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Old 23rd May 2012, 02:36 AM   #1
gbcomp is offline gbcomp  United States
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Default 28+28v transformer lm3886

Hey,
I am going to build the chipamp.com lm3886 kit to drive 8ohm speakers and I am wondering a few things. I want to try and get the most out of the chip without causing issues.

I know that the 3886 should run just fine with a 25+25v transformer (+33v -33v after rectification) but will using a trasformer with 28v secondaries (+38v -38v) pose any issues to sound quality of the chip or endanger the chip (the data sheet says it should be fine but I want to make sure). This way I can get the most out of the chip with the 8ohm load.

Will the other components of the chipamp.com kit be able to handle this voltage.

The transformer that I was looking to use is this one from antek Antek - AS-3428 I realise that it also has 15+15v outputs but I will simply not connect them and cap them until I get around to finding a use for them, assuming that this does not cause a problem.

If it is not safe or smart choice I will just go for the 25+25v transformer.


Also I know that using that much voltage will create a lot of heat, With the 38v supply would a 4''x 6''x 1.25''high heatsink be enough for both chips? or should I use one heatsink of that size for each?

Thanks in advanced for any input, it all really helps!
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Old 23rd May 2012, 04:53 AM   #2
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The LM3886 can work till +41V to -41V DC on 8 ohm impedance speakers. However the 8 ohm speakers have a nominal impedance of 8 ohms and it can be lower sometimes thereby overloading the amplifier leading to activation of protection circuitry inside the chip which in turn leads to a lot of noise.

The ideal and safe transformer voltage for 8 ohm loads is 24-0-24V AC which gives you around 33-35V DC after rectification and filtering. VA rating of 300 to 500VA.
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Old 23rd May 2012, 09:53 AM   #3
Ar4 is offline Ar4  Latvia
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I was using +-40VDC for LM4780 and paralelled LM3875's. 6ohm load. Worked fine, although I didn't push it to maximum. 1/2 of max volume was too much for my ears already.
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Old 23rd May 2012, 04:04 PM   #4
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The recommended rail voltage depends on the load that the amp must drive. I advise you not to exceed the recommended voltage unless you know what you are doing (e.g. WRT heatsinking). Read the datasheet carefully.

-Charlie
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Old 23rd May 2012, 04:35 PM   #5
AndrewT is offline AndrewT  Scotland
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A 25+25Vac transformer is going to give a lot more than +-33Vdc when feeding a chipamp.
A 230:28+28Vac transformer is very likely to overvoltage a chipamp in worst case "normal" operating conditions.

Expect upto ~+-43Vdc when mains voltage is high. That exceeds the 84Vdc limit for the 3886 when signal is present.
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Old 23rd May 2012, 05:18 PM   #6
Puffin is offline Puffin  United Kingdom
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I run a pair of LM3875 amps on 30+30 secondaries, into regulators as per Decibel Dungeon and have 45vdc per rail. I have had no problems at all. Speakers are 8ohm.
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Old 23rd May 2012, 05:39 PM   #7
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Do you meAn to say that the Texas instruments and national semiconductor data sheets are wrong ? 42V per rail is the absolute maximum according to them.
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Old 23rd May 2012, 09:58 PM   #8
Puffin is offline Puffin  United Kingdom
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Maybe the grid was having a surge on the day I measured (several years ago)
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Old 23rd May 2012, 11:11 PM   #9
gbcomp is offline gbcomp  United States
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I ran a simulation in multisim and I was getting about 38v per rail after rectification and diode drop from a 28v ac source. I am also assuming that this would drop under amp load. I really want 35v per rail running into my amp.

Should I just go with the 25+25v transformer and take the hit on output power?
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Old 24th May 2012, 02:47 PM   #10
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Quote:
Originally Posted by gbcomp View Post
I ran a simulation in multisim and I was getting about 38v per rail after rectification and diode drop from a 28v ac source. I am also assuming that this would drop under amp load. I really want 35v per rail running into my amp.

Should I just go with the 25+25v transformer and take the hit on output power?
Yes you should.

You will anyway get about 39V if in the UK (can't see where you are from here). The change in power will be insignificant but the chip will be a lot cooler.

Edit: I see you are in the States, so you can probably get away with 28V. (Our mains is still mostly higher than the 220V it's specced at. In fact the ref is 230 in UK which is close enough for Euro compliance. Yours doesn't have a problem so the transformers will be wound just for US.) So, go with what you have.

Last edited by ChristianThomas; 24th May 2012 at 02:51 PM.
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