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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

enter image description here

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?).

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  • \$\begingroup\$ But that circuit does not provide impedance of 1k to phone. DC impedance is 2k. For AC it is less than 200. \$\endgroup\$
    – Justme
    Jun 17 at 15:21
  • \$\begingroup\$ @Justme: Correct. The 1k resistors have next to no influence on the impedance as seen by the phone. \$\endgroup\$
    – JRE
    Jun 17 at 15:22
  • \$\begingroup\$ to increase attenuation, increase R5, reduce R4 \$\endgroup\$
    – Neil_UK
    Jun 17 at 15:31

2 Answers 2

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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.

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  • \$\begingroup\$ thank you. First I must say that I implemented the circuit on the image that I attached on the post (you kindly provided me in another thread) direct from the speaker to the smartphone and it worked fine BUT the sound is clipping when I try to record so I think I just need more attenuation. According to what you said here, I just need to replace the 1k ohm resistor to 10k ohm. Right? Just changing this you think it will attenuate more? \$\endgroup\$
    – user314164
    Jun 17 at 18:56
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    \$\begingroup\$ Yep. Change the 1k to 10k. \$\endgroup\$
    – JRE
    Jun 17 at 18:57
  • \$\begingroup\$ I made another question about this exactly same circuit! Would you mind taking a look at it on this link electronics.stackexchange.com/questions/624130/… ? \$\endgroup\$
    – user314164
    Jun 19 at 2:23
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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.

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