LM3886 kit DC grounding - diyAudio
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Old 10th February 2012, 06:18 PM   #1
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Default LM3886 kit DC grounding

after alot of digging around in this forum i've come up with this design. Please let me know if this is even close to what i need. I want to make sure it's correct before i solder anything. As you can see I'm still waiting on a few parts.
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Old 10th February 2012, 06:31 PM   #2
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That is a nice and clean setup. Others do it differently, but I have experienced a bit of hum/buzz in some setups with the common ground also tied in to the AC ground wire. You may isolate the star ground point (and other taps to the chassis for that matter) from the chassis ground. Just my thoughts.
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Last edited by whaleman; 10th February 2012 at 06:34 PM.
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Old 10th February 2012, 06:58 PM   #3
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Thanks for the quick reply whaleman. I am a lil worried about the humming issue these kits seem to have. I also have that horrible "pop" when powered off. I thought i read somewhere on this forum about putting a mylar cap across the main switch to prevent it....... I'll have to look some more
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Old 11th February 2012, 01:49 AM   #4
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So can i add a headphone jack to what i have and just use a rotary selector switch or do i have to use a heavy duty dpdt rocker?
I already have the rotary switch and plan on using it to switch between rca and 3.5mm inputs.

I'm actually running out of room in this case... I still have to mount the transformer I do have one other option for the location of the trans. and possibly a vu meter, i'll have to post up a pic when i get home tonight.
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Old 11th February 2012, 07:32 AM   #5
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Add a 100 Ohm 1 W resistor in series between each amp channel and the headphone jack as current limiters for the short that occurs, when the headphone is plugged in and out.

A rotary switch is OK for source selection.

Only one wire must go from the star point to each amp board. Connect it to the power supply input GND terminal.

The speaker ground should either connect directly to the star point or to the PCB. From THD point of view the PCB is better. From hum point of view the star point is usually better.

Connect the input grounds to the same point as the potentiometer grounds, and since they have a metal case, connect that to ground, too. Ground at the PCB first. Only if you have hum, and grounding the speakers to the star point does not cure that, move the input ground from the PCB to the star point.

Do not use the chassis as a lead, neither for input nor for speakers. The goal is to keep the chassis as free from current as possible.

With that PSU board it will probably work best to separate the star point from the chassis, so that the GND screw on the output side of the PCB effectively becomes the star point. Then use the GND screw on the PSU input side to connect to the chassis and the transformer. Join PE to the point, where the PSU GND is connected to the chassis.

Here is a quick and dirty sketch, where only the ground is drawn. Start with version 1. If you get hum, use version 2. If hum persists, try version 3.

Click the image to open in full size.

In all cases make sure that the source and the amp are fed from the same outlet to keep the loop through PE small.
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Old 11th February 2012, 04:51 PM   #6
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Wow! thank you for the very specific response pacificblue. Your explanation makes complete sense and i do not know how i didn't catch it on my own now.

I'll give it a shot when i make myself available and let you know what i come up with.
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Old 11th February 2012, 09:33 PM   #7
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Note that any switch used for alternating between speaker and headphone operation must obviously be able to carry all the speaker output current, i.e. 5+ amps.

For an amp that typically drives 4..8 ohm loads fine, 10 ohms of output series resistance for the headphone jack should easily suffice, and not influence frequency response as much. Noise level and gain may turn out to be a bit high though - a gain of 26..29 dB is usually OK for 600 ohm cans, but more sensitive ones tend to be fine with 0..14 dB.
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Old 11th February 2012, 09:39 PM   #8
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So does this look anything close to what i need to do or am i beating my head on the floor???



If photos or specs are needed let me know and i"ll post what i have and to the best of my knowledge
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Old 12th February 2012, 09:00 PM   #9
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I think I'm going to start a new build thread or blog on this project.... got to get some better pictures of the progress i have so far.
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Old 12th February 2012, 09:01 PM   #10
djoffe is offline djoffe  United States
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Default more grounding info

check here:

The PAT-4 Pages

look for "Library of Grounding Problems"

Douglas Self's Power Amplifier Design book is a very good reference also...
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