What to measure in audio project

[andy257]

hi all,

i have designed and built a little audio project. It uses A/D and D/A converters and a small amplifier. I wish to see how well it is performing but i am unsure of what i should be looking to measure to establish how well it perfroms. I have a sig gen and oscilloscope so i was thinking i can def do frequency response.

Any other ideas for perfromance paramters i can measure?

Thanks

Andy

 

[crutschow]

If the signal generator outputs a triangular wave you can do a rough check of the output linearity. The output ramps should be straight lines, the same as the input. You can increase input gain of the oscilloscope to view only a portion of the waveform at a time to improve the observational sensitivity.

 

[audioguru]

If you make a notch filter then you can measure harmonic distortion and signal to noise ratio.

 

[andy257]

hi audioguru,

i have filters in my project. An anti alias and a reconstruction filter, one lowpass the other bandpass. Can i still measure SNR and harmonic distortion?

thanks

 

[Nigel Goodwin]

hi audioguru,

i have filters in my project. An anti alias and a reconstruction filter, one lowpass the other bandpass. Can i still measure SNR and harmonic distortion?

thanks

Of course you can, but you still need a notch filter, very sharp, and very steep, plus a low distortion sinewave oscillator, and an AC millivoltmeter (or a scope).

 

[andy257]

hi nigel,

i have the equipment covered, why do i need a notch filter? and where might i put it in my system to measure those parameters.

also i am worried i might fry my amplifier. The amp has a 1/8 speaker output jack and a 1/8 headphone jack. The amp is capable of 2W output into 4 ohm speaker.

My plan was to buy (or make) a BNC to 1/8 jack lead and connect it up to my scope. Likewise with the input, only it will be BNC to phono jack this time. But i am wondering if i will have to terminate it with 50 ohms and whether i will overload my amp at the output with no speaker attached.

Thanks

 

[Nigel Goodwin]

hi nigel,

i have the equipment covered, why do i need a notch filter? and where might i put it in my system to measure those parameters.

It's part of a distortion meter, you use the notch filter to remove the original signal, and what's left is distortion and noise.

also i am worried i might fry my amplifier. The amp has a 1/8 speaker output jack and a 1/8 headphone jack. The amp is capable of 2W output into 4 ohm speaker.

My plan was to buy (or make) a BNC to 1/8 jack lead and connect it up to my scope. Likewise with the input, only it will be BNC to phono jack this time. But i am wondering if i will have to terminate it with 50 ohms and whether i will overload my amp at the output with no speaker attached.

A scope isn't 50 ohms, it's 1,000,000 ohms, and you don't need a load (assuming you want to test it without a load?).

 

[andy257]

It's part of a distortion meter, you use the notch filter to remove the original signal, and what's left is distortion and noise.



A scope isn't 50 ohms, it's 1,000,000 ohms, and you don't need a load (assuming you want to test it without a load?).

The only load i was thinking of was the scope instead of the speaker. As you say the scope has a high input impedance so it shouldnt matter.

Can you guess what my next question is?

Will a simple RC or LC notch filter work ok, id rather not go into building an advanced notch filter.

thanks

 

[Nigel Goodwin]

The only load i was thinking of was the scope instead of the speaker. As you say the scope has a high input impedance so it shouldnt matter.

Can you guess what my next question is?

Will a simple RC or LC notch filter work ok, id rather not go into building an advanced notch filter.

No, it needs to be a VERY sharp active filter - but do you really want to measure it's distortion?, and do you have a low distortion sinewave oscillator?.

 

[andy257]

no its not really necessary, i was just trying to think of certain parameters i could measure its perfromance with the instruments i have at hand. If it requires specialist equipment then i will give it a miss. I have access to a sine wave oscillator, i would have to dig out its manual to see if its low distortion.