After hooking up the circuit to my headphone (ATH-CKS1000), I was found the sound level was incredibly loud (not enough to damage my hearing/headphone though..), with some calculations, I found out the reason:
- Note that the output is 2Vrms, U1/U2 is formed as an active I/V, converting IOUT-, IOUT+ to voltage.
- At -FS, IOUT- gives out -2.3mA, IOUT+ gives out -10.1mA; At +FS, IOUT- gives out -10.1mA, IOUT+ gives out -2.3mA
- So for example, at -FS, U1 will gives out 750Ohm*(-2.3mA)=-1.725V, and U2 will gives out 750Ohm*(-10.1mA)=-7.575V
- You can refer to the following table for more details:
- U3 is configured as a differential amplifier, using -FS as an example again, U1 output -1.725V; U2 output -7.575V; Output of U3 = (270R/560R)*((-7.575V)-(-1.725V))≈-2.82V, formula:
- You can also confirm the findings from the above table.
- Output is ±2.821V, which is ≈1.995Vrms
- Notice there is R7 (100Ohm) at the output, for my 16Ohm headphone connected in series with it, the output stage turns into a voltage divider network:
- So my headphone will receive MAX 2.821V * (16/(16+100)) ≈0.3891V
- Power delivered to my headphone: 0.3891^2/16=9.462mW, which is less than the maximum input power 100mW. But don't get fooled by this! 9.462mW is already REALLY LOUD, you do not need the full 100mW!!
- The sensitivity of my headphone is 106dB/mW, so for 9.462mW, the actual output loudness is: 106+10*log(9.462)≈115.76dB SPL
- Taking the dB SPL chart from here, you can discover that 116dB SPL is already quite loud:
I have also made a small calculator here, you can try out different values:
| Audio System Output | |
| Vrms out | |
| Series resistor (R7)/Total output impedance | |
| Headphone | |
| Impedance | |
| Sensitivity (dB/mW) | |
