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Old 24th April 2006, 10:02 PM   #1
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Default bridging 2x paralleled LM4780 per channel - few questions

I am looking to build a flexible medium-power amplifier. I would like something in the neighboorhood of ~100 watts per channel at 8 ohms so i am looking into the AudioSector LM4780 kit. my understanding is that when paralleled a 4780 will give the same power at 8 ohms as a single 3886 would because each 3886 'half' of the 4780 would see a 16 ohm load, which would be summed between the two amps. well i need more power than that - i'm in university and i have keg parties to throw. the house-party aspect of the amp's possible use has me thinking that i may need to be able to drive low impedance loads if i use multiple speakers per channel, but not all the time. so to have high power even at 8 ohms, i am inclined to think i should bridge a pair of paralleled 4780s so that i can have lots of headroom, and more when i run a lower impedance. to clarify: most of the time i will be running a single pair of speakers for everyday listening, and i want to be able to drive them with plenty of power. in some cases i might be driving 2 or maybe even 3 pairs though, so i don't want to bottom out either.

how large of a transformer am i looking at here? i wanted to build a dual-mono power supply with a single transformer using dual 25 VDC secondaries. i was thinking 500ish VA and lots of capacitance(20,000+ uF per channel) with an RC snubber at the end. should i go for a larger transformer?

how should i go about this? what method of bridging is recommended?

thanks.
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Old 25th April 2006, 07:23 PM   #2
Bazukaz is offline Bazukaz  Lithuania
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Hi,
Bridging paralell LM4780s is basically a BPA-200 amp , isnt it ?
Google for BPA-200.Also , search www.national.com for overture design guide excel spreadsheet.

Trafo should be twice as powerful as amp's output.Almost half of power is lost as heat in output transistors.
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Old 26th April 2006, 01:56 AM   #3
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I tried very recently to do the same thing and it was EXTREMLY difficult although I am not up to par with some of these guys on here. And from what I have seen/heard so far...this has never been successfully done.

If you accomplish it or see of a finished amp that used this structure--Please do share!

The following 2 threads will progress you greatly in your inquiries!
Paralleling LM4780 chips?

LM4780 ESL Amp

Im not sure if these will come up as links, but you can search them by title.

REALLY-have a look!

And good luck to you--I hope you can do it and share your success with us!

Dominick in New Jersey
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Old 28th April 2006, 11:19 AM   #4
sangram is online now sangram  India
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I am running exactly the same beast. Though the paralleled amps have huge offset (~200 mV), once they get bridged the offset drops to nearly nothing, around 20 mV.

Each channel (2 chips) runs off a single 440 VA transformer, and it's nice having an amp that does not clip while blowing your ears off. I assume the system is good for about 150-180 watts per channel, is 4-ohm capable, and due to pure dual mono design, images beautifully.

Each channel of a 4780 produces a little less than a 3886, but not half, about 8 watts less. Once parallelled they acquire the capability to drive lower loads at higher supply voltage. And it's nice having a system with more current than you need.

Dominick had a few problems getting his to work, but mine worked right out of the box. I had a few hassles with connecting the transformer to the bridges, but once that worked out I was fine. I also experimented with a single feedback loop for both halves of the chip, but there was a lot of oscillation so I used independent loops. The offset is high but overall mangeable.

About capacitance, I just use the 1500 uF that is supplied with the kit. I tried 20,000 uF per chip but the sound became instantly duller, so I dropped them from the system. As it is there's enough of bass, and the PS has plenty of reserve for my in-room listening (-40 to -25 dB). Maybe if you're regularly running them near their limits you shold think about more capacitance, but with a 500 VA transformer for each channel you should be fine.

As for bridging I have not yet built my adapters, I use the balanced output of my Delta 66 to drive the amp. That pushes up DC offset to about 100 mV, but still works for me. I was initialy planning a fully kitted out preamplifier, but now am leaning toward an ultra-quiet media PC with professional soundcard (110dB+ SNR and dynamic range) and balanced outputs.
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Old 28th April 2006, 03:49 PM   #5
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Sangram,

Just to be clear-I never did get my set up working. The best I got was VERY distorted sound.

But I also was configuring a little different than what you are talking about.

I tried to parallel two paralleld LM4780 chips togeter. So essentially parralleling 4 3886's!

I did not ever try bridging!

Anyway, If you were successful doing that as well-please do tell!

Dominick
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Old 28th April 2006, 04:58 PM   #6
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Default parallel lm4780

Its actually not that difficult to parallel these chips I posted a circuit that allows you to dial out the DC offset of the non-inverted chip and nobody showed any interest The cicuit allows for 1% resistors and no FSB cap, then matching can be done easily.
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Old 28th April 2006, 06:13 PM   #7
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well,

Maybe all that is just above my skill level then.

But thanks for all you help tiltedhalo-You have held my hand through other issues!

Dominick
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