Building passive computer speakers

[lti]

I have two Peerless W3-LK76-PBK drivers that I want to use as computer speakers. The built-in amp in my old Compaq is the only amp I have that is capable of powering speakers with such low impedance (the actual impedance seems to be about three ohms). I plan on putting these speakers in small sealed boxes made of 1/2" plywood. The boxes will have internal dimensions of 4"x6.5"x2.5". Bass reproduction isn't an issue since these drivers don't produce much bass (less than all other 3" full-range drivers I have heard).

Unfortunately, the speakers are not actually shielded and have strong magnets. Is it possible to add magnetic shielding?

Are there any other problems with this design? I have the drivers and tons of wire, but I don't have any other parts yet.

[dumpystig]

Re: Building passive computer speakers

How about lining the inside of your boxes with aluminium kitchen foil? I reckon there has to be at least some benefit from doing this.

[kaboom]

Re: Building passive computer speakers

Aluminum foil won't block the static field from those magnets. Unless you are close to a CRT monitor or magnetic media of any kind, you don't even need shielded drivers.

Znom is 4 ohms. DCR is 3.4 ohms, but the amp feeds them AC- you've got the reactance of the voice coil with increasing frequency. And the characteristics of the loudspeaker system will have an effect on the impedance as well.

Any idea on the output power of that amp? It's something to keep in mind when you're starting with 85 dB W/M drivers. You may not need much with 'near-field' 'puter speakers.

-Paul

[dumpystig]

Re: Building passive computer speakers

Yeah you're right, I'm a fuckwit, duno what I was thinking

[dood]

Re: Building passive computer speakers

Do a search for bucking magnets on Parts Express. They cancel out the magnetic field that the magnets on the driver produce.

[lti]

Re: Building passive computer speakers

Aluminum foil won't block the static field from those magnets. Unless you are close to a CRT monitor or magnetic media of any kind, you don't even need shielded drivers.

Znom is 4 ohms. DCR is 3.4 ohms, but the amp feeds them AC- you've got the reactance of the voice coil with increasing frequency. And the characteristics of the loudspeaker system will have an effect on the impedance as well.

Any idea on the output power of that amp? It's something to keep in mind when you're starting with 85 dB W/M drivers. You may not need much with 'near-field' 'puter speakers.

-Paul

The speakers will be placed next to a CRT monitor. One speaker will be placed close to the computer.

The impedance seems to be less than four ohms. Any amp that is not designed to power speakers with an impedance lower than 4 ohms overheats or produces distorted sound.

The amp is based on a TDA1515BQ IC and is powered from 12V. The output power should be around 5 watts into a 4 ohm speaker.

Do a search for bucking magnets on Parts Express. They cancel out the magnetic field that the magnets on the driver produce.

These drivers appear to already have bucking magnets, but they don't do much.

[kaboom]

Re: Building passive computer speakers

No, the impedance is four ohms. The DCR is just that. Also, the coupling caps will limit what you can get with decreasing frequency. They are 1000uF in the app ckt in the datasheet.

Don't sweat the impedance. Measure the primary of a large power transformer or induction motor. Usually less than an ohm to a few ohms- a dead short right? But their impedance is higher. Impedance is made up of reactance and resistance.

Look for other drivers that have the shielding bell over the magnet. You don't necessarily need 4 ohm drivers if you do the math and determine you can reach your target SPL, based on the power you have and drivers's sensitivity. If the output rating of that amp is accurate, the following is true: 8-ohm drivers with 89 dB/W sensitivity will play as loud as the ones you suggested, -provided- your box makes them happy.

You could always make your own mu-metal cans, but that would cost more than the drivers!

-Paul

[lti]

Re: Building passive computer speakers

I have started to build the boxes for these speakers. I have one almost complete. I just need to cut a hole for the driver and find some terminals. Also, I remembered that I suck at woodworking.

If I had the money to spend on better drivers, I wouldn't be trying to make these drivers work.

[lti]

Re: Building passive computer speakers

I have one box finished and the glue is drying on the other box.

These will replace my cheap computer speakers.

[lti]

Re: Building passive computer speakers

I finished building these speakers. They sound as good as I expected.