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Speakers 16 ohms, 8 ohms, what's the catch ?

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rompelstilchen

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Hello,

I bought this amp

https://www.aliexpress.com/item/Fre...32808014301.html?spm=a2g0s.9042311.0.0.z2nGmn

it uses a PAM8406

"5W Output at 10% THD with a 2Ω Load and 5V Power Supply at Class-D Mode
3W Output at 10% THD with a 4Ω Load and 5V Power Supply"

I have :

8ohms 1W speaker (tried 1 ,then 2 in series, terrible sound)
8ohms (0,4W) tested : crap low sound
16ohms (0.2W) speaker : a bit better sound, but not powerfull
16ohms (1W) speaker : best solution so far, but does not match amp perfectly

is it ok if I dont use a speaker in perfect accordance to the amp specs ? what is the risk ?

thanks
 

Re: speakers 16 ohms, 8 ohms, what's the catch ?

The amp specs 4 ohms, and you're using twice that (And more). Maybe you should first try the recommended impedance.
 

Re: speakers 16 ohms, 8 ohms, what's the catch ?

The amp should work fine with a 16 ohm speaker, but the maximum power output will be about 3/4W.
(Maximum power is inversely proportional to the speaker impedance for a given maximum output voltage.)
 

Re: speakers 16 ohms, 8 ohms, what's the catch ?

rompelstilchen said:
5W Output at 10% THD...
3W Output at 10% THD...

I have :

8ohms 1W speaker (tried 1 ,then 2 in series, terrible sound)
8ohms (0,4W) tested : crap low sound
16ohms (0.2W) speaker : a bit better sound, but not powerfull
16ohms (1W) speaker : best solution so far, but does not match amp perfectly

is it ok if I dont use a speaker in perfect accordance to the amp specs ? what is the risk ?
Click to expand...

Apparently, the harder you drive your amp, the worse it will sound. 10% THD will sound terrible. What are your intentions? To build a ghetto blaster? Or, to have high-quality sound?
 

Re: speakers 16 ohms, 8 ohms, what's the catch ?

Let us assume that the amplifier has Z impedance and the output voltage is 5V and we focus on the RMS power at const distortion.

At 2R (Ohm) load, the current will 5/(2+Z) and the power delivered to the load will be (25/(2+Z)^2)*2 and you are told that this is 5W. We get Z as 1.2- this appears as reasonable value. We have assumed that it can output full supply voltage but that may be wrong.

At 4R load, the power delivered to the load will be 25/((4+Z)^2)*4 which is 3.5W- you claim that it can deliver 3W- that sounds reasonable.

At 8R and 16R, the power delivered to the load (with the same assumptions) will be 2.4W and 1.4W - the values are approx but indicative.

To get decent power with a 16R speaker load, you will need greater supply voltage.

Manufacturer will specify a low speaker impedance because (i) that gives higher power and (ii) lower operating voltage.

Higher current may give greater distortion- you have to make some compromise.

Most people can hear 10% distortion - some claim that they can even make out 1% distortion. Most amplifiers will give you a graph (they are tough to calculate)

16 Ohm speaker is giving a decent sound (distortion wise speaking): this is expected.
 

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Re: speakers 16 ohms, 8 ohms, what's the catch ?

c_mitra said:
...

To get decent power with a 16R speaker load, you will need greater supply voltage.

...
Click to expand...

then why do I get the best sound with the highest load ?

talking about RMS

I wanted to drive a digital led(ws21...) reading the output straight from the power amp

audio_measure.gif

I want to turn on the led proportionaly to the output of the amp to the speaker

- I am not sure if the input of the above circuit should be connected to both +- output of the amp or between one output(+) and the gnd ?

- I am not sure of the way I calc my analog value
Code: 
    int audio=analogRead(A0)-(1023/2); // remove center 2.5V
    audio=(audio>>3); // divide value to store to 8 bit storage
    int audioRMS=audio*audio;
    if(audioRMS<10) // threshold
      audioRMS=0;
    lightUpLed((byte)audioRMS);
 

Hi,

Code: 
    int audio=analogRead(A0)-(1023/2); // remove center 2.5V
    audio=(audio>>3); // divide value to store to 8 bit storage
    int audioRMS=audio*audio;
    if(audioRMS<10) // threshold
      audioRMS=0;
    lightUpLed((byte)audioRMS);

My comment:
* RMS means: Root Mean Square. But the only thing I see in your code is "square". I don´t see "mean" and I don´t see "root".
* instead of (1023/2) use --> 512. It is mathematically more correct and you avoid to calculate with floating point values (because 1023/2 is no integer value)
* You subtract half range of a 10 bit value. (then the range should be -512 ... 511)
* then you shift it right 3 bits. This is not correct with negative input values. I assume you wanted to divide by 8 --> then the output range is -64 ...+63 .. but this is only "sign +6 bit" resolution. (not useful for audio dynamic)
* then you square this value: this gives a range of 0...4095 (12 bit not signed)

Klaus

- - - Updated - - -

Hi,

My comment on your circuit from post#6:
* For DC bias: just use two good quality 1% resistors from the same production batch. No need for a potentiometer.
* If you want to build an RC low pass filter with the 4.7nF capacitor, then you need to add a series resistor.

Klaus
 

then why do I get the best sound with the highest load ?
Click to expand...

I did mention. Highest impedance is NOT highest load. In the example I worked out, the highest load will be approx 1.2R. Lower resistance will give higher current but lower power. Higher resistance will give higher voltage but lower power.

The distortion is dependent on current (NOT voltage; most common semiconductor devices are current devices whereas olden era tube sets were voltage driven devices) and distortion may well be lower with lower current.

At 16R, the power output is less and the current is low and distortion is lower and sound is good- it is difficult to understand?

Common speakers have a voice coil that moves between the pole pieces of a permanent magnet. Lower current means lower excursion of the diaphragm and better linearity.

The above description is semi-qualitative but intuitive (I guess).

- - - Updated - - -

I want to turn on the led proportionately to the output of the amp to the speaker
Click to expand...

The LEDs are basically current devices and they work under almost constant voltage. Therefore to modulate the light output of the LED you need to modulate the current. NOT the voltage (that may kill the LED).

Simplest way will be to use a voltage to current converter (with sufficient compliance). Your program is wrong in the sense (say audio = 100); audioRMS = 10000; (byte)audioRMS =???

Better to use a simple to sample and PWM a LED.
 

Your very cheeeep aliexpress amplifier is powered with 5V and has an output from each speaker wire of about from +0.4V to +4.6V. But doesn't the WS21... LED strip use 12V?
Please post the datasheet of the WS21... LED strip.

Your speakers all have a very low power rating so they must be small (bad sound) and cheeeep (bad sound) but some will sound better than others.

The PAM8406 amplifier might not be able to drive a speaker and power some LEDs at the same time.
 

Audioguru said:
Your very cheeeep aliexpress amplifier is powered with 5V and has an output from each speaker wire of about from +0.4V to +4.6V. But doesn't the WS21... LED strip use 12V?
Please post the datasheet of the WS21... LED strip.

Your speakers all have a very low power rating so they must be small (bad sound) and cheeeep (bad sound) but some will sound better than others.

The PAM8406 amplifier might not be able to drive a speaker and power some LEDs at the same time.
Click to expand...

the led is a digital(serial) led powered with 5v, see the chematic, it's not linked to the amp, the mcu sends serial data to the led

- - - Updated - - -

@KlausST : thx, I'll check this out
 

Don't you want to rectify and filter the audio so that a fluctuating DC voltage representing the audio level is given to the MCU?
Does the MCU light the LED brighter with higher audio levels then dim the LED with lower audio levels?
What determines the LED color that lights up?
 

Audioguru said:
What determines the LED color that lights up?
Click to expand...

If you have a 2D display, you can send the audio signal to a freq-to-voltage converter and send the signal to the X. Also the same audio can be sent to another peak detector and use that on the Y axis. You will need to fiddle with the time constant.
 

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