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Old 10th January 2005, 05:26 PM   #1
Mr Teal is offline Mr Teal  Canada
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Default Overvoltage to LM4780

How dangerous is it to have too high a voltage feeding the LM4780. A toroid I'm looking at to power a stereo 6 channel (tri-amped), 6 chip paralleled GC is has an unloaded voltage of 31.6V. According to my little windows calculator, that's 43.5VDC, higher than the chip is rated for.

Would the extra volt and a half over the rating cause issues? According to the specs, with the secondaries in parallel, the output is 29V @ 20A. 38.8VDC after rectification. I wouldn't mind avoiding the complication of a regulated PSU, but the other option would be to regulate it down to 36V or so.

I definitely want to avoid spike visiting me, has anyone tried this? If so, what were your results?
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Old 10th January 2005, 11:27 PM   #2
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IIRC the LM chips can tolerate higher voltages with higher impedance loads and with NO signal applied. If you have several chips you can experiment... if the first one pops, install another with a regulated supply or unwind a few volts off the transformer's secondaries if possible. Perhaps a little resistive load of the power supply would lower the voltage enough to be tolerable. Perhaps the quiescent current of your circuits combined would be enough.

The safest route of course is to look at ANOTHER transformer.

I am pretty sure this has been discussed before...be creative with your search parameters.
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Old 11th January 2005, 12:50 AM   #3
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You can also try adding some extra diodes to drop the voltage.
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Old 11th January 2005, 01:16 AM   #4
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figure the PDMax and delta-T and heat sink appropriately -- you can get a lot out of these chips if you keep them cool (hint -- try a fan!). if you dont heat sink adequately you will wind up with power output well below what you anticipate since the SPIKE will kick in.

SPIKE isn't a bad thing, it sure beats soldering these babies!
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Old 13th January 2005, 11:42 PM   #5
Mr Teal is offline Mr Teal  Canada
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Well, I decided to go with a dual secondary 600VA 27V unit. That'll be within the rating of the chip, even unloaded.

My PS will be a 600VA 27-0-27
2 35A bridge rectifiers
4 68,000 50V 105C Nippon large can filter caps (2 per rail)
10uF and 100nF on the amp board.

The amp will have 6 seperate boards with 6 chips, one per channel (each 4780 is paralleled).

A question about grounding though. Most people suggest a star ground where everything's returned to the same point, usually at the caps 0V point.

I'd planned to run 12 guage wire to each amp board from the cap common, and then have the line input, speaker output and feedback grounds on each board go to that 0V input. Should that work as well?
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Old 14th January 2005, 12:18 AM   #6
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It might.

Try it.
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Old 16th January 2005, 08:17 PM   #7
Mr Teal is offline Mr Teal  Canada
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Here's the amp board. It's fairly wide at about 2 1/2 inches, but slightly less than an inch deep. I left the extra area between the caps and the chip to make sure it was easy to screw the retaining bar into place.
Rm is 8.2k
Rb's are 1k 0.1%
Ri's are 1k 0.1%
Rf's are 20k 0.1%
Rin is 47k 1%
Rout's are 5W 0.1Ohm

C1 & C2 are 10uF
C3 & C4 are 100nF
C5 & C6 are 1nF


What say you all? Does the grounding scheme look good? are the traces thick enough?
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Old 17th January 2005, 01:25 PM   #8
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thomas997 wrote:
Quote:
You can also try adding some extra diodes to drop the voltage.
Good idea!!
Yes 3 power diodes each per rail. will drop 2...3volts/rail.
No schottky or FR diodes...
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Old 17th January 2005, 02:19 PM   #9
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Quote:
What say you all? Does the grounding scheme look good?
Yes and no. The feedback ground should not pass through the filter capacitor tie-point on the way back to the center tap input. You will get hum this way. Not just a little bit, either.

You already have a top-layer trace running to pin 3 of the 4780; disconnect it there and run it back to the POWER connector. If you have the time, I suggest making two runs for a HQG, one for the input signal and one for the feedback resistors. Since those caps are relatively small, and the charging currents will likewise be small, the speaker ground is fine where it is.
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Old 17th January 2005, 02:44 PM   #10
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RMute is going to dissipate about 0.25W -- make sure you size it correctly.

If you are going to use surface mount, then the 100nF caps could be SMT also. Mount these as close to the power pins of the LM4780 as possible.

you might also want to check the diameter of the pads and drill setting for the LM4780's pins -- these are 21mils wide, so a standard DIP pad opening (drill diameter) is going to be too wide. Set the drill for these at 25mils.
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