Op Amp for speaker circuit

[Sadlercomfort]

Hi Guys,

I'm trying to find a simple circuit design which amplifies sound from an audio jack, to two 8ohm speakers. I only have quad LM324 op amps.

Thanks,
Ash

 

[BobK]

The LM324 is not capable of driving a speaker. Do you have and NPN and PNP transistors? You can add on a push-pull output stage to the LM324 to power a speaker at < 1Watt pretty easily.

Bob

 

[BobK]

Here is a circuit that will give you 1/4 W output to an 8 Ohm speaker from a 9V battery, using an LM324, 2N2222, 2N2907, and a few other components. Gain is set by the ratio of R8 to R4 (in this case 10). Showing input of 0.2V p-p 1000Hz sine wave with 2.0V p-p output. If you use 2 9V batteries, you can eliminate the virtual ground and the output capacitor, just make the ground the junction of the 2 batteries, and you get a full 1W out. But the transistors will probably get hot!

Bob

 

[(*steve*)]

For even higher efficiency, there are some class D amplifier chips that can give you 2 x 3W from a 5V power supply without anything getting hot!

For peanuts you can get a module using them on eBay. These use PWM to drive the speaker. As such, the losses are even lower than class B.

Possibly not useful for Ash, but interesting for anyone wanting a small cheap amplifier running from a low voltage rail.

The datasheet makes interesting reading.

 

[Sadlercomfort]

My 8ohm speakers are 3w, I also have two 6ohm 2w speakers I can use. How will a change in speaker load effect the circuit?

I must admit the module looks nice and easy. Making the device portable would be nice, so I'd like to use a 9v battery and use a 5v voltage regulator if necessary.

What are other types of amplifiers? I have no idea why I brought lm324's.

 

[BobK]

Having speakers rated at higher wattage than the amp is no problem, in fact it is desirable since it means you cannot destroy the speakers.

You would not want to regulate the voltage down. All that does is wastes power and limits the power you can get out of the amp.

The LM386 is a popular amp that can drive 8 Ohm speakers directly. It will get you about 1/2 W at 9V (although that is listed at 10% distortion, whereas the amp I posted has < 1%, so it can probably also put out 1/2 W at 10% distortion).

As Steve suggested, there are a lot of pre-built modules out there, on Ebay for instance, that are cheap and have great specs. If you look for a class D it will get you the most power at a given voltage and the most efficiency, so your batteries last longer.

Have fun.



Bob

 

[Sadlercomfort]

Thats something I didn't know about speaker wattage, thanks Bob. I've also learnt that higher gain/volume can increase distortion.

I like the voltage range on the LM386 and I can see why it might be popular.. I'll grab some of those to experiment with, aswell as the PAM8403 module.


Thanks Guys.

Ash

 

[Rick L]

The LM386 makes a great little amp, power it with a 9 volt battery and you have a great little portable amp. There are different versions of the LM386N-4 is capable of more wattage than the others. I have built a couple portable guitar amps using the LM386. Sometimes distortion is desirable

 

[Sadlercomfort]

There are different versions of the LM386N-4 is capable of more wattage than the others.

Brilliant, I'll be able to test the effects of the different types if I install the IC in a socket. Definitely worth learning about. There's the 0.25w, 0.5w and 0.7w, all fairly cheap on ebay if stamped from china. Everythings a learning experience

 

[BobK]

After seeing the chip Steve linked to, I would go with that. It will be twice as efficient and put out more power. I would use 4 NiMH rechargeable cells to power it.

The prices for the ones you linked are very high. On Mouser it is $1.02 for the LM386-4 single quantity.

Bob

 

[Sadlercomfort]

Yeah I've brought that module aswell. They are cheaper on Mouser but its a case of paying a £12 delivery charge. Great though, if your bulk buying.

 

[Sadlercomfort]

Hi Guys,

The PAM8403 module has arrived. I have a supply of 4 1.5v AA batteries, with an LED indicator and appropriate current limiting resistor to power the module.

The supply voltage is a steady 5.75v and I need 5v to drive the module, so I created a voltage divider where R1 = 33Ω and R2 = 220Ω.

The problem is that the sound is choppy. A 22uF capacitor just makes the choppy sound faster. If I reduce the volume it works fine and sounds great. I have thought maybe the voltage divider is affecting it, but I'm not sure.

There is a slight voltage drop from the voltage divider when the speakers play, varying from a 0.2 - 0.6 voltage drop.

 

[Harald Kapp]

A voltage divider is not the proper way to create a volteg that is supposed to deliver noticeable power. Any current drawn from the divider will lower the voltage even more (you see this as the "0.2 - 0.6 voltage drop". You may not see even more drop at higher frequencies, your meter will be too slow to display dips at more then a few Hz. This leads to an unstable power supply to your amplifier which in turn creates that "choppy" sound.

A good way to reduce 5.75V to 5V is a low drop voltage regulator.

An easy way, which may work as well in your case, is to drop 0.6V...0.7V via a simple diode in series with the power supply (in V+, anode towards supply, cathode towards load). The 22µF shall be place after the diode, next to the amplifier. If 22µF is insufficient, try 100µF or more.

 

[Sadlercomfort]

I found a nice diode, my voltage dropped to about 4.85v. Perfect, I increased the capacitor to 100uF anyway which seems to steady the voltage.

Sounds great, I hadn't heard of low drop voltage regulators before. So I've Googled it for future reference.

Thanks