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Topic: Low voltage ~2 Watt at 8Ω audio amplifier (Read 2489 times) previous topic - next topic

privateRob

Hi guys!

My current setup is an
* Arduino Uno
* 7.4V Lipo
* 95dB 8Ω 5W speaker (mono!)
* and a LM386N-4 amplifier.

The amplifier is set up with gain=200 and runs directly at 7.4V.
This would be about 1Watt and 40-50dB, right?

The speaker is already buzzing/humming because there is no potentiometer between the Arduino and the amp-input. Otherwise the sound quality is really bad and the sound volume also would be too low.

My plan is to use a stronger amplifier, which should
* increase the sound volume
* increase the sound quality by eliminating the buzzing/humming (The wav files themselves aren't in a good quality anyway. So no need to improve the overall sound quality in this way.)
* run at max 7.4V
* 1.5Watt+ with 8Ohm speakers

I already had a Stereo 2.8W Class D amp from adafruit (https://www.adafruit.com/product/1712) which was loud enough ("8Ω speaker at 1.5 Watt") for my project and did a good job. I also would use it BUT I want to use as few external/assembled products as possible which also should be cheaper.
Also, if I use an amplifier with more Watt output, the buzzing/humming will we reduced too, right?

The Mono 2.5W Class D Audio Amplifier (https://www.adafruit.com/products/2130) also seems to be completely sufficient I guess (no stereo or I2C needed!), since it also has "1.5W at 8Ω" but I can't find a standalone chip.

privateRob

#1
Oct 29, 2014, 03:05 pm Last Edit: Oct 29, 2014, 03:07 pm by privateRob
Ohh, I forgot to mention something!

The problem I have, is, that I couldn't find an amp which would fit my needs. Either they have a too high min-voltage supply or I can't figure out, if they will provide the power and therefore the sound volume I need.

Also it will show if I understand all the relevant things or if I have said/done something completely wrong.
:)

DVDdoug

The supply voltage is the limiting factor, and you don't need nearly that much gain! 

Power = Voltage Squared / Resistance

From a 7.4V battery you can ideally get 7.4V peak-to-peak with a regular class A-B amplifier, or twice that with a bridge amplifier.  (Real-world, non-ideal, amplifiers can't provide the full supply voltage to the load, so you'll loose a volt or so depending on the amplifier)

7.4V peak-to-peak  is 3.7 peak.  Myltiply that by 0.707 to get the RMS (assuming a sine wave).   That's 2.6V RMS across an 8 Ohm speaker, and that works-out to about 0.85W maximum.  You can double that by using a 4-Ohm speaker.

Or, a bridge amplifier will double the voltage for about 1.7W into 8-Ohms or 3.4W into 4-Ohms.


The Arduino puts-out 5V peak-to-peak, so a gain of 5 with a pot to control the voltage should be plenty.

Quote
Also, if I use an amplifier with more Watt output, the buzzing/humming will we reduced too, right?
No.  Less gain, lower input impedance (don't leave the amplifier input floating), and a regulated or filtered power supply will help to reduce the noise.

privateRob

Thank you very much for your awesome reply!!! :)

It will take me some days to do the research for the new information. I'll post following questions as soon as possible.

smeezekitty

Quote
7.4V peak-to-peak  is 3.7 peak.  Myltiply that by 0.707 to get the RMS (assuming a sine wave).   That's 2.6V RMS across an 8 Ohm speaker, and that works-out to about 0.85W maximum.  You can double that by using a 4-Ohm speaker.
Or if you use an impedance transformer.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

privateRob

#5
Nov 04, 2014, 11:36 am Last Edit: Nov 04, 2014, 11:45 am by privateRob
Thanks for your help guys!

First, I'll get another lm386n-4 to create a bridge amplifier and see if it's sufficient (what it absolutely should be).
I'll leave the options to increase the volume by getting a new speaker or by using an impedance transformer open at the moment.

Lowering the gain to 20 by removing the connection between pins 1 and 8 decreases the audio quality* and also the volume. I'll have to do some research for that issue. ^^ (maybe it's because of floating in/outputs and a capacitor and a reeeally high (or low) resistor will fix this. I will test it asap)

Quote
...regulated or filtered power supply will help to reduce the noise
I could increase the audio quality* by adding a capacitor between + and -

* As mentioned, increasing the sound quality = eliminating the buzzing/humming.

Grumpy_Mike

Quote
Nevertheless I could increase the audio quality* by adding a capacitor between + and -
Reducing pickup is best done by lowering the input impedance of the amplifier. The simple way to do this is to put a shunt resistor across the input. The attached works well for me in projects:-

privateRob

Thank you Grumpy_Mike!

I added a capacitor to the the input and also a resistor which i grounded. It improved the sound quality quite well. Also it allows me to remove the gain controlling capacitor, as mentioned by DVDdoug, which improved the sound quality too.

So, now I'm just waiting for some ordered parts which then will increase the sound volume. Yeah! :)

Meanwhile I will try to figure out where the clicking noise at the end of the sound file is comming from ^^ Maybe it's in the file, maybe it's some noise in the connections. At the beginning, I could also hear the sd card working ^^ but after changing some components and wiring, it has gone.

Thanks again guys! A highly appreciated your help!

Grumpy_Mike

Quote
Meanwhile I will try to figure out where the clicking noise at the end of the sound file is comming from
My guess it is a discontinuity at the end of the file. Load the file into Audacity and make a very short fade out at the end of the file.

CrossRoads

I'd suggest a switching boost regulator to give you more voltage to with, and then an amp that could run from a 9-12V supply for example.
Parts Exress has many low-priced amps that could work well
http://www.parts-express.com/cat/audio-amplifier-boards-modules/3464
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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