Amplifier (fan) cooling circuit mod - diyAudio
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Old 25th March 2016, 11:45 PM   #1
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Default Amplifier (fan) cooling circuit mod

I'm trying to figure out how to slow down the fan of an old PA amp (not just an amp, otherwise I wouldn't bother )
to make it more suitable for home use.
The easiest way would be replacing the fans but in this case we are talking about two 24VDC blowers from Rotron with cooling designed strictly around them (not enough room to use a Papst or even an axial fan).

The amplifier has temperature sensors and a LED panel showing the current temp between 40 and 80 degrees C but it never reaches 40*C.
Even at idle the fans are unbearable for home use, there is a slow/normal setting but it doesn't do anything with idle speed.

I'd like to modify the circuit to lower the base speed without butchering the PCB and if possible, make the mod reversible.

The schematic showing 1 channel's cooling, temp. sensors are not visible on this pic although probably not relevant since we are talking about the lowest speed setting.
Both channels are fully separated and have their own power supply.

Click the image to open in full size.

Last edited by czolgista778; 26th March 2016 at 12:03 AM.
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Old 26th March 2016, 12:01 AM   #2
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You would have to do some trial and error on resistors R836 and R837. Try replacing them with 120 ohm each as a starting point. 7 watt as original will be more than enough.

It would be worth checking if the fan is running full speed all the time first. Maybe due to a fault. Measure the voltage across it.
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Last edited by richie00boy; 26th March 2016 at 12:05 AM.
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Old 26th March 2016, 12:20 AM   #3
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Thanks for the reply, I was thinking about doubling the value on those 7W resistors too.

I already replaced all LM339's that control the slow speed and some other functions but that didn't make a difference. The fan(s) do slow down a little bit after startup.
Shame that this amplifier is such a PITA to work on - no access to measure anything when it is running.

Last edited by czolgista778; 26th March 2016 at 12:25 AM.
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Old 26th March 2016, 12:28 AM   #4
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You could also replaced that complex circuit with a dedicated speed fan controller with NTC as sensng device, the IC is the MIC502. See here for an exemple although the IC doesn't work at 24Vdc

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Old 26th March 2016, 12:34 AM   #5
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Hi Eric,

I also considered putting an external PWM module between the fan output and the fan itself.
The innards of that thing are so cramped that it would be difficult to fit another PCB in it.

I will keep the MIC502 idea in mind, though.

Last edited by czolgista778; 26th March 2016 at 12:38 AM.
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Old 27th March 2016, 07:01 PM   #6
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I soldered two test leads from the fan output and with the fan connected the voltage is 17V and it steadily rises to 20.3V as the amp warms up.

Doesn't sound correct to me as the amp stays barely warm all the time.

Last edited by czolgista778; 27th March 2016 at 07:04 PM.
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Old 27th March 2016, 08:12 PM   #7
JMFahey is offline JMFahey  Argentina
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Fender Bass amplifiers use a very simple and reliable fully mechanical system.
Will cut and paste schematic here when I find it, but in general the idea is:

1) use a heatsink attached NO (Normal Open) bimetallic thermal switch, rated around 50C , in series with fan , so it does not turn a bit under 50C
Instant home peace.

2) fan has a series power resistor connected, so even when activated by first thermal switch, it runs at 1/2 to 1/4 speed, enough to move air around but not enough to make audible sound unless you apply your ear to it.

3) said low speed resistor has a second thermal switch, this one again NO, and rated, say, 65 or 70C .

Works like a charm and outperforms way more complex systems, at the same time being way more reliable.
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Old 28th March 2016, 03:15 AM   #8
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Be careful running fan slow otherwise it might stall and your amp will cook.

I have always used low noise fans in my amp designs.
You could use two 12 volt fans with a high power 12v zener in series.
Or even run the two fans in series.
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Old 28th March 2016, 04:00 AM   #9
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Default Fan speed controlled with NTC on heatsink.

Here is a schematics that works great and only takes a few components, the motor will never stall. Have a look.

Attached Files
File Type: pdf Fan temp control.pdf (77.2 KB, 48 views)
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Old 28th March 2016, 09:16 PM   #10
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After a more thorough inspection i decided to stick with the original circuit.
There is no room for any different circuit - everything is made perfectly to size in this amp.
(Yet big thanks for posting the schematics, I will surely keep those in mind for future).

The amp won't cook as it has multiple temperature sensors and a thermometer on the front panel. At around 80*C on the O/P devices it automatically shuts down.

This is what I mean with no room for mods:

Click the image to open in full size.
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