Audio Quandries (sp)

[Kingpin094]

I will start this off by saying that I am not an audiophile, so be nice. I am not a noob to electronics
just to making audio base circuits.

This is on of those projects that came up out of the pile of stuff and a general desire to take a
break from the other 12 projects that are currently going on. It started off by wanting to add some
tunes out in the garge to replace the little crapy FM radio that came with a Black and Decker cordless drill.
I had a pile of bose computer speaker systems that looked like I would be able to plug into the garage
computer (I know, I know electronics dork...) but they run on SPDIF. Since the computer doesn't have
digital outs I thought "Hey I am an electrical engineer I can just bypass the input and go stright to the amp!!".
Novel idea and given enough boards I could probally get it right but only had two tries and used them both up.

So know I have speakers and no amp.... hmm. Lets check the parts box to see what I can find. Low and
behold I have some LM4755's and LM1875's from a previously abandoned project.

Attached is the preliminay circuit digram for the 6 speaker system. The input will be from a coumpter sound card.
Assuming that voltage is ~1Vpp? Not sure crappy oscilloscope doesn't do really will with random signals.

Anyway I had some questions and I know there are some audio guys out there so here we go:
1) In the circuit I was going to put each channel of the input to 3 different outputs. A front, back and
sub for left and right. In order to get the input to go in all 3 places to I need to buffer input?
2) Is there going to be a problem in tying both inputs to the LM4755s together but still having seperate outputs?

Before I go off and start building does anyone see anything that might be an issue?

Thanks to all who reply,
kingpin094

 

[Leftyretro]

Looks workable to me. Usually the chip data sheets are the best resource to use. Be sure to work out the heat sink requirements. Here is a link to what I consider the best DIY audio site on the web. The articals and project sections are very informative and useful.

**broken link removed**


Good luck and keep us posted.

 

[audioguru]

The capacitors in the filters have their positions reversed for a sharp Butterworth filter.

The filter circuits must be fed from a low impedance buffer that can also be the buffer for the other amps.

Add some gain into the buffers since your music might not be as high as 1V.

The power at clipping into an 8 ohm speaker is only 5W per channel.

 

[Kingpin094]

After experimenting with a few things I have come up with the following revised circuit.

Let me know what you guys think.

 

[audioguru]

All four of the OPA2134 opamps did not have a DC reference voltage of 0V.
I added 220k resistors to ground to give the reference voltage to the inputs of the first opamps and I DC-coupled the filter opamps so they have a DC reference voltage and a very low source impedance.