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Old 16th May 2008, 03:48 PM   #1
diyAudio Member
 
Join Date: Feb 2007
Location: Quebec
Post gainclone LM4780 problem ?

Hello

I build gainclone with LM4780 parallel. (mono 120w) (audiosector PCB)

I have good power supply when loaded I always have 75VDC
I don't have noise or hummm at speaker output but...

*first problem: When I put the sound loud it goes on PROTECTION Is it possible that my heatsink is too small ?

link to view my heatsink for each lm4780
http://cgi.ebay.ca/Aluminum-Heat-Sin...QQcmdZViewItem

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*Second problem: I ALWAYS HAVE 166mVDC at the speaker output
I have only 1.0mVDC at the power supply output.
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*Third problem: I put ALPS 50K pot at input and at the POT output I add coupling cap 2.4uF SOLEN it's okay or not ?

Because when I turn the sound at minimum the lm4780 is very very HOT !


P.s. when I add 2.4uF coupling cap is it possible that it kills my low frequency.

thanks in advance for your hints

nicK

_________________________________________________
My power supply is:
500VAC toroidal avel lindberg output 2x25vac
4x 100uF elna Cerafine CAP on power supply PCB
4x 5400uF elna LPO for audio
4x 4.7uF WIMA MKP non polarized
on PCB snubber is: solen 0.01uF non polarized + 0.5ohm 2W

the lm4780 have only audiosector standard parts.

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Old 16th May 2008, 04:51 PM   #2
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Default pics

pics of my gainclone here :
http://profile.imageshack.us/user/gainclone/


I think the problem 2 and 3 was associated because when I put 50K POT at maximum I have 100mVDC speaker output and in minimum volume I have 166mVDC


nicK
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Old 16th May 2008, 05:05 PM   #3
AndrewT is offline AndrewT  Scotland
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Am I allowed to say "******* Hell"!!!!! and that word starts with F and finishes with k.

But to be constructive:
where is the Safety Earth connection to all and every exposed conductive part?

Leave your parallel chipamp with separated PSU for now.

Get a simple chipamp with simple nearby PSU working and understand what you're trying to achieve.
Then maybe you can progress to this abortion.
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Old 16th May 2008, 05:36 PM   #4
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I don't understand ??? MY POWER SUPPLY HAVE EARTH GROUND !!!
and my amplifier section TOO !


the problem is my ALPS 50k POT and the LM4780 PROTECTION.

THAT'S IT !

read my first post I don't have HUMMM trouble or noise in speaker or in power supply ! it's AMPLIFIER section trouble.


thanks !
I search for answers please

nicK
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Old 16th May 2008, 05:55 PM   #5
AndrewT is offline AndrewT  Scotland
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all right, I'll ask the question again, but in more detail.

How many metal parts are exposed on the outside of your two box mono amplifier?

How many of these exposed parts are permanently connected to the Safety Earth?

Are there any non metal parts that can conduct electricity to the outside of the cases?
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Old 16th May 2008, 11:20 PM   #6
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Interesting that you have 75 V DC under load. With a 2x25V transformer you should get something like 2x33..34 V with no load, and about 2x30..31 V under load.

According to the design guide you need 0.86 K/W for 8 Ohm speakers and 0.05 K/W (!) for 4 Ohm speakers with the 2x37.5 V. Your heatsink doesn't look that efficient.

If you have 75 V and use 4 Ohm speakers you might also reach the current limit.

Concerning the DC voltage at the output and the heat issue at low volume: did you respect the recommendation on page 16 in the datasheet to use 0.1% tolerance resistors for the gain setting (Ri and Rf)?
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Old 17th May 2008, 12:17 AM   #7
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Location: Sydney/Australia
hi nickthevoice,

Heatsinks should have their fins vertical. Think of the air flowing over the fins and imagine where it goes. Mounting the heatsinks like you have will more than double the size required.

If you can't mount them vertically, the next best is to have the heatsink horizontal with the fins pointing up.

Having said that, I think you may have another problem and when you solve that your heatsinks may be OK for normal volumes if you speakers are 8 ohms and reasonably efficient

Did you short the inputs to ground to make DC offset measurements? If you didn't you will find your measurement will drop considerably.

Andrew is talking about the safety earth not ground earth. I measure the resistance between all metal surfaces and the earth wire on the back of the IEC socket. It looks (in your picture) that the front and back panels are metal and are not connect to safety earth. It seems to me people in 230V/240V countries are a lot more concerned about safety earthing issues, rightly or wrongly.

BTW: What is the supply voltage in Canada?

regards
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Old 17th May 2008, 01:15 AM   #8
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no no my box is only WOOD and ACRYLIC front, back and top.

but the parallel lm4780 accept 4ohm load this is not my problem ?

rail to rail voltage supply range: 20V - 84VDC for Lm4780 why I have problem with my 75volt ?

Thanks !

nicK
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Old 17th May 2008, 03:23 AM   #9
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Default problem

First 75VDC is pushing that top 10% supply range, dissipation goes up as voltage goes up, check Nationals design notes and dissipation figures.
Two the heat sinks are way way to small for a lm4780, again check Nationals app notes on heat sinks.
Third the pot if not separated by a cap will introduce DC into the input. A 4.7uf should not kill your bass at all?? should be smooth to around 5Hz. Get bigger heat sinks, and run the cap between your pot and the amps 1K feeder and 22K shunt, if your designs is built around the data sheets common values.
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Old 17th May 2008, 07:29 AM   #10
AndrewT is offline AndrewT  Scotland
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what about the number of metal connectors and terminals and connector shells that come to the outside of the case?
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