Reducing fan noise on a pro amp

[whell]

My Carver PM - 900 will now be permanently retired for home duty. Since it is fan cooled, I thinking I can install a resistor on the red power lead to the fan, and reduce fan speed and noise. What value of resistor do you think would work for this?

Thanks!

 

[avionic]

Or find a quieter fan..Also it is amazing what cleaning and lubricating the original fan will accomplish.

 

[roggom]

Dont know the voltage of the fan. Either way, it would be best to find a quiet alternative fan that is rated at the same voltage and cfm. If you are dead set on reducing the speed, then get a voltage regulator, (that is if this is a 12 or 24 volt dc fan)
This guy has an example for a cpu application http://fans.cuzuco.com/. Of course use caution with electricity it may be lethal blah blah blah.

 

[roggom]

Avionic you beat me to it again, I gotta get outside more. LOL

 

[avionic]

You can find some pretty damn quiet ball-bearing fans now a days.

 

[PakProtector]

hey-Hey!!!,
I had a PM900 and its fan was one loud SOB. After I got the rest of it fixed ( blown TIP36 driver transistor and most of the intermediate PS traces ), I put in a pair of RadioShack muffin fans( smallest 12VDC, to be rigged in series). I tweaked the dropping R in low power mode so they barely started, and left the high power to deliver full voltage. That amp runs the fan at V_1 until one of the thermal switches trips and IIRC shorts the dropping R and raising the voltage to V_2. The second thermal switch disables the amp( it is set a bit higher than the first ).

Anywhoooo, I made a small bracket to join the two fans together and then hung them both from that bracket right up against the cover. It has seen service in a multitude of theater installations since I traded it for a pair of consec. serial numbered MC75 mono amps half a decade ago.

The schematic/manual is available online if you look, if you can't find it email me and I'll send it to you. Don't just put in a resistor at the fan, as both low and high power will be dropped. IMO, you'd want just enough low noise airflow to keep it cool enough 98% of the time; it takes a lot of power output/heat generation to get it to engage high-power mode.
cheers,
Douglas

 

[whell]

Douglas -

I really don't have the technical skill to do as you've described. I was hoping a simple job like splicing in a resistor, which is about at the top end of my comfort zone, would get the job done.

Replacing the fan seems like a job in itself. The existing fan appears to have a resistor mounted directly on it, and then the fan draws power directly from the circuit board, which might suggest that it is integral to the operation of the amp as Douglas describes. Maybe I'll try the cleaning / lubricating route first.

 

[avionic]

If they are indeed 12 volt fans ..you can drop the voltage a bit. I use twin 12 volt computer fans to cool my MOSFET amps which I have dropped the voltage down to 8 or 9 volts.

 

[whell]

If they are indeed 12 volt fans ..you can drop the voltage a bit. I use twin 12 volt computer fans to cool my MOSFET amps which I have dropped the voltage down to 8 or 9 volts.

Would I use a resistor for this? What value would work?

 

[PakProtector]

If they are indeed 12 volt fans ..you can drop the voltage a bit. I use twin 12 volt computer fans to cool my MOSFET amps which I have dropped the voltage down to 8 or 9 volts.

The PM900's fan supply is more like 24V, perhaps 27. I can't find the file, and I can't find the folder with the hard copy either...dammit! It'll turn up soon enough.

Whell, There is a single resistor in the PC board to change in order to tune this. You can do it with the original fan and keep the functionality. IIRC you can get to it by pulling the covers off the bottom and top. The amp has a solid board, and a bit of solder wick and an Iron will free the original resistor.
cheers,
Douglas

PS: Dude! you're in Livonia, just grab that amp and come pay me a visit, I'd be happy to help.

 

[Loud & Clear]

I would have to have the voltage, and the current of the fan. Then we can use some ohm's law, and get a better R value. It would also tell us what size in wattage resistor you need. I didn't read all the posts. Is this a variable speed fan??? I've made circuits to make a continues fan variable w/heatsink temp. As the statement claims above this. There is a resistor in the circuit already. That could be changed, and slowed down a bit R=E/I. By the way, Gotta be talking DC here... :scratch2:

EDIT: If the fan were 12 volts, and a standard size. You could replace it with a 24 volt unit, and run half speed. Just a thought... Might have to change the resistor to double the current rating though. If it is currently a 1/4 watt resistor, replace it with a 1/2 watt piece or higher, of the same value R of course...

 

[PakProtector]

I would have to have the voltage, and the current of the fan. Then we can use some ohm's law, and get a better R value. It would also tell us what size in wattage resistor you need. I didn't read all the posts. Is this a variable speed fan??? I've made circuits to make a continues fan variable w/heatsink temp. As the statement claims above this. There is a resistor in the circuit already. That could be changed, and slowed down a bit R=E/I. By the way, Gotta be talking DC here... :scratch2:

EDIT: If the fan were 12 volts, and a standard size. You could replace it with a 24 volt unit, and run half speed. Just a thought... Might have to change the resistor to double the current rating though. If it is currently a 1/4 watt resistor, replace it with a 1/2 watt piece or higher, of the same value R of course...

hey-Hey!!!,
The fan circuit is indeed running on DC. IIRC it is half-wave rectified off the lowest voltage AC supply( in that amp there are three with a common CT). I don't remember all the details, but I spent quite a bit of time under that one's hood until I got it running properly... and then I went on to tubes...heh-heh-heh!!! That Mag-Field amp was quite a bit lighter than the valve amps.
cheers,
Douglas