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Old 8th November 2008, 03:00 PM   #291
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Default Re: Finished my First 3875!!!

I usually prefer the LED to be less bright, and adjust it for brigtness in a circuit. Although your calculations look OK, 1k5 seem to be a bit on a low side. I would start with 20K or so and then adjust.

IEC ground connects directly to the chassis. CHG connects to chassis as well, but the star ground is not there: it's at the amp boards: http://www.diyaudio.com/forums/showt...69#post1518369


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Originally posted by mikje
It goes up and down with the volume. I did the star ground as Peter advised. When I touch the RCA connectors, one or both, the noise goes away. I don't have the RCA's mounted yet, as I only have everything on a 12" X 12" board since I just finished assembling it. Any ideas?
When the hum goes up and down with volume it could indicate that it's generated by a source. Even without chassis, touching RCAs should not be affecting anything. Check the link above, it shows proven way to connect grounds.
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Old 9th November 2008, 11:04 AM   #292
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Hi Peter

Thanks for your reply. I have built the amps up as dual mono - connecting PG+ and - and V+ and - exactly as shown in post #31. Other than to ground the earth from the IEC to the chassis, there seems no need for any further connections.

CHG is connected to the chassis via the pcb to SG to the ground side of the input jack, so no need for separate connection.

Please could you confirm this as I'm going to try powering them up

Cheers

Matt
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Old 9th November 2008, 03:36 PM   #293
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If I understand it correctly, the amp's ground is connected through RCA to the chassis and you still need to connect IEC ground pin to chassis. I guess, that should work.
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Old 12th November 2008, 12:08 PM   #294
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Hi Peter

I've tested one mono block. The voltage at v- and v+ is approx 29v and 33v. DC offset is around 6mv.

This seems quite a bit different from your measurements. The heatsink gets hot quickly with no load attached. The transformer is a nuvotem 230v - 2x 25v.

Would the bigger trafo account for the readings I get, or do I need to look at my soldering? I'm thinking with a load attached, I won't get anywhere near the heat in the chip?

Thanks

Matt
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Old 12th November 2008, 12:42 PM   #295
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Sorry, should have added that trafo is rated 300VA

Ta
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Old 12th November 2008, 01:02 PM   #296
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You should have closer to 35V with no load. Sounds like there may be something connected incorrectly.
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Old 12th November 2008, 01:19 PM   #297
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disconnect the chips and measure the output voltages of the PSU.
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Old 13th November 2008, 12:36 PM   #298
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Pg+ to V+ and PG- to V- on both amps now reads approx. 0.020vdc. I'm really confused. LED does not light - negative terminal connects through square pad. Have visually checked all solder joints and touched them up.

Whats the go guys?

Cheers as usual
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Old 13th November 2008, 02:55 PM   #299
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Check the fuse.
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Old 14th November 2008, 07:27 PM   #300
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Hi,

This is my first soldering experience and finally, after purchasing some months ago an lm3875 kit from audiosector for my "reference" amp and an lm1875 kit from chipamp for my computer amp, I am now ready for the first test (the lm1875 amp, it is cheaper, so let's destroy it first).

The lm1875 is in stereo version.

The amp is now placed on a plywood panel for testing. The heatsink is a piece of copper 1cm thick. A first question: should I connect the power supply ground to the main AC earth?

The power supply seems to work fine: I measure 25VDC as should be with a transformer of 18VAC and the lightbulb tester lights up just for a while when I switch on.

I only connected the amps and I measured between out and ground out 20mV in one channel and 30mV in the other one.

Then I added a 50K pot and the RCA inputs and now I measure at amp outs -10mV and -15mV when volume is at minimum and +10mV and +15mV when volume is at maximum.


Can I connect the testing loudspeakers and a source?

Thank you for your help

Renato
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