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Old 7th October 2006, 09:23 PM   #1
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Unhappy Trouble shooting LM3875 and NOSDAC

Hi,
I just finished building Peters Daniels LM3875 and NOSDAC. Before hooking it up to my speakers it tested the DC offset across the terminals which read at 12mV per side stble. I thought great, let me hook it up and see what happens.
Well there is sound, a little closed in but probaly more as a result of not burning in the caps yet. There thing is the volumne is very low. I have a pair of 88db speakers that I had no problem driving with a CLass D 20W amp before. I can listen with the gainlone but just not very loud
So don't believe the problem lies with the DAC as if there was i would get no sound at all. It must be something with the amp.
I have little experience troubling shooting but this is what i have so far:
1. Audio Sector LM3875 kit
2. 2 PLitron 120 V 22VAC toroids; one per side
3. I checked the rectified coltage after the PCB- 65 volts per side... wierd???? AT that voltage surley the chip should have blown?

Any ideas on how i might trouble shoot this and what the likley source of the problem might be?
Cheers
A very lost and confused newbie
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Old 7th October 2006, 09:50 PM   #2
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I was measuring across the wrong terminals i get 34.5 volts which seems fine. Is it possible that the gain will increase as the amp and DAC burn in?
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Old 8th October 2006, 03:08 AM   #3
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check the amp with some other source first. then at least you will know where the problem is.
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Old 9th October 2006, 02:50 PM   #4
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No problem with the source. I have a couple of questions that may help trouble shoot though:
1. I placed a 10ohm 1 watt resisotor between the star ground and the chassis to limit "dirty" current feedback from any other component. ANy chance that this may recduce volume and what is the typical resisotor used in this posistion?
2. Is there and way to test a gainclone board with a multimeter to see what the gain is so that i can compare this board to others?

Thanks
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Old 9th October 2006, 05:47 PM   #5
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Quote:
No problem with the source.
What was suggested was to compare the volume between the NOSDAC and another source. Some NOSDACs have a lower output than say a typical CDP.

Add a 220n cap in parallel with that 100R resistor (star ground to case) but that won't affect the volume level.
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Old 10th October 2006, 03:36 AM   #6
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Nuuk,
Any suggestions on how to get more gain from a LM3875 or more specifically Peter's boards?
Ho might i go about testing the wattage / gain of my particular board?
And finally could adding a buffer help with the gain at the risk of adding coloration?
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Old 10th October 2006, 08:17 AM   #7
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You can change the input/feedback resistor ratio to get a bit more gain providing you don't go outside the limits set in the datasheet for that chip.

As many prefer the GC with an active buffer or gain stage, that may be a better way to go! Try a simple one like the one I built to go with a class-T amp. You can find full details here .

You may well be satisified with that but if not, there are any number of circuits to try!
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