splitting the LFE - AVS Forum
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post #1 of 24 Old 06-01-2014, 12:58 PM - Thread Starter
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     Currently I’m running 2 subs in my HT.  I’d like to run 4.  Right now I have the single line level (LFE) output on the amp split and running the divided signal to the two subs.   My question is right now I notice what I think is a reduction in bass output when I split it once.  Is it actually a reduction in bass output from splitting the output on the amp or is it just wave cancellation in the room?  Is there any way I can test the signal level on the sub out from the amp?

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post #2 of 24 Old 06-01-2014, 01:39 PM
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I've split the lfe signal as many as 5 times in the past with no issues. If you are noticing a reduction in output when adding a second sub it is most likely phase related as you noted. Try changing the phase settings and see if it helps.
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post #3 of 24 Old 06-02-2014, 04:31 PM
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Originally Posted by dpc716 View Post

I've split the lfe signal as many as 5 times in the past with no issues. If you are noticing a reduction in output when adding a second sub it is most likely phase related as you noted. Try changing the phase settings and see if it helps.

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the sub "hears" the voltage, which is not divided in a parallel connection. Assuming the subs have reasonably high input impedances, there is so little current demand that splitting cannot cause a real problem with, probably, any reasonable number of subs a person could actually fit into a room. At 2 volts (high for a receiver's sub output) into 20,000 ohms (low to middling for an input stage - I don't think a megohm - 1,000,000 ohms, is unheard of) the receiver is being focred, FORCED, I say, to put out 0.0002 watts. Yes! Fully two thousandths of a watt. And one thousandth of an amp. Put two of those subs in parallel and the power demand skyrockets to .0004 watts. Fully .0002 amps. Just not a problem
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post #4 of 24 Old 06-02-2014, 08:03 PM
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Originally Posted by JHAz View Post

the sub "hears" the voltage, which is not divided in a parallel connection. Assuming the subs have reasonably high input impedances, there is so little current demand that splitting cannot cause a real problem with, probably, any reasonable number of subs a person could actually fit into a room.
+1. If you parallel enough sub amp inputs the summed low impedance will load down the AVR sub output driver, but that would probably take at least 6 subs.

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post #5 of 24 Old 06-05-2014, 11:00 AM
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I have noticed a drop in preamp output when Y-jacking on two different systems. I think when you do that the inputs have weird interactions with each other too. The best way is to have active buffers between each sub that isolate the inputs, kind of like a headphone amplifier with multiple outputs. If you have one of those lying around you can use it instead of splitters to see if the problem is still there or not. The cheaper headphone amps tend to be a bit noisey but it shouldn't matter that much on the low end.
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post #6 of 24 Old 06-05-2014, 11:21 AM
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I have used one output for 8 subs with success.

AVR-Yamaha A830
amps-5 Adcom 555 in 850 watt monoblock mode
sub amp-Sanway FP14K
LCR-Dual stacked BFM DR-250's
Surrounds- Dual stacked BFM W10's
subs-12 SI 18's ported 6hz.
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post #7 of 24 Old 06-05-2014, 01:39 PM
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I have noticed a drop in preamp output when Y-jacking on two different systems. I think when you do that the inputs have weird interactions with each other too.
Sorry, that is simply incorrect. See JHAZ's post above.
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post #8 of 24 Old 06-05-2014, 01:49 PM
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I have noticed a drop in preamp output when Y-jacking on two different systems. I think when you do that the inputs have weird interactions with each other too. The best way is to have active buffers between each sub that isolate the inputs, kind of like a headphone amplifier with multiple outputs. If you have one of those lying around you can use it instead of splitters to see if the problem is still there or not. The cheaper headphone amps tend to be a bit noisey but it shouldn't matter that much on the low end.

The difference Aiui is that phones are passive and powered subs are not. I have over time taken plenty of heat around here from folks who do not grok this distinction in the opposite direction. Different cables can result in measurable and audible differences when they are connecting say a high output impedance guitar pickup to a guitar amp. Shifts the resonant frequency of the system. It's why Hendrix liked coiled guitar cables with his post-CBS neutered strat pickups. Rolled off some highs. Irrelevant unless something is terribly wrong in the context of a relatively low output impedance preamp out into a high input impedance sub amp.
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post #9 of 24 Old 06-05-2014, 11:47 PM
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Look, I ain't making it up. 2 times Y-jacking outs got screwy with me. The rest of the time I used Y jacks it was fine. I didn't get schematics on the inputs and test cables and do the math so I don't really know what the reason was.

Wouldn't the signal load change from all of the different input impedances combining in parallel? So if the number drops and there is a dc blocking cap on the amp's input for example, it would create an RC filter with a faster rolloff down low, right? I'm not saying that's what happens every time but it is one possibility where there would be degradation. I just don't see hooking 19 subs up to one jack being an audiofile kind of thing to do.
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post #10 of 24 Old 06-06-2014, 12:26 AM
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Look, I ain't making it up. 2 times Y-jacking outs got screwy with me. The rest of the time I used Y jacks it was fine. I didn't get schematics on the inputs and test cables and do the math so I don't really know what the reason was.
OK. So you split the signal and got some sort of effect, but you don't know what it was or what caused it. So it could have been an error on your part, not the general principle and that's what's being argued, so to say that you need to add a multiple line driver in all or even most cases is wrong. I've done it before many times with no effect, and I know because I've measured it.
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Wouldn't the signal load change from all of the different input impedances combining in parallel?
The input impedances of the amps in parallel will combine in the same manner as drivers in parallel, only the impedances will be about 1000x higher.
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So if the number drops and there is a dc blocking cap on the amp's input for example, it would create an RC filter with a faster rolloff down low, right? I'm not saying that's what happens every time but it is one possibility where there would be degradation.
It would not be a cap in the amp's input but the source's output. However most LF points in most gear is low enough not to be audible even if it was brought up an octave or more and then would only be noticeable as a slight reduction in LFE on some movies, and almost never on music.

I just don't see hooking 19 subs up to one jack being an audiofile kind of thing to do.[/quote]Hyperbole doesn't help. No one suggested that. But splitting a signal to two (or even four) sub or power amps, will not cause a drop in level, nor provide any real issue in almost all cases.
As for audiophiles, they do lots of stupid stuff and generally don't understand any electrical theory, so it not 'seeming audiophile' is a positive.
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post #11 of 24 Old 06-06-2014, 12:34 AM
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I care about single digits when it comes to a sub so it would effect me. I'm telling my experience, sorry you don't agree with it.
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post #12 of 24 Old 06-06-2014, 12:51 AM
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Originally Posted by Shreds View Post

I care about single digits when it comes to a sub so it would effect me.
It would for me to.
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I'm telling my experience, sorry you don't agree with it.
I'm not disagreeing with your experience, but your suggestion of a line driver. In most cases, it will simply NOT be necessary. That's what I'm trying to get through. I have been designing and building linear electronics for a long time, so I come with a strong theoretical background and lots of experience. Your problem might have been user error or a really oddball set of devices, far from the norm (electrically). However, without any measurements it simply may have been that the placement and/or setting caused the fault that you heard and not simply splitting the signal.
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post #13 of 24 Old 06-06-2014, 02:26 AM
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It is also possible that the signal will be distorted without buffers depending on the equipment. I don't need your resume to realize it either.
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post #14 of 24 Old 06-06-2014, 02:37 AM
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It is also possible that the signal will be distorted without buffers depending on the equipment. I don't need your resume to realize it either.
You made the claim. Prove it for non pathologically bad sets of gear.
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post #15 of 24 Old 06-06-2014, 02:37 AM
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I run four subs on two outs through a mini DSP. No issues whatsoever. Only time I notice a drop in level is if I add filters to the Mini DSP (i.e. reducing peaks on the graph), but that is something entirely different.
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post #16 of 24 Old 06-06-2014, 02:45 AM
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Yeah, I made the claim. Don't have to prove it. Just adding my two bits. Everybody reading it can do what they please. You are getting in my face about it and it's starting to get irritating Mr. Coolaid.
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post #17 of 24 Old 06-06-2014, 03:22 AM
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Yeah, I made the claim. Don't have to prove it.
Tradition is whoever makes the claim has to provide the evidence. In other words, you have nothing.
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post #18 of 24 Old 06-06-2014, 07:30 AM
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And apparently you have a menstrual cycle. Where's all of this data on your end from your extensive tests then cause you haven't done anything other than treat me like I kicked your dog down the steps.
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post #19 of 24 Old 06-06-2014, 08:24 AM
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And apparently you have a menstrual cycle.
Cheap attempt at an insult.
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Where's all of this data on your end from your extensive tests then cause you haven't done anything other than treat me like I kicked your dog down the steps.
What I am telling you is the application of basic electronics. I feel no need to keep reams of data to prove time and time again that 2+2=4.
Your emotional response to the situation is your problem and it seems that you can't bear to be questioned. I'm more than happy to be proven wrong and learn something, but have yet to see anything from you that I can learn from on this subject.
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post #20 of 24 Old 06-06-2014, 08:31 AM
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I used 5 y splitters for 8 subs on 4 amps(2 channel each) and no problems whatsoever.

AVR-Yamaha A830
amps-5 Adcom 555 in 850 watt monoblock mode
sub amp-Sanway FP14K
LCR-Dual stacked BFM DR-250's
Surrounds- Dual stacked BFM W10's
subs-12 SI 18's ported 6hz.
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post #21 of 24 Old 06-06-2014, 09:09 AM
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So you guys are basically telling me that there is no way to load down a preamp's output, increasing distortion. I disagree. I am not conducting an educational class on it. Are you done talking yet or do you want me to keep repeating the same thing?
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post #22 of 24 Old 06-06-2014, 09:38 AM
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So you guys are basically telling me that there is no way to load down a preamp's output, increasing distortion. I disagree.
I have about a hundred channels or SS power amp here. None has a Zin of less than 10k. So I could put 5 of those 10k Zin units in parallel and drive them with a typical cheap opamp the 4580 a very common and typical unit at the lower end of the market and get the following result.



Image is from the TI datasheet. Note the 1-2V region which is what most power amps need to drive them to full output. None of the distortion there is audible. This easily fulfils the OP's requirement of 4 units in parallel and I'd put money on being able to do 10 in parallel with not much worse, and certainly not audible performance.
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I am not conducting an educational class on it.
Good. You don't have the knowledge or the skills required.
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Are you done talking yet or do you want me to keep repeating the same thing?
Feel free, but repeating the same incorrect thing will not make it any more true.
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post #23 of 24 Old 06-06-2014, 09:38 AM
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So you guys are basically telling me that there is no way to load down a preamp's output, increasing distortion. I disagree. I am not conducting an educational class on it. Are you done talking yet or do you want me to keep repeating the same thing?

I did not say that, I am saying I ran 8 subs on one lfe output with no problems and I measured my output and THD to be very low.

AVR-Yamaha A830
amps-5 Adcom 555 in 850 watt monoblock mode
sub amp-Sanway FP14K
LCR-Dual stacked BFM DR-250's
Surrounds- Dual stacked BFM W10's
subs-12 SI 18's ported 6hz.
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post #24 of 24 Old 06-06-2014, 10:36 AM
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Loading an op amp increases distortion and can cause the device to overheat. Sounds like my point. If you have different amps all with different input Z, the resistance can drop a lot faster than if they are all the same amp. What if you run a short cable to one but a long cable to another with a lot of capacitance? What if the output of the preamp is of a poor design? Every situation is different. I pointed out that most of the time there was no problem at all in splitting a signal that way but sometimes there is. That is my point. Sometimes there is. Yeah, it isn't most of the time, I GET IT. Problems are possible though. Actively distributing a signal rules them out.
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