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Old 5th August 2009, 10:53 PM   #1
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Default LM3886 in parallel

Hi

I was using only one of my 2.1 DIY amp to push my subwoofer, since the Onkyo receiver was doing all the work, bu the I found out that I can use 2 LM3886 in parallel to push my subwoofer, duplicating the power.

Theorically now, I am using 100w instead of the 50w, but what is the benefit of this? I didn't notice any difference in the sound at all. The only difference is that now they are a little bit warmer than before ( maybe becouse of the 2 amps working, duh!).

I was expecting that the sub was going to be a little more louder, but it is just the same volume.
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Old 5th August 2009, 11:04 PM   #2
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Doubling the power should ideally only give you a 3dB difference in maximum output level. Depending on the speaker etc. you'll probably get a little less than 3dB which, due to the logarithmic nature of human hearing, is only a barely perceptible difference. Again, we're talking about _maximum output level, which you may or may not ever reach during regular listening - possibly only during loud bass drum hits or explosions in movies.

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Old 5th August 2009, 11:16 PM   #3
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And making they work in a bridge circuit is the same thing?
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Old 5th August 2009, 11:28 PM   #4
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This article here explains it better than I can:
http://en.wikipedia.org/wiki/Bridged...led_amplifiers

-j
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Old 6th August 2009, 11:04 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
if you have a chipamp delivering a maximum output of 60W into 8ohms, then it is effectively driving the speaker with a 21.9Vac signal (~31Vpk).
Putting two of these chipamps in parallel gives the same drive voltage to the speaker, i.e. 31Vpk.
The amps try to deliver equal current to the speaker load, so each amp effectively deliver half the current that a single chipamp would do.
The parallel chipamps driving a 16ohm load (what each of the parallels sees as a load) will probably achieve a maximum output of ~32Vpk (+0.3dB).
If you can hear that increase of 0.3dB you will be doing very well.

Don't even think about bridging the chipamps.
Each chipamp in the bridged pair will see an effective load of half the speaker impedance.
A 60W into 8ohm amplifier will see a 4ohm load and try to send that maximum signal of 31Vpk into the 4ohm load. It will shut down repeatedly due to over current and over temperature.
It might just about manage 28Vpk on a transient. That is equivalent to ~56Vpk into your 8ohm speaker. About 200W into 8ohm.
You are going rapidly into melt down.
You will need to redesign the amplifier to allow it to be bridged.
Just to put you off a bit more all these chipamps are crippled by the very limited peak current they can deliver and this gets worse as the chips heat up.
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Old 9th August 2009, 03:31 AM   #6
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i guess u may have more hearable difference if you have 4 ohm subwoofer rather than a 8 ohm woofer. just check your subwoofer impedence
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Old 9th August 2009, 03:56 AM   #7
lgreen is offline lgreen  United States
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Default confusion

i think there is confusion on the difference between paralleling the amps and bridging the amps. There will be no change in gain with parallel.
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Old 9th August 2009, 04:30 AM   #8
infinia is offline infinia  United States
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Default LM3886 in parallel needs high precision

Quote:
Theoretically now, I am using 100w instead of the 50w, but what is the benefit of this? I didn't notice any difference in the sound at all. The only difference is that now they are a little bit warmer than before ( maybe because of the 2 amps working, duh!).
[/B]

The reasons for paralleling is the presumed ability to drive a 4 ohm speaker with same rail voltages of an 8 ohm, IF the gains are perfectly matched. If not then the mismatched voltages produced will create circulating currents in the amp chips causing heat and inefficiencies. Tiny voltage mismatches can produce huge currents across the small output ballast resistors (typ 0.1 ohm). Probably best to tweak the final gains to within 0.1% or less. The initial tolerance build up using 4x 1% resistors is not good enough. The circulating currents may be the cause of your running hotter. Parallel operation s/b really done by extra precision and not for newbies to achieve the 3dB power gain.

circulating currents with 0.1%
Ipk =0.1%*30V /(2*0.1)=1.5A still high
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Old 9th August 2009, 06:32 AM   #9
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Default LM3886 parallel circulating current

Scratch calc above is 1% not 0.1
See AN -11192 fig6 http://www.national.com/an/AN/AN-1192.pdf


Using four ultra precision 0.1% resistors for 2 gains.
estimate initial tolerance using root sum of squares method ~final gain toler between amp outputs = 0.2 %

calculate circulating current using Vo = 34Vpk undistorted

Ipk = 0.002*34V/(2*0.1 ohm) = 0.34 Amps peak
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Old 9th August 2009, 07:00 AM   #10
infinia is offline infinia  United States
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Default Re: LM3886 in parallel

Quote:
Originally posted by Denis.BR

I was expecting that the sub was going to be a little more louder, but it is just the same volume.
Unless you boost the power supply voltage to max for 8 ohms
And drop your speaker impedance, it will be the same maybe worse (Vgains matched?). If you just add another sub in parallel you can see the most gain in volume and power.
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