Speaker/Stereo Amplifier Question

[thakidd]

I need a new set of speakers for my computer as the previous ones bit the dust after cat took claws to them. I have an old stereo amplifier that works. It is suppose output to a pair of 8 Ohm speakers which bit the dust along time ago. This stereo is 15 years old so there is not a switching option to step the stereo amplifier down to any other ohm setting.

A friend of mine gave me a set of 5 surround sound 3 Ohm (each) speakers. I hooked a speaker up to each channel (left/right) and noticed a distinct buzz at low volume. I am wondering if I could hook two of the 3 Ohm speakers up to each channel of the stereo amplifier to minimize the buzzing sound. I am also wondering what I need additionally to cancel out the extra 2 Ohm (Speaker 1: 3 Ohm + Speaker2: 3 Ohm = 8 Ohm - 6 Ohm = 2 Ohm left).

Total n00b on speakers etc so any input is greatly appreciated.

If the above is not the problem, I would love to know what other steps I could take to stop the buzz and use at least two speakers which the stereo amplifier.

[Lance]

Is it a sort of mid frequency static like buzz or a low hum?

[DJPhil]

I am wondering if I could hook two of the 3 Ohm speakers up to each channel of the stereo amplifier to minimize the buzzing sound. I am also wondering what I need additionally to cancel out the extra 2 Ohm (Speaker 1: 3 Ohm + Speaker2: 3 Ohm = 8 Ohm - 6 Ohm = 2 Ohm left).

Speakers are often listed at their minimum resistance, which is a bit higher than the DC resistance, but their impedance varies drastically over the audio range. Here's a Wikipedia article to get you started.
You can certainly use two speakers per channel, and it's not worth worrying too much about the extra two ohms. It's safe to tinker with, at any rate, as the risk of damage increases as the impedance drops. A solid state amp (just about everything nowadays) will be damaged by a short and indifferent to an open circuit, and tube amplifiers are largely the opposite. Small amplifiers will have trouble delivering power to a low impedance load, and I believe operating at lower volumes begins to bring out the distortion at the frequencies the speaker presents the lowest impedance (low end and mid minus the resonant peak).
If it doesn't sound right no matter what you might want to try hitting a thrift store and looking for different speakers.

Hope that helps.

[thakidd]

Its a low hum with a small bit of static. It got better after I hooked two speakers in series but its still quite noticeable especially when no sound is currently playing.

So the current hook up is as follows:

Amplifer (+) --- Speaker1 (+)     Speaker1 (-) --- Speaker2 (+)     Speaker2 (-) --- Amplifer (-)

[DJPhil]

Its a low hum with a small bit of static. It got better after I hooked two speakers in series but its still quite noticeable especially when no sound is currently playing.

That's not likely to be related to the speakers. Sounds like 60Hz pickup from the front end of the amp. Disconnect the amp from the computer and see if you still get a hum. If you pull the cord from the computer and touch the end you should hear a loud hum/buzz from at least one channel. Make sure the volume isn't turned way up, it'll be loud! If this sounds roughly similar to your hum, you've found your trouble.
To fix this, try increasing the level coming out of the computer until it's as high as you can get it without distortion. This should help by swamping the interference with a stronger signal. If nothing you do rids you of the interference then it might be a bad ground connection (or a dirty circuit board, or a dozen other things) within the amp.

So the current hook up is as follows:

Amplifer (+) --- Speaker1 (+)     Speaker1 (-) --- Speaker2 (+)     Speaker2 (-) --- Amplifer (-)

That's just fine. Standard loudspeakers have no polarity, but the terminals are marked so that both channels are in the same phase. If they weren't in agreement it could cause some odd distortion and canceling effects on any part of the signal that's sent to both channels. This usually makes vocals sound quiet or really weird. It doesn't hurt anything, just sounds awful.

[Lance]

Yup, that hum is 60Hz pickup. There are ways to filter it out using inductors or ICs.

[thakidd]

Ahh, it must be the amp. Unplugged the PC from it and hooked a different cable up to the dvr and the buzz came back. I also tried switching input channels with the same result. It is 10-15 years old (Marantz PM-100). The unit is in a server cabinet with network cables, power cables, switch, kvm, dvr, and a printer in the vacinity. I will try pulling it out and see if its picking up interference. I will also take a look inside if that doesn't work.



@Lance: Any pointers on where I might find info about filtering?

[Lance]

Have you tried a search engine?

To help reduce it, I'd say try and keep power lines away from your audio lines as much as possible. Also try to take loops out of power cables, leaving them in a loop creates an inductor. To my understanding not a huge one.

60hz hum is tricky to get rid of. I had fun with trying to solve that issue while building an amp in high school. I just ended up leaving it alone, because the noise was pretty quiet.

[Kiriakos-GR]

About the hum issue , some times it helps if you reverse the mains plug .
But it depends on the wall plug socket ,  In Europe it can be reversed easily. ( two pin sockets )

  

[thakidd]

@Lance: Thx for the link. Right after I saw your post, I got busy and have found a few options. I am about to pull it and connections out of the cabinet to see if that fixes the problem. Hopefully it does as it does appear to be kinda difficult to get rid of that hum (from what I've read so far).

@Kiriakos: I believe this isn't the issue in my case. The amp's plug is two prong (no group connection). This particular unit has a larger pin for one side of the plug then the other. I am guessing this was built in to prevent the exact problem you are talking about.

I did make one discovery...I had the amp plugged into the same UPS unit that most of the PCs and peripherals are plugged into. When I pulled the amp plug to check Kiriakos' suggestion, I decided to plug the jack back into a separate surge protector. The hum has significantly decreased but is still there. I'll report back when I have the results of the 'out of the server cabinet' test. I will also test plugging the amp into its own outlet on the other side of the room. Don't know if it could effect it, but maybe it is due to all of the crap I have plugged in on two 8 port UPS.

[williefleete]

what is the earthing like on the amp and computer, because computers use switchmode powersupplies they need an earth reference, if you have no earth you have a rather live case which is at least half mains this may be where your hum is coming from, it may be because you have a voltage difference between the earth on the PC and the earth/0 volt line in the amp and the hum is coming from a mains side line or bad power caps modulating the DC out of the switchmode or something

[thakidd]

@WillieFleete: You know, I have been scratching my head at why the amp does not have a ground included. I have a Yamaha receiver in the living room that does require the third ground pin. I guess they figured it was not needed back in 1989? I am wondering now if maybe I should ground the amp case to fix the problem. It is metal so I am assuming that the amp boards ground themselves in one way or another to the amp's case. Off to Google I go to check out your idea.

All of the PCs however are properly grounded. All of the PCs use Antec ATX standard supplies and all supplies are no older than 3-4 years old. They are are properly grounded. Some of the peripherals, however are not, such as the switch, kvm, router, etc. Those devices should not matter directly as they are not part of the audio system.

[Lance]

@WillieFleete: You know, I have been scratching my head at why the amp does not have a ground included.

Then your choice is clear. Build a new amp!