Hi Paul, Gary, Marv, Robin, Kim, and all,
> In a couple different amplifiers, I have gone to the hardware and
> purchased some rubber washers. I place these on the mounting screws
> of the blower giving the assembly a little cushioning instead of
> direct metal-to-metal contact. It calms down the vibration and some
> of the noise.
For a long time I have wanted to do that. But it would mean removing the
blower and installing it outside the cabinet, because in this amplifier
the blower is mounted through the chassis, with the motor above and the
air cage below the chassis. There is no room left between the blower and
other parts to allow any soft mounting. And I would like to keep this
amplifier as one compact unit, not having a hose, cable and blower
dragging behind!
But yes, I agree that this vibration dampening mount should help a lot,
because it seems that a lot of the noise is being radiated by the
chassis and the covers.
Gary,
> A couple of things you might try to help: A filter of open cell foam
> sheet placed over the inlet of the blower will reduce some of the
> noise, maybe a lot of it.
I fear it would reduce the airflow more than the noise!
> If there is a wall behind the amplifier you can place a sheet of
> thick foam closed cell material on the wall behind the area where the
> blower inlet is. This will keep some of the noise from reflecting off
> the wall.
I will try that. The blower sucks in the air from below, through an
opening in the bottom cover. The amplifier stands on tall feet, so I can
place a sound absorbing mat there. It might absorb some of the higher
frequency noise, at least.
> You can place a temperature switch above the tubes where the air
> exits to switch a capacitor or resistor in series with the blower
> motor to run it slow until the heat rises then the switch shorts out
> the capacitor and runs the blower full speed.
I will try if this induction motor really can be speed-controlled that
way. I thought it would either run at near-synchronous speed, or stop!
But rather than sensing the temperature of the air, I prefer the simpler
approach of running the blower at full speed during TX, and switching to
low speed 20 or 30 seconds after TX ends. That avoids having to place a
sensor close to the tube plates, in that crammed space.
> If you have a smaller high speed blower then changing to a larger
> slower speed blower may be quieter and still give the same required
> amount of air and back pressure.
This thing has an induction motor, and I have 50 Hz in the power line,
so it probably runs at 2800 rpm or so. Actually it SHOULD run from 60
Hz, and I have to derate the tubes a bit for the reduced airflow at 50 Hz!
A larger blower would have to be fitted outside. Maybe I do that. At
home I could use an external, slow and low noise blower, and leave the
other installed with a switch. When operating way from home, I can take
the amp along without dragging anything, and accept the higher noise...
> Another thing to try, place a large hose on the inlet of the blower
> and run it down below the desk. Place a couple of bends (curves) in
> the hose. That will reduce the noise.
I'm not sure it would help a lot in my case. The largest noise sounds
like mechanical scraping, and singing of the blades, and seems to be
radiated by the whole structure.
Marv:
> First, have the armature and impeller dynamically balanced,
> preferably together.
I'm not aware of any workshop that could do this for me. The only
balancing places I know are those that balance car wheels, and they
surely can't work on this small thing! Also, I assume it should have
been balanced at the factory. But your comment gives me an idea: I will
take apart the blower and carefully clean it. It might have dirt
accumulation causing imbalance.
> Isolate the fan mounting on some rubber gasketing and be sure to
> ground the enclosure afterwards for RF. Isolate the mounting feet of
> the amplifier enclosure with rubber.
That would be nice, but as stated above, there is no room to do this!
Kim:
> I think the answer to your first question depends on whether or not
> the noise is truly noise only from the movement of air or if it's a
> combination of vibration/AC noise and air noise.
It's a combination of mechanical and air flow noise, but the most
disturbing noise seems to come from vibration of the blower blades. It's
a high, mechanic ringing. If I knock at the blades with a screwdriver, I
get the same sound, that's how I know it's coming from there. There is
also a metallic scraping noise, but the bearings are fine, and
re-lubricating them didn't significantly change this noise.
The air flow noise also is composed of several parts. One is the normal
"shh" flow noise, about which I probably can't do anything. But there is
also another variable sound, denoting turbulent air flow. It has a
random pattern, and is much more disturbing than the uniform flow noise.
> The AC motor in the fan was simply noisy: the blades seemed to
> vibrate, the fan made parts of the case vibrate, and even the
> bearings seemed to be a bit noisy.So, I replaced it with a DC muffin
> fan with a higher CFM rating,
At my job I have replaced several AC muffin fans by DC ones, for the
same reason. The torque-mode vibration of some small single-phase
induction motors is very high. The DC motors fare a lot better, thanks
to their multi-pole construction and electronic commutation.
But in my amplifier, 50 Hz hum and its harmonics is apparently not a
significant part of the problem.
> I had a few problems related to how these work: they run with a DC
> input, but they have internal inverters that generate AC at a few
> kHz.
They use Hall effect switches to switch on and off the coil groups in
the stator. The rotor is a multipole permanent magnet.
> I found that RF would stop this inverter, so I had to RF shield the
> fan.
That's interesting!
> Having doen so, it is lots quieter that it used to be: all I hear now
> is noise due to the movement of air.
I should try that. Maybe my blower blades are getting exited by motor
vibration. But it sounds more like they are excited just by the passing
air.
> Nevertheless, first you must identify the noise source. I'm guessing
> that you have some kind of squirrel cage blower, since you need a
> reasonbale pressure.
Yes. Centrifugal type, axial inlet, tangential outlet.
> Are the bearings noisy?
They are friction bearings, not ball bearings, so actually they should
be less noise than ball bearings! Re-oiling them didn't help, so I
wonder whether my scraping noise is actually any scraping, or is just
another resonance mode of the blades! The rotor turns quite freely, in
any case.
> Does the blower fan itself vibrate or sing? Is there some kind of
> vibrational resonance?
Most definitely yes to both.
> AC noise/hum?
No, or at least not at an objectionable level.
> Or it is it simply the sound of lots of air going through small
> openings?
That too, but the other components are stronger and more disturbing.
> As for your second question: it depends on the tube: some can be run
> without cooling and some cannot. In any event, I don't think you'd
> want to stop the blower as soon as you stop transmitting, since there
> is some delay in heat transfer due to thermal diffusion. Even though
> you've stopped, the tube exterrnal and seal temperature is likely to
> continue to rise without cooling airflow.
OK. So the best course of action in this regard would be to switch the
blower to a lower speed, not completely off, with a delay of perhaps 20
or 30 seconds, after each transmission. I will see if I can convince
that induction motor to run slowly but reliably with a capacitor in series!
Well, thanks to all who gave ideas! I have a lot to try now. Maybe the
last resort will be eliminating the blower and immersing the whole
amplifier in oil! :-)
Manfred.
----------------------------
Visit my hobby website!
http://ludens.cl
----------------------------
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|