No output of a LM3876 GainClone??? - diyAudio
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Old 16th January 2003, 11:40 AM   #1
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Default No output of a LM3876 GainClone???

Hi!

Today I started another attempt on a GainClone, using two of my LM3876T samples floating around.

I took both Thorsten's and the minimzed schematic for the 3875, simply exchanging the pins for the proper ones of the 3876.

It didn't blow up, I measured DC on the output - about 1.2 mV, so very OK - connected it to a speaker and signal, and nothing comes out...???

Or am I doing something wrong on the input side? I am not using a pot since volume regulation is done by my computer, so I simply left the pot out of the schematic, connecting the input via 2.2uF and 10k R to pin 9 of LM 3876 (Vin-).

Any ideas, before I blow up my IC by doing something very wrong...???

Thanks,

Arndt
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Old 16th January 2003, 02:59 PM   #2
Philo is offline Philo  United States
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I suspect you haven't run a small voltage (>0.5Mv) to leg 8 to turn off the mute feature this chip has. The LM3875 IC does not have this feature so it isn't talked about in the other threads. Refer to this datsheet for how to turn the mute off. The important part is the component description of Rm and the electrical characteristics of the IC.
http://www.national.com/ds/LM/LM3876.pdf
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Old 17th January 2003, 08:02 AM   #3
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Hi!

Yeah, me idiot! I thought of it myself, but not before I fried one more IC...

I got the 3876 instead of 3875 because I have ordered the IC-Amp-PCB from ESP, and I totally forgot about the differences (causing the first two samples to fry because I even did not change the schematics for the different pin layout)...

Now it works, and I can tell you that it works even with a 30-0-30 toroid (which was advised not to use in another thread), and does not run very hot without any heatsink, simply an aluminium casing with a heasink-like layout (Conrad).

I will have to eliminate some more components, and then test it again, right now I think it does not sound as well as my modified Marantz amp from 1978...

Ciao,

Arndt
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Old 17th January 2003, 09:07 PM   #4
2002tii is offline 2002tii  Norway
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Default 2*30V ac

My 2*24V trafoes gives me +/-40V dc at idle.
How high are your dc at idle with 2*30V ac???
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Old 17th January 2003, 10:14 PM   #5
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Default Re: 2*30V ac

Quote:
Originally posted by 2002tii
My 2*24V trafoes gives me +/-40V dc at idle.
How high are your dc at idle with 2*30V ac???
I'm afraid there's something wrong on this DC at idle.

If your transformers are 24VAC, the most DC you should get from them is 32.2VDC at idle.

If you get 40VDC you probably start from around 29.6VAC.


Carlos
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Old 17th January 2003, 10:22 PM   #6
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The 24V spec is probably based on rated load. The LM3886 amp isn't much of a load so the transformer voltage is quite a bit higher than the spec 24V.

This is one reason why you need to be generous with the voltage specs on your filter caps.

MR
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Old 17th January 2003, 10:26 PM   #7
2002tii is offline 2002tii  Norway
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Your right.
I have just measured a little.

I have 239V ac connected to the 220V ac input on the trafo instead of the 240V ac input.
So my 24V=30V.

I`m going to fix that now!!!

Power supply: 2*30V ac, 12*5800uF/63V

I`m sorry if I have misled somone here!

I do now have a stereo set playing music from my computer.
Tomorow (or maybe tonight..) I`m going to test it on my B&W 801 (Matrix)...
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