the stock blower I can't do that easily, because it has an induction motor that 
develops very little torque when running below its near-synchronous speed. So I 
can't make it run at a low, but sufficiently stable speed, by adding series 
resistance. It either runs at full speed, or it tends to nearly stop. And I 
don't feel like building a variable frequency drive circuit for it...

As you see, I'm in writing mood today. Sorry for that! :-)

Manfred
XQ6FOD

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