[Amps] Blower noise

[Jim W7RY]

I use Bose QC15s... And I LOVE them.... But this is the replacement for 
them..

https://www.bose.com/products/headphones/over_ear_headphones/quietcomfort-25-acoustic-noise-cancelling-headphones-apple-devices.html


73
Jim W7RY


-----Original Message----- 
From: Manfred Mornhinweg
Sent: Tuesday, August 11, 2015 9:13 AM
To: amps at contesting.com
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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