need help

[bob91343]

I think it should work fine without the filter. If you want to compute a filter, there are online setups that allow you to enter the impedance, cutoff frequency, and such, and will tell you inductance and capacitance needed. But I don't think you need it.

However, I see the audio output is coupled through a 2 microfarad capacitor. Knowing that the 71A can only put out probably less than a Watt, you need to come up with the optimum load (speaker?) impedance for this stage. The lazy man's way would be to allow the capacitor to make a filter with the load such that the corner frequency would be, say, 100 Hz or even 50 Hz. So you calculate the reactance of the 2 microfarad capacitor at that frequency and that will tell you how many Ohms speaker to use.

Let me see, my memorized rule is that 1 microfarad at 1 kHz is 159 Ohms. So 2 microfarads would be 80 Ohms. But it needs to be at least one tenth that frequency, making for a load impedance on the order of 800 Ohms. I would suggest they expected the user to use a 2000 Ohm speaker or headphone.