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.
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.
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?
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?
Pre-amp is exactly as described in post 28. Works great for several weeks then output drifts into a DC offset. Preamp is feeding Anaview ALC0300-1300 power amps. Amps are DC coupled.
Power amp datasheet states a few hundred millivolt offset will cause shutdown. Both pre-amp channels show same offset which I suppose is to be expected.
Can I eliminate the offset permanently or do I need to add coupling capacitors and if so, where best to place: Between input buffer & PGA2320, PGA2320 & DRV134 or at output of DRV134? Thanks.
Power amp datasheet states a few hundred millivolt offset will cause shutdown. Both pre-amp channels show same offset which I suppose is to be expected.
Can I eliminate the offset permanently or do I need to add coupling capacitors and if so, where best to place: Between input buffer & PGA2320, PGA2320 & DRV134 or at output of DRV134? Thanks.
Hi Andrew. Thanks for response. Post 28 schematic shows input buffer to PGA2320 volume control then output from this feeds directly into DRV134. LME49720 used in place of OPA2134, otherwise exactly as schematics.
So I need to add a speaker protection board to each power amp. But how is DC error correction done? My understanding of your post 26 is that the offset should not exist?
So I need to add a speaker protection board to each power amp. But how is DC error correction done? My understanding of your post 26 is that the offset should not exist?
Have measured preamp (no inputs/outputs connected). A whopping 7.5v offset at balanced output (hot to cold). Both channels measured same. Hot to ground measured 7.5v, cold to ground <0.5v. Power supply measured +14.2 / -15.2v. Readjusted power supply to +/- 15.2v. Re-plugged power supply to audio board. both hot & cold outputs measured -80mv to ground but only 8mv to each other. Input & output at PGA2320 measures 0v. I'm reluctant to put back into use until I've solved it but since I seem unable to provoke the fault to reappear I'm struggling on how to prevent a recurrence.
Jiggled connections but cannot reproduce fault.
Jiggled connections but cannot reproduce fault.
Thought it polite to let you know -
did what I should have done first, removed power supply board from case, examined under magnifier.
240R voltage setting resistor, lead was loose in a volcano of shiny solder. Must've taken the slightest disturbance to make open circuit then, when testing, to re-contact and test ok.
Oh well, one for experience.
did what I should have done first, removed power supply board from case, examined under magnifier.
240R voltage setting resistor, lead was loose in a volcano of shiny solder. Must've taken the slightest disturbance to make open circuit then, when testing, to re-contact and test ok.
Oh well, one for experience.
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