LM3886 in bridge mode with Single supply polarity - Schematics neded - diyAudio
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Old 8th August 2013, 03:27 PM   #1
fab is offline fab  Canada
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Question LM3886 in bridge mode with Single supply polarity - Schematics neded

Hi

I saw the Single Supply configuration suggested in the LM3886 datasheet.

Does anybody have a readily available validated design implementation of a LM3886 audio amplifier in bridge configuration mode (LM3886 x 2 and NOT parallel) for single power supply polarity of +37.5Vdc to get about 48 Wrms into 8 ohms full range speaker (without any crossover) ?

No major turn ON or OFF thumps since there would be no relay used to connect audio signal to speaker.
NO SMPS allowed and the DC power supply of +37,5 V dc is already provided.

Thanks

Fab
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Old 9th August 2013, 03:32 AM   #2
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without looking at the schematics too hard...
Build 2 single-supply amps and invert the input to one...
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Old 9th August 2013, 03:37 AM   #3
fab is offline fab  Canada
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Originally Posted by sofaspud View Post
without looking at the schematics too hard...
Build 2 single-supply amps and invert the input to one...
Yes I know the principle but what about the output caps and the power supply middle point virtual ground and so on.... I mean a practical circuit that works without a flaw.

Thanks

Fab
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Old 9th August 2013, 03:50 AM   #4
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The ps virtual ground could be easily made the same for both amps.
Output DC shouldn't be a problem... if you have measurable offset a single cap (per channel) should take care of that (and should be expected in any single-supply circuit).
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Old 9th August 2013, 04:43 AM   #5
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It could be tricky to avoid the turn-on transient with a single supply and output cap. When bridging two outputs, I may be wrong (I haven't actually done this) but I think the two outputs would somewhat track each other during the turn-on time output cap charging moment, leaving a relatively small differential, which is what the speaker sees. If the output caps and any negative feedback related caps are matched to 1%, and the input half supply node is the same for both amp circuits, I wouldn't be surprised if there was no audible turn-on transient. Linkwitz (at Linkwitzlab.com) bridges the 3886's for the woofers in his Pluto speaker system. The schematics are posted (if you look long enough). That might be a good starting point for some part values, but he does use a bipolar supply.
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Old 9th August 2013, 11:40 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by fab View Post
Hi

I saw the Single Supply configuration suggested in the LM3886 datasheet.

Does anybody have a readily available validated design implementation of a LM3886 audio amplifier in bridge configuration mode (LM3886 x 2 and NOT parallel) for single power supply polarity of +37.5Vdc to get about 48 Wrms into 8 ohms full range speaker (without any crossover) ?

No major turn ON or OFF thumps since there would be no relay used to connect audio signal to speaker.
NO SMPS allowed and the DC power supply of +37,5 V dc is already provided.

Thanks

Fab
to get to your target of 48W into 8ohms you need each half of the bridged amp to be able to deliver 24W into 4ohms.
That needs an output of ~28Vpp and 3.46Apk.
This is achievable from a supply of +37.5Vdc, 0Vdc
All you need is dual polarity signal, i.e. a balanced signal, to feed the two amplifiers.

Use the mute/unmute to turn on both chips simultaneously.
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Old 9th August 2013, 11:43 AM   #7
AndrewT is offline AndrewT  Scotland
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Bob,
you are probably right.
The two output caps should charge up at the same rate and that way the speaker sees the same voltage on both leads with the resultant zero noise during switch on.

But I am only guessing. It needs to be tested.
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