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Old 25th May 2004, 09:59 PM   #1
azira is offline azira  United States
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Default Crackling/Static on my GC.

I decided to finally put together a LM3875TF NIGC since I had a few chips I've been meaning to use and a 24VCT Transformer on hand.

I built it by clipping the NC pins and soldering the components directly onto the pins like a lot of people do. Since the parts stores were closed, I used what I had on my bench. Basically I meant to follow the "Typical" schematic in the National datasheet. The only stereo volume pot I had was 100K so I put a 10k in parallel with it to set the input impedance to my amp around 10k. Notibly, I somehow forgot a 1k resistor from the + input to the pot and I only had a 10uF nonpolar cap on hand instead of 22uF for the feedback path.

Right now my powersupply is literally aligator clipped together from the transformer to the rectifier to the caps to the chips. The "star ground" is a bunch of aligator clips clipped together and I'm driving it from the headphone out of a boom box.

The input signal to begin with is noisy, I can hear CD spinup and some humming when I use it on my Zen (and likewise on this thing).

Here's the problem. When at low volume, it seems to sound ok, but when I turn the volume up to around 60%, there is obvious static, some hum and crackling best heard in the vocals. If I unplug the source the amp is dead quiet at any volume level. The hum and spinup sounds don't concern me since they are present in the Zen, the static and crackling are new.

What did I do wrong? Thanks.
--
Danny
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Old 25th May 2004, 10:34 PM   #2
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First of all, check the datasheet for LM3875

Second: Connecting a potentiometer like you have done is a no no because the DC path from the LM3875 isn't secure. If the potentiometer is the slightest bad you may get an interrupted current path which means inconnected inputs => not good! in fact a death sin.

The medcine for this is a pull down resistor directly from the non-inverting input down to ground. If you have 100 kohms potentiometer, the resistor should be 220-470 komhs. I recommend though that you change your potentiometer to 10-25 kohms at the most. If you change the potentiometer to something lower the pulldown resistor can also be lowered, like 100 kohms or so.

You may also have problems with your output. Have you forgotten your zobel network and L//R filter?

My strongest hunch is that you have oscillations which occur above a certain output level.
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Old 25th May 2004, 10:48 PM   #3
azira is offline azira  United States
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Quote:
Originally posted by peranders
First of all, check the datasheet for LM3875

Second: Connecting a potentiometer like you have done is a no no because the DC path from the LM3875 isn't secure. If the potentiometer is the slightest bad you may get an interrupted current path which means inconnected inputs => not good! in fact a death sin.

The medcine for this is a pull down resistor directly from the non-inverting input down to ground. If you have 100 kohms potentiometer, the resistor should be 220-470 komhs. I recommend though that you change your potentiometer to 10-25 kohms at the most. If you change the potentiometer to something lower the pulldown resistor can also be lowered, like 100 kohms or so.

You may also have problems with your output. Have you forgotten your zobel network and L//R filter?

My strongest hunch is that you have oscillations which occur above a certain output level.

Check the datasheet for what?

I used the first schematic in the datasheet called "Typical Application". That schematic does not include a zobel. It also doesn't include a pull down resistor. Hence the reasons they are not in my schematic.

Thanks for the tips, I'll try a different source to verify it's not a source problem, add the pull down resistor, and see if I can get a O-scope to check for oscillations.
--
Danny
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