# Amplifier question

1. Nov 13, 2009

### m718

I've made a single opamp amplifier for a mic but it has this problem, when I play a very low
200hz tune the output of the opamp shows 6 mV and another 200hz tune at higher volume shows a 600 mV output, the problem is that if the loud tune is ON the low level tune doesn't make any difference on the output. Shouldn't the output go from 600 mV to 606 mV ?
And each tune is played by a seperate speaker.

2. Nov 13, 2009

### Bob S

Are you summing the two input signals through a summing junction (the negative (inverting) input of an operational amplifier? This should sum the two amplitudes. See thumbnail.
Bob S

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Last edited: Nov 13, 2009
3. Nov 13, 2009

### Staff: Mentor

Are the two tones in phase? If not, then you will not get a different peak value in general.

4. Nov 13, 2009

### m718

I have the two sounds coming from two seperate speakers but I have just one mic recieving both and the mic is connected like this: + and - of OPAMP to ground thru 1K, 100K - to OUT,
dynamic mic to ground and + of OPAMP

I also tried different frequencies for the two sounds.

5. Nov 13, 2009

### m718

but if your speaking into a mic and there is background noise all the sounds are not in phase how come in that case all the sounds add to the amplifier output?

6. Nov 13, 2009

### Bob S

The velocity of sound is about 1100 ft per second, and the wavelength of 200 Hz is about 5.5 feet. Try moving one speaker about 1 to 3 feet closer and/or further away from the mic.
Bob S

7. Nov 13, 2009

### m718

I get the same results .
could this have somthing to do with the dynamic range of the opamp I read somewhere the there is a minimum amount of signal that the op amp can amplify in the presence of a much larger signal?

8. Nov 13, 2009

### m718

if out of phase signals don't increase the peak voltage would I still see a power output increase
and if yes how would I measure the power output of the opamp?

9. Nov 13, 2009

### Staff: Mentor

I like Bob's suggestion the best. Drive both speakers with the same tone source, and move the speakers to adjust the phasing at the mic. You should be able to see some delta in the peak amplitude as you move the quieter speaker to in-phase and out-of-phase positions.

10. Nov 13, 2009

### m718

but I need this to work with out of phase signals too so if I can't get the peak voltage to change, does the power output change? or anything else to show that the small speakers is ON?

11. Nov 13, 2009

### Staff: Mentor

Can you tell us a bit about your application? Maybe we can make some suggestions, if we know what you are trying to do overall, and what the nature of the signals is.

12. Nov 13, 2009

### uart

Out of phase signals cancel. That's just how audio signals combine, it has nothing to do with you're amplifier. It's already happened in the air before it even gets to your microphone so get over it (or state more precisely what you're trying to achieve so someone can help you).

BTW. If the signals are not the same frequency then they are are additive, but not in the simple way you're expecting. If you have a 600mV (rms) tone and a 6mV tone at another frequency then the combined rms voltage is $\sqrt{(600^2 + 6^2)}$ which is only about 600.03mV. Note however that although this looks almost unchanged (from the original 600mV) it does however contain precisely the sum total of the power of the 600mV tone and of the 6mV tone.

13. Nov 13, 2009

### Bob S

If you combine two tones of slightly different frequencies, you will get sidebands of the difference frequency and the sum frequency. The amplitude will have hills and valleys in it with the max an min being about the sum and difference of the peak amplitudes of the two frequencies.
Bob S

14. Nov 13, 2009

### Staff: Mentor

Only if the combination mechanism is non-linear. The microphone picking up multiple audio tones is a linear mechanism.

15. Nov 13, 2009

### Bob S

I seem to get a 10-Hz beat frequency in the amplitude between a 500 Hz and a 510 Hz tone (2:1 ratio).
Bob S

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16. Nov 13, 2009

### m718

I tried this in simulation (TINA TI) two voltages, 600 and 6 connected to one opamp the total shows 606mV. And if in rms the total comes to 600.03 would the peak be 606mV?

I made a gaussmeter and I noticed that in the presence of large signals it didn't sense smaller magnetic field changes that it would when it was not in a strong field, its a coil sensor meter, so I was wondering why it works like this.

17. Nov 14, 2009

### m718

I did a new test and it doesn't even have to be 600mV, the 6 mV signal can't be seen even with a 30 mV background signal.

Last edited: Nov 14, 2009
18. Nov 14, 2009

### m718

maybe its my test equipment I will measure the output another way and see what happens

19. Nov 14, 2009

### uart

Again let me summarize. Ideally 600 mV tone + 6 mV tone gives :

1. 606mV if same frequency and in phase.
2. 594mV if same freq and out of phase.
3. sqrt(600^2 + 6^2) = 600.03 if either same freq and quadrature phased or different frequencies.

Last edited: Nov 14, 2009
20. Nov 14, 2009

### uart

Expected is sqrt(30^2+6^2) = 30.6mV, so just how accurate is the measurement anyway?