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Old 8th April 2008, 07:26 PM   #1
SRMcGee is offline SRMcGee  United States
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Default 100 Watt Chipamp Recommendations

Folks:

I'd like to build a chip amp for my daughter. I have constructed four LM3875-based integrated amps to date, but all have been mid-powered (about 45 watts/channel into 8 ohms). On 86 dB speakers, it doesn't take much volume before clipping is audible on those amps.

For this next project, I'd like to build something a bit more powerful. The audio inputs into this new amp will all be single-ended and the speakers will probably have a nominal impedence of 8 ohms. I'd like to shoot for an integrated stereo amp capable of producing 90 to 100 watts/channel.

Can anyone offer a recommendation? I'll look for PCBs once the requisite chip has been determined.

Many thanks,
Scott
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Old 8th April 2008, 07:43 PM   #2
CJ900RR is online now CJ900RR  Sweden
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128W into 8ohm at +/-30V according to Nationals Overture Design guide. First board is up and playing now. Sound is excellent
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Old 8th April 2008, 07:55 PM   #3
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Quote:
Originally posted by CJ900RR
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128W into 8ohm at +/-30V according to Nationals Overture Design guide. First board is up and playing now. Sound is excellent

Is this three in parallel? If so, how did you figure 128W in to 8 ohms at +-30v?

Assuming ~28v peak
28v peak = 19.8v rms?
19.8v rms in to 8 ohms = 49W rms?
28v peak in to 8 ohms = 98W peak?

That's how I understand it anyway. It's may be wrong..
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Old 8th April 2008, 09:07 PM   #4
CJ900RR is online now CJ900RR  Sweden
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Quote:
Originally posted by markiemrboo



Is this three in parallel? If so, how did you figure 128W in to 8 ohms at +-30v?

Assuming ~28v peak
28v peak = 19.8v rms?
19.8v rms in to 8 ohms = 49W rms?
28v peak in to 8 ohms = 98W peak?

That's how I understand it anyway. It's may be wrong..

Yes it's 3 in parallel. I filled in the values I have used on the board in Nationals XLS-dokument (http://www.national.com/appinfo/audi...gn_Guide13.xls) and didnt bother to calculate by my self at all. Dont know if their calculation is correct.
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Old 8th April 2008, 09:24 PM   #5
juma is offline juma  Serbia
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For 8 Ohms speakers bridged topology might be a better choice (although it serves me great with my 4 Ohms speakers).
You can see the details here:

http://jrexton.ecv.ms/GC3.htm
http://jrexton.ecv.ms/JFET-Buffered-opa1632-Bridg.gif
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Old 8th April 2008, 09:25 PM   #6
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Quote:
Originally posted by CJ900RR



Yes it's 3 in parallel. I filled in the values I have used on the board in Nationals XLS-dokument (http://www.national.com/appinfo/audi...gn_Guide13.xls) and didnt bother to calculate by my self at all. Dont know if their calculation is correct.
That figure is for a 2.6 ohm load, not 8 ohm
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Old 8th April 2008, 09:44 PM   #7
juma is offline juma  Serbia
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Quote:
Originally posted by markiemrboo


Is this three in parallel? If so, how did you figure 128W in to 8 ohms at +-30v?

Assuming ~28v peak
28v peak = 19.8v rms?
19.8v rms in to 8 ohms = 49W rms?
28v peak in to 8 ohms = 98W peak?

That's how I understand it anyway. It's may be wrong..

Your understanding is pretty much on the spot. Extensive paralleling of chips improves current availability <=> more power on low impedance loads (2-4 Ohms).
Higher impedance loads are better served with more voltage - bridge topology (but there still has to be enough current to support the demand - Ohm's Law can not be cheated)
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Old 9th April 2008, 03:19 AM   #8
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Default Re: 100 Watt Chipamp Recommendations

Quote:
Originally posted by SRMcGee
Folks:

I'd like to build a chip amp for my daughter. I have constructed four LM3875-based integrated amps to date, but all have been mid-powered (about 45 watts/channel into 8 ohms). On 86 dB speakers, it doesn't take much volume before clipping is audible on those amps.

For this next project, I'd like to build something a bit more powerful. The audio inputs into this new amp will all be single-ended and the speakers will probably have a nominal impedence of 8 ohms. I'd like to shoot for an integrated stereo amp capable of producing 90 to 100 watts/channel.

Can anyone offer a recommendation? I'll look for PCBs once the requisite chip has been determined.

Many thanks,
Scott
Hi Scott!
I'm wondering if your daughter is a musician? Otherwise 107db is a little bit intense (ear damage) for home use. Anyway, about 120 watts will do it.

Easier: Your current amplifiers with 91db speakers (quite a lot of them at Parts Express) will also do that same 107db. Just a thought. Personally I like that option best because "lively" speakers tend to do more intense dynamics, which is somewhat more helpful to modern recordings.

For a few amplifiers worth of production, PCBs are somewhat unnecessary due to the innovation of "Experimenter's Board." That's just like veroboard (phenolic) except with circular solder pads at every hole (avoids the usual tangle). Its nice because you can make adjustments to the layout. Its purpose is actually testing before ordering PCB boards, but it works well for home amplifiers too.

That's all I can think up this late in the evening. Have fun!

EDIT: The reason that I was thinking about speakers is. . . the next step after exposing most speakers to more than 100 watts, is a purchase of replacement speakers, so might as well just skip the speaker cooking step and get 91db speakers instead.
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Old 9th April 2008, 06:25 AM   #9
CJ900RR is online now CJ900RR  Sweden
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Quote:
Originally posted by markiemrboo


That figure is for a 2.6 ohm load, not 8 ohm

Ah I see. I read it wrong. But they can be bridged
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Old 9th April 2008, 08:45 AM   #10
CJ900RR is online now CJ900RR  Sweden
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Quote:
Originally posted by markiemrboo


That figure is for a 2.6 ohm load, not 8 ohm

But then again... isnt that the purpose with paralleling them? That each LM3886 see's only some-what-ohm, in this case 2,67ohm/IC. They share the power dissapation equally. The result is a higher output. In the case of 8omh it might be more efficient to use a bridged or a bridged/parallel config...
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