Speaker Amp

[napalm]

I just got two speakers and a subwoofer, but no amp/head unit (no sure of the term). I'm looking to build one from scratch. I don't need it to play CDs or even have a volume control, I'll be hooking these up to my computer. It just has to supply enough power so I can use the system.

This is the exact system here - **broken link removed**

That one has the head unit, I just have the two speakers and amp as pictured here http://i14.tinypic.com/66vhdg0.jpg

The speakers are unmarked except for a sticker that claims the speaker system impedance rating is 6 ohms. Luckily, because RadioShack sells the product, the user guide and technical specifications are online **broken link removed**

I know how speakers and headphones work. The wires leading out of them look like this http://i8.tinypic.com/4lqe9vr.jpg

I'm going to convert the bare wires into phono (RCA) plugs. That'll make it easier to disconnect them from the amp. I know what all the bare wires are for, but why does the sub have an additional wire with phono plug running out of it? Is it just to supply more power to the sub? Perhaps the sub has a built in amp? I'll have to take it apart and check.

Anyways, what I need is a a circuit diagram. It needs to have a 1/8 jack so I can run an audio cable from my computer to the amp. Coming out of the amp it needs two phono jacks for the left and right speaker, and two phono jacks for the sub. The diagram can have other connections, it doesn't matter. I can figure how to substitute a phono jack instead.

What I'm having trouble doing is boosting the power. From what I know right now, I can receive the audio from the 1/8 jack, split the two channels, and send each one to their respective phono jack. I guess for amp, rather then sending it to the phono jack, I'd have to send it to a transistor switch and that does something and now I'm lost...

I need to boost the power and I need to figure wtf I'm doing with the sub (convert the stereo to mono and feed the whole thing into the sub?). A complete diagram would be really nice, but just a diagram to boost the power and a little advice on the sub is all I need. I'm not sure what the watt rating is. According the RadioShack, its 400watt total (125watt per channels x2). Maybe someone could look at the specs and tell me whats up.

As far as the circuit, I looked through these two sections
http://www.discovercircuits.com/A/a-amplifiers.htm
http://www.discovercircuits.com/A/au-audio.htm
But I'm not really sure what to look for

Well, now that I've taken up this much of your time, any help?

 

[RadioRon]

You are ambitious. Here is an example of an amplifier kit that you could use between PC and speakers. You would need one amp for each speaker, and you would have to find or build a +/-30V power supply with enough guts to power all three amps. They also have some power supply advice in an application note link.
**broken link removed**

This is just an example, there are many makers of modules or kits like this that would work for you.

Here are some more:
http://www.siliconchip.com.au/cms/A_100503/article.html
**broken link removed** (scroll down to see many)
**broken link removed**
http://www.dckits.com/audio.htm (scroll down)


As for the subwoofer, you can either find a specific subwoofer amplifier kit, of which there are some out there, or you can simply put a low pass filter inline with the input of the power amp. Of course, you have to choose a power amp that puts out power at a low enough frequency, like down to 20Hz. I would feed the subwoofer section by simply tapping off one of the two stereo channels. I find that the bass energy is often included in both channels so this works ok. The low pass filter idea would need a circuit and perhaps you can find one or someone can suggest something. A cutoff frequency at around 90Hz might be ok. Don't they sell crossover networks for this purpose?

 

[napalm]

I checked out that kit, quite cool. It looks like I'm going to buy a kit rather then build it from scratch, which is fine. I'm looking at some of the stereo amps (just two mono amps in the same box). **broken link removed** is 25$ and **broken link removed** is 60$, I'm pretty cheap, is 100w really worth it? I don't throw parties or anything. I really want someone to look at the specs I posted, because I'm not sure what the speaker rating is. I think each speaker is 125w at 6ohm, but I can't be sure. That said, it seems a waste to build a 50w amp for a 125w speaker. The 100w need dual 50vdc power supplies and the 50w needs dual 24vdc power supplies.

I'm wondering if the sub has a built in amp. That may be what the rca phono plug is for, feeding a low level signal to an on board amplifier. The sub has an on/off switch with a red led to signify on/off status. Is this any indication to the existence of an on board amp? I would open it, but there are no screws, just wood glued together (typical short term manufacturing).

 

[audioguru]

The side speakers have an amplifier with "125W" per channel with 10% distortion. The sub-woofer has an amplifier with "150W' at 10% distortion.
The entire system has a max draw of only 265W from the mains. So maybe the amplifiers are 40W plus 40W plus 55W. Then the total amplifier output power is 136W and the amp heats with the other 135W.

You need two 40W (or less) amps for the side speakers and one 55W (or less) amp for the sub-woofer. You also need a active crossover system so that freqencies above 120hz go to the side speakers and frequencies below 120Hz are added into mono and go to the sub-woofer.

 

[mneary]

This is going to be for your computer station? 50W per channel is a lot. 100Wpc is only 3db higher.

The sub has an on/off switch with a red led to signify on/off status. Is this any indication to the existence of an on board amp?

The sub has an on board amp if it has a source of DC power.

50W is probably enough for me, but since my amplifier came with 100Wpc and I never clip it (I think), I don't really know for sure.

Most people spend most of their listening time in the 1 watt range. 50W gives you a lot of headroom to prevent distortion and/or clipping.

 

[napalm]

You need two 40W (or less) amps for the side speakers and one 55W (or less) amp for the sub-woofer. You also need a active crossover system so that freqencies above 120hz go to the side speakers and frequencies below 120Hz are added into mono and go to the sub-woofer.

Alright, thank you very much. I'll get the 30W stereo amp and the 50W mono amp kits. Now, where do I go about acquiring said "active crossover system"?

Thanks, you were very helpful.

 

[RadioRon]

If the subwoofer is a powered type, you won't need an amp for that. You should find this out first. If it does, I wouldn't bother with any crossovers or anything, and just count on the speaker frequency response to help limit the low bass only to the subwoofer.

You should give us a photo of the place where the connections go into the subwoofer, so we can agree or disagree with you. Then you might want to test the subwoofer. You would have to provide DC power to it and then feed an audio source into it and listen to what comes out. If nothing comes out, maybe it doesn't have an internal amp after all.


I hope you know about "phasing" your speaker connections.

 

[audioguru]

Look in the owner's Manual. The sub-woofer is shown with only two wires like the other speakers. No RCA plug with an extra wire.
Somebody added the RCA plug.
So I doubt if the sub-woofer has electronics inside.

 

[Speakerguy]

LM3886 makes a great little chip amp. Order a rail of 5 LM3886TF (the insulated/isolated ones) chips from NatSemi as samples. They take a while to get to you but hey, they're free and you get two extra to blow up if you make a mistake. They are basically BIG power opamps. 68w into a 4 ohm load IIRC.

Get 3 of the $6 amplifier boards from www.chipamp.com. The components are easily found at digikey or elsewhere.

Make your own power supply with surplus parts, etc. You'll need +/-30VDC or thereabouts. Specsheet will tell you everything.

As for a heatsink, somone always has something on sale somewhere (ebay, forum for sale threads, etc). I could lop you off a big hunk from our stock but that's against work policy.