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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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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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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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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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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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Old 13th April 2018, 08:48 AM   #25
Lex vs Kingpin is offline Lex vs Kingpin  Turkey
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Quote:
Originally Posted by CharlieLaub View Post
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.
Hello,

I was wondering whether you are still enjoying the amp. I am also thinking to pick up one for dual 1260w subwoofers (4ohms - 300w rms) to be used in bridged configuration.

Best.
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Old 20th April 2018, 08:39 AM   #26
TBTL is offline TBTL  Germany
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Quote:
Originally Posted by CharlieLaub View Post
+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.
This assumes a constant resistance of the fan, regardless of speed, which is not the case. In reality, fans have a lower resistance when standing still. So the voltage drop across the fan will be lower than calculated and the fan might not (reliably) start at all.
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Old 10th August 2018, 07:19 AM   #27
thomass77 is offline thomass77
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Join Date: Aug 2018
Default My KM1700

I love the KM1700. It is extremely fast, quiet and powerful.
I added a 300 ohm, 5W Xicom ww resistor in series with each fan. This quieted them down to acceptable levels and should not result in cooling problems with my home use. After 18 months, so far so good.
The 1700 uses two Ametherm NTC Inrush Surge Suppressors to reduce the turn on surge caused by the approximate 1.5kVA toroidal transformer and four 10,000 uFd electrolytic caps. The amp has dual secondaries and is dual mono from there.
The very high quality pcbs use a mix of surface mount and through hole components. I replaced all of the electrolytics in the amp with Nichicon Muse andFine Gold and Elna Silmic II audio grade types.
I believe the 1700 employs current feedback judging by a JRC transconductance chip in use. This undoubtedly contributes to the high slew rate, in the neighborhood of 500 v/usec as mentioned in a Gernan blog.
Four op amps for balanced to single ended conversion. etc. are TI 5532 8 pin dips.
All wiring is 18 guage or heavier and well routed
There are trimmer adjustments for dc offset and quiescent current bias.
The input level pots appear to be of high quality, but I cannot discern the manf.
I added a sheet of a/c filter foam to the entire inside front panel. I was not able to find my preferred Filtrete cut to size sheets at Home Depot or anywhere else this year.
There is no flux residue on either side of any of the pcbs.
This amp has a continuous output of approx. 125 wpc with 6 dB of dynamic headroom to yield 500w on peaks. I plan to add four 4700/63 Nichicon Fine Gold ecaps to each of the four supply caps.
This is a killer bargain among power amps. If not known, I would estimate its retail price at $600-$800 easy.
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Old 10th August 2018, 01:23 PM   #28
thomass77 is offline thomass77
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That should read the transformer has dual secondaries.
Also one 4700/63vdc Nichicon Fine Gold electrolytic cap. In parallel with each 10,000 uFd/63vdc main supply filter cap for longer peak reserve.
I also twisted off the red and black knobs on the binding posts to access the banana plug. They are not the kind with a pull out center safety plug.
Simply twist the binding post knobs hard fully counterclockwise and they will strip off, revealing a much more useful banana plug.
Can't stress enough how amazing the transient response is with the 1700. Behringer credits the ON Semiconductor output devices, but there is more to it.
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Old 12th August 2018, 02:01 AM   #29
ViennaTom is offline ViennaTom
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Quote:
Originally Posted by chris661 View Post
Hmmm...

FWIW, 1/8th power is pink noise with a 9dB peak-to-average ratio run so the amplifier is just clipping. Sine wave testing is still technically only half-power, since it has a 3dB peak-to-average ratio. If you want to really cook an amplifier, you'll need square waves, where the peak-to-average ratio is 1.

Chris
On a purely ohmic load, square waves at full power level would not cook the amp itself - if the amp is Class AB or H - but put high load on it's PSU. The amp itself would not be very much stressed since there is almost no voltage drop at the transistors. Sine wave testing would put way more heat to the transistors and less load on the PSU.
In case of purely ohmic load and Class D amp, both the amp and the PSU get stressed more with 0dB square instead 0dB sine.
Real loads are not purely ohmic and that makes the whole situation even more complicated - with a BIG advantage to class D over AB and H in that it has higher efficiency AND can recycle instead of just burn reactive energy and therefore get away with a less powerful PSU for same output.
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