How to choose speakers for audio?

[MrNobody]

Hi,
I am trying to generate voice by using PIC's PWM module. Which means that the max Vpk-pk of the voice is 5V. However, I anticipate that it will mostly be around 4V. I connect the output of PWM to a 2nd order RC lowpass filter with a Fc of 2kHz and then to a speaker similar to **broken link removed**. However my speaker is only 8 Ohm and 0.25 Watt. Using the PWM, I generate a sine wave of 400 Hz but the sound that comes out from the speaker is quite soft. When I increase the frequency, the sound comes out abit louder but it is still quite soft.

My initial idea is to connect the output of the lowpass filter to the amplifier stage of this circuit (only the U1A). The supply for the opamp will be 9V. The output of the U1A will connect to the speaker.

Having said that, is there any other better way to make the sound louder..? What should be the specs of the speaker..?
Thanks..

 

[Nigel Goodwin]

You can't feed a speaker directly from a preamp or filter, you need a power amp.

So use a power amp IC, such as an LM386.

 

[MrNobody]

Thanks..
Attached is the schematic of LM386 with a gain of 20 (taken from datasheet).
Lets say if I use that circuit with Vs of 5V, if I connect the output of RC lowpass filter to Vin (Vpk-pk of 4V), won't the output of the audio amplifier be 5V for most of the time because of the gain factor of 20..? I mean, It won't be able to amplify the voltage any further than 5V right..?

Sorry, I don't quite understand.. What is the advantage of using the audio amplifier..?

Another question is what is the purpose of the 10K potentiometer at Vin (in the schematic)..? Is it to bias the Vin so that it is Vs/2..?

Another question is, lets say i use headphone such as **broken link removed**, do I still need the power amp or can I donnect the headphone directly to the output of lowpass filter..?

 

[audioguru]

You need a power amplifier to drive the 8 ohm speaker or drive the 32 ohm headset.
The datasheet for the LM386 shows that its output power into an 8 ohm speaker at clipping is only 0.113W which is very low but will be loud in the headphones. The tiny speaker has poor sensitivity and poor low frequency response. Cutting high frequencies above only 2kHz will sound horrible.

 

[MrNobody]

Thanks audioguru..
Given that the PWM frequency is 80 kHz, and the voice signal is below 4 kHz, it would still be safe to cut it above 4kHz right..?
I was just worried of the ripples.. Worried that if I cut it too high then ripples will occur.. Hmm, what Fc value do u recommend..? And is a first order filter sufficient or do I need to go for second order filter..?

Oh yeah.. one more thing i need clarification.
Attached is a part of the datasheet that shows 3 circuits with different gain (20, 200 and 50).
In the datasheet, it shows that the gain for 1kHz is between 26dB to 46dB (approx 20 to 200 gain).
What I don't really understand is how can an input of 4V (output from lowpass filter) experience such a high voltage gain when the Vs is only 5V and not 80V or higher (20 gain x 4V = 80V). I don't really understand the concept..
Can you please explain.. Thanks..

 

[MrNobody]

Below is the schematics mentioned in the post above..

 

[audioguru]

Your input to the LM386 has a max of 4V p-p and the max output from an LM386 into a speaker is only 3V p-p. So connect the LM386 with a gain of 20 and turn down the volume control.

A CD plays voices perfectly because its frequency response is flat to 20kHz.
FM radio plays voices almost perfectly because its trequency response is to about 15kHz.
AM radio plays voices not good because its frequency response is to only about 3kHz.
a telephone sounds bad (what did you say?) because its frequency response sometimes is to only about 2kHz.

The response must be good to at least 14kZ for the "S" letter in speech to sound correct.

Your PWM frequency is 80kHz so a 4th order Butterworth filter will reduce it to -48dB which is quite low when the lowpass cutoff frequency is 20kHz.

 

[MrNobody]

Thanks..
I forgot to mention that eventhough the PWM frequency is 80Khz, the actual voice data is only 4kHz.. So, i guess even with a cutoff frequency of 20kHz, the sound will still be sound bad...

I'm wondering.. if I use the 20 gain circuit but I turn down the volume control, then, the purpose of using the power amplifier is not really to amplify voltage right..? It is more for impedance matching is it..?. So that more current can pass to the headphone..?

 

[audioguru]

A 4kHz data rate is very poor for voices. But try it. Maybe hearing only vowels is good enough.

What did you say? What did you say? What did you say? What did you say?

 

[MrNobody]

Sorry..
Umm.. I mean.. I still don't really understand why we need to decrease the output of the lowpass filter from 4 Vpk-pk to around 0.15V pk-pk so that it is able to be amplified by a gain of 20 to cause an output voltage of 3V pk-pk (Vs = 5V). Isn't that an attenuation of -0.75 (3V/4V) because the output voltage is lower than the input voltage? How does it help in amplifying the sound and make it louder..?
I am abit lost there.. Can you please explain.. Thanks..

 

[crutschow]

I'm wondering.. if I use the 20 gain circuit but I turn down the volume control, then, the purpose of using the power amplifier is not really to amplify voltage right..? It is more for impedance matching is it..?. So that more current can pass to the headphone..?

Yes, the purpose of the power amplifier is to supply the relatively large current needed by the 8 ohm speaker which the PIC's output can't provide. It's a power amplifier, not necessarily a voltage amplifier.

 

[MrNobody]

Hi..
Lets say, I intend to use **broken link removed**, can I still use the 20 gain amplifier circuit..? If possible, i hope to be able to interchange between using a speaker and using a headphone..
Thanks..

 

[audioguru]

The 800W (WOW) computer speakers might blow out all the windows in your neighbourhood.
They don't have any bass but voices will sound pretty good. Your 4kHz sampling will sound horrible.
They have an amplifier built-in, 1W or 2W per channel. You don't need another amplifier.
They say there is an earphone plug but they mean earphone jack.
They are powered from USB, not from an adapter.

 

[erosennin]

800W PMPO...

USB Powered, = 2.5W Maximum (5V *500mA)
- Losses/Driving
= 2W total output...

 

[MrNobody]

The 800W (WOW) computer speakers might blow out all the windows in your neighbourhood.
They don't have any bass but voices will sound pretty good. Your 4kHz sampling will sound horrible.
They have an amplifier built-in, 1W or 2W per channel. You don't need another amplifier.
They say there is an earphone plug but they mean earphone jack.
They are powered from USB, not from an adapter.

Sorry.. the sampling frequency is 8kHz.. but the cut off freq is aprox 3kHz.. I tried to record voice signal at 8kHz sampling frequency using 8 bit using Adobe Audition (audio software) and tried playing out the sound, of course the sound is not as good as sampling at 44kHz but it is ok i guess..

I plan to have a USB connector on the device so that I can power the speaker using that connector. Since the speaker uses USB, i guess the current required is only around 500mA or so and using a 9V battery would be fine i guess..?

Well, the computer soundcard is able to drive the earphone as well as the speaker so I jst tought of having the same functionality.If I don't include a power amplifier in the device and only rely on the computer speaker's internal amplifier then the sound would not be loud enough for the headphone.. Anyway, since the speaker also have its own volume control, then I guess it would be safe to have a seperate power amplifier inside the device. I mean, if it somehow manages to blow the neighbours' window as what u pointed out, i guess i can decrease the volume using the speaker's control..

Anyway, what do you guys think about it..?

 

[audioguru]

I was joking about its "800W" blowing out windows because its power is only about 0.6W per channel. The manufacturer lies when they say 800W. It will be as loud as a cheap clock radio.
The output from its earphones jack is more than enogh to drive earphones very loudly.

 

[erosennin]

I realise that AudioGuru, but I think it's funny nonetheless.
Look at all the crap we've come to "PMPO", Peak, and all this other garbage that manufacturers are now quoting for speaker output.
Anyone who didn't realise that you could only get at absolute maximum 1W per channel out of those deserves to be ripped off by them.

 

[MrNobody]

I was joking about its "800W" blowing out windows because its power is only about 0.6W per channel. The manufacturer lies when they say 800W. It will be as loud as a cheap clock radio.
The output from its earphones jack is more than enogh to drive earphones very loudly.

I thought you were serious.. lol
Anyway, I bought those speakers ages ago, coz they were cheap.. And yeah, they r not really that loud anyway..

Anyway, thanks for everything guys..