Sorry for the constant questions but i have constant questions coming up with not many answers.


Is it common to have a low pass filter,high pass filter on the final stage of a audio amp output to speakers/earphones or is best between preamp and poweramp stages ?


How do i calculate the series output capacitor value for the final output stage of a audio amp which connects to 8ohm earphones i know the lowest freq i want to be 100hz ?


Looking for a simple adjustable low pass/high pass filter network schematic that will filter out freq below 100hz and no higher than 2khz for use in a audio amp circuit i just cant figure out how to combine the two filters into one if you get what i mean ?




@Audioguru and others thx for your time you have helped me alot with your answers to my questions on these forums.

 

Is it common to have a low pass filter,high pass filter on the final stage of a audio amp output to speakers/earphones or is best between preamp and poweramp stages?

Audio amps do not have lowpass and highpass filters unless the filters must remove loud hum and hiss. Their high frequency response might normally roll-off at 100kHz so the response at 20kHz (the highest frequency most people can hear) is flat with no roll-off.
The coupling capacitors block DC but pass low frequencies down to 4Hz so that 20Hz (the lowest frequency most people can hear) is flat with no roll-off.

How do i calculate the series output capacitor value for the final output stage of a audio amp which connects to 8ohm earphones i know the lowest freq i want to be 100hz?

Since you can hear down to 20Hz then your cutoff frequency of 100Hz will remove more than two octaves of bass sounds.
The formula for the cutoff frequency of a series capacitor feeding a resistor to ground is 1 divided by (2 pi RC). So a 0.1uF (100nF) capacitor feeding a 16k resistor cuts frequencies below 100Hz. But all the other coupling capacitors also remove low frequencies.

Looking for a simple adjustable low pass/high pass filter network schematic that will filter out freq below 100hz and no higher than 2khz for use in a audio amp circuit i just cant figure out how to combine the two filters into one if you get what i mean?

Music has audible frequencies as high as 20kHz. Voices produce important consonant sounds as high as 14kHz. If you cut frequencies above 2kHz then music will sound very dull and voices will be difficult to understand.

If you want reduced bass and reduced treble sounds then you can use a highpass filter (a coupling capacitor) feeding a resistor to ground, then a buffer that feeds a lowpass filter (a series resistor then a capacitor to ground).

 

hey audioguru, can you tell me if I busted my cheapy speaker? at the higher volumes of a headphone output (85%+) it's cutting in and out. I was trying out the second circuit in this thread and hooked it up without a resistor on the top and no capacitor on the output. oops

 

hey audioguru, can you tell me if I busted my cheapy speaker? at the higher volumes of a headphone output (85%+) it's cutting in and out. I was trying out the second circuit in this thread and hooked it up without a resistor on the top and no capacitor on the output. oops

The second circuit is this thread is a single self-biased transistor which is a preamp not a power amp.
You removed the collector resistor and did not use an output coupling capacitor so you connected the speaker directly to the battery.
If the battery is 12V then a current of 1.5A will flow in the speaker which is 18W. Maybe your cheapy speaker is blown up.

 

It was 3.3v and it's a 2w 4ohm speaker. I was mainly wondering if that behavior was typical of a broken speaker. I assume so since it should be able to handle a headphone output but just making sure... I didn't test it much before that

Is it possible it's drawing too much power for the mp3 player? I figured the speaker was the problem since headphones still work at high volume, but I don't know anything about speakers

 

It was 3.3v and it's a 2w 4ohm speaker. I was mainly wondering if that behavior was typical of a broken speaker. I assume so since it should be able to handle a headphone output but just making sure... I didn't test it much before that

Is it possible it's drawing too much power for the mp3 player? I figured the speaker was the problem since headphones still work at high volume, but I don't know anything about speakers

Headphones are the same as little speakers except they are usually 32 ohms per ear instead of 8 ohms like most speakers.
Headphones are very close to your ears so a small amount of power in them makes them very loud.
Your "preamp" transistor cannot drive a 4 ohm speaker which is almost a dead short.

Your preamp circuit does not show the resistor values for us to see if it overloaded your MP3 player, but probably not. The 4 ohm spreaker will overload your MP3 player and maybe blow it up if it is connected to the headphones output.

You connected your speaker directly to the 3.3V supply so its DC current was 3.3V/4 ohms= 0.83A if the supply is strong enough. The power was 3.3V x 0.83A= 2.7W so maybe you blew it up.

A defective speaker does not cut in and out. A weak power supply or overheated transistor can cause an amplifier to cut in and out when it is overloaded.