summing amp

[sofaspud]

Since the current in an op amp summing amplifier "goes around" the op amp, what are the important specs to consider in choosing the IC? I have in mind an audio subwoofer application.

[Intimidator#3]

one of the members here has this site that may be worth looking at http://www.electronet.dyndns.org/electr ... ctures.htm

[sofaspud]

I had looked at that page previously and it seemed to be a dual voice coil driver. I acquired 1 of a two-way speaker pair and I was thinking of disconnecting the tweeter and using the rest as a sub for a pc multimedia setup. Mostly I was wondering to what extent the chip noise gets summed with the signal at the output, and how close my technical understanding was to correct.

[Mike]

The speaker used in that project was actually a single voice coil, 8-ohm speaker, which is rare for car drivers. I have upgraded that speaker, though, to a 15" dual 4-ohm voice coil speaker which in running in series (8-ohm). It is made by Lanzar/Eclipse. The box says Eclipse, but the driver looks identical to a 10" Lanzar driver. I'm going to update my site with new pictures soon.<p>Are you talking about the preamp to sum the left and right inputs? If so, I have always wanted to know how, but I really don't think it is neccesary. I just take the signal for the sub from either left or right. With bass, it's usually the same in both channels, so summing the channels together is not neccesary.<p>What type of speaker is it? (size, wattage, impedence, etc.) I don't know how much power you are looking for here, but a TI OPA549 chip works really well in a subwoofer. One will power the speaker up to about 75W RMS, assuming it's a 4-ohm, or two will produce around 150W, assuming it's an 8-ohm. These chips can tolerate low impedence loads better than national's chips (at least in my opinion, and don't have as much trouble driving a 4-ohm in bridge or 2-ohm in single chip mode, but they will need a large heatsink. The only other problem is you can't go much over a 42-44VCT transformer, because the max voltage is only +/-30VDC.<p>My sub uses a bridged 549 powered by a 175W transformer. It sounds very good and has a lot of power, and it's only being run on a 175W transformer, limiting power output. If I were to upgrade it, I could get a lot more out of it.

[sofaspud]

Ah ha. Well I saw the 2 output amps and figured it was DVC. This speaker I have is 10" 8 ohm and probably only about 20W or so. I'd be happy driving it with 5 to 10 watts max.
I have some cookbook subwoofer L+R circuits, summing amps, and mixers. I was going to study what their differences are because it seems to me they all perform the same basic function - 2 channels into 1.

[Mike]

Yes, the amp is designed to run in bridge mode, or, if you really wanted to you could run each coil seperately, but wouldn't get as much power as with bridge.<p>I don't know what type of sound you want, because although it is 20W, you can make it sound like a lot more, with porting, bandpass boxes, radiators, etc. Sealed will produce very controlled bass, though nor as efficiently as the others. Ported will get louder sound but at the expense of control of the bass. Bandpass is for loudness, but only at certain frequencies and is very very thuddy sounding. Radiators are another thing to look into. They act like a port, but since they move like a speaker, you don't have to deal with tuning and only getting optimal sound at a certain frequency. You also get the control of a sealed box. They aren't that badly priced either. I believe they are around 20 bucks at parts express for a 10"<p>Anyway, back to the amp. If you want to, you can use that summing circuit, but as I said, it honestly doesn't make much of a difference. If you are looking for around 20W, I would say use either a LM1875 at full power, or a single OPA549 running from a 30-40VA transformer so if you ever change the speaker down the road your amp is ready for it. I only suggest this because there is very little difference in the cost of the chips, and you can most likely get samples if you want of either.<p>Let me know if you need a PCB layout for either a 1875 or a 549 chip, I have both. I also have a pcb layout for the bridge 549 amp I use in the sub on my website. Other options are lm3875 (56W, 8-ohm only) or LM3886 (68W, 4 or 8-ohm). Again, I've got PCBs for all of them.<p>-Mike

[sofaspud]

I appreciate the offers, but what I was really hoping for was someone to chime in with summing amplifier info and also how that circuit compares with a standard L+R sub circuit or mixer.
The speaker is already mounted in a ported box. I hooked it up to a tabletop FM radio through an old mono 5 watt tube PA amp I have and it sounded just fine. I'd like to combine stereo channels for sub use though. I'd hate for Ron Carter to be soloing on the wrong channel!

[MrAl]

Hi Sofa,<p>One of the most important specs for audio work
is the noise spec. Offset and stuff like
that doesnt matter unless you're planning
to direct couple.<p>What exactly are you trying to do...
design a preamp or a full driver complete
with output stage to drive the speaker?<p>Take care,
Al

[sofaspud]

I acquired that 2-way spkr cabinet and got the idea that it could be made into a sub for a pc. I would just need a L+R preamp and a power amp. It may or may not need a low-pass filter. I'm a dedicated parts scrounger so this was more a "use whatcha got" project than a hi-fi design.
I did take a look at some subwoofer circuits and mixers and found that, yes, they are all summing amps. My understanding of summing amp operation led me to believe that the signals are summed at the input, but op amp noise would get summed with the signal at the output. It seemed to be sort of an unusual circuit in that the signal goes around the op amp rather than through it. I hope I got that right.