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I have a Peavey PV 2600 amplifier (1800W RMS) that I'm going to use in my home theater, but the 24V 80mm fan is much too loud and noisy. I want to replace it with a silent fan (Noctua NF-A8 PLX), which only come in 12V.

They Peavey outputs two voltages: 16V for normal operation, and 24V when it detects higher temperatures. There does not appear to be any in between or ramping, it's just one of the two discrete values.

I would like to keep this feature with the Noctua fan, such that at 16V the Noctua sees ~8V (pretty similar to running the Noctua at 12V with one of their 'low noise adapter' resistors), but when the amp puts out 24V the Noctua sees ~12V.

Because this will not be stuffed in a rack or driven very hard, cooling airflow isn't as critical as it would be in the amp's typical use case, and so the reduction in CFM won't be critical. However, even the Noctuas do make SOME noise at full speed (16.1 dBA), but less with their low noise adapters at ~8V (12.9 dBA), I'd only like to have the fan running at full speed when the amp needs it, not constantly.

  1. What would the implications of calculating a single resistor value be that would be perfect for the 24V full load? I could provide the Noctua 12V, measure the current and thus derive a 'nominal' resistance while running, and add this resistor in series to drop 12V off the 24V. So if it's 40 mA, then I'd add a 300 ohm 1/2 watt or more resistor. However, at 16V, the fan now becomes less of a load, so could it potentially receive a higher voltage than it should, or such a low voltage that it stops spinning?

Option 1

  1. Most suggestions on using a 12V fan in a 24V system are to use a voltage regulator, which if I did so I would lose the two speeds and just run at a constant full power, which is undesirable to me (at least with a fixed regulator).

Option 2

  1. Could I instead do a voltage divider circuit? If so, what would the nominal resistance values be to run an x mA constant / y mA startup fan, that has somewhat varying power requirements?

Option 3

  1. If none of the above work, are there other solutions perhaps? Is there a circuit where I could use a regulator to provide exactly 1/2 Vin?

  2. If there are no viable options without tampering with other circuits in the amplifier, are there brushless DC 24V fans that are guaranteed to be as quiet or quieter than Noctuas?

I don't think adding a second fan in the amp somewhere is too much of an option:

Peavey PV 2600 Amplifier

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closed as off-topic by Andy aka, Daniel Grillo, Dave Tweed Dec 1 '15 at 13:29

This question appears to be off-topic. The users who voted to close gave these specific reasons:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Dave Tweed
  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – Andy aka, Daniel Grillo
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ You could make a voltage-controlled regulator using LM317 or whatever and the 16/24V as the controlling voltage. \$\endgroup\$ – Fizz Nov 18 '15 at 9:56
  • \$\begingroup\$ Could you use two Noctua fans in series? I'm not sure about the implications of this. \$\endgroup\$ – Icy Nov 18 '15 at 10:02
  • \$\begingroup\$ I could, but I might have to take a ~2V hit because of the regulator (24Vin = 22Vout, 16Vin = 8Vout), or try to find something more rail-to-rail. \$\endgroup\$ – Ehryk Nov 18 '15 at 10:07
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    \$\begingroup\$ 555 timer, 50% duty, an L-C to produce a smooth DC... \$\endgroup\$ – JonRB Nov 18 '15 at 10:23
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    \$\begingroup\$ Why are you worried about 2V loss in regulator? You want 12V output not 24V \$\endgroup\$ – Icy Nov 18 '15 at 10:30
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You can use a current boosted op-amp design with a voltage divider on the input. something like:

schematic

simulate this circuit – Schematic created using CircuitLab

The op-amp is powered directly from the fan supply, and always biased at half the input voltage. The BJT acts as a current boost for the op-amp, and will of course drop the same power as the fan, so you will need a transistor capable of taking at least as much current as required by the fan, and probably need to mount it on a heat sink.

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  • \$\begingroup\$ How would I determine the need for a heat sink? Could you recommend some good starting components? Would there be all of the 24V available at the output, or would it be more like 23.3V? The fan only requires ~30-40mA of current (measured), with a 70mA 'Maximum' rating noctua.at/… \$\endgroup\$ – Ehryk Nov 18 '15 at 10:14
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    \$\begingroup\$ The output would be very nearly exactly half the input voltage 12V for 24V input, 8V for 16V input. The power dissipated by the transistor (worst case) would be 0.07 * 12 = 0.84W - Transistor data sheets have temp rise / W specifications. Parts are not very critical, try OP71 for the op-amp and TIP41 for the NPN transistor \$\endgroup\$ – Icy Nov 18 '15 at 10:23
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    \$\begingroup\$ You'll likely also want a snubber diode in parallel to the fan. That'll protect the transistor when it turns off. The fan is an inductive load so you should have the same safety measures as if you switch a relays. For additional fun add a 33µF to 47µF capacitor in parallel to R2, this will give you a ramp up/down feature for almost no additional cost. \$\endgroup\$ – Nils Pipenbrinck Nov 18 '15 at 14:52
  • \$\begingroup\$ And add some decoupling capacitors. \$\endgroup\$ – Icy Nov 18 '15 at 14:55
  • \$\begingroup\$ Does the last update to the schematic include the 'snubber' diode and decoupling capacitor (100nF)? If not, where do they go? \$\endgroup\$ – Ehryk Nov 18 '15 at 17:43

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