So heres what i decided on for the comp speakers, now how to connect? - diyAudio
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Old 7th October 2003, 01:22 AM   #1
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Default So heres what i decided on for the comp speakers, now how to connect?

I decided on using 5 x LM1875 for the front, center, and rear and a LM3886 for the sub. Now, I don't know how to connect it. I assumed that the front and rear would be a stereo plug, and the center would connect with a mono plug. But I went to best buy, and all of them use a stereo plug for the center. How do I connect the system to the computer, and do I have to worry about any crossovers for the center, or does the SC do it for me?

Thanks, Mike
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Old 14th October 2003, 11:26 PM   #2
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hey dude

It very much depends on the soundcard, although the most common 5.1 channel sound card topology seems to be 3 x stereo 3.5mm jacks for the 1) front 2 channels, 2) the rear 2 channels and 3) the LFE and centre channel.

Now, the issue can be a little annoying here, the LFE channel is not the same as the sub woofer channel, (unless your soundcard drivers/software will mix it for you) the other 5 channels are full range, so you would probably have to have 5 two way crossovers, crossing over at (opinion alert!!) no greater than 100hz, then use an op amp mixer to combine the sub channels and LFE channel.

I really recommend this setup, i got something similar just built, using Lm1875's (4 channel biamped!) Remember u can use a surface mount feedback resistor!

Nik
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Old 18th October 2003, 02:47 AM   #3
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Thanks Nik, but when the LM1875s arrived, I thought of an even better use for them, to make a 4-channel stereo amp that can be switched into 2 channels bridged. For the PC speakers, I changed my mind on 5 channels since my sound card is only a 4.1. So, I am using a LM4766 for the front and back (only 2 are needed since it is a stereo chip) and I can get about 40W per channel. For the subwoofer, I was thinking about using a LM3886, but am not sure on it. If I were to use the 3886, would it work better for this application in normal or bridged mode?

Finally, about connecting the sub amp. Should I feed it off of the front or back? And, how do I combine the Left and Right signals without shorting them together?

Thanks for your help, Mike
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Old 18th October 2003, 01:24 PM   #4
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Hi Mike

Hmm, so with regard to the LM3886 it depends on your application as to which mode will give best performance. If you plan to use a high effeciency sub design like a ported configuration with a high effciency driver then i think you will find normal mode (~56 watts) will give more than acceptable SPL (based on my observation of some videologic crossfires which uses a 40w sub amp + ported woofer to achieve some very pleasing volume levels). However if you plan to use a lower efficiency design such as a sealed (maybe equalised) configuration using only one driver, then bridged mode may become necessary/desirable. (Remember though when in bridged mode the amps will only see half the drivers impedence!!)
Also there is nothing to stop you building a two 'normal mode' amps, and having a sub driver each for them, which is IMHO a more flexible way of doing things, plus it gives you the option of stereo subs etc.

Tell me what sort of sub you have/want and i can give u some better advice than that

Where the sub input should come from, all 4 channels sounds like the only way to me, to do this you can use 4x 2way crossovers, like two of this one (re-configured for around 100hz) .. http://sound.westhost.com/project09.htm not at all complicated, but does require a lot of boring repitition (i suggest quad opamps to make your life easier) and can easily be made on strip board or bought as PCB's, then the sub/low outputs can be summed in a number of ways, a simple one being 4x22k reisitors (1 per channel) to a single point, into a non-inverting op amp buffer, and from that buffer to your sub amp(s)

This probably sounds like a lot of work, however building a good quality 4 channel amp with a sub is!!

Hope this helps, Nik
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Old 19th October 2003, 02:40 AM   #5
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My subwoofer is a 10-inch 4-ohm model. I right now drive one of these speakers from a single 3886 and it is in an about 15inch wide and tall by about 8 inches thick wood cabinet made out of 3/4 inch cherry wood. There are no ports and it is sealed. I have no sub out on my amplifier so I use the headphone out. My crossover isn't that great, since no matter what I do, I just cant seem to get a Passive line level x-over to get a low enough cutoff where I don't hear voices. I right now use a 25K potentiometer, and a 2uf capacitor. You can easily understand voices. It sounds pretty good, but doesn't go as loud as I would like before distorting.

I know though that the 3886 cannot drive a 4-ohm load in bridged mode. I may just spend the money and buy an 8-ohm speaker, since the bridged 3886 should make a drastic improvement. Or, what about a 4-ohm resistor in line with the speaker? Or I have some of those audio output transformers from RatShack. Would those do the trick?

What chip would you reccomend that will work better than the 3886? Can the LM3875 in normal mode sound better than the 3886? And could it drive 4-ohm in bridged?

Thanks again for your help, Mike
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Old 19th October 2003, 02:42 PM   #6
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First of all for this application the LM3886 is probably the better chip given that it has a higher max power output, for a subwoofer application it is unlikely (IMHO) that the small differences between the 3875 and 3886 would be noticable (unless you're really picky) and no, I'm pretty sure that the 3875 would not be happy driving a 2ohm load, although I have not tried it.

By the 'sound' of your sub woofer filter, it would seem that your current LM3886 amp is doing a lot of extra work that is either inaudiable or unwanted, and thus if you used a better filter you could probably have higher, undistorted SPL's.

I would actually suggest putting the sub and associated amps on hold for the moment. Instead construct a high order active crossover as previously suggested and also construct the sattellite's and amps. Then see if your current sub + amp can 'cut the mustard' if not at least you know what you need, rather than having to predict what you need.

Also i dont recommend putting a 4ohm resistor in series with a 4ohm cone, it will more or less halve the power to the speaker! With regard to the output transformers, i have no idea as i've never used them before, sorry perhaps another member may be able to help here.

Hope this helps, Nik.
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