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Old 23rd February 2015, 04:51 AM   #1
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Default output resistors for paralleled amplifiers

I have a quest to use a Kenwood KA-894 to power a subwoofer
stereo receiver is rated at 100Wx2 into 8 ohms
theoretically this should mean 200W at 4 ohms with outputs in parallel(and mono input)
what resistance should my series resistors be to prevent the channels from fighting each other?
I'd really like to do it this way as the other option is two 8 ohm coils or two dual 4 ohm subwoofers which would cost significantly more.
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Old 26th February 2015, 01:44 AM   #2
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nobody has any advice to offer?
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Old 26th February 2015, 02:50 AM   #3
Struth is offline Struth  Canada
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Hi Guys

No comment because it is basically a bad idea.

Any series resistance you add will reduce damping of the woofer and there will still be some current flowing between the outputs. The Rs simply reduce it from many amperes to a much lower value.

Since bass is directional from the observer's perspective, you would benefit from running two subs that are separated as left-right. The whole "point-one" thing is flawed.

Note that MRI machines and similar do use paralleled amplifiers with current-sharing resistors between the outputs to drive the coils. Currents are very high as impedance is very low, and the losses through the Rs are not insignificant but allow the use of off-the-shelf amplifiers and just a tiny drop in drive if an amp fails. But that is an industrial application.

Have fun
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Old 26th February 2015, 02:50 AM   #4
jcx is online now jcx  United States
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1 ohm is a good start - 2 in parallel are then 0.5 Ohm and you still have not too bad damping, only ~ 1 dB loss

if you need more damping then you have tradeoffs - either hack inside the amp to include the current sharing R inside a common feedback loop or drop the value some more risking wasting power in the amps fighting each other due to DC offset or gain mismatch

I wouldn't want to do it without schematic, measurement tools to verify result but then i r a engineer
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Old 26th February 2015, 03:09 AM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by Pjtruslow View Post
nobody has any advice to offer?
Even if it gives twice the power, that's only 3dB louder before clipping.
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Old 26th February 2015, 03:38 AM   #6
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Quote:
Originally Posted by jcx View Post
1 ohm is a good start - 2 in parallel are then 0.5 Ohm and you still have not too bad damping, only ~ 1 dB loss

if you need more damping then you have tradeoffs - either hack inside the amp to include the current sharing R inside a common feedback loop or drop the value some more risking wasting power in the amps fighting each other due to DC offset or gain mismatch

I wouldn't want to do it without schematic, measurement tools to verify result but then i r a engineer
thanks to both of you

I don't think the loss in damping factor would help, and two decent subs would be out of my price range, so I think this: Dayton Audio DVC310-88 12" DVC Series Subwoofer is in my near future. it's not cheap, but it's a driver good enough to warrant building a custom box for it so it can really sing.
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Old 26th February 2015, 04:58 AM   #7
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No No No . If you have a receiver rated as 100w per channel into 8 ohms it will be about 200w per chanel into 4 ohms for each chanel. Placing the chanels in parallel will not give you a single chanel with more watts. The place you might need run the chanels in parallel is if you wanted to run a 2 ohm speaker.

Now you could use it in bridge mode and get close to 400w into an 8 ohm speaker but that is a lot more complicated and is another story.
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Old 26th February 2015, 06:02 AM   #8
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Originally Posted by woody View Post
No No No . If you have a receiver rated as 100w per channel into 8 ohms it will be about 200w per chanel into 4 ohms for each chanel. Placing the chanels in parallel will not give you a single chanel with more watts. The place you might need run the chanels in parallel is if you wanted to run a 2 ohm speaker.

Now you could use it in bridge mode and get close to 400w into an 8 ohm speaker but that is a lot more complicated and is another story.
so you're suggesting that I can run a 4 ohm load on each channel on an amp that was only rated for 8 ohm use and probably lacks the heatsinking to handle the additional heat and beefy power supply to cope?
I could bridge, as inverting one input wouldn't be an issue for me, but running at 4 ohms per channel or 8 ohms bridge would be drawing DOUBLE the current the amp was designed to handle, and it wasn't even designed to be a subwoofer amplifier anyways.

paralelling the outputs with a 4 ohm load would share the load on each channel so that each channel "sees" the 8 ohm load it was designed for, but I'm thinking a seperate 8 ohm load for each(either an 8 ohm DVC or 2 8 ohm subs) would be the best idea to simplify things.
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Old 26th February 2015, 08:56 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by woody
No No No . If you have a receiver rated as 100w per channel into 8 ohms it will be about 200w per chanel into 4 ohms for each chanel. Placing the chanels in parallel will not give you a single chanel with more watts.
Yes yes yes. 100W per channel into 8 ohms will give you about 200W per channel into 4 ohms - provided that the output can provide the current. However, paralleling both channels will definitely give you 200W into 4 ohms as this does not require any more current per channel than separate 100W into 8 ohms.

You could alternatively use bridge mode and get 200W into 16ohms, but that is another story.

Using current-sharing resistors is a valid idea, as they will ease any problems of slight differences between the channels. 0.5R might do, although even this will reduce damping factor to only 16.
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Old 26th February 2015, 02:34 PM   #10
Struth is offline Struth  Canada
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Hi Guys

Best to look up what the amp is actually rated to drive in terms of load impedances. Most multi-channel amps only rate the channels for 8R. If they do not give a 4R rating it is unsafe to assume it can drive such a load.

Paralleling channels assumes the gains are exactly the same, which is bad to do.They will have nominally the same gain, based on resistor tolerances, but that tiny variation will pretty much guarantee that the outputs will work against each other. Generally, bridging amps to drive higher load impedances is safer than paralleling them to drive lower impedances., but even in the bridge mode the amps must be rated for such operation inasmuch as a signal inversion is required for one channel input.

Among the many problems with the modern "sub-woofer" is the fact that the enclosure is almost universally too small. This makes shipping and assembly cost low and profits high for the manufacturer, but you simply cannot subvert physics and expect good sound. This dollar-driven approach relies on the fact that solid-state power is cheap and one can use massive electrical energy to overcome the inefficiency inherent in a too-small cabinet. Most subs are truly just "woofers in a separate box" and when you realise that you must conclude that the use of a single woofer for stereo+ is simply wrong.

It does not help that music fashion today pushes over-emphasised bass. Movie bass is primarily for effects and is nonmusical. What your "sub" has to do depends on how you use the system. For either purpose, two subs is far better than a single box even if just to reduce driver distortion.

There is a psychoacoustic phenomenon that many people miss out on based on the use of multiple sound sources. Assuming that the main issue is to have loud bass to have a "body loudness" effect, a single source must be operated at high SPL to achieve this, depending on how well it can couple energy into the room. Two sources can inject energy into the room a lot easier, and you get the body-loudness without ear-shattering ear-loudness. The sense of being "inside the sound" is greater with two or more sound sources, even when those sources are fed the same electrical signal. So, again, I would encourage you to go with a second sub and more manageable power for each.

Have fun
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