my first try at a LM3886 layout, any comments/suggestions?

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#224
edjosh23 said:
neutron7, what does the VCT mean? I saw the 300Va 35V-0-35V and I know that would be fine for the chips.

Thanks,

Josh
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Josh,

VCT means volts center tapped. If an tranformer is a 80VCT, this implies 40vac-0-40vac. A centertapped transformer is a transformer with a single secondary winding with the a wire coming off the middle of the winding acting as a center tap.

35-0-35 will be too much for a chipamp setup, unless you use regulation to drop it down quite a bit.

Another thing is that often times centertap transformers will actuall have seperate secondary windings with the middle wires tied together to form a centertap. If you peel back the covering on the transformer, you might discover this. I have seen people say that some of the centertap transformers on ApexJr can be modified to have dual secondaries.

--
Brian
 
#225
SheldonD said:
RE TRansformer at A1

there were a number of RS transformers 100VA DUAL 25v
at A1 I got one for 10$ as it was unpriced. I don't know if one could get the same price.
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Is 100VA DUAL 25v enough? Recommended transformer is 330VA 25V (dual), I think...

Could anyone please compare VA's, Volts and potential power output of 3886 setup, so that it's easier to make right selection between different models/prices?
 
#229
tomtik

I pointed out that with REGULATION AND 3875 that there was sufficient power to drive my inefficient speakers with loud music.

Under these condtions the voltage at the amp pins was stable under load, the regulation was under 1%. IIRC.

However the xfrmer got quite warm, but not hot. this was with 63ufd and 100 nfd at the chip pins and 11,000 mfd at the PS

The sound was very clean both hi lo and mid. Better than standard NIGC 3875 configuration.

Conservatively the power xfrmer should be about double the amp watts,

For 3886 this single xfrmer will not be enough but 2 should be more than adeqiuate. The price is certainly right.
 
#230
homer09 said:

I emailed A1 asking if they had hammond toroidals or Piltrons in surplus as Peter suggested but received no reply. Since your from toronto, why dont you call them and post their response here?
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I've called them up. They do not carry Plitrons anymore and will not in the near future as the guy suggested. But they have some turoidal tranformers.
All their Hammonds are listed on the website.


SheldonD said:
RE TRansformer at A1

there were a number of RS transformers 100VA DUAL 25v
at A1 I got one for 10$ as it was unpriced. I don't know if one could get the same price.

I made a 3875 amp using lm318 regulator and it worked very well.
Click to expand...

To your comment about having transformer with double the output power requirement for amplifier, did you mean that for two channel amp, 50W (average) each (LM3886), I would need 100W per channel, which can be supplied by 200W dual tranformer?

Do they label these as 200VA dual?

So I might get some 225VA Dual 25V for the LM3886 stereo kit I've bought. Is it going to be ok?

Thanks
 
#231
tomtik

the 3886 is 68 WATT ACCORDING TO THE SPEC SHEET

and according to upopa's measurement he gets 68% out of his xfrmr for his amp.

so I believe 2 x 100 VA should do this.

If you want 200 watts then it will not work.

I know that people are going hog wild on xfrmr capacity I believe this is not necessary, but either regulation of perhaps snubber
PS woill be required, in any case sufficient PS capacity.

The xfrmr i originally suggested is a buy at VA/$,
I spent almost 100$ for plitron 400VA, 400VA for RS 100VA is only 40% of this cost.
 
#232
Here is a closeup of the layout for the feedback capacitor. If you wish to use it, put the bottom leg of R3 into hole 3, and install the capacitor. If you wish to use the amp without it, install the bottom leg of R3 into hole 2.

--
Brian
 

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#237
m0tion said:
Anyone monitoring temps on these LM3886 chips? What are your full-load temperatures after about an hour or so?
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Wouldn't it vary on how big of a heatsink you use?

I used to use two CPU heatsinks, but they just didn't cut it. They would get so hot that i could ALMOST not keep my finger on them.

So i switched to using a heavy duty aluminum case with fins ($12) and it works wonders, they just get warm after several hours.

i am using this case with excellent results:

http://www.fairradio.com/cases.htm
5 9/16" Wide, 5 lbs sh, #ENC-8-6, $12.00

An externally hosted image should be here but it was not working when we last tested it.
 
#238
Sure, there are lots of variables that will effect the temperature of the chip, the major ones being what kind of heatsink (mass, composition, surface area) you're using, the airflow around it, and the ambient temperature. I'm just trying to get a general idea for what most people are experiencing because I think I'm experiencing above average temperatures.
 
#239
m0tion said:
Sure, there are lots of variables that will effect the temperature of the chip, the major ones being what kind of heatsink (mass, composition, surface area) you're using, the airflow around it, and the ambient temperature. I'm just trying to get a general idea for what most people are experiencing because I think I'm experiencing above average temperatures.
Click to expand...


And those above avg. temps would be???
 
#240
I saw it get up to around 60C before I pulled the plug, volume wasn't even that high. That was when I was using some thermal tape to bond the chip to the heatsink. I switched to using some real thermal interface material (Artic Alumina) and a piece of electrical tape to bond the chip to the heatsink. With the setup in the picture below the max I've seen is 47C and thats under full load.

Link to Setup Picture
 
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