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Old 22nd May 2004, 04:00 PM   #1
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Question Nasty, voltage reduction question for LM3886

Greetz good people!
I have been reading the forum for a while
and I only have good things to say about it. Many nice people!

Now trying to build the LM3886 amp for the 2nd time; The first one worked already, but it became rather hot, because I used a +-46VDC power supply, which has not changed since. Now i'm trying to get some advices about the issue. I cant afford to buy another transformer, and this is a rather good one, comes from a fried Sanyo STK chip amp. Problem is the slightly higher voltage.

The trafo secondary itself gives 40Vac, I can measure 46Vdc after rectification and the 4700uF buffer elcaps.
I was thinking a lot what kind of voltage reduction should I use except the winding kindof tinkering (dont have expert tools).

By the way, how disadvantageous is to supply +/- 46 volts for the LM3886? I bet it is too much.

Does anyone have some good tips or experiences about what to redesign in the amp circuit or what kind of power solution to use? I'm using the GC plan can be found
here .

Voltage regulator?? Emitter follower? How much do they waste?

I have some 33V zdiodes here and a few BJT transistors, maybe they could be useful.

Please, if anybody has met the problem of too high voltage for the LM3886 GC, share the wisdom with me.

The first one sounded good, but at high volumes but I'm afraid it could sound even much better with the correct voltage without the SPiKe eventually being activated...

Blesses go to everyone...
Thanks!
neocorteX
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Old 22nd May 2004, 04:25 PM   #2
groyne is offline groyne  India
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hey,
the 3886 datasheet says the max you can give the ic is 42 volts, and it's usually advisable to leave about 2-3 volts headroom, so you should, at best provide only about 38-39V. 46 is waaaay too much!!!
1. use a simple resistor divider n/w with precision resistors.
2.construct electronically regulated power supply(give a search on google).

hope this helps.
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Old 22nd May 2004, 05:13 PM   #3
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Default Proper 3886 GC voltage

Thank you! But the solution you wrote makes the circuit have a huge decrease in efficiency i guess.

Electro experts,

isnt it possible to build something more or less simple to drop the excess 6-10V voltage, or even better would be to have it changing along with the amplifier's power output? - i mean in a lower range, but never 46V.

Can someone please give me some links where can I find proper information on modifying an amplifier for a given (higher) voltage?
What is an emitter follower exactly? Does anyone think it is applicable solution for my problem? I dont feel like going twice as expensive, so something simple and agile would be nice.

May it be possible without wasting Earth's precious energy sources ?
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Old 23rd May 2004, 02:58 PM   #4
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If you want a quick and dirty solution you might be able to take several diodes and put them in series to drop the extra voltage. Keep in mind you need diodes to handle the proper amount of current, and of course you're going to have power dissipation. The power disspation can be calculated pretty simply by 0.7v*Current for each diode. I'd suggest using several TO-200 package diodes (nice small package), on semiconductor might be a good website to find some free samles. Also I might suggest putting a 0.1uF cap in parallel with the diodes, doesn't need to be high voltage, but to quiet them down some. This may help you get over the hump quickly, but next time i'd suggest using a power transformer with lower voltage secondaries. If you want to run the amplifiers at maximum voltage rails i'd suggest a center tapped secondary transformer at 60VAC (rms). 48VAC is a nice number too, but will not get you to the maximum rails. Hope this helps.
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Old 23rd May 2004, 04:17 PM   #5
groyne is offline groyne  India
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as far as i know, an emitter follower has a voltage gain of nearly 1(but less than 1). it has high current gain though, so i don't see how this will help you in any way, since your voltage will not decrease.

like super audiobuff says, you could use diodes/resistors to achieve a calculated drop, but then you'd lose out on efficiency.

correct me if i'm wrong, but wouldn't a 40V zener work here??
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Old 24th May 2004, 12:17 PM   #6
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Thanks for the advices, I found out theres no way to efficiently reduce the voltage without going switch mode (which is not cheap, especially if I dont wanna visit the shop again).

Meanwhile I've found another trafo in my garbage, its slightly smaller (2x3A I measured on DC with a rectifier) but it has 25V ends. Now I'm thinking about making a dual capacitance multiplier, what about the idea? Will LM3886 sound better with it than the simple unregulated (graetz-4x2200uF elcap) PSU?

The problem is, I only have a lot of various NPNs at home, but no PNP complementaries for them. Is it possible to implement the dual (+/-) capacitance multiplier circuit based on this ) project using only NPN transistors?

wish ya success...
neocortex
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