Calculate gain and cut off freq

[Jay2OR]

Hi,
im speakers for my phone i need some help

its a dual stage amplifier circuit using a TL071, to form an active low pass filter with an upper cut-off frequency of between 18 and 25 kHz with and gain matched to the output for my phone, and an OPA551 current amplifier with unity gain.

I checked the datasheet for the OPA551 and found Max current out is 200mA
Speaker impedance is 8 ohms
i loaded a continuous tone on my phone and measured,
Max volume = pk-pk 2.68V
Medium volume = pk-pk 100mV
Low volume = pk-pk 10.30mV

how will i work out the gain? i tried this
200mA x 8 = 1.6V

max volume gain = 20xlog(1.6/2.68) = negative 4.4dB
im not sure if this is right

and then on medium and low i did the same thing and got 24dB and 43dB

[pebe]

If you use the OPA551 its 200mA maximum current will be the pk-pk current swing of the waveform. That's +/- 100mA from the centre line of the waveform (= 70mA RMS) and that will give you a maximum power of only 40milliwatts in an 8ohm speaker. Is that enough for you?

You don't say where you measured the maximum volume as 2.68Vpp. Was it measured across the earphone socket?