Amplified PC speaker set volume issues

[electron]

So I have been using PC speakers (amplified) for TTS for quite some time. I basically cut off the stereo plug, and ran the signal over cat5e between the home automation server and the location of the speakers. This has always worked, but there was always a little hum (sounds like a ground loop issue). Another funny side effect: I can hear a loud buzz every time a UPB command gets sent onto the power line (sure is helpful with troubleshooting!)

So this weekend, I decided to take a closer look since it's starting to drive me crazy, and I noticed the stereo wire isn't just simple a 2-wire cable. Both the positive and negative side contain a 2nd non-insulated wire outside the actual insulated signal wire. So just for fun, I connected the bare copper wire to signal wire, and now the hum is gone. However, volume seems to be much lower now, so I am guessing this isn't the right thing to do (I am no audio expert!).

Can anyone suggest what the bare copper wire is supposed to be connected to? Thanks!

 

[WayneW]

A pair of stereo speakers should need at least 3 wires, maybe 4 if they separate grounds. The common 3.5mm plug used for PC audio (same as for headphones) has 3 connections; Left, Right & Ground. The cable to the speakers may have the ground as a shield wire (similar to coax). Basically, the bare copper should be your ground connection.

 

[electron]

The male 3.5mm plug I purchased only had 2 solder points, I guess I need to find something better. Thanks!

 

[Dave X10]

The male 3.5mm plug I purchased only had 2 solder points, I guess I need to find something better. Thanks!

A mono-plug?

 

[electron]

I bought it a long time ago, I am fairly sure it was a stereo plug, but I have my doubts now.

 

[KentDub]

A pair of stereo speakers should need at least 3 wires, maybe 4 if they separate grounds. The common 3.5mm plug used for PC audio (same as for headphones) has 3 connections; Left, Right & Ground. The cable to the speakers may have the ground as a shield wire (similar to coax). Basically, the bare copper should be your ground connection.

Essentially you shorted out the wire. The hum isn't gone, it's probably just too quiet to hear it. Unbalanced audio wires (no negative) are not suited well for long cable runs (more than 10ft). You have a few options. First, you can put a low-pass filter (around 80Hz) at the receiving end of the cable. This will remove the 60Hz hum from the power lines you hear. Unfortunately, if this is connected to a decent sound system you will observe a loss of bass frequencies. The other option is to convert to balanced wires. You said that you were using Cat5, so if you have an extra pair of wires you can do it. The positives should be twisted with the negatives in a balanced configuration. With balanced cables you can do a several hundred feet run with no hum or noise. At the receiving end of the cable the negative and positive signals are combined to cancel out any noise that was picked up over the cable run.

I would highly recommend going balanced if you can afford it. I have my entire sound system run on balanced wires, probably around .5 miles of audio cables, crisscrossed with power lines, and everything sounds as if they were on 1ft cables!

For any of these devices you put them at the receiving end of the cable, with the exception of the balanced/unbalanced converters, those go at both ends.

Low pass filters: http://en.wikipedia.org/wiki/Low-pass_filter
Hum killer (built-in low pass): http://www.rdlnet.com/product.php?page=108
DYI ballanced to unballanced & back: http://www.jensen-transformers.com/an/an003.pdf
Unballanced/Ballanced converter example: http://www.mcmelectronics.com/product/555-...ELAID=220570784
I have two of these, they provide 8 channels of bi-directional ballanced to unballanced conversion, grab one for yoru equipment room and use the smaller units for the EOL: http://www.bhphotovideo.com/c/product/3037...PRO_Direct.html

Hope this helps,
Kent

 

[JimS]

I recently ran CAT5 from a downstairs sound system to some amplified computer speakers in a bedroom upstairs. Probably about 50 or 60 foot run. I decided I would try without baluns first. I ran each channel on it's own pair and used the other wire in each pair for the return. I also connected the other wires in the cable to ground. It does have a little noise - not so much hum - more like hiss. But it is low enough to not bother. I had thought of making some op amp balanced line drivers and receivers or using some baluns. MCM has some fairly low cost ones at about $12 per channel if I remember correctly. Got the link to MCM stuff from another post on this board.

This is used for background music/talk radio and I would also like to use for TTS. Need to figure out how to connect so they can be used for both. Also thinking of switching power to the speakers for controlling which rooms get TTS.

 

[roussell]

I do essentially the same thing, but I tie the left and right inputs together for mono-ish audio and send it over RG6 coax. I've got a couple of 50+ foot runs and no hum.

Terry

 

[electron]

What did you do with the ground connections?

 

[KentDub]

What did you do with the ground connections?

For audio cables the ground is always connected to the shield.

Dan - I don't think you have a ground-loop; Ground loops have very, very loud hums. Noise and/or low-volume hums are caused by interference picked up over the length of the cable run.

Terry - You solution kind of works. Essentially what is happening is you are doubling the amplitude of the signal when you convert to mono, thus drowning out the hum. This only works if the source signal is mono to begin with. The problem is still there, just masked better. The RG6 coax also has much better shielding against interference than a 1/8" mini cable or unshielded CAT5.

Dan - What are your gain stages set at? You should have the source device set at about 95% to 98% maximum volume (don't set it to 100%, you risk overdriving the output). Then you should control the actual volume you hear at the speaker. Driving a signal with a higher signal amplitude will improve the SNR and mask the noise/hum better. Imagine doing the opposite, having the source set at 30% gain and the speaker at 300% gain. In that horrid scenario you are tripling the amplitude of the noise.

 

[electron]

I used to run the source around 90%, but had to boost it to 100% (with the speaker set bumped up to almost max as well) just so I can still hear TTS. I am about to fix this, but the idea of RG6 is tempting, since I do have a spare run in that same location. I do like the idea of combining the stereo signal into a mono run, so I can power a second set of speakers.

 

[KentDub]

I used to run the source around 90%, but had to boost it to 100% (with the speaker set bumped up to almost max as well) just so I can still hear TTS. I am about to fix this, but the idea of RG6 is tempting, since I do have a spare run in that same location. I do like the idea of combining the stereo signal into a mono run, so I can power a second set of speakers.

Sounds like you may need to get a headphone amplifier also to boost the signal. The outputs on a computer are very weak (line level is the actual term). The signal is enough to drive headphones, but really not speakers. The speakers should be Active if you arn't going into an amplifier (active in this context means self-amplified -- i.e. they have a wallwart). If you already have active speakers you still may need a headphone amp, it could be the distance of the run has too much resistance and you need to shove more power down it. RG6 has a ticker copper core than Cat5, and will not lose as much power for longer runs.

 

[electron]

They are indeed powered speakers. The volume was fine before I started screwing around with this, so it's user error (was too desperate to get rid of that hum).

 

[KentDub]

They are indeed powered speakers. The volume was fine before I started screwing around with this, so it's user error (was too desperate to get rid of that hum).

Oh, right, I forgot... You shorted out your cable.. Move the ground back to the shield and don't short it out with your signal wire. You'll get your volume back.

 

[electron]

I just went to look at my 'handy' work, server side, and it looks like I soldered it to an RCA connector, which I guess is part of the problem. Time to go get stereo 3.5mm plugs.