Portable speaker project. LM831!!!

[kaning]

So the goal of this project is to build portable speaker (0.5w-1w) fed by internal li-po battery and charged by USB. i will use 5v step-up converter or not depending on amplifier IC.

and this is the first point of this thread. after looking for some amplifier datasheets i found this perfect one LM831 where with 3v you can get 440 mW power output, wich is the best i've found. and with 5v you can go up to 0,9W.

datasheet :**broken link removed**

the thing is i cant find it anywhere!! its obsolete and for some reason its not in market anymore...and i also couldnt find anything similar to this... lm836 is not even close to this. can someone help me here with something this good on market?

also... on ebay i found this
**broken link removed**

do you guys think this is reliable?? many thanks in advance

 

[Audioguru]

The obsolete LM831 with a 3V supply produces a horrible sounding 440mW into an 8 ohm speaker with 10% distortion. Have you ever heard how bad 10% distortion sounds??
Its output is only about 200mW just before clipping which is almost no output. A cheap clock radio produces at least 500mW.

The expensive IC from ebay might be a fake one.

I looked for a similar amplifier IC the TDA2822M but it is also obsolete today.

Texas Instruments have many modern class-D amplifier ICs but most are in a tiny surface-mount package.

 

[kaning]

after some help i found pam8403 d-class amplifier. its a 2 channel stereo amplifier with 2x 3w output with 5v supply

many thanks

more problems will come up this project so i will keep it open

 

[Audioguru]

after some help i found pam8403 d-class amplifier. its a 2 channel stereo amplifier with 2x 3w output with 5v supply

3W into 4 OHMS when its output is clipping like crazy with 10% distortion that sounds horrible. Its outputs are 2W when it begins clipping.
The output power is slightly more than half with 8 ohm speakers.

 

[kaning]

many thanks again. after somereading on datasheet i want 5v, 8ohm 1% thd to get best quality possible, i dont mind getting lower power output. what its bugging me is how to get that configuration.

they mention about 4ohm thd 10%, 4ohm thd 1%, 8ohm thd 10%, 8ohm thd 1% but they dont mention how to get each configuration. imagining i want 8hm thd 1% how should i proceed?

Also im getting some problems understanding what pin 4 and 13 are for (PVDD???) and also how to get calculations for Ri value since the equation mentions a Rf value?? tips would be apreciated,many thanks

 

[Audioguru]

Graph #2 in the datasheet shows with a 5V supply the output from each channel is about 1.2W into 8 ohms with about 0.03% distortion. An output level that is slightly higher causes clipping.
The gain is 24dB which is a voltage gain of 16 times.
When the gain is reduced to 18.5dB in graph #4 then the distortion is slightly less.
The datasheet says the max Rf is 85k and the minimum Ri is 15k.

The datasheet tells you the functions of pin 4 and pin 15 are Power VDD and might be the positive power to the outputs of each channel.

 

[kaning]

Yes i already have seen that graph, and it was from there i got confused because then how its possible to get 10% thd with 8ohm and 5v supply? What are the things that influence thd between 1% and 10%? Load and voltage supply are the same, volume (current)?

And about Rf same thing, because there is no Rf on pam8403 circuit. Its only used for Ri formula calculation? How we define Rf?

By the way, thanks a lot. U have been great help

 

[Audioguru]

Yes i already have seen that graph, and it was from there i got confused because then how its possible to get 10% thd with 8ohm and 5v supply? What are the things that influence thd between 1% and 10%?

Simply turn up the volume control too high (or make the voltage gain too high) then the output goes as high as it can and produces square waves.

And about Rf same thing, because there is no Rf on pam8403 circuit. Its only used for Ri formula calculation? How we define Rf?

The datasheet is confusing maybe because this company never made an audio amplifier IC before.
The circuit will not work without Rf so it is probably inside the IC.

 

[kaning]

after some calculations and assuming max Rf value and minimum Ri value, the gain is 24dB, so i will buy 18k and 25k ohm resistor for Ri and test to see better option.

they also mention something about using filters for EMI (eletro magnetic interference). how to choose right ferrite bead? just curiosity because probably i will not use those..

thanks for the help. i will keep updating this thread

 

[Audioguru]

25k is not a standard resistor value so use 18k for the input resistors.
A 10k to 50k volume control should be connected to the input of each input coupling capacitor and they should have a logarithmic audio taper to match the sensitivity response of our hearing.

The datasheet tells you about the ferrite beads but you must calculate their value.

 

[kaning]

Hi again. About volume control can it be connected to the output of the amplifier?

 

[BradtheRad]

This website shows a similar project. It breaks it down into sections. It explains how each contributes to overall success.

The volume control goes at the input. To avoid erratic DC swings, you add two capacitors.

http://formesyn.com/SudburyRC/coxamp.html

 

[kaning]

Ok thanks a lot, ill give it a go...

I tried at the output and it worked. I tried with and without capacitors and its easy to notice the diference in sound quality

 

[Audioguru]

A volume control connected between the output of a power amplifier and a speaker:
1) The volume control burns out.
2) The volume is either full blast or very low like it is switched.
3) When the volume control is turned down a little then the speaker sounds "boomy" like a bongo drum.

The hand sketched circuit in the Sudbury link shows the polarity of the input capacitor on the LM383 power amplifier connected with backwards polarity. The polarity is correct on the datasheet.
The LM381 does not need this input capacitor.