Power supply for amps

[audioguru2]

The LM3886 with a dual 28V power supply and driving a 4 ohm speaker with 68W is about as easy a circuit as it gets, and works extremely well.
I doubt that you will hear any difference in loudness between the 68W amp and a 100W amp. Your hearing's response to loudness is logarithmic, so 10 times the power sounds only twice as loud. Most people can barely hear a difference in loudness of twice the power.

 

[frankwas]

Thanks once again for all your replies guys!!!

I think I have finally decided what I'm gonna do. I am going to build the two velleman kits, and then two LM3886 amps. I am going to go with Rod Elliots design and his power supply. But in his notes he says that his power supply is only good enough for one dual opamp. Does he mean that it will only be string enough for a single LM3886?

Another question I have is heatsink ratings. Is lower oC/W better or how does it work? I am going to use fans on my heatsinks anyway, so will these values be that significant?

If Rod's psu design can pull two 3886's then I can really save a lot of money cause I can get a 160VA, 25-0-25 toroidal transformer for really cheap.

TA!!!

 

[audioguru2]

I am going to go with Rod Elliots design and his power supply.

The LM3876 is designed to drive an 8 ohm speaker, not the 4 ohm speaker you said you will use.

But in his notes he says that his power supply is only good enough for one dual opamp. Does he mean that it will only be string enough for a single LM3886?

An LM3876 is a power amp, not a little opamp. His circuit uses a couple of 680 ohm resistors feeding 15V zener diodes to power a dual opamp for a preamp or something.

Another question I have is heatsink ratings. Is lower oC/W better or how does it work? I am going to use fans on my heatsinks anyway, so will these values be that significant?

Yes, the lower is the degrees/W number of the heatsink then the cooler will be the amplifier ICs. The datasheet and the application note for the amplifier ICs shows how to calculate a heatsink's rating and even gives recommended numbers. Heatsinks have a degrees/W rating without a fan and with a fan having a certain airflow.

If Rod's psu design can pull two 3886's then I can really save a lot of money cause I can get a 160VA, 25-0-25 toroidal transformer for really cheap.

A 25-0-25 transformer makes about positive and negative 35VDC. Each LM3886 amp gives 50W to an 8 ohm speaker with that power supply. Aren't you using 4 ohm speakers? The amps are about 70% efficient so two amps need a total of 143W which is 143VA from a transformer for 8 ohm speakers.

 

[frankwas]

Ok.

But in the LM3886's datasheet it says it puts out 68W on 4 ohms at 28V. Will my IC be damaged if I use the 25-0-25 160VA transformer with two 4700uF caps and a 400V bridge rectifier as in the diagram?

I am really confused right now.  ???

 

[audioguru2]

But in the LM3886's datasheet it says it puts out 68W on 4 ohms at 28V.

Correct. With a positive and negative 28VDC supply, that is made with a 21-0-21 transformer.

Will my IC be damaged if I use the 25-0-25 160VA transformer with two 4700uF caps and a 400V bridge rectifier as in the diagram?

It would try to protect itself if the volume is high. It might be destroyed or just sound very bad. When it sounds bad then it might blow a tweeter.
The transformer would be overloaded if it supplies two amplifiers having 4 ohm speakers.
The applications note shows how to parallel two LM3886 ICs to drive a 4 ohm speaker from a positive and negative 35VDC supply from a 25-0-25 transformer.

 

[frankwas]

Oh ok. I understand.

But let me ask this, it might sound very strange: I'm only going to power, or need, 40W rms out of each amp and probably not constantly. Won't that give the transformer a little room to breathe? Or would you recommend that I get another transformer?

 

[audioguru2]

You won't have anything to limit the power to only 40W. You won't even know when the power reaches only 40W. If the supply voltage is too high for the amplifier's load then the current will also be too high. Voltage times current= power. Power makes the amplifier IC get hot. The heatsink with a fan can take away only as much heat as per the power rating of the IC.

One inexpensive 25-0-25 transformer is perfect for two LM3876 amplifiers driving 8 ohm speakers. If you use 4 ohm speakers then either parallel ICs are needed and a single transformer per channel, or a lower voltage transformer with two LM3886 amplifiers.

 

[frankwas]

Ok.

I have another quesiton. You said earlier that I can use a 21-0-21 transformer. This will work perfectly for a 4 ohm load then? How many VA's must the transformer be?

I apologise if we are going in circles. 

 

[audioguru2]

The datasheet shows an LM3886 with a positive and negative 28VDC supply feeding 68W to a 4 ohm speaker.
Amplifier ICs are about 70% efficient, so two amplifiers will draw 194VA from a transformer.
A 21-0-21 transformer will make positive and negative 28VDC.

 

[frankwas]

Oh ok.

The closest that i can get to that is a 18-0-18. Will that be ok?

 

[audioguru2]

A transformer that is 18-0-18 will make a positive and negative 23.4VDC supply. The LM3886 will give about 50W to a 4 ohm speaker.

 

[frankwas]

Ok. Will this transformer do the trick?

230V to 18-0-18V 2x4.44A 160VA

Or must I get another one?

Can I still use Rod's design for the LM3886 just as it is with the PSU I mentioned earlier with two 4700uF's and a 400V 35A bridge rectifier?

I also have another question about pots. What is the difference between a linear and log pot and which one will work best for this project?

Thanks.

 

[audioguru2]

Two 50W amplifiers will need a transformer of at least 143VA so the 160VA transformer will be fine.

Two 4700uF capacitors would be the minimum value for 1 amplifier. Use two 10,000uF capacitors for the stereo amplifier.

The voltage to the bridge rectifier is the peak of 18V which is only 25.5V. It is a waste to use a bridge rectifier rated for 300V. A 50V or 100V bridge rectifier would be fine.

A linear pot is for adjusting a DC voltage. A logarithmic (audio taper) pot is a volume control and it matches the logarithmic response to loudness of our hearing.

 

[frankwas]

I built the first prototype today and it worked really well!

Except, I have a lot of noise as soon as signal is fed into the amp. Can the be because the gain is too high? Or what can I do to remoce the noise?

Thanks!

 

[audioguru2]

The noise from these excellent IC amplifiers is so low that you would hardly hear it, and it would be all the time, not just with signal. The gain is only 23 and is recommended.

The pcb layout and supply bypass capacitors are very important to prevent the IC from oscillating, which could occur with signal. The pcb ground trace from the speaker should not flow into the input ground trace.

Frequently the datasheet has a recommended pcb layout but not this one. Maybe its applications note does or maybe there is a project with it on the web.

 

[frankwas]

Oh ok.

I think that it's my layout that is giving the problems. Do you perhaps know of a pcb layout for this circuit?

I really appreciate your input!!!

 

[frankwas]

The pcb ground trace from the speaker should not flow into the input ground trace.

How do I seperate my input ground from my speaker ground? Where do I get ground for my speaker and/or signal?

TA.

 

[MP1]

You should post your schematic and board layout if possible so that you can get more exact help.

MP

 

[frankwas]

I would just like to say thanks guys for all your input!!

I have built one of the amps so far and it sounds great!!!! I will post some pics when I have completed the whole project, and ahen I get my bro's cam

Thanks once again!

 

[audioguru2]

I have built one of the amps so far and it sounds great!!!!

Nice going, Francois. ;D
How did you fix the noise problem?