Convert speaker level to microphone level

Question

I have a digital piano without line output (not even earphone output) but I would like to record what I am playing. So I decided to convert the speaker level to mic level so I can record the audio in my smartphone (using the bultin TRRS input of my smartphone).

My piano speaker is 4 ohm and 10v. I need to bring it down to 0.01v (mic level? I think smartphones accept 0.1v?). A really smart and nice guy told me, in another thread (in which I had a similar problem), how to bring line level to mic level with this circuit:

https://electronics.stackexchange.com/a/620993/314164

It worked PERFECTLY! It has 10:1 attenuation and impedance of 1k which is the recommended to smartphones. Now I need to increase this attenuation and I know I can easily do this changing the 1k and 100 ohm resistors, however that will mess with the 1k impedance recommended to smartphone.

So how can I do this? Do you know the values of the resistors that I need to change (the capacitor will have to be changed?).

Answer 1

The 1k resistors don't matter to the recording phone. They set the impedance that the electric piano "sees."

Assuming that you will be connecting the "Earphone_Left" and "Earphone_Right" lines to the hot side of the speakers, you will need to change R1 and R2 to 10k. More might be needed, but 10k is a good starting place.

Connect the "Earphone_Ground" to the common ground of the electric piano.

You don't need to change the value of the capacitor. It depends on the impedance of the phone.

The messier question is "Which way does the capacitor go?"

The original circuit was intended for use between Android phones. On those phones, there's no DC on the headphone lines but a small DC on the microphone line so that the "+" side of the capacitor goes towards the microphone.

Your electrica piano may have DC on the speakers. Measure the voltage between the speaker hot and ground. Do NOT play a note. Just have it turned on and ready to play. If there is a DC voltage between the speaker and ground, then the "+" of the capacitor will go towards the electric piano. If there's no DC, then the "+" goes towards the microphone.

Answer 2

You can use a "DI" (Direct Injection) box that are designed to do just this. There are two types, active and passive. The passive ones typically have a transformer internal which isolates the signals. The grounds typically go through a switch called "Ground Lift", when open the input and output are gavallinacly isolated. Masny have a built in actuator that allows you to cut the levels so you can use a higher level output device such as a speaker to a low level input such as a microphone. They can be gotten for less then $40.00 to several hundred dollars each. Nice thing they come nicely packaged with the appropriate jacks etc. Use the term "DI box" to find a large variety of them. Your favorite music store probably has some in stock.