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Old 9th December 2009, 08:10 AM   #1
Plammox is offline Plammox  Denmark
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Default Bridged LM4780 kit: 5 Ohms (Min) at 300Hz = smoke?

Hello all (and Peter in particular)

I'm in the process of building a pre and a power amp and procured most of the parts including the LM4780 kit. Now, since I'd like to take advantage of the balanced output of my preamp, meaning I want to bridge the LM4780s and run them in balanced mode.

Now, the loudspeaker kit that I'm planning to get (Jantzen audio MTM kit) has a minimum impedance of 5 Ohm @ 300Hz.

The application note AN-1192 from National claims the BR100 config can drive a 4 Ohm load (@ +-20V supply). Is this completely analogous to the balanced configuration option of the LM4780 kit?

Best regards,
Lars
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Old 9th December 2009, 10:10 AM   #2
AndrewT is offline AndrewT  Scotland
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a 3886 and half a 4780 is shown to give 35W into 4r0 when fed with +-20Vdc.
When bridged (balanced) this gives 70W into 8ohms.

To take account of voltage sag in your power supply the quiescent supply voltage is likely to be ~+23Vdc.
Each half of the bridged amp will dissipate a maximum of 27W and typically >10W during normal listening.
That means your 4780 chip must dissipate >20W normally and <=54W at worst case conditions.
That will require a big heatsink if you don't want Spike to cut current transients when the chip is hot.
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Old 9th December 2009, 12:00 PM   #3
Plammox is offline Plammox  Denmark
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Hi Andrew and thanks for your answer.

<=54W for a component in such a small package that on top has to be electrically isolated from the chassis seems like a bad idea. A more compelling option seems to be bridging two parallel coupled LM4780 boards for one balanced channel.

Are there any examples of people having done so with the Audio Sector kit?

Lars
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Old 9th December 2009, 02:40 PM   #4
Plammox is offline Plammox  Denmark
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...or (I didn't really describe it correctly above) having two LM4780 boards each in parallel configuration, but each board running complimentary to the other (balanced mode), meaning one board providing the "forward" current and the other one sinking it.

Is this completely mad? Constructive feedback (even if the idea is barking mad) is appreciated.
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Old 9th December 2009, 03:11 PM   #5
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I've been using bridged LM4780 for years in my workshop system with Audio Technology drivers:Flexunits 5 H 52 15 06 SD

As you can see, there is not much heatsinking in my simple chassis, yet with moderate sound levels, or really pushed for a reasonable period, it does not create any problems.

Without trying simple balanced configuration first, I would strongly discourage you from going into complicated and questionable from sonic standpoint parallel/bridged configuration.

You always have a choice taking advice from people who actually build that stuff, or merely theorize about specs

More info about bridged configuration can be found here: AudioSector-chip amp kits, dacs, chassis
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Old 10th December 2009, 03:44 PM   #6
Plammox is offline Plammox  Denmark
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Quote:
Originally Posted by Peter Daniel View Post
You always have a choice taking advice from people who actually build that stuff, or merely theorize about specs
Exactly. Which is also why I addressed you, Peter.

On closer thought, I think my choice of speaker kit is the problem. This being my first power amp I will build it, run the MTMs at low output power and build some bigger iron bigger next time

Thanks,
Lars
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