Hi friends,

Long time reader yet first time poster here on the forums.

I'm quite a newbie with analog electronics and could use some help trying to understand a seemingly basic concept.

I'm working on a small project that allows me to test (hear and see via a level meter) the audio input from a variety of sources (line level audio, mic level and high impedance "instrument/guitar" level).

I believe I have a grasp on the proper circuits to handle the line level and instrument level inputs, yet I'm struggling with the low level 10mV microphone level input.

I'd hoped to use the same dual op-amp (MCP 6002) to handle this amplification, and using a switch to change R2 of the non-inverting feedback loop, thus changing the AV gain to the appropriate level.

Yet, I'm sure I'm looking at this all wrong.

My first question addresses IF this is the right path?
Additionally, is it better to design a separate op-amp circuit to hand each "type" of audio input, using a switch to engage the signal path to the appropriate amplifier circuit?

Thank you all fr any guidance and apologies for the newbie approach.

-Nicholas

 

I am not an audio engineer expert but my guess is that this is the way it is done.
You need audio amplifiers with fixed gain settings, usually in steps of +3dB (x1.41 voltage) or +6dB (x2 voltage).
You also need to pay attention to the amplifier input and output impedances. You want high impedance input and low impedance output. 600Ω output impedance is a common audio industry standard. 1mW into 600Ω is a standard 0dBm.

Reference:
https://en.wikipedia.org/wiki/Line_level

 

Additionally, is it better to design a separate op-amp circuit to hand each "type" of audio input, using a switch to engage the signal path to the appropriate amplifier circuit?

Using an analog mux into a unity G buffer should result in a general purpose interface.
Follow that with amplifier. Or IA with G control. Layout is crucial to minimize crosstalk.

https://www.analog.com/media/en/training-seminars/tutorials/mt-072.pdf

https://www.analog.com/media/en/tra.../designers-guide-instrument-amps-complete.pdf

http://www.ti.com/lit/ds/sbos002/sbos002.pdf



Regards, Dana.

 

Using an analog mux into a unity G buffer should result in a general purpose interface.
Follow that with amplifier. Or IA with G control. Layout is crucial to minimize crosstalk.

Thank you, Danadak. Very helpful!

A few quick questions and clarifications:

1) Is the jellybean HC4066/4053 a viable mux option for this application?
2) When dealing with the high impedance nature of a guitar/instrument signal in addition to the the low impedance nature of line level/mic level inputs, do I need a buffer in front of the MUX also? If using a non-inverting op-amp, is it necessary to compensate for input impedance? If so, how in the application?
3) I'm assuming the unity gain buffer on the output of the mux is necessary/best practice when heading to a gain stage?
4) I'm reading through the AD note you passed along in regards to a low noise VGA solution to easily adjust gain of the amplifier which follows the mux/buffer stage. I see some voltage ref points (I think). Is this circuit possible with a single supply?

Thanks again for any help, Danadak

 

1) Is the jellybean HC4066/4053 a viable mux option for this application?

Yes.

2) When dealing with the high impedance nature of a guitar/instrument signal in addition to the the low impedance nature of line level/mic level inputs, do I need a buffer in front of the MUX also? If using a non-inverting op-amp, is it necessary to compensate for input impedance? If so, how in the application?

No, just one Voltage Follower on Mux out.

3) I'm assuming the unity gain buffer on the output of the mux is necessary/best practice when heading to a gain stage?

Yes in general. One could do a noise analysis. Ir. take G in first stage to min noise.
But todays OpAmps low enough noise follower should be OK.

4) I'm reading through the AD note you passed along in regards to a low noise VGA solution to easily adjust gain of the amplifier which follows the mux/buffer stage. I see some voltage ref points (I think). Is this circuit possible with a single supply?

Page 8 of ap note shows a single supply approach.


Regards, Dana.

 

You selected a "general purpose" opamp instead of a low noise audio opamp. You will get lots of rummmmble and hissss when the gain is high for a microphone input. Many audio opamps also have a high impedance Jfet input for a guitar pickup.

 

You selected a "general purpose" opamp instead of a low noise audio opamp. You will get lots of rummmmble and hissss when the gain is high for a microphone input. Many audio opamps also have a high impedance Jfet input for a guitar pickup.

Thank you, AudioGuru! Good catch! I have experimented with the TL082 for the guitar input due to the high impedance nature. I THOUGHT the MCP6002 would be a good choice for audio but it seems that not the case. Do you have any other recommendations on single-supply happy low noise OP AMPS?

I've considered 5532/5534, LM358, OPA series...

Thank you!

 

ANY opamp works with a single-supply when its (+) input is biased at about half the supply voltage and input, output and feedback coupling capacitors are used.

A TL082 is also a "general purpose" opamp but a TL072 is a TL082 selected for low noise. The TL07x and TL08x opamps have a serious problem called Opamp Phase Inversion where the output suddenly goes as high as it can if an input voltage becomes within a few volts from the negative supply which is ground with a single-supply. Then the distortion is bad and sounds weird.

The LM358 dual and LM324 quad opamps have awful crossover distortion and a poor slew rate that cuts high audio frequencies. They are also very noisy. The NE5532 and NE5534 audio opamps are old but still pretty good but the NE5534 oscillates at a high frequency if its gain is less than 3.

I think a OPA2134 is an excellent modern dual audio opamp. Its minimum supply is 5V when it barely works but it works well with 9V. It has Jfet inputs.