I would like to have LM3875 bridged, do you have schematic? - diyAudio
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Old 14th September 2006, 03:57 PM   #1
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Default I would like to have LM3875 bridged, do you have schematic?

Please, post it to me.

I made a fast search and i have found LM3875 bridged but using an auxiliary Op amp in the input...when i want to try them alone, without auxiliary parts.

I have two units with pretty red boards and i am thinking to install them inside a low powered subwoofer that i have, using the active filter that is already there and the speaker that is already there too.

The sub woofer speaker, included in the enclosure, will be protected using series resistors, the power transformer will go inside the speaker enclosure and the LMs will be cooled using external back mounted heatsinks.

The full power, 8 ohms speaker output will have new connectors to feed sound for two 4 ohms speakers, connected in series and distant from the amplifier.

The internal speaker will be just monitoring, having a small sample of the entire power.

I made mods in the electrolitic condenser and my units are reproducing very low frequencies..going flat down 10 hertz.

Thanks in advance.

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 14th September 2006, 07:04 PM   #2
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Upupa Epops has the one, for LM4780.
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Old 14th September 2006, 07:31 PM   #3
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Default I am afraid, Pavel, that this chip will not be perfect with 4 ohms.


Because when we connect two channels in bridge mode...the 8 ohms speaker, will be felt by each amplifier, as beeing 4 ohms in the place of the 8 ohms speaker load connected between them... i think each amplifier may "perceive" 4 ohms.

I am not absolutelly sure about that....please confirm.

People use to say that LM3886 is more adequate to 4 ohms loads.

I am afraid that this will not work fine.

regards and thanks,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 14th September 2006, 07:48 PM   #4
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Quote:
i think each amplifier may "perceive" 4 ohms. I am not absolutelly sure about that....please confirm.
Visualise it like this: when one amp in the bridge is outputting, say, 8v, the other amp will be driving -8v, so the load will have 16v across it. If the load is 8 ohms, that results in 2A passing through it thereby giving the appearance of a 4 ohm load.

Bridging is an ideal method for increasing power output when the running from low supply rails, but you are still be limited by the device’s current delivery capability (the LM3886 can typically drive 11.5A whereas the LM3875 is limited to 6A). Paralleling multiple devices is the only way to increase the maximum current.

Nice one,
David.
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Old 14th September 2006, 09:52 PM   #5
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Default Thanks Daatkins, i understood you clearly

This limiting current in the LM3875 will ruin my bridge amplifier..i was observing the data charts, and it is limited to 55 watts over 8 ohms...the protective circuits, current limiting circuits will ruin my sound.

I think it will turn compressive....and a little distorted when high current drain will be needed and will be supressed during protection moments.


I eas studying data sheet and i decided to give up related to bridge them.

thenks,

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 14th September 2006, 09:52 PM   #6
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Default Thanks Daatkins, i understood you clearly

This limiting current in the LM3875 will ruin my bridge amplifier..i was observing the data charts, and it is limited to 55 watts over 5 ohms also...this is the power limit...

The protection circuits, including the current limiting circuits will kill the sound quality.

I think it will turn compressive. the audio will turn compressed, and dinamics will be limited..also a little distorted when high current drain will be needed and will be supressed during protection moments.

I was studying data sheet and i decided to give up related to bridge them.

I do not need anymore help related how to bridge them.

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 15th September 2006, 06:53 AM   #7
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Hi Carlos,

This should be close enough:

http://www.national.com/appinfo/audi...ation_Note.pdf

regards
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Old 15th September 2006, 07:24 AM   #8
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Bridged, parallel...
http://www.national.com/an/AN/AN-1192.pdf
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