i'm building a project with small gothic lamps that i printed (resin). I am adding a speaker to the lamps that has a diameter of 40mm (size is important to me) and the ones i found are 2W and 8ohm. The project will be powered by a 5v batterypack. The batterypack is in a small plastic (weatherproof) container. In that container i will install an (yet to be determined model) Arduino. As want to use 4 of these speakers on the arduino, i will need one (or more) amplifier(s). I have two restrictions: budget and size. Both are not huge for this project (not 100 plus euros).
I found a solution that includes a LM386 ic. However.. the datasheet states that at 6v the output is 250mW (and us will be using 5V). That would mean that i have to use about 32 (250mW x 32=8W) of these ic's. I'm sure i'm missing an important clue to the right solution as this one isn't it.
Thank your for any thoughts on the matter.
(i am not a electrician but i like to learn from these kinds of projects)
Then you have to use 32 speakers as well. Speaker power is limited by the voltage applied to their impedance. With double voltage you get 4 times power.
You can't just add amplifiers like that. You can't have more than one amplifier per speaker.
The speaker power rating is the maximum. It doesn't require 2W. You can use less power and it just won't reach it's maximum volume.
In general, a 2W speaker should be able to handle a 2W amplifier that's hitting 2W on regular program peaks (voice or music) with a much lower average.... It might burn-up with a constant 2W tone.
In case you don't know Ohm's Law, it defines the relationship between voltage, resistance (or impedance) and current as Current = Voltage/Resistance. Resistance (and impedance) is "the resistance to current flow". Usually the voltage and resistance/impedance are known (or "controlled") and current is the result.
Power (Watts) is calculated as Voltage X Current, or from Ohm's Law you derive Power = Voltage squared / Impedance, which is handy for amplifier calculations.
So the voltage & impedance determine how much power you can get from an amplifier. Since audio is AC we need to know the RMS voltage. With a 5V power supply we can (theoretically) get 5V peak-to-peak which is about 1.75V RMS. That's about 0.4W into 8 Ohms. For more power you need more voltage or lower impedance.
If you put two 8-Ohm speakers in parallel you get 4-Ohms for double the power. i.e. Since the speakers are getting the same voltage each speaker is getting 0.4W for a total of about 0.8W. If you put four 8-Ohm in parallel that's 2 Ohms or 4 times the power, but most amplifiers can't drive a 2-Ohm load.
Or, a bridge amplifier can give you double the RMS voltage from the same power supply and as DrDiettrich says, that's 4 times the power.
Thank you for the DrDiettrich lesson! Two questions still so i understand it better:
i can follow the part that you convert to 1.75RMS but isn't 1.75RMS/ 8 ohm about 0.2W in stead of 0.4? Or is it 0.4W because AC is peak to peak and the outcome (0.2W) is one of two peaks?
i hope i got it right when i conclude: build a stereo bridge amplifier of 2W and on each channel place 2 x 2W 8 ohm speakers in paralell ?
It's not so easy to calculate. Consulting the LM386 data sheet distortion explodes above 250mW. So it depends upon your audience how much distortion is acceptable. A bridge amplifier can double the output voltage swing, but at the cost of distortion and limited by the acceptable chip power consumption. Consider that the then doubled output voltage results in 4 times output current. Also the reduction of the supply voltage reduces the pp output swing considerably, see Figure 3 in the data sheet.
My feeling from the internal circuit is a current limited output stage that won't work well with speaker impedance < 4 Ohm. Feel free to build a bridge amplifier from it and find out quality and thermal limits yourself.
I'ld like the sound to be clear (little to none distortion) but it doesnt has to be super HQ. I understand that power is an issue so i can only solve that by changing the powersupply so a higher voltage will be supplied. The powersupply that i'll need depends on the amplifier that i need to power 4x 2W 8 ohm speakers (i really need these for their exact size -diameter 40mm).
Thank you DrDiettrich, i'm aiming to use a stereo amplifier from that very site.
I, however, want to make change to this amplifier to mono. Is that possible? And if so; what change should i make?
I am no technical person and am learning to read and build the schematic as i go. I believe i see two potentiometers in this schematic (VR1/1 and VR 1/2). If i would connect both inputs together, could i also lose one of the two potentiometers?
I don't really understand the dotted line in the schematic. I presume its just nice to know info. Am i reading it wrong?
I decided to build two stereo amplifiers; just because i can use these in the future for other projects (who knows i want stereo than).
Just one last question: in the right hand upper corner of the schematic of the stereo amplifier, there is the power supply part. It powers all three LM386 on leg 6. If i build two of these amplifiers on one pcb board, could these amplifiers use the same 'power supply' schematic? So would i only have to use the 6v batteries, only one 470uF Electrolytic Capacitor, only one led and only one 1k ohm resistor to power the two amplifiers?
Many thanks for all your help. I'm building my first PCB from scratch with your help.
