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Discussion Starter #1 (Edited)
I have loved my QSC K10's since I bought them 4 months ago but sometimes- and I swear it's really ROOM temperature that causes them to come on more than anything else (long story that's largely out of my control)- the fans are simply too loud. I've tried a new "quieter" fan, but it just doesn't get me where I want to be.

Now I've jammed 105+db, bass heavy, music through these for 30 minutes and the fans haven't moved, and the amp plate has remained cool, so I'm really not concerned with ~ halving the rotational speed of these things. I have read many have disabled them completely and gone years with no issues. I'm not willing to go that far, so...

The fans are 12V and marked 2.10W, so I figure that's .175 amps, seeing 2.10/12 = .175.

SO then if the target voltage is 7 volts (to ensure they start)...

QSC fan voltage: 12V

QSC fan amperage: 0.175A

V=IR (Ohm's Law), so 12 = 0.175R => R = 12 / 0.175 = 68.571

QSC fan resistance: 68.571 Ohms

Target voltage: 7V

V=IR (Ohm's Law), so 7 = 68.571I => I = 7 / 68.571 = 0.102

Target amperage: 0.102A

Voltage drop across resistor: 12V - 7V = 5V

V=IR (Ohm's Law), so 5 = 0.102R => R = 5 / 0.102 = 48.98

Target resistance: 48.98 Ohms

Power dissipated by resistor: 5V * 0.102A = 0.51W


So then I'm assuming a 50OHM 1 watt resistor inserted into the pathway should sort me? My question is, if I can only find 2 watt resistors at 50 OHMS ( that I can have in a couple days, ;) ) I'm still OK, correct? That's (the wattage rating) just concerning heat dissipation?

Thanks for any help here, this is really not my strong suit!

James
 

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I have loved my QSC K10's since I bought them 4 months ago but sometimes- and I swear it's really ROOM temperature that causes them to come on more than anything else (long story that's largely out of my control)- the fans are simply too loud. I've tried a new "quieter" fan, but it just doesn't get me where I want to be.

Now I've jammed 105+db, bass heavy, music through these for 30 minutes and the fans haven't moved, and the amp plate has remained cool, so I'm really not concerned with ~ halving the rotational speed of these things. I have read many have disabled them completely and gone years with no issues. I'm not willing to go that far, so...

The fans are 12V and marked 2.10W, so I figure that's .175 amps, seeing 2.10/12 = .175.

SO then if the target voltage is 7 volts (to ensure they start)...

QSC fan voltage: 12V

QSC fan amperage: 0.175A

V=IR (Ohm's Law), so 12 = 0.175R => R = 12 / 0.175 = 68.571

QSC fan resistance: 68.571 Ohms

Target voltage: 7V

V=IR (Ohm's Law), so 7 = 68.571I => I = 7 / 68.571 = 0.102

Target amperage: 0.102A

Voltage drop across resistor: 12V - 7V = 5V

V=IR (Ohm's Law), so 5 = 0.102R => R = 5 / 0.102 = 48.98

Target resistance: 48.98 Ohms

Power dissipated by resistor: 5V * 0.102A = 0.51W


So then I'm assuming a 50OHM 1 watt resistor inserted into the pathway should sort me? My question is, if I can only find 2 watt resistors at 50 OHMS ( that I can have in a couple days, ;) ) I'm still OK, correct? That's (the wattage rating) just concerning heat dissipation?

Thanks for any help here, this is really not my strong suit!

James
Use a voltage divider, not just a resistor in series.
 

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Discussion Starter #3 (Edited)
^ Thanks, please forgive my ignorance here, but isn't a voltage divider simply a series of resistors (or at least, a simple one)?

If that's the case, what is gained by, say, running (2) consecutively, over one? I'm just try to make this as easy as possible especially since I'm strapped for room on this plate amp.

Thanks again
James
 

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Discussion Starter #5 (Edited)
^ I have yet to find a suitable option. I'm all ears if someone is aware of one: 12V 50 mm x 50 mm x 10 mm. Last one I used claimed half the RPMs and was noticeably louder.

thanks

James
 

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Discussion Starter #6

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calcs look right to my eye.


it seems the voltage divider allows precise control of the voltage that will go into the fan without knowing the actual resistance of the fan.


this is interesting:
http://www.acousticpc.com/acoustic_...le_speed_controller_with_molex_connector.html


not sure if it would work in your speakers though.


could also wire a switch to go around the series resistor if you ever intend to run them full out:


 

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Discussion Starter #9
^ Thanks LTD. Honestly, I'm growing tired of this issue already. I'm really kind of pi$$ed about ordering 7 of the last fans only to find they were worse, and I won't be getting all of that cash back. Now, I'm looking at $100 more for some "really quiet" fans?

I'd really like to try this resistor and see what I get. I don't have to monkey with removing the old fans (x7) replacing them (x7) and the stock ones actually seem pretty-well built- they just spin at 6,000 RPMs it appears.

Going to order the 2 watt 50 OHM resistors and see what's what.

thanks everyone
James
 

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Discussion Starter #10

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^ Thanks, please forgive my ignorance here, but isn't a voltage divider simply a series of resistors (or at least, a simple one)?

If that's the case, what is gained by, say, running (2) consecutively, over one? I'm just try to make this as easy as possible especially since I'm strapped for room on this plate amp.

Thanks again
James
Not necessarily. It's often an L-pad, with one resistor in series and another in parallel.

en.wikipedia.org/wiki/Voltage_divider

The reason I mentioned voltage divider is because I have seem this done for noisy fans.
 

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Discussion Starter #12
Not necessarily. It's often an L-pad, with one resistor in series and another in parallel.

en.wikipedia.org/wiki/Voltage_divider

The reason I mentioned voltage divider is because I have seem this done for noisy fans.
Thanks- I know it's not necessarily, I was just using two resistors in a series as an example.

Thanks for your help.

James
 

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Discussion Starter #14
I just found a spec sheet on the stock fan and found a "starting voltage" of 10.8 V's.



Boy I hope that's not really the case. I've heard that this spec is usually quite higher than what's actually needed.

James
 

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Discussion Starter #15
k10's pretty sweet?
They are freaking phenomenal as far as I'm concerned. The fan's a pisser but when I ran them in an "ordinary" room (read ~70 degrees) I couldn't get the fans to come on. But when I moved them back into my living room (which is prolly 80+) they come on frequently. I have a ridiculous HVAC situation right now that requires me to heat my living room up to a stupid temp (again, long, dumb story).

So I'm not even convinced that in a PROPER environment that they'll even be coming on often, but I've reached my breaking point for the current situ and I really don't have much fear dialing them back.

The dynamics, image and otherwise low-noise/quiet operation of the K-10's is really something.

I considered DIY'ing a bunch of SEOS' but when I found these 1000W powered units for under $700 (really about $650 for me), I simply couldn't pass them up.

James
 

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Yikes... Lots of confusion here.

The series current limiting resistor is the right idea, but may not be the right value. A voltage divider is a very bad idea and won't work.

The right way to size the resistor is to use an adjustable power supply and determine the voltage and current of the fan at the speed you want it to run. If you're set on 7V you need to determine the current draw of the fan at 7V. It isn't likely to be the same as the draw at 12V. Then you use 5V/i = resistor value.
 

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Yikes... Lots of confusion here.

The series current limiting resistor is the right idea, but may not be the right value. A voltage divider is a very bad idea and won't work.

The right way to size the resistor is to use an adjustable power supply and determine the voltage and current of the fan at the speed you want it to run. If you're set on 7V you need to determine the current draw of the fan at 7V. It isn't likely to be the same as the draw at 12V. Then you use 5V/i = resistor value.
This.

However, you might find it easier to modify the thermistor circuit.
 

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can one of you guys clarify that? I thought that is what he did?


12v fan @ .175 amps is spec
r=v/i 12v / .175 amps= resistance = 68.5 ohms
drop the voltage to 7v and the current drops to .10 amps.
r=v/i again 5/.10 = ~50ohms


?
 

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Well, the spec may not be right. The fan isn't a resistor. It's unlikely to have a constant resistance over voltage. Hence why you need to measure the actual current draw at the target voltage and use that to size the resistor.
 
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