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Old 9th March 2014, 11:23 AM   #21
AndrewT is offline AndrewT  Scotland
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Describe and/or draw your final Grounding Scheme for those that could benefit from a summary.
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Old 9th March 2014, 05:55 PM   #22
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OK. Here goes. I thought about a drawing but decided a wordy description is easier and better allows for explanation. Sometimes it can be difficult to see how to adapt a diagram to individual projects.
Preamplifier first - power comes from a toroidal transformer (scrounged from an old amp), secondaries had a few turns removed to give 2x18v. The wire removed was then rewound to give an additional 8v secondary. A custom transformer for free!
Circuits are built on stripboard (I have found Maplin FR4 stripboard to be excellent).
The circuits are built separately (modular):
Analogue circuit (PGA2320 plus LME49720 input buffer and 2 x DRV134 balanced drivers.
Power supply; +/-15v analogue supply, 5v digital and 5v for input relays.
Input selector; 6 relays, 5 pairs RCA and one 3.5mm stereo jack.
Ready made Arduino and LCD display.
The 2 xlr output sockets are chassis mounted and pin 1 is connected to its body therefore directly to chassis.
The short piece of balanced cable from socket to analogue pcb has its screen connected to pin 1 only, not connected to the pcb.
The RCA (pcb mounted) inputs have their shells linked by pcb trace. The screened cable between the input pcb and the analogue pcb has its screen connected to this trace and the 0v (unbalanced signal ground) trace of the analogue pcb.
The 5v power (+5/0v) connects to the Arduino then to the PGA2320 digital side.
The +15/0/-15v power connects to the analogue pcb and supplies the PGA and 3 other chips.
Because I was using stripboard, I needed umpteen link wires. I rearranged the 0v link wires in a straight line from top to bottom of the pcb rather than as originally I had them scattered around the pcb. Digital and analogue 0v were joined under the PGA2320 (which is mounted on a breakout board on top of the stripboard). Close by the PGA2320 a stout wire was run from this 0v to the chassis/mains star ground.
This arrangement would appear to follow the principles of the article that XLR screens are connected to chassis, RCA screens follow the signal.

Power amplifier obviously simpler. PCB grounded to chassis through the mounting screws & spacers.
XLR input socket pin 1 direct to socket body & therefore chassis. Short internal balanced cable screen connected to pin 1 only, not to pcb. Mains cable earth screwed to chassis.
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Old 1st June 2014, 08:45 PM   #23
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A hiccup in the smooth running of preamp.
Power amplifiers giving loud thump (1 beat per second) also loud hum.
Checked output from preamp: 2v dc on DRV134 outputs! Oh No! - symmetrical power supply giving +15 & -13volts. Supply regulation is by LM317/LM337 and cermet trimmers,initially set within .01v, but had drifted.
Should I - 1, replace potentially dodgy trimmer with an identical part; 2, replace with a multiturn pot (easier adjustment); 3, replace with 1% fixed resistors & forgo fine adjustment?
Is it worth monitoring power supply with the Arduino & displaying an error message if supply drifts by say 0.1v?
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Old 2nd June 2014, 02:37 PM   #24
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There is nothing magic about 15.000 Volts, any reasonable voltage will do. The Plus & Minus power supplies do not need to be matched to 0.1 Volt. Even a 2 Volt difference will not be audible. You have some bigger problems.
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Last edited by Speedskater; 2nd June 2014 at 02:41 PM.
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Old 2nd June 2014, 07:30 PM   #25
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Thanks Speedskater, but the imbalance in the supply appeared in the DRV134 output.
After re-adjustment of the supply, the audio balanced output was free of DC and everything now works fine. Just looking to avoid a repeat performance. I did have a 2 volt imbalance.
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Old 2nd June 2014, 08:30 PM   #26
AndrewT is offline AndrewT  Scotland
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There is
Quote:
.........some bigger problem...........
The unequal voltage should not affect the output offset, except when one rail gets very close to the reference ground.
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Old 2nd June 2014, 10:27 PM   #27
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Andrew, there must be more to the circuit schematic than we think. Or the trim pots were doing something really strange.
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Old 4th June 2014, 07:32 PM   #28
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Schematic is as attached. Courtesy of Mark Hennessy and TI datasheet.
LME49720 op amps used. Output from PGA2320 straight into DRV134.
Power supply is on a separate board. Standard LM317/LM337 circuit.
Apart from 470n input caps, all is dc coupled.
Working fine at the moment.
Attached Files
File Type: pdf scheme.pdf (54.4 KB, 10 views)
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