# Question on how to improve my analog audio amplifier circuit

Joined Aug 23, 2018
7
Hello everyone!

My original intention was to sample analog audio via the analog pin of the Arduino Uno Microcontroller. For testing purposes I used my phone as the audio source and plugged in a 3.5mm AUX cable I extracted from some old phone headphones. Then I used the R speaker audio wire to measure the output voltage on maximum sound volume when playing a 1kHz test sound sample by using an oscilloscope. The output was a 400mV peak to peak sine-wave signal.

Then I proceeded to design a pre-amplifier for the input signal. First, a 1μF capacitor was used to block any DC noise of the output signal. Then, I decided it would be a good idea to first offset the signal to 0, by using a voltage devider, which would output a voltage range from 0 to 400mV. This new signal is then connected to the non-inverting input of an op-amplifier, which amplifies the signal by a factor of 12, by employing another voltage devider, whose output is connected to the inverting input of the amplifier. The final output, which has a range from 0-4.8V is then connected to the analog pin of the Arduino. The circuit is shown in the picture.

Because I am quite inexperienced in electronics, I wondered how I can improve this circuit to make it suitable for a wider range of use. For example, when analysing the output voltage of the AUX cable alone and playing several different melodies, the output voltage depended wasn't always the same and amplitude ranged from 200-250mV. I guess the same would happen if I had plugged the AUX into another phone? Maybe a potentiometer combined with the first voltage divider would help?

Besides that feeding negativne voltage into the analog input of the Arduino is a potential danger in my case. I have thought of using a zener diode, but those negative voltages would not exceed more than a Volt, which is not anywhere near the breakdown voltage of zener diodes.

#### crutschow

Joined Mar 14, 2008
33,365
What is the op amp and what is powering it?

#### Ian0

Joined Aug 7, 2020
8,948
Bias the input to half supply by using equal value resistors instead of 2.4k/100Ω.
Then reduce the amplifier gain at DC to unity by inserting a capacitor in series with the other 100Ω resistor.

#### LowQCab

Joined Nov 6, 2012
3,595
Multiply the values of all of the Resistors by 100.

You are putting too much Load on the Input-Source, and the Output of the Op-Amp.

Is your Op-Amp a "Rail-to-Rail" design ?

More Gain would mean better Digital-Resolution.
.
.
.

Joined Aug 23, 2018
7
What is the op amp and what is powering it?
Hello,

The op-amp I am using is the LM324 and is powered with +/-15V. Later on I plan to substitute it with a 5V rail-to-rail one, since the final circuit is supposed to be in form of a PCB shield for arduino

Joined Aug 23, 2018
7
Bias the input to half supply by using equal value resistors instead of 2.4k/100Ω.
Then reduce the amplifier gain at DC to unity by inserting a capacitor in series with the other 100Ω resistor.
Thank you for your suggestion. The added capacitor would cause the amplifier gain to be more frequency dependant, right? And it would cause only the AC component of my signal getting amplified? Or is there any other benefit I'm missing?

Joined Aug 23, 2018
7
Multiply the values of all of the Resistors by 100.

You are putting too much Load on the Input-Source, and the Output of the Op-Amp.

Is your Op-Amp a "Rail-to-Rail" design ?

More Gain would mean better Digital-Resolution.
.
.
.
Thank you for your reply. By heavy load, did you mean that due to low resistance values, I draw to much current into the op-amp and into the Arduino?

#### Ian0

Joined Aug 7, 2020
8,948
I was
Thank you for your reply. By heavy load, did you mean that due to low resistance values, I draw to much current into the op-amp and into the Arduino?
i wasn’t so worried about that, as the output was probably intended to drive 32Ω headphones.
In other circumstances, I would agree with @LowQCab

#### DickCappels

Joined Aug 21, 2008
10,104
Your 1 uf capacitor and 100 ohm resistor to ground make a 1.5 kHz high pass filter. If you want intelligible speech to come out you need to increase the resistor values so as to not eliminate so many low frequencies. Take @LowQCab's advice and multiply the resistor values by 100.

The corner frequency is ƒc = 1/(2πRC).
This is the frequency at which the signal is attentuated by about 30% and is reduced by 50% every time the frequency is halved.

https://www.electronics-tutorials.ws/filter/filter_3.html