speakers for juke box

[cider]

I need some descreet speakers for an nsm wizard juke box.
It has a 4 ohm 250w per channel valve amp and eats speekers for a past time due i guess to mis matching.
The 16inch 4 ohm pa speekers i am currently using are fine but the other half wont let me have them in the newly decorated lounge

Do i need 4 ohm speekers? any sugestions greatfully received

 

[ColJack]

2 x 8's in parallel = 4 ohms..

 

[johnblack99]

Hi cider

If it is burning speakers the first thing to check for is for DC on the amp output (speaker wires). There should be none in a perfect world but up to 100millivolts would be believable. To check them use a multimeter on DC voltage setting. Measure between the Positive and Negative terminals with the speaker disconnected and NO source playing. Start with your meter on high range and work your way down in case it has rail voltage there. You don't want to add a fried meter to your woes.

As with everything be aware that there is enough electric in there to kill you twice. Are you sure it has a valve amp in it? That makes it even more deadly, up to 1000 volts can be found in those. If unsure about this contact a professional and let them take a look at it.

The impedance mismatching is more likely to kill an amp rather than a speaker.

For the same sound pressure an amp has to give nearly twice as much power into a 4 ohm load as an 8 ohm with the resultant extra heat and stress on components. It's much more complicated than that, but that's about all I understand of it! If the amp is designed to work with 4 ohm speakers all is good, some however will be marked as 8 ohm and that's where your troubles will begin.

How loud are you listening to it? If you are clipping the amp (running it flat out) that will kill speakers too.

If you can make your own speakers you can get them passed for style considerations and have a far nicer sound quality than PA speakers in a lounge sized room will give you. That's unless your lounge is huge and you are doing Motorhead at concert volumes. There are designs that fill that gap too http://www.passdiy.com/pdf/KleinHorn.pdf

Check out http://www.diyaudio.com/forums/ for inspiration. You may have to register to see some of the photos.

John

 

[ericmark]

As already said you are looking at impedance not resistance and pick up a book on transmission lines (Wires to speakers) and you would be amazed at the calculations required.

Once you start to turn up the power then you start to look at band pass filters and sending different frequency sounds to different speakers. Often these also remove any DC component.

You often find DC is put on the transmission line to power items and it is expected that de-coupling will be used.

You need to read the details of amp. To have 250W one would expect it to be correctly matched and not simply plugged in.

 

[cider]

thankyou gentlemen, def a valve amp (seems to be one each side) I will take care poking about inside (im used to fixing boilers live).
I think previous speakers have been destroyed by running flat out for long periods, but have ony ever run one set of 8 ohm at a time.
Might i be better of running 2 8ohm sets?
I would love to make some speakers if i had the time.
Always thought it would be great to conceal them in the base of a sofa, having the subs pointing down at the floor and tweeters in the arms pointing out

 

[johnblack99]

Hi Cider.

Have you had the chance to check for DC offset on your amps?

Do you have any paperwork with the jukebox? I would be wary of manufacturer power output claims, a genuine 250W per channel running flat out would put your windows through in a domestic room size. I would like to see a schematic if you have one just out of interest.

A lot of the advertised power output claims are made with misleading measurements and certain amp destruction if you attempted it yourself. One way to check. When you are in there, note the transformer primary fuse rating and work out how many watts can get into the power supply. It can't put out more wattage than it can get in. I assume you know about discharging capacitors before going near them. As I say, valve amps have step up transformers and very high voltages around the place. Be careful with it.

Speaker sensitivity is a far more relevant figure to be looking at over peak power handling. Google your speakers and see if you can find figures for them. Theres a good explaination of it here...

http://stereos.about.com/od/stereoscience/a/amppower.htm

I have a diy stereo chipamp using two LM1875s that puts out all of 20W per channel in my "best" system. It's feeding a pair of Bib speakers and that can go too loud for a reasonable sized UK front room but still sound stunning with only moderately efficient CSS FR125S drivers.

These are Bibs..... http://www.zillaspeak.com/bib-pics.asp

Dead easy first build and a superb sounding speaker. Must be against the back wall, preferably in the room corners. Placement is very important with them.

If you have some spare time there is a very informative site for all things hifi including building your own stuff here.... http://myweb.tiscali.co.uk/nuukspot/decdun/

Check out the "gainclone" pages on there and have a peep at the Arcam A60 modification article. If you can poke about in boilers and solder you can build far better sounding amps and replace the ones in the juke box for not that much cash. Pair that to a suitable speaker design that you both like the size and styling of and work from there. Everyone's a winner.

John

 

[cider]

Have loads of info and just read 2 x 200 watts at 2ohm