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Old 21st March 2013, 09:17 AM   #1
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Default PGA2311 pre-amp static discharge problem

I've built a PGA2311 based pre-amp like this one on eBay.

Works very well and sounds good but is extremely sensitive to static discharge which causes the pre-amp to "crash" and make the most appalling speaker destroying loud noise. It's like hum and buzz at full volume.

The "crash" can be triggered occasionally simply by touching the volume knob and more frequently by the static discharge from vinyl LP's

I've tried earthing the case but it hasn't made any difference.

Any suggestions to fix this problem are most welcome!!
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Old 21st March 2013, 01:07 PM   #2
marce is offline marce  United Kingdom
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If the case is correctly grounded it should not be a problem as it is then I would suggest that the grounding scheme has a high impedance connection, probably a star point, run large seperate ground wires between points to improve and lower the ground impedance.

A schematic of the grounding would help give a more qualified answer.
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Old 21st March 2013, 04:06 PM   #3
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Thanks for the reply Marce. Here's a schematic for the board:

Click the image to open in full size.

And here's my pre-amp:

Click the image to open in full size.



There's no obvious grounding point on the board. In fact the commercial assembled versions of the pre-amp on eBay do not appear to have any grounding.

Last edited by activexp; 21st March 2013 at 04:08 PM.
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Old 21st March 2013, 04:13 PM   #4
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I've had similar problems in similar circumstances. Check the grounding between the PGA digital ground connection, and the digital ground of your ucontroller and rotary encoder setup.

If there's an issue there, static discharge can look to the controller as a digital input stream. The fact that it happens when you touch the knob points that way. This can send all kinds of random commands to the PGA with the results that you see - random levels, quickly changing, sounding like hell breaking loose

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Old 21st March 2013, 04:33 PM   #5
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Yep. Have the same problem wit that kit. Now I just avoid touching and use the remote...
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Old 22nd March 2013, 05:06 AM   #6
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Check continuity between different chasis parts.

Gajanan Phadte
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Old 22nd March 2013, 07:09 AM   #7
marce is offline marce  United Kingdom
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Yes anodising is a good insulator.
What concerns me is the damage that the discharges are doing to the actual devices, it can destroy chips.
I would affix a nice thick wire from the front panel to the safety earth at the mains input, also a thick GND wire to connect the two boards together, minimising the GND (0V) impedance, the aim is to create a low impedance path for the ESD energy to discharge down.
I would be interested to any links to the PCBs used as this problem is probably down to far from optimal PCB layout, and or things lacking in the circuit layout such as protection devices for ESD. It is srtrange to hear of these problems these days, they were more common in early to mid 80's!
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Old 22nd March 2013, 09:25 AM   #8
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Quote:
Originally Posted by marce View Post
Yes anodising is a good insulator.
What concerns me is the damage that the discharges are doing to the actual devices, it can destroy chips.
I would affix a nice thick wire from the front panel to the safety earth at the mains input, also a thick GND wire to connect the two boards together, minimising the GND (0V) impedance, the aim is to create a low impedance path for the ESD energy to discharge down.
I would be interested to any links to the PCBs used as this problem is probably down to far from optimal PCB layout, and or things lacking in the circuit layout such as protection devices for ESD. It is srtrange to hear of these problems these days, they were more common in early to mid 80's!
Will give it a try, thanks for the suggestion

During my 50 odd years tinkering with audio gear, I recall RF breakthrough being an issue way back when, a lot less dangerous than chip-killing static discharge.
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Old 22nd March 2013, 10:57 AM   #9
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It is preferred to not connect the safety earth wire, as it (earth connection) will give you ground loop problems.

Gajanan Phadte
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Old 22nd March 2013, 02:19 PM   #10
marce is offline marce  United Kingdom
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No it may give a ground loop problem, but ground loops are solvable.
What do you mean do not connect the earth wire to the unit, dangerous and under no circumstances should it be ommited from ANY DIY gear.
And while I am on a roll, we have a major proble to solve ESD, no point worrying about possible ground loops.
Ground loops are caused by some high impedance point in the 0V (GND) scheme of a system, Henry Ott, Tony Waldron etc have plenty of info on interference grounding of audio systems.
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