LM3875 SMPS problem pretty sure its my grounding arrangement - diyAudio
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Old 22nd March 2007, 08:09 PM   #1
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Default LM3875 SMPS problem pretty sure its my grounding arrangement

Hi all

I am posting yet again. I am having more problems with my amp

I can get it to work. But only with an i-pod. As soon as I plug in another source it goes badly wrong.

So far I have tried it with my pc, my i-pod and my dac in a box

What is making it go wrong is when the ground from another device is attached. The amp just starts spewing out hums, pops and then cuts out for a few seconds. Then it does the same again ad infinitum

The smps are 4 skynet 8080's rigged to give 24v a la decibel dungeons instructions. I have isolated the smps from ground. However as I see it the ov line in the smps is created only because the smps modules are isolated from earth. When I connect the rca cable the 0v line is no longer floating and my troubles begin.

Am I making sense.

I realise I have probably got something totally wrong, perhaps someone could educate me?

Blair

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Old 22nd March 2007, 08:42 PM   #2
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I should have made it clear that I tried cutting the ground on the RCA connector. Then the amp worked with my DAC however I also got loads of noise and interference from a radio station.

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Old 22nd March 2007, 11:53 PM   #3
BWRX is offline BWRX  United States
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Have you tried removing the resistor and cap between earth ground and 0V? You don't want to cut the ground between your signal source and 0V.
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Old 23rd March 2007, 12:25 AM   #4
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Yes I tried that.

Didn't solve the problem at all I am afraid.

On top of that I have just fried my brand new dac in a box dac707.

Think it can be fixed though. Looks like a bulged cap and possibly a damaged dual op amp. Bugger!
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Old 23rd March 2007, 12:27 AM   #5
BWRX is offline BWRX  United States
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Do you have a conventional supply to try with your amp circuit? Are you sure your amp circuit is wired up properly?

You could try just using two supply modules for +/-12V rails.

Sorry to hear about the fried DAC! That's no fun when components bite the dust.
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Old 23rd March 2007, 12:43 AM   #6
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I may try going back to the 12v+/- however when I had it running the other night on the ipod it was sounding absolutely glorious, the bass response especially was amazing and detailed. I have two amps per channel wired in a by amp arrangement.

I'm loathed to go back to the lower power.

I am fairly sure I have the amps wired as per the schematics I displayed above.

I'll take suggestions from anyone and their dog to try and avoid having to drop the amps available.
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Old 23rd March 2007, 06:57 AM   #7
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You have it wired wrong. Take a look at this document. It has diagrams of the basic connections for a Gainclone in it.

http://audiosector.com/nigc_kit-users_guide.pdf

Also take a look at this diagram. It looks similar to the one you posted but there are some differences. Where is the referencing resistor to signal ground off the feedback loop on yours? I havent tried to build an inverted Gainclone yet but I hope this info helps.

http://myweb.tiscali.co.uk/nuukspot/.../nixgccd2l.gif
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Old 23rd March 2007, 03:38 PM   #8
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Hi G

I have poured over the document that you linked to. My grounding arrangement certainly is not the same as that one. Though I think that it is correct to the schematic. I may have taken the diagram a little too literally.

Looking at the document the 0 lines are connected directly to the amp from the transformers, the signal ground is attached via seperate cables at a single point in the amp. On my amp stages there is a point where the 0v line, a cable to the signal ground, the speaker return and the power caps all meet. Just the way it looks on the diagram.

Is this where you think my problem lies?

I am finding it a bit hard to equate the instructions for smps to the standard instructions. how different are they?

Quote:
Also take a look at this diagram. It looks similar to the one you posted but there are some differences. Where is the referencing resistor to signal ground off the feedback loop on yours?
The referencing resistor joins pins 3 and 6 on mine. I am pretty sure that is not the problem. The diagram you linked to is a t networked amp. I dare not even think about making one of those if I can't get this right!

I think I have nothing to lose in trying to run the grounds differently, the fact that it is a ground connection to another device that sends the amp into a tailspin makes me think that.

If anyone else has ten pence worth to chuck in, please do so, It can only help, I am confused enough by the grounding of the amps. I have read all the threads around grounding and the only thing I have learned is that I am not the only one!!!
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Old 23rd March 2007, 06:25 PM   #9
AndrewT is online now AndrewT  Scotland
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Hi,
Q1, is the amp wrongly wired?
Q2, is the grounding wrongly wired?
Q3, is the smps wrongly wired?
Q4, is the SMPS interacting with the amplifier?

We don't know until you dump the smps and temporarily fit a conventional transformer+rectifier+smoothing caps.

Then we should find if the fault is in the SMPS or in the rest of the arrangement.
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Old 23rd March 2007, 08:03 PM   #10
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I think I can answer the questions. I got it working on a soundcard. One channel so far.

I moved my bypass filter onto a piece of wire, leading from the power ground to earth. previously it tied the 0v line to earth further down the chain.

It tells me one of three things were wrong.

1. The filter was faulty, I perhaps damaged the cap at some point.

2. The wiring was wrong.

3. Having 4 amps running through one filter was overloading it.

It was a bit of a guess trying it, I read the document that G sent to me and sort of cludged the idea from there. I dont personally understand why tying the 0v to earth through a seperate cable should make a difference?

The other thing that I did was to add a 10ohm load to the 5v line of the smps but I dont think that has anything to do with it.

Part of me wants to experiment to get to the reasoning behind it, part of me wants to leave well enough alone. I hate the fact I lost the DAC. Even if I am pretty sure I can fix it, I was looking forward to hearing it. I looked inside and the new dac in a box looks quite a bit better than the old inside.
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