How to Reduce Rectified Voltage in Power Supply

[hongrn]

To all amp guys, I need some help. My LM3886 goes into thermal shutdown because my toroid cranks out 27VAC instead of the specified 25VAC. That makes it almost 38VDC going into the amp board. It'll play for about 5mns, shut down, and resumes playing after I let it cool down. And yes, the boards are heat-sinked. The datasheet calls for Vcc = +-35V at the most, so the question is, how to I drop 3 volts? I don't want to get another toroid, so do I need a voltage regulator here? Please be real specific with your explanations here, because of my learning curve. Thanks.

Hong

[hipcheck]

Re: How to Reduce Rectified Voltage in Power Supply

What Toroid do you have? Do you have a link to the datasheet? Also, what power supply did you build? This would really help me to help you.


For a regulated supply look here:

http://www.firstwatt.com/pdf/art_zv3.pdf

Simple mosfet follower will drop about 4V without any zener diodes. You could use zeners to set a operating voltage also. Simple to put together on perf/vero board.

I would suggest setting the voltage to operate the amp @ a lower voltage, maybe @30V to 32V +/- max. You have some variables like AC line voltage being higher or lower, and different speakers having different apparent loads on the chip, you would not want to operate " at max" long term. The glory of DIY is the ability to overbuild our projects to not have these problems.

[dcibel]

Re: How to Reduce Rectified Voltage in Power Supply

You can unwind the secondary winding on your transformer to lower output voltage. ;) Honestly, anything you add to drop the voltage will be less than 100% efficient, dissipating the loss as heat so likely requires another heatsink in your project. Also I would suggest that even if you do drop 3 Volts, your still gong to have problems. I don't know where you got 35V max supply for LM3886. They are rated maximum voltage of 84V from V- to V+, which would be 42V rails. They will work fine with your supplied voltage, but will prefer an 8 ohm load. I would suggest that either your heatsink is very inadequate, the chips are not properly mounted to the sink, or something else is wrong with your amp. What is your heatsink temp at idle? Are you using the T or TF package?

[benchtester]

Re: How to Reduce Rectified Voltage in Power Supply

To all amp guys, I need some help. My LM3886 goes into thermal shutdown because my toroid cranks out 27VAC instead of the specified 25VAC. That makes it almost 38VDC going into the amp board. It'll play for about 5mns, shut down, and resumes playing after I let it cool down. And yes, the boards are heat-sinked. The datasheet calls for Vcc = +-35V at the most, so the question is, how to I drop 3 volts? I don't want to get another toroid, so do I need a voltage regulator here? Please be real specific with your explanations here, because of my learning curve. Thanks.

Hong

The simplest method is to use the forward voltage drop in diodes (0.7 volts average for silicon, but may vary greatly with part number). You might need diodes (used for AC rectification) in a T-220 package that you can heat sink as well. (Power is current amps times 0.7 volts per diode.) Just put 4 or 5 in series.

You might look into voltage regulation. A simple transistor circuit can be designed or Linear Technologies has high power voltage regulation (but they don't inherently take the voltage in and some protection circuitry would be in order).

[Pete Schumacher]

Re: How to Reduce Rectified Voltage in Power Supply

What you need are bigger heat sinks for the amps. Either that or some low speed, super quiet fans.

[AJ]

Re: How to Reduce Rectified Voltage in Power Supply

What you need are bigger heat sinks for the amps. Either that or some low speed, super quiet fans.

+1

Into 8ohm loads you should be ok with +/-38Vdc rails with enough heatsinking. Double check your mounting arrangment (single screw with washer or bar across the center of package?) and set up a small fan to see if you can keep it out of thermal shutdown longer. If that doesn't work, there's something else wrong.

If you're driving four ohm loads you will need to drop the power supply down to around +/-28Vdc. A new toroid would really be the simplest route.

[AJ]

Re: How to Reduce Rectified Voltage in Power Supply

$22 solution. https://www.antekinc.com/details.php?p=662

[Æ]

Re: How to Reduce Rectified Voltage in Power Supply

The simplest method is to use the forward voltage drop in diodes (0.7 volts average for silicon, but may vary greatly with part number). You might need diodes (used for AC rectification) in a T-220 package that you can heat sink as well. (Power is current amps times 0.7 volts per diode.) Just put 4 or 5 in series.

That was my thinking also. However, he should go and look at the specs and see how much power supply voltage should or should not be used and how much heat sinking is recommended.

[clydethecat]

Re: How to Reduce Rectified Voltage in Power Supply

You could go to a CRCRC filter in the power supply:

[hongrn]

Re: How to Reduce Rectified Voltage in Power Supply

Guys,

Here's the set up: Avel 330VA 25-0-25. LM3886TF kit. As you can see, the heat sink is big enough because the temperature on the LM3886 was 93F when the amp shut down (I used a laser temp probe), so my thinking is the voltage. Load was 8ohm speakers. Thanks for all your replies. I'll look into the diode solution to attempt dropping the voltage, otherwise I'll have to get a new toroid as suggested.

[Pete Basel]

Re: How to Reduce Rectified Voltage in Power Supply

What sort of load are you using?
I take it that when it shuts down after 5 min it is after driving it hard?

Have you considered oscillations?

That heat sink is really boarderline if you are using low impedance loads.

[hipcheck]

Re: How to Reduce Rectified Voltage in Power Supply

Ok what pcb's do you have?
What schematics?
what test equipment do you own?
When the amp shuts off what conditions: was it hooked up to a source, driving a speaker? Or was it on a test bench solo?

We will get it fixed,

Steve

[hongrn]

Re: How to Reduce Rectified Voltage in Power Supply

The amp shuts down during music play with a pair of 8 ohm speakers. This is the kit I used, sorry, no schematics. I'll try to mount each amp board on a separate heatsink to see if that works better. And no, I don't have an oscilloscope. thanks guys for your help.

Hong

[hipcheck]

Re: How to Reduce Rectified Voltage in Power Supply

Hong, did you buy the pcb's already full, or just bare pcb's and stuffed them yourself?

I am thinking oscillation, also.

Do you have a multi-meter? If so can you place a 10k resistor between the speaker + and speaker - terminals. Do not hook up a speaker or source, just short the inputs. Make a video of the multi-meter readings in millivolts. That could show us what the amp is doing.

Here is a simple oscillation tester: http://www.diyaudio.com/forums/attac...n-detector.gif

If LED's light up you have the problem.

Steve

[NickS]

Re: How to Reduce Rectified Voltage in Power Supply

Are there decoupling capacitors on the pins of the chips between V+ & Gnd and V- & Gnd?
I've seen some of these use a 2.7 ohm/.1 uf zobel across the output to control oscillations.
Is it possible these are counterfeit chips?