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Old 8th February 2014, 07:54 PM   #1
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Default Help with LM4766

Hi,

I want to make an amp with the LM4766 chip(2x40W). I've read in the data sheet that I need to power it with a -/+30V voltage. Since it is my first project I don't really know what transformer should I use. I was thinking of a transformer with two voltages of 30V, but what power rating should it have?

As an example, this is what i have in mind
TST160/016 INDEL - Transformer: toroidal | TME - Electronic components

is it any good?
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Old 9th February 2014, 03:24 AM   #2
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This transformer is not suitable, as will +-42V and is greater than the maximum power supply suitable transformer ~ 2x18V . http://www.tme.eu/en/details/55152-p...630954a8cee779

Last edited by rulez alex; 9th February 2014 at 03:32 AM.
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Old 9th February 2014, 03:30 AM   #3
Mihkus is online now Mihkus  Estonia
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Voltage is being amplified by 1,414 after filtering...
So for around 30V DC you need 30/1,414=~21,2V AC
But whats the resistance of the speakers your going to drive with this amp?
I belive you have to reduce the supply voltage.
Maybe for 4 ohm Id choose dual 15V and for 8 ohms Id go for 20V AC
And yes, you need 2 voltages. like 2x20 20-0-20
But you should check page 14 on datasheet it shows you what you need to know..
Ive built one of those... It started smoking at 4 ohms -+28V so... It cannot dissipate all that heat.
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Old 9th February 2014, 08:37 AM   #4
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Quote:
Originally Posted by Mihkus View Post
Voltage is being amplified by 1,414 after filtering...
So for around 30V DC you need 30/1,414=~21,2V AC
But whats the resistance of the speakers your going to drive with this amp?
I belive you have to reduce the supply voltage.
Maybe for 4 ohm Id choose dual 15V and for 8 ohms Id go for 20V AC
And yes, you need 2 voltages. like 2x20 20-0-20
But you should check page 14 on datasheet it shows you what you need to know..
Ive built one of those... It started smoking at 4 ohms -+28V so... It cannot dissipate all that heat.
Ok. I'm beginning to understand things a little better. How do I determine the amount the voltage is amplified with? It is something standard or do I calculate it from the datasheet?

The speakers I'm going to use will have an impedance of 8ohms and from what you've told me I've come up with this transformer.

TST100/010 INDEL - Transformer: toroidal | TME - Electronic components

Still, I'm not really sure about the power rating. I know that it should be higher than the amp needs, but by how much. I've guessed that since the output power of the amp will be at around 60-70W, I should get a transformer 30% more capable of that power.

Also, I want to thank you for the help. As a beginner it is very hard to find the information everyone takes for granted.
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Old 9th February 2014, 09:49 AM   #5
sangram is offline sangram  India
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That would be the absolute maximum I would use on the 4766. I've run with 30V rails and very easy speakers, and it ran super-hot. At 25V and a large heatsink it runs properly warm. You will end up between 26 and 27V, which is higher than I would use but tolerable. I would choose 16 or 18V transformers. The power dissipation is proportional to the square of supply voltage, so a small change in voltage affects the dissipation significantly.

AC to DC conversion is 1.414, minus the voltage drop across the diodes. VA rating of the transformer for commercial amps is usually 0.6 to 0.8 times power output - they assume that you will never hit full power on both channels and that filter caps will take some of the load - a reasonable compromise to keep costs down. Higher quality products may stretch this to 1.0 or 1.3. DIY builders generally build at 1.5x or 2x. I would say VA sizing is based on the way you look at the project.
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Old 9th February 2014, 03:52 PM   #6
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Originally Posted by sangram View Post
That would be the absolute maximum I would use on the 4766. I've run with 30V rails and very easy speakers, and it ran super-hot. At 25V and a large heatsink it runs properly warm. You will end up between 26 and 27V, which is higher than I would use but tolerable. I would choose 16 or 18V transformers. The power dissipation is proportional to the square of supply voltage, so a small change in voltage affects the dissipation significantly.

AC to DC conversion is 1.414, minus the voltage drop across the diodes. VA rating of the transformer for commercial amps is usually 0.6 to 0.8 times power output - they assume that you will never hit full power on both channels and that filter caps will take some of the load - a reasonable compromise to keep costs down. Higher quality products may stretch this to 1.0 or 1.3. DIY builders generally build at 1.5x or 2x. I would say VA sizing is based on the way you look at the project.
So, 2x17V @2.93A should be fine? At this voltage the amp will output around 30W per channel, so the transformer power rating is 1,6 times that of the amp.
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Old 12th February 2014, 04:14 PM   #7
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One other thing. Regarding the schematic(see atttchment), the VccA and VccB pins have the same supply from the main Vcc line?

Same goes for the audio input? +In_A and +In_B have the same source?

I've put together an Eagle schematic. Can somebody take a look and tell me if it's any good? (i've uploaded via skydrive. Couldn't make an attatchment for a .sch file).
Attached Files
File Type: pdf 0s8gwd6fxfdz1733yer850805fky.pdf (602.5 KB, 4 views)
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