LM1875 Amplifier Watts

[Suraj143]

Guys I made a subwoofer amplifier using LM1875.The sound is good but the volume level is low for a subwoofer for home use.

I use a 12-0-12 AC transformer.when it convert to DC it will give 16.9V.

I also attached the pdf.

Please give me ideas to bring this amp to a descent sound level.

 

[picbits]

The datasheet says it will give you up to 20 watts with a +\- 25v power supply so you would want to use a higher voltage than the one you are using to get a higher volume.

 

[audioguru]

Your transformer is 24V center-tapped so its peak voltage is 16.9V or 34V and rectified and filtered it can produce +14.9VDC then the amplifier has an output of only 2.5W, or plus and minus 15.9V then the amplifier has an output of only 10.5W.
Increasing the power to 20W (+3dB) will barely be noticed since 10 times the power sounds only twice as loud.

You probably need an amplifier IC and transformer that provide much more power unless you live in a paper house.

 

[picbits]

..... or a more efficient speaker for your subwoofer.

 

[Suraj143]

Thanks picbits

Thank you audioguru.I'll increase my transformer to 18-0-18 AC so it will give nearly 25V DC to the IC.

I noticed when I increase the volume & the bass level the voltage drops down when it punches the bass beats.

I build a 2.1 system,it has 3 LM1875 IC's.One for subwoofer & the other twos for the satellites.I used a 2A transformer.The subwoofer is 8 ohms.

What is the recommended RMS watts you suggest for a subwoofer design?I mean for home use not a high end one

 

[Suraj143]

Guys I can give you any information what you require ex:subwoofer enclosure details,satellites,power supplies,I also have oscilloscope as well.

I want to beat ready made 2.1 subwoofer systems ( below 150$ budget ones).I have deeply researched on the net most of them used below 20W rms to their subs.So my one is more powerful than them.

 

[picbits]

10 watts RMS is sufficient for general music with a low to medium volume.

My subwoofer on my office speaker system is 25w RMS (true - not made up) and is pretty loud and clear. The subwoofer/amp on my home cinema is 500w RMS (possible but I feel a little optimistic) but certainly isn't 20 times the volume.

As Audioguru says though, you need a lot of extra power to get a small amount of extra sound.

You could try some extra capacitors on the output of your power supply but a 16v transformer at 2A isn't going to give you any more than 32 watts of total power output so you may well run into problems with three LM1875 ICs driven off the single transformer.

 

[Suraj143]

10 watts RMS is sufficient for general music with a low to medium volume.

My subwoofer on my office speaker system is 25w RMS (true - not made up) and is pretty loud and clear. The subwoofer/amp on my home cinema is 500w RMS (possible but I feel a little optimistic) but certainly isn't 20 times the volume.

As Audioguru says though, you need a lot of extra power to get a small amount of extra sound.

You could try some extra capacitors on the output of your power supply but a 16v transformer at 2A isn't going to give you any more than 32 watts of total power output so you may well run into problems with three LM1875 ICs driven off the single transformer.

Very nice informations.I really like those stuff Thats what I'm also going to say.25W RMS is really a descent feeling sound.I need to take my design to that level.

I'm planing to put a 18V - 0- 18V/ 5A transformer

 

[picbits]

Are you using 8 ohm or 4 ohm speakers for the subwoofer ? If you are using 8 ohms, you can put two in parallel and run them from a single LM1875

 

[audioguru]

Are you using 8 ohm or 4 ohm speakers for the subwoofer ? If you are using 8 ohms, you can put two in parallel and run them from a single LM1875

You don't get more power output like other amplifier ICs. instead you get more distortion and overheat the IC.
Its output at clipping with a plus and minus 25V supply into 8 ohms is 20W. Its heating is 19W. It needs a "rather large heatsink" as is said in the datasheet.
Its output at clipping with a plus and minus 25V supply into 4 ohms is a total of 20W. Its heating is 32.5W which needs a huge heatsink plus a fan.

 

[picbits]

Ewww - my bad.

 

[Suraj143]

Hi Audio & picbits

Ok I understood.I'll test my design by changing the transformer & adding a large heatsink & let you know.

By the way can you recommend me an IC which can suitable for a subwoofer amplifier ?

Thanks

 

[audioguru]

First you need to determine how much power you want from an amplifier. You didn't DOO DAT.

Changing the transformer and adding a large heatsink will produce a tiny increase of loudness. It is not worth it.
Texas Instruments have a few high power amplifier ICs.

 

[4pyros]

Maybe you should start with your subwoofer.
How many watts can it handle?
Than on to your system, how loud is it?
You sould match your sub to the rest of the system.

 

[Suraj143]

Very nice.

My sub model is RSW80 - Roadmaster

40 watts Total RMS

200 Watts Total Instantaneous Peak Power

 

[4pyros]

My sub model is RSW80 - Roadmaster

What kind of box do you have it in?
What are you using for a cross over and at what frequency.

 

[Suraj143]

I have a 0.75 Cu/F enclosure build by myself.It has a 2 inches bass hole.

What you mean by cross cover?

The whole scenario is I'm still checking with that 12V transformer which still outputs only 10W which is too low.

I must change the transformer first.

 

[4pyros]

I have a 0.75 Cu/F enclosure build by myself.It has a 2 inches bass hole.

The box is supposed to be sealed not ported.
The bigger the box the lower it will go.
You need a cross over to prevent the sub from getting high frequencies.

I must change the transformer first.

Without the proper box and cross over you will never get good bass.

 

[Nigel Goodwin]

The box is supposed to be sealed not ported.

It rather depends on the design of the cabinet - sub-woofers commonly use ported cabinets.

However, a port isn't just a random 'hole', it needs careful design.

 

[audioguru]

It is not a sub-woofer, instead it is a low power 8" ordinary woofer that costs only 25 bucks or less.
It is rated to go down to 40Hz which might be at -10dB. At a reasonable -3dB it might go down to 70Hz or 80Hz.
The manufacturer says to use a sealed little enclosure.