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Old 1st October 2006, 07:34 PM   #1
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Question Lm4780


Hi guys,
I'm new to this stuff <mostly>, but have contructed 2 amps. 1 a sterio lm3876 non inverting <esp> and 1 sterio LM3886 non inverting using Brians boards. Both sounded ok, but looked very bad. Since improving my DIY electronics skills, as well as wading through tonnes of gainclone sites I've decided to run with 2 LM4780 <each in paralell, giving me my 2 channels out of 4 LM3886's> using audiosectors gold boards.
Anyhow, I have a couple of questions before I begin construction, so if anyone out there can help it will be well recieved.
So:
1/ I have 2 150x50x50 finned heatsinks going begging and wondered if these would be adequate?
2/ Do said sinks have to be isolated from the chassis?
3/ I have 2 toroidal 25x25 300va transformers, which gives me roughly 36 volts, with 4 ohm speakers, would this give me a decent output?
Any help would be awesome!
Scott.
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Old 2nd October 2006, 01:45 AM   #2
Leolabs is offline Leolabs  Malaysia
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Quote:
1/ I have 2 150x50x50 finned heatsinks going begging and wondered if these would be adequate?
What's the weight???

Quote:
2/ Do said sinks have to be isolated from the chassis?
Not all the time.But most sinks should be electrically isolated from the chips.

Quote:
3/ I have 2 toroidal 25x25 300va transformers, which gives me roughly 36 volts, with 4 ohm speakers, would this give me a decent output?
It depends on the driving capability of the amp.
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Old 4th October 2006, 01:52 AM   #3
lgreen is offline lgreen  United States
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Hi wotsascott, I just made one of these amplifiers using BrianGT boards.

That is you said-
Quote:
with 2 LM4780 <each in paralell, giving me my 2 channels out of 4 LM3886's>
So I take this to mean 1 LM4780 per channel (2 parallel LM3886 per channel, 4 total), which is what I did.

Quote:
1/ I have 2 150x50x50 finned heatsinks going begging and wondered if these would be adequate?
Answer- No, Mine are about the same size but are very heavy/massive and are only adaquate because I am also using a fan to bring cool air over the sinks. They don't get hot in normal use but when the vol. goes up they need the fan.

Quote:
2/ Do said sinks have to be isolated from the chassis?
Answer- Depends. The back of the chip is the negative power supply rail voltage and you don't want to shock anyone who touches your chassis. So you MUST electrically isolate the chip from the heatsink OR isolate the heatsink from both the chassis and from being touched by anybody or anything. Its normally easier to isolate the chip from the sink, and its much safer practice. Also, if you are going to use 1 transformer per chip each negative rail will not be identical, and you don't want DC (even a little offset) going from one heat sink to the other, which would be very bad. I think you really need to have the sinks isolated from the chips. If so, don't need to isolate from the case but it never hurts.

Quote:
3/ I have 2 toroidal 25x25 300va transformers, which gives me roughly 36 volts, with 4 ohm speakers, would this give me a decent output?
Answer- YES! I have used a single 330 VA 25x25 and you have two 300VA, so yours are better. Mine is very powerful and drives 4 ohm speakers quite well; I would not worry about this aspect of the design, make sure you use some good filter caps. Note that you will get quite a bit of heat because you are at 36 volts (see the datasheet) so this is another reason why your heatsinks are too small.
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Old 4th October 2006, 04:48 PM   #4
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Default Ta very much

Hi one and all!
Thankyou for your help, it is much apprieciated.
Does anyone know where I can get the correct heatsinks?
I've got a 12 dc fan which is standard 80mm size, but I'd rather get the right sinks?
Extra thanks to 'lgreen' and I'd like to say how effective and cool looking some of the creations on said website are; nice one!
What gauge thickness shielded wire for the internal connects do you guys use? Can someone give me the site for 'decent' diy hifi supplies?
Until the next time?
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Old 5th October 2006, 11:21 PM   #5
lgreen is offline lgreen  United States
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Default Re: Ta very much

Quote:
Originally posted by wotsascott
Hi one and all!
Thankyou for your help, it is much apprieciated.
Does anyone know where I can get the correct heatsinks?
I've got a 12 dc fan which is standard 80mm size, but I'd rather get the right sinks?
Extra thanks to 'lgreen' and I'd like to say how effective and cool looking some of the creations on said website are; nice one!
What gauge thickness shielded wire for the internal connects do you guys use? Can someone give me the site for 'decent' diy hifi supplies?
Until the next time?

Hi, good luck with his; glad you like my odd constructions. I'm not in England so cannot help on the heat sinks and parts. Here I spent a lot of time looking for electronics surplus stores and they have saved me a lot of money for parts, things like wire, and hardware screws, bolts, cases etc...

Lets see if I can answer some of your build questions- I have found that big wire is not necessary in these gainclones, 18 gauge is fine for mostly everything and 20 or 22 or 24 works for most non-powerline situations. I have bought a lot of silver-teflon wire on ebay and its perfect for the job.
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Old 6th October 2006, 06:36 AM   #6
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lgreen wrote:
Quote:
you don't want to shock anyone who touches your chassis. So you MUST electrically isolate the chip from the heatsink OR isolate the heatsink from both the chassis and from being touched by anybody or anything. Its normally easier to isolate the chip from the sink, and its much safer practice.
????
SHOCK ? How ? Aren't we using a transformer ?
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Old 6th October 2006, 07:57 AM   #7
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I get most of my heatsinks from ebay, buying new is very expensive.

http://electronics.search.ebay.co.uk...pZ1QQsacatZ293

The top two would be ideal for a 4780
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Old 6th October 2006, 02:14 PM   #8
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Default Hmmmm

Well, I'll check out ebay for wire and heatsinks and let you know how I got on!
My enclosure for the project arrived today, a standard 19 in, 2u, black type which I think I'll spray once I've cut the holes I need.
I'm having serious problems cutting sheet metal accurately and neatly; any idea's? The only sure fire way is to go get it done at the local metal worker, but is very expensive and I've already faced the wrath of 'she who must be obeyed' for spending too much; yikes?

It seems to me that the electronics part is a lot easier that the cosmetic part of the diy amp? Anyone else feel like this?
Anyway, thanks again guys, see ya!
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Old 6th October 2006, 05:22 PM   #9
lgreen is offline lgreen  United States
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Quote:
Originally posted by sivan_and
lgreen wrote:


????
SHOCK ? How ? Aren't we using a transformer ?

Yes, but the back of the chip carries the negative power supply rail, and if its touching the heat sink, which is touching the case, the case will also carry the negative power supply rail. This must be avoided to avoid the risk of shock.

And yes, the mechanical part is much more difficult than the electrical part.
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Old 9th October 2006, 10:10 AM   #10
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Default In Business!

Thanks once more to 'lgreen' for the advice! Ordered some of the silver teflon wire of ebay and found some sinks matching the one's on the LM4780 data sheet. I now have all I need to start the project!
Still stuck on the metal cutting bit though?
Any suggestions as to what sort of speakers to use?
Cheers, ta ta!
Oh yeah, certainly don't want the chasiss linked to the negative rail, so I'll insulate the chips from the sinks, nice 1!!!!
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