LM4780 sounds murky in bridged mode - diyAudio
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Old 9th October 2006, 04:53 PM   #1
alexcd is offline alexcd  United States
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Default LM4780 sounds murky in bridged mode

I recently made some PCB's for LM4780 chips. I was very happy with the sound from each channel seperately and get about 88W transients without a problem. It sounded glorious, excellent, clear, smooth and bright. As good as a solid state mcintosh with less balls....deep, rich harmonics, etc. I was very impressed.

I then wired each chip in bridged configuration per the spec sheet and it sounded terrible. It was murky, there was a dc offset (a turnon on thump, albeit quite small) and it current limited like a b!tch..... soft crunch on bass transients (with inefficient 4 ohm speakers.) I have a strong feeling that thermal circuit is a real shot in the foot even though I'm not even approaching anything hot. Does anyone else have more insight?
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Old 9th October 2006, 05:05 PM   #2
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Default Re: LM4780 sounds murky in bridged mode

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Originally posted by alexcd
inefficient 4 ohm speakers
There you go - the apparent 2 ohm load is too low. Current limiting kicking in. Maybe thermal, too.
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Old 9th October 2006, 05:09 PM   #3
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McIntosh LS360 speakers... no idea of impedance curve but they are 4ohm rated is all I know.
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Old 9th October 2006, 05:14 PM   #4
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Rated at 4 ohms could mean they dip as low as two ohms.

A bridged amp "sees" half the impedance you have connected. Even being generous at a 4 ohm minimum impedance you're still taxing the amp with an apparent 2 ohm load.

With those speakers I'd go for paralleling the amps rather than bridging.
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Old 9th October 2006, 05:16 PM   #5
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Has anyone tried them in bridge mode with "4ohm speakers"? How about parallel mode? I want some insight before I rewire the output.
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Old 9th October 2006, 07:47 PM   #6
traw is offline traw  United States
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i've made bridge BR100's (as an1192 dubs it) running 4 ohms with good success. one person used it to run maggies, another 4ohm north creak speakers... however, i ran the voltage low, +/-22 volts. I'd think 28 max 8 ohms may 6ohms, need to stay under 25 for 4 ohms.

also... seems gain (11 per leg) setting of the app note a touch low. curious to try again with maybe 15 per leg and off this monster industrial smps supply i picked up, +/-24v, over 500w i'd guess given size and mains fuse amperage rating.

i tended to like the bridge over the parallel. while the parallel had great bottom end, seemed the bridge sound tighter. casual impressions only.... but, in general for my own listening on my speakers/habits, single chip does fine thus did not go much further.
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Old 9th October 2006, 07:52 PM   #7
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I was going to attempt bridge/parallel (2 chips) but that seems out of the question now.

My rails are very close to max (+/-42V)
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