split RCA or no?

[chjade84]

I hear people saying they use a Y splitter to merge their LFE channel to both inputs on an amp that just has L and R RCAs, but is that really necessary? It seems like if all the low frequency effects are coming from the one input, and it's only hooked up to one subwoofer, that it would play from just that one input - that it wouldn't need both. Is that right, or since it is expecting two channels does it output about half from each input?

[jjohnson]

This is assuming some things....

Stereo input into speaker system with stereo output into two top speakers and mono output into subwoofer(s)
Either the crossover will combine the stereo channels to create the mono output or you will have to combine the two signals with a summing amp (expensive) or Y-cable (cheap) for mono output to the amp.

Crossover with mono output
Input: L R Output: L HF, R HF, Mono LF

Crossover with stereo output
Input: L R Output: L HF, R HF, L LF, R LF

You could also be asking about how to parallel the amp - that would use a Y-cable to send a single output to 2 amp channels.

[chjade84]

...

lol

Ok, well that kinda went over my head. The "top speakers" part especially. I don't think I've ever heard that term before.

I have an Onkyo TX-SR606 receiver with a single "subwoofer pre-out" and a Foster WF-100k amplifier for my sub with L/R RCA inputs. Should I run to Radio Shack and get a Y adapter, or can I just plug into the Right channel and be fine?

[jjohnson]

Sorry...... top speakers = mains or non-subwoofers

For your setup I would just plug it in.... Looking at the info on the amp it will just combine it anyway so why go to the hassle of splitting it. I was thinking more PA or pro audio and not really home theatre.

EDIT:
I would also disable the crossover since the receiver should be taking care of sending only the LFE to the output. But do use the subsonic filter as a HP for the sub.

[chjade84]

Ah, thanks for the clarification.

[SoundInMotionDJ]

The Y-cable is OK for splitting a signal, but not for combing a signal. See this for more:

http://www.rane.com/note109.html

--Stan Graves

[Ron K]

Simply take the sub out of the reciever right into either the L or R of the Foster Amp.

According to the Foster schematics the L/R inputs (RCA or Hi Level inputs) are immediately summed at the inputs anyways.Although not the best way to do it they use a simple resistive network to complete the sum. Yuk! Probably much better off using just 1 input anyway.

To clear one thing up about summing two signals.You can do so with a "Y"as long as the two signals are the same in all respects. Phase/waveform etc.A simple Y sum will work passed through a resistor. Of course any asymmetrical waveforms will reek havoc in the output but by and large most LF sums are banking on the L and R content to be equal!!! Granted it's not the best way to do things and only works when the signals are equal in all aspects. Just a little twist of logic there! It doesn't work well at all when the two signals need to be mixed! In fact it's usually quite horrid!

[Fish]

To clear one thing up about summing two signals.You can do so with a "Y"as long as the two signals are the same in all respects.

This is absolutely and categorically incorrect: it's all about the hardware circuitry of the outputs and has nothing whatsoever to do with the signal. The Rane note linked above explains why.

And yes, I've had mono audio played back from laptops that were improperly summed with a Y cable... the distortion was absolutely atrocious until I unplugged one side of the Y cable.

[Gregory East]

The note says to not sum signals and then goes into great depth on how to sum signals with networks of resistors. Go figure.

[Ron K]

To clear one thing up about summing two signals.You can do so with a "Y"as long as the two signals are the same in all respects.

This is absolutely and categorically incorrect: it's all about the hardware circuitry of the outputs and has nothing whatsoever to do with the signal. The Rane note linked above explains why.

Wanna bet? As long as the signals are symmetrical they "can" be summed with a Y through "resistors".I clearly stated that in the post.It's done quite often in amp circuitry as it is with the above mentioned amp. Bass signals tend to be recorded "mono" so the signals in the "bass" region are generally symmetrical.That's why it "can be" and "is" done. When the signals are asymmetrical the summing still works but not without phase cancellations and such.For basic applications the resistive sum does indeed work but not without issue in full range on most signals. If it's just 2 low end signals for a mono bottom it works fine.Two mics running vocals you probably wont have much luck.Two stereo signals from recorded material and the result would most likely be pretty bad indeed.

Rane's discussion on the matter is based around summing say two low end signals and basically for the guy hooking up his PA. It's not all as simple as the Rane paper but a good guide if you dont know any better or are just learning!One of the things Ranes note indeed does mention is the use of this for "bass" summing and the reason is because it is indeed signal dependent.

From the Rane note:

One of the most common examples of tying two outputs together is in "monoing" the low end of multiway active crossover systems. This combined signal is then used to drive a sub-woofer system.

Vocals for instance are generally asymmetrical waveforms and will reek havoc in the above configuration especially in the mid bands. Stereo signals are as well.

The Rane white paper is mostly for people doing general hookups not for people designing amps or other circuits or working outside the box so to speak.

Here's a print of the summed circuit.
Notice the low level input L and R summed through a common - and 2-12K resistors through a Cap and then off to an opamp! No need for any opamp summing just a simple "Y" at the input and add resistors.

And yes, I've had mono audio played back from laptops that were improperly summed with a Y cable... the distortion was absolutely atrocious until I unplugged one side of the Y cable.

That's because you were summing asymmetrical waveforms and most likely the combined voltage exceeded the max value that could be used in that circuit, hence the need for the resistors or a more pro level preamp!

[SoundInMotionDJ]

Notice the low level input L and R summed through a common - and 2-12K resistors through a Cap and then off to an opamp! No need for any opamp summing just a simple "Y" at the input and add resistors.

Read the Rane note...it provides schematics for a passive summing box - both balanced and unbalanced. What it does not mention is the more than 10dB of signal loss that will happen through one of these boxes. That, and the Y-cable from the shack doesn't come with any resistors...

--Stan Graves

[Ron K]

Notice the low level input L and R summed through a common - and 2-12K resistors through a Cap and then off to an opamp! No need for any opamp summing just a simple "Y" at the input and add resistors.

Read the Rane note...it provides schematics for a passive summing box - both balanced and unbalanced. What it does not mention is the more than 10dB of signal loss that will happen through one of these boxes. That, and the Y-cable from the shack doesn't come with any resistors...

--Stan Graves

I read the Rane note and I understand it. However my comments on the entire matter was in reference to the OP regarding "his" amp. The schematic I posted "is" in fact "his" exact piece of gear and exactly how it is done.The remainder supporting this very scheme that is used quite often in the industry.

I'm not suggesting you go out and wye all your mics together.I stayed on topic in reference to the OPs amp and set-up and clarified it a bit further. Some people like to look under the hood not just drive the thing.

[Bill Fitzmaurice]

I hear people saying they use a Y splitter to merge their LFE channel to both inputs on an amp that just has L and R RCAs, but is that really necessary?

No. The reason why the amp has L/R inputs is for the rare occasion when it's being driven by the tape outs of an old 2 channel stereo receiver that doesn't have an LFE out. If driven by an LFE out the receiver has already summed the L/R.