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Old 23rd March 2004, 11:03 AM   #1
sobazz is offline sobazz  Denmark
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Cool 1st PCB - critique, please!

Well, finally I've finished my first PCB (ehh.. silk screen not completed yet). It is a 2 channel inverting OPA549 amp with 24db/okt. Linkwitz Riley crossover. The OPA549s are mounted on daugtherboards along with the gain setting resistors and local ps decoupling caps.

IC1 is a standard single opamp. IC2 and 3 are dual types. The opamps should all be pin compatible with OPA134 and OPA2134 - that is - the standard pin configuration.

IC4 and 5 are LM317/337. And the regulator circuits are Fred Dieckmann's posted very recently. Depending on the results he get I'll go with this regulator. It is unlikely to fit a more complex one on the board.

The amp will be powered from a 18-0-18 120VA toroid and supposed to be mounted in a loudspeaker cabinet. The board measures 116x85mm.


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What would an appropriate C/W value for a shared heatsink be? The minimum impedance of the drivers are around 5.5ohm. And what about the regulator IC's? They will regulate the voltage down to 15-16v from 25v. But the crossover will not draw more than 50mA maximum.
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Old 23rd March 2004, 02:50 PM   #2
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1.) where are the outputs or power ICs?
2.) are heatsinks on the regulators?
3.) C1 looks like an eye - 1 circle - 1 pad. why?
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Old 23rd March 2004, 03:04 PM   #3
sobazz is offline sobazz  Denmark
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1. The power IC's are mounted on seperate daughterboards. The daughterboards are connected to the pads on the left and the right side. The heatsink for the OPA549 will be mounted above the board due to limeted space.

2. No heatsinks are on the regulators yet. But space is reserved. As I wrote, I have yet find out what rating is needed.

3. Hehe... C1 is a 1uF PP capacitor. It was not in ExpressPCB's library, so I added it myself. The pad to the left of the yellow circle (on line with the iris... ) is the second capacitor pin. The pads are obviously too small. I'll have to correct that.
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Old 23rd March 2004, 04:55 PM   #4
Alcaid is offline Alcaid  Norway
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How about posting schematics so we know what we're looking at?
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Old 23rd March 2004, 05:13 PM   #5
sobazz is offline sobazz  Denmark
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... I knew someone would ask. I'm working at it.
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Old 23rd March 2004, 08:18 PM   #6
sobazz is offline sobazz  Denmark
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The crossover components are now named. Here's the schematic for the crossover. It's a basic 24db/okt L-R.

The regulator was posted by Fred Dieckmann a couple of weeks ago. You can find it here.

The power supply for the power opamps is trivial. It consists of nothing but the toroid, two 25A IC rectifiers and the electrolytics placed on the board. Each channel has a rectifier and to electrolytics. The crossover is powered from the high-pass power amp supply.

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Old 23rd March 2004, 10:59 PM   #7
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u3, -Vee pin is not connected. at least not from what i can see.

where does the signal enter and where does it leave the board? i assume it leaves on the vertical strip, but where does it enter.
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Old 23rd March 2004, 11:05 PM   #8
sobazz is offline sobazz  Denmark
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Fixed, thank you.

What about generel comments on track width, risk of stray capacitance/inductance etc? I have no theoretical background and no practical experience, so this is sort of a shot in the dark...

The signal enters at W1. Signal ground at W2.
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Old 24th March 2004, 04:40 AM   #9
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r5 on the PCB, located at far right, seems to be connected to the inverting input instead of the non-inverting input.

you may want to do a netlist check, either software or manully.

on the schematic, confirm ALL connections are correct.
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Old 24th March 2004, 04:44 AM   #10
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regulators will dissipate 0.5W each if what you say is correct. this is fine for a metal tabbed TO220 case without heatink, so i'd say heatsinks are not needed.
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