Design an audio power amplifier

[PhysicoRaj]

yes that is OK, the reg's in that circuit were just a progression for if a well regulated supply was needed ... feeding IC's for example

Thanks

 

[PhysicoRaj]

This amp IC (in the attachment) , is expected to deliver around 8W to an 8ohm load. That means the rms output would be 8v.

1) looking at the circuit, the gain must be decided by the 150k & 4.7k divider, which gives a gain of ~30. (Am I right?)
I learned the headphone jack has a max output of 1v rms. This means the output of the amp would be 15v at half input. This exceeds the figure given by the amp manufacturer. Am I wrong with the gain?

2) What is the use of those diodes going from the output to the supply? Are they a kind of protection? Do I need to incorporate them compulsarily?

Thanks.

 

[PhysicoRaj]

The amplifier :

 

[davenn]

1) looking at the circuit, the gain must be decided by the 150k & 4.7k divider, which gives a gain of ~30. (Am I right?)
I learned the headphone jack has a max output of 1v rms. This means the output of the amp would be 15v at half input. This exceeds the figure given by the amp manufacturer. Am I wrong with the gain?

so you can adjust those resistor values to give a useable gain
Also I think I mentioned in an earlier post, you should have a volume control in the input for gain adjustment so that you don't overdrive the input to the IC and cause distortion

2) What is the use of those diodes going from the output to the supply? Are they a kind of protection? Do I need to incorporate them compulsarily?

these diodes are for the protection of the IC from voltage spikes on the output
Note for a single rail PSU, as you are using, here is a valid connection for them ...

also note, this chip is capable of around 12-14W for a 4 Ohm load and up to 18W for a 8 Ohm loadDave

 

[PhysicoRaj]

Thanks. I am worried about the output because I have to run my 8ohm speaker at 8w, without knowing what the amp is ousting at the moment.. (can't measure the output on my DMM since it's not a sine). I'll have to play with the volume control and gain, and hope my load won't smoke off.
Is there a way to know the output?

 

[davenn]

Is there a way to know the output?

audio wattmeter
oscilloscope
measure the voltage across a known load resistor ( 8 Ohm)Dave

 

[meBigGuy]

Is there a way to know the output?

Depends on how accurate you need to be, what response you require, etc. Not sure what your goal is.

For example, you could build a true RMS detector with a threshold comparator and an LED, or add a VU meter, or just detect high output peaks, or whatever.

 

[PhysicoRaj]

Thanks! I downloaded a 1kHz sine beep. I'll play it in a loop and check the rms across the load. Then I hope I can work out the gain etc.

 

[meBigGuy]

LOL --- ok, you were just trying to get the gain aprox. correct. I thought you wanted continuous monitoring for some reason.

 

[PhysicoRaj]

:D well it'd be nice if I had some continuous monitoring, but the only thing I have (and could afford to) is a DMM.
It's enough because I just want to know when the amp is at min/max/optimum, so that I can hook up the speakers at the correct operating point.

 

[davenn]

I just want to know when the amp is at min/max/optimum, so that I can hook up the speakers at the correct operating point.

that sounds a really strange statement

Dave

 

[PhysicoRaj]

that sounds a really strange statement

Dave

I'm not sure if the amp is going to blindly deliver 8w to 8ohms it's a function of input voltage. So to check the limit I'll have to vary both the input (volume control) and the gain, check the output on an 8 Ohm load {not sacrificing my speakers, maybe another old speaker ;)} get it to around 6w undistorted, then connect the speakers. That's what I meant.