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Old 16th January 2004, 12:15 PM   #1
macce78 is offline macce78  Finland
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Default Reducing output voltage

Hello!

I'm building an amp based on the LM3886, and now (I think) I've got some problems with the voltage supply. The supply gives out about 44 V wich is a bit too much for the chip (isn't it?). The amp works pretty well for a few minutes, and then the chip blows.
Is there any simple way to reduce the voltage to say 35-40 V, or do you know any othe chip that can handle that much voltage?
I'm using a 230V/2x30V 260 VA toroid in the power supply. I already thought of reducing a few turns from the secondary windings but found it impossible, because the toroid lies in a plastic "cup" wich is filled with some sort of white glue.
The easiest (and the most expencive) way would probably be to buy a new toroid, but that's the last option.
Any suggestions?
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Old 16th January 2004, 12:27 PM   #2
dhaen is offline dhaen  Europe
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If there is a centre hole in the toroid, you can add some turns, and connect in series - out of phase. It will need 2 sets of new windings.
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Old 16th January 2004, 12:35 PM   #3
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....you could use a pair of voltage regulators to get the voltage down to 35-38 volts......i use lt1083.........
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Old 16th January 2004, 12:37 PM   #4
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Quote:
Originally posted by dhaen
If there is a centre hole in the toroid, you can add some turns, and connect in series - out of phase. It will need 2 sets of new windings.
in series with the primary -- this is the best solution
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Old 16th January 2004, 12:41 PM   #5
dhaen is offline dhaen  Europe
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I meant the secondaries.
There will be considerably fewer turns (though much thicker).
There are safety issues to address when messing with the primary too.
Remember that macce78 is in a 220V country.
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Old 16th January 2004, 01:18 PM   #6
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if you put in more "secondary" turns you increase the ratio of secondary to primary, ergo hoc the output voltage goes up.
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Old 16th January 2004, 01:25 PM   #7
macboy is online now macboy  Canada
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Technically, +-44V is too much. The chip is spec'd for up to +-42 V, so you aren't too far out of range. If, however, you are using the chips in a bridged amp and/or you are trying to drive a load of less than 8 ohms, then you definitely will run into problems with that voltage. 8 ohm would be the absolute minimum for a single chip, and you can forget about bridged altogether.

The chip can survive +-44 V, but the key is reducing the power dissipation so that it will remain happy. To do this, you need to design the amp so that less current flows (for a given output voltage). Less current translates directly into less power dissipation in the chip. There are two ways to accomplish this: us a higher impedance load, or use more than one chip in parallel.

If you use two (or three or four) chips in parallel, they will each provide a fraction of the current output, and therefore each chip heats up a lot less.

If you are using a 8 ohm load, I would recommend two chips in parallel. Using just one might be OK, but only if it has a very large heatsink and is in the normal (T) not isolated (TF) package. If you are using a four ohm load, consider Three chips or maybe even four. You can drive an almost arbitrarily low impedance by just paralleling more and more chips (and as long as your power supply is up to it). Refer to National's application note AN1192 for schematics of parallel amps. (it is not as simple as connecting the pins of multiple chips in parallel - that could result in disaster).

You didn't mention how big your heatsink is. You are using a heatsink aren't you? It is not optional.
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Old 16th January 2004, 01:31 PM   #8
SY is offline SY  United States
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Quote:
Originally posted by jackinnj
if you put in more "secondary" turns you increase the ratio of secondary to primary, ergo hoc the output voltage goes up.
Depends on the polarity of the new winding wrt the old one. Connect one way and it boosts, connect the other and it reduces, right?
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Old 16th January 2004, 01:37 PM   #9
dhaen is offline dhaen  Europe
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Quote:
Originally posted by dhaen
If there is a centre hole in the toroid, you can add some turns, and connect in series - out of phase. It will need 2 sets of new windings.

Quote:
Originally posted by jackinnj
if you put in more "secondary" turns you increase the ratio of secondary to primary, ergo hoc the output voltage goes up.
Although I wasn't clear enough about which winding, I did mention the phasing
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Old 16th January 2004, 01:44 PM   #10
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This could be a great chance to try a voltage regulator with the chip amps. You can try something like the power supply regulator for the Zen version 4 (www.passdiy.com) and use it as a capacitance multiplier. That will drop 4+ volts per rail.

Jeremy
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