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Opinion on the Behringer KM1700 Class-H amp
Opinion on the Behringer KM1700 Class-H amp
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Old 31st October 2017, 04:32 AM   #21
CharlieLaub is offline CharlieLaub  United States
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Opinion on the Behringer KM1700 Class-H amp
I been running two 4R subwoofers lightly with the KM1700. So far, not bad, not bad at all. The fans are there but pretty unobtrusive (with the amp right up front and not hidden, etc.). I have two new 12V "quiet" fans on order to replace the two 24V stock fans. Hopefully I can simply connect the two new fans in place of one of the old. The current and voltage specs of the new and old fans seem to indicate that is totally possible. The airflow with the new fans will be about half of stock, but because the amp will be used in a home audio setting instead of for PA duty and will have an open airflow path my guess is that it will be fine. I might try some thermal measurements before and after the mod if I have the opportunity.
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Old 31st October 2017, 01:14 PM   #22
cliffforrest is offline cliffforrest  United Kingdom
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Opinion on the Behringer KM1700 Class-H amp
As an aside to Charlie, The DC fans as used in personal computers can be readily and reliably slowed down by simply adding series resistors.

I found in the past that 12v PC fans would start reliably with 7v applied, with almost zero subsequent fan noise. A 50r series resistor will get you there!
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Old 1st November 2017, 05:06 PM   #23
CharlieLaub is offline CharlieLaub  United States
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Opinion on the Behringer KM1700 Class-H amp
Quote:
Originally Posted by cliffforrest View Post
As an aside to Charlie, The DC fans as used in personal computers can be readily and reliably slowed down by simply adding series resistors.

I found in the past that 12v PC fans would start reliably with 7v applied, with almost zero subsequent fan noise. A 50r series resistor will get you there!
+1 on this approach. The series resistance you want to use will depend on the fan's resistance so this is by no means "universal". Instead, if you can get some data on the fan you can calculate the series resistance value. It should be rated 1W or more.

Here is how to determine the resistor value:
1. Get the MFG specs of the fan. You want to be able to calculate the fan motor's resistance, Rfan. You can do this from its rated voltage and current consumption, which are typically included in the specs, using V=IR --> Rfan=Vrated/Irated.
2. Make note of the minimum operating voltage, Vmin, specified by the MFG. This is the voltage at which the fan will reliably start.
3. The equation V=Vrated*Rfan/(Rfan+Radd) can be used to calculate any Radd that you would like to use. Note that V must be greated than Vmin! The fan speed (and likely the airflow) is proportional to V/Vrated.

The quietest operation occurs at the minimum speed, which will be obtained with the fan running at Vmin. This will also provide the least amount of AIRFLOW through your amp. A quiet fan that moves too little air will increase the thermal stress on the components and may cause the amp fail sooner (have a shorter lifetime) and it may go into thermal protection when driven hard. Choose wisely.

Another consideration is the back-pressure that the fan sees. If your amp is installed in a cabinet with little room for airflow, the fan must work against the resistance created by the restricted flowpath and this will reduce the fan efficiency and its total airflow. Most "quiet" or "silent" computer fans have very low static pressure ratings and will quickly lose efficiency even with very little airflow restriction. This is why you often find noisier fans in pro audio amps: they can operate into higher static pressure and have a higher no-load airflow capability. These are needed when the amps are mounted in tight rack cases that are hot. In a home application, you can provide better access to cool air and the average power demand on the amp will likely be lower, so a slower and quieter fan is very likely sufficient.
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Old 2nd November 2017, 10:02 PM   #24
CharlieLaub is offline CharlieLaub  United States
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Opinion on the Behringer KM1700 Class-H amp
I just replaced the two stock fans with "quiet" (12dBA) 12V computer case fans. The stock fans were rated at 33dbA, so the new ones are about 20dB quieter. Nice. I thought I would share my experience in case anyone else wants to try it.

There are two 24V fans on the amp. Each is fed by its own 2-wire V+ and GND wires from a regulated DC supply on the output device PCB. Since computer case fans are 12V and you need two of them, you can simply wire them in series and power the pair via one of the stock fan leads. Simple! I cut off the unused fan lead at the fan and disconnected its molex connector from the PC board but left the wires in place (they route under the PCB). After soldering the new wiring and re-insulating the connections I tested everything with the cover off before putting the cover back on.

NOTE that there are quite a few screws to deal with: to remove the cover you remove screws through the top at the front and rear, and from each side just below the top. Additionally, there are four screws on the bottom and four more inside at the corners of the output device PCB. You must remove all of these because the heat sink and output device board assembly needs to be slid forward about 1/2 inch to get the stock fans out. All wiring can remain connected during this process.

All-in-all this was not too bad of a job and now the amp is practically silent. In a very quiet room with no other sounds to mask noise I can barely hear the fans from about 3 feet away when I listen for it. The amp does not produce any hum or noise otherwise. It's plenty quiet to use in a home setting.
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Last edited by CharlieLaub; 2nd November 2017 at 10:07 PM.
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