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Advice on long subwoofer signal cable runs from switch box

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Advice on long subwoofer signal cable runs from switch box

PostPosted: Mon Sep 16, 2019 2:01 am
by DC-Choppah
I built a simple switch to allow me to turn two remote subwoofers on/off. These powered subwoofers are in two different listening rooms. Unfortunately this is NOT pro-audio stuff, but good quality Hi-Fi kind of stuff.


One subwoofer is only 5' from the box, but the other is 50' away in a different room.

The signal is the low frequency only signal from the playback system that has its own bass management - subwoofer output signal that is intended for a powered subwoofer.


So I am concerned with picking up 60 Hz hum on the long line. So I am deciding what kind of cable to use here.

I am also wondering what I should have the subwoofer see when its switch is off. A load? or a short? An open?


I did a little experiment and I ran a 50' long, regular speaker cable between the switch and the subwoofer. The hum is loud.

Then when I open circuit the switch the hum is extreme. WoW!!! Like 60 Hz feedback!!!

Now when I use a short at the switch, the hum completely ceases.

Is this going to be possible? Can I really shield the signal cable enough so that I can run a 50' signal to an active subwoofer using the unbalanced RCA connectors.

Or do I need to convert these connections to Pro-Audio kind of stuff and use a balanced line to the subwoofer?

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Mon Sep 16, 2019 6:40 am
by ef37a
Ideally DC you would use transformers on the output and input to balance the line as you say but, it is a very low frequencies that transformers perform at their worst in terms of distortion and wobbly frequency response. These problems are made worse because of the non-zero output impedance (I am betting!) of the sub woofer drive circuit. Even if you purchased very expensive, high grade transformers* the drive circuit will be a limiting factor.

The alternative is electronic balancing . I would build a symetrical drive amplifier using say an NE5532 and another one 'tother end to unbalance the line. If you used 4 core screened cable you could send power down it for the remote amp. Does nobody make such a thing?

But, if you have 50ft of two core screened cable handy you could try what I call "shielded return". At one end, a mains earthed device, the shield and the cold core are connected together. At the remote end the shield is not connected but the cold core goes to the signal connector shield, e.g. the sleeve of a TS jack.

*And ideally you would use a proper, "600 Ohm" driver transformer and a 10k "bridging" traff at the sub end. Not cheap if you want top quality.

Dave

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Mon Sep 16, 2019 6:55 am
by Wonks
You won't get any screening on standard speaker cable. Even a short length will pick up hum.

All you can really do is try some screened instrument cable, fit that with RCA connectors and see what it sounds like.

Unbalanced connectors aren't meant to be used for long runs, so will suffer noise issues, but screened cable will be a lot better than unscreened.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Mon Sep 16, 2019 9:48 am
by Hugh Robjohns
DC-Choppah wrote:The signal is the low frequency only signal from the playback system that has its own bass management - subwoofer output signal that is intended for a powered subwoofer.

I presume these are unbalanced line-level audio connections, not speaker-level?

So I am concerned with picking up 60 Hz hum on the long line. So I am deciding what kind of cable to use here.

You don't have a lot of choice, unless you want to invest in some line isolating transformer units or active balance boxes to run the main length of the cable in a balanced format. So use standard single-core screened cable -- instrument cable, if you like.

I am also wondering what I should have the subwoofer see when its switch is off. A load? or a short? An open?

Definitely short-circuit, connecting the signal line to the (destination) ground.

I did a little experiment and I ran a 50' long, regular speaker cable between the switch and the subwoofer. The hum is loud.

Not surprised. Speaker cable is not screened and you will probably have an almighty ground loop too...

Then when I open circuit the switch the hum is extreme. WoW!!! Like 60 Hz feedback!!!

Not surprised... you have a 50-foot VLF radio aerial connected to your powered subwoofer!

Now when I use a short at the switch, the hum completely ceases.

:D Funny, that!

Is this going to be possible? Can I really shield the signal cable enough so that I can run a 50' signal to an active subwoofer using the unbalanced RCA connectors.

It's certainly not ideal, and you're almost certainly going to have to intervene with the ground path because of the ground-loop problem. I'd experiment with breaking the cable screen connection at subwoofer end (keep it intact at the source amp end). You could link the cable screen to the RCA-phono plug sleeve at the subwoofer via a 0.1uF capacitor if you like to maintain some RF screening, but that would avoid the inevitable ground-loop and should minimise the hum problems.

[quote Or do I need to convert these connections to Pro-Audio kind of stuff and use a balanced line to the subwoofer?[/quote]

Making the long run balanced would make things easier... but would also involve some expense in transformers... so I'd see how you get one with the lifted screen connection as a first step.

H

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Tue Sep 17, 2019 5:26 am
by DC-Choppah
Hugh Robjohns wrote:
DC-Choppah wrote:The signal is the low frequency only signal from the playback system that has its own bass management - subwoofer output signal that is intended for a powered subwoofer.

I presume these are unbalanced line-level audio connections, not speaker-level?

H

Correct

Specifically, the subwoofer output of this.

http://pdf.outlawaudio.com/outlaw/docs/RR2160_OM_19.pdf

Connector R25.


Subwoofer is this https://us.kef.com/speakers/home-theate ... oofer.html

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Tue Sep 17, 2019 5:39 am
by DC-Choppah
ef37a wrote:Ideally DC you would use transformers on the output and input to balance the line as you say but, it is a very low frequencies that transformers perform at their worst in terms of distortion and wobbly frequency response. These problems are made worse because of the non-zero output impedance (I am betting!) of the sub woofer drive circuit. Even if you purchased very expensive, high grade transformers* the drive circuit will be a limiting factor.

Thanks for that Dave. The point of this connection is to preserve the high quality bass management of the receiver. I need to get that signal 50' away to the subwoofer somehow. So if the transformers are going to alter that signal, then I would like to try something else.

I did try using an inexpensive isolation transformer and got a large drop in the signal level going into the sub, and the hum was only 90% gone, but still there.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Tue Sep 17, 2019 7:57 am
by ef37a
Neither of those specifications are very helpful. There is no figure for the sub output level or its source impedance for the reciever and no figure for the input sensitivity or impednace of the sub woofer (shame on you Kef! Used to be better than this) All it says for the sub is "RCA phono input, speaker level" Does that mean a phono socket AT speaker level or are there two inputs? I cannot find a good enough photo to see.

If there is indeed a separate "speaker level" input that could be happy fed from unscreened speaker cable but the drive would have to come from a very low impedance source, a small headphone amplifier might work. Something like the Behringer HA400 and you could just parallel the L/R inputs and outputs to get half the nominal output impedance (100 Ohms or so at a guess) .

Considering how cheap it is to provide a balanced input I am surprised Kef have not done so.

Dave.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Tue Sep 17, 2019 8:24 am
by Wonks
You won't just have the sub in a room on its own, so you must have some main speakers in there. How are they supplied with a signal?

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Wed Sep 18, 2019 1:15 am
by DC-Choppah
The B&W speakers that go with the sub are happiest when they don't need to produce the deep bass. The highs are clearer, especially the cymbals, hi hats. This is one of the main reasons why I prefer to use a sub for 'Hi-Fi' listening. It is what happens to the highs actually that is most important. The bass is great too.

So I need to split off the low end and send the rest of the signal to the B&Ws.

Unfortunately my Kef subwoofer does not have this kind of filter output. You have to feed the Kef the full range input and let it filter it, but then there is no output to send to the B&Ws. Darn. The Kef also has stereo RCS inputs for the low signal and they say to use the left for mono, which is how I how hook it up.

So enter the Outlaw receiver that has its own very good bass management, and I have my signals to send to both speakers - high pass to the B&Ws, mono low pass to the Kef. Lif is good.

Now do that in two rooms each with their own stereo speakers + sub. That's me.


This would all be so much easier if I didn't actually hear all this stuff.


I feel like I am the first guy to run two rooms where each each room has its own subwoofer. I actually couldn't find anything. Uncharted territory I guess?!?


I just got a 100' roll of shielded 2 conductor + shield cable. Gonna go for it and run it through the walls and see if I can get rid of the hum once installed using your guys various techniques.

BTW there is definitely a 10' radius loop in the line because of the way the cable has to run through the basement. With the regular speaker cable it really does sound like 60 Hz feedback and gets louder as it sits!

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Wed Sep 18, 2019 10:01 am
by Sam Spoons
You mention 'regular speaker cable' again, I'm assuming you mean shielded single cored signal cable like this

Image

not speaker cable like this?

Image

Powered speakers don't use speaker cable externally, they need shielded signal cable.

I suspect you do know this but its another example where using the wrong terminology could confuse a newb looking at this thread in the future.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Wed Sep 18, 2019 10:09 am
by Hugh Robjohns
ef37a wrote:...you could try what I call "shielded return". At one end, a mains earthed device, the shield and the cold core are connected together. At the remote end the shield is not connected but the cold core goes to the signal connector shield, e.g. the sleeve of a TS jack.

*And ideally you would use a proper, "600 Ohm" driver transformer and a 10k "bridging" traff at the sub end.

This 'shielded return' idea is only of any use at all if there is a transformer or a differential receiver at the destination end. Without one of those, this scheme achieves nothing at all! It will still have ground-loop problems and interference rejection might even be degraded...

H

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Wed Sep 18, 2019 10:22 am
by Wonks
'Regular speaker cable' clearly means unshielded twin core cable, the worst thing to use.

I'm still unsure about the layout and equipment used. Can I assume that the main speakers in the basement are fed by the main upstairs amp, using normal speaker cable, but that the sub is active, so needs a domestic line level feed to it?

You say the amp unit has got sub/main speaker frequency management built in to it? So it's passing a low pass signal to the sub which itself has a low pass filter built-in? I can see problems here with a frequency drop at the crossover point.

If the crossover points are the same, then the bass just above the crossover point will drop off at double the rate so you'll get a notch where frequencies are missing.

If the amp's crossover point is significantly higher than the sub's, then you then get a big frequency gap between the mains and the sub.

If the amp's crossover point is significantly lower than the sub's, then the sub's LP filter will work well, but you then get an area where both the sub and the main speakers are producing the same frequencies so you'll get a hump in output at that point.

You ideally need to know the sub's LPF frequency and slope, be able to set the main speakers' output HPF to have the same frequency and slope as the sub, and pass a full-range signal to the sub.

Do you know what all the filter frequency points are, and if the ones in the amp are adjustable at all?

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 12:31 am
by DC-Choppah
Wonks wrote:'Regular speaker cable' clearly means unshielded twin core cable, the worst thing to use.

Right! Or we say 'extension cord', or perhaps 'lamp cord'. It is just two wires next to each other. I am starting with that. We buy this here at hardware stores as 'speaker wire'. It looks like thin 'extension cord' cable. This kind of cable is creating massive hum for me. I have some 2 conductor shielded. Gonna try that.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 12:42 am
by DC-Choppah
Wonks wrote:'
I'm still unsure about the layout and equipment used. Can I assume that the main speakers in the basement are fed by the main upstairs amp, using normal speaker cable, but that the sub is active, so needs a domestic line level feed to it?

Right!

The sub is fed by a line level low-frequency-only signal from the amp.

I believe the frequencies have been sorted out and the crossovers are correct. The music itself sounds good. The hum however is the remaining problem. If the hum was gone I would be happy!

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 12:49 am
by Hugh Robjohns
DC-Choppah wrote:This kind of cable is creating massive hum for me.

Yes, of course it does. That much is inevitable. The signal wire has no screen in that cable configuration, so all local electrostatic fields will induce noise currents which will add to the wanted signal.

I have some 2 conductor shielded. Gonna try that.

Good plan. You don't need a second inner conductor for an unbalanced connection, so you can either wire the two inner cores together, or use one of them for the signal connection and wire the other with the screen. I'd start with the screen connected at both ends, because I think the subwoofer is a ground-free device. From the images I've found it appears to have a fig-8 style two-core mains cable, without a ground connection... In which case a ground-loop is very unlikely.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 1:09 am
by DC-Choppah
The Outlaw receiver and the both subwoofers are double insulated. No ground wire on the A/C plug of any of these components.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 7:41 am
by ef37a
In general if you are using two core screened cable as single core it is better to tie the "cold" core to the screen and use a single hot core.

This gives the minimum core to screen capacitance, not an issue for a sub woofer feed but can help in other applications such as a guitar cable.

The configuration also greatly reduces microphonic thumps from cable impacts in the latter usage. The screen/cold core can of course be dissed if an earth loop is experienced.

Dave.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Thu Sep 19, 2019 9:28 am
by Hugh Robjohns
DC-Choppah wrote:The Outlaw receiver and the both subwoofers are double insulated. No ground wire on the A/C plug of any of these components.

Oh wow! That's unusual! Normally, the receiver/amp is the only grounded unit in a hi-fi system, which makes a lot of sense... I don't think I've ever come across a system which has no solid earth reference at all!

Oh well... at least that means you won't be troubled by ground-loops via the mains supply. There could be other issues to resolve, but let's see what happens when you wire up the subwoofer's line feed with the appropriate screened cable first.

H

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Sat Sep 21, 2019 12:28 am
by DC-Choppah
Wonks wrote:Do you know what all the filter frequency points are, and if the ones in the amp are adjustable at all?

Both the amp and the subwoofer crossovers are adjustable. I have them both set at 80 Hz.

Although the subwoofer signal wire runs through the basement, the listening room is on the ground floor and is a 25’ x25’ room with vaulted ceilings and cork flooring and one wall full of windows. I also record instruments in this room as it has a very appealing sound.

Re: Advice on long subwoofer signal cable runs from switch box

PostPosted: Sat Sep 21, 2019 10:22 pm
by DC-Choppah
Hugh Robjohns wrote:
DC-Choppah wrote:
Oh well... at least that means you won't be troubled by ground-loops via the mains supply. There could be other issues to resolve, but let's see what happens when you wire up the subwoofer's line feed with the appropriate screened cable first.

H

Success. Using this cable, with black wire connected to the screens on both ends solves the hum problem.

https://drive.google.com/open?id=1rp6Dt ... Z3djOa1o65

The subwoofer switch needs to be SPDT with the wires shorted together when off to get rid of the hum. If left open circuited at the switch it still hums (faintly) with the shielded cable.