Why remove DC bias only to read-add later - Mic amplifier

Thread Starter

Antonii

Joined Apr 17, 2020
5
I was reading this thread and I don't understand why not add the voltage divider on the mic (instead of the 1K resistor)? That way the capacitor can be completely removed. I'm sure there's a reason and I would like to know it.

(shoot got typo in thread title)
 

ronsimpson

Joined Oct 7, 2019
2,389
You want to know why 1MF.
The op amp is set for a gain of 30.
The DC level at the output of the MIC is unknown and unpredictable. For the sake of talking, lets say there is 1mV of audio and hopefully 3V of DC. The op-amp amplifies the 1mV of signal to 30mV. That is good. If there was no capacitor, the DC would also be amplified by 30. If the DC=exactly 3V and you compensated for that all is good, but if the MIC was running at 2V or 4V or the DC changed with temperature, then the error would also be amplified by 30 and sent the output voltage too high or low to operate. The capacitor removes the DC and lets through only the ac signal.
1664139422239.png
 

WBahn

Joined Mar 31, 2012
27,395
But in that circuit (Post #2), isn't the DC level of the LM358's noninverting input undefined?

I don't think it's safe to assume it's 0 V, but even if it is, having an AC signal right at the negative supply rail would concern me. I haven't worked with the LM358, so maybe it is designed with this in mind.

Wouldn't it be better to put a divider on the non-inverting input that would bring the DC level of Vout to mid-range? Since it has a gain a bit over 31, perhaps bias it to about 0.1 V? Then put a decoupling capacitor between the output and whatever comes next.

The thing I don't like about that is that the bias level is gain-dependent and it doesn't take much error to rail the amp.

Seems like the way to go might be to change it to an inverting configuration and bias the non-inverting input to mid-supply.

Or, with the current circuit, to put a cap in series with the 330 Ω resistor to let the cap turn it into a voltage-follower at DC (and then set the bias point at mid-range).
 

ronsimpson

Joined Oct 7, 2019
2,389
There are many reasons why that circuit will not work. I thought about fixing that but if you only want to know why the cap. I started out simple.
Added -6V. Added 10k on pin-3 to set voltage to ground. 1664143884679.png
 

LowQCab

Joined Nov 6, 2012
2,506
It can also sometimes be beneficial to adjust the 1K Microphone-Bias-Resistor to
achieve closer to half the Supply-Voltage to reduce distortion at high Input-Volume-Levels.
Quite often this Resistor is arbitrarily chosen as "close-enough" for government work.
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Thread Starter

Antonii

Joined Apr 17, 2020
5
@
You want to know why 1MF.
The op amp is set for a gain of 30.
The DC level at the output of the MIC is unknown and unpredictable. For the sake of talking, lets say there is 1mV of audio and hopefully 3V of DC. The op-amp amplifies the 1mV of signal to 30mV. That is good. If there was no capacitor, the DC would also be amplified by 30. If the DC=exactly 3V and you compensated for that all is good, but if the MIC was running at 2V or 4V or the DC changed with temperature, then the error would also be amplified by 30 and sent the output voltage too high or low to operate. The capacitor removes the DC and lets through only the ac signal.
View attachment 277020
I see what you are saying. I was refering to the @ericgibbs schematic. In that one he uses a voltage divider to offset the voltage that was just filtered by the cap.

Why not move that divider on the mic side, remove the cap and just ... use it on the opamp? I imagine that if you use a battery as source then you avoid voltage drifts (or the opamp being powered by the same source the drift would be cancelled).
I also imagine that if the voltage at the mic swang wildly this is exactly the signal that anyway we wanted to amplify, no?
 

WBahn

Joined Mar 31, 2012
27,395
@


I see what you are saying. I was refering to the @ericgibbs schematic. In that one he uses a voltage divider to offset the voltage that was just filtered by the cap.

Why not move that divider on the mic side, remove the cap and just ... use it on the opamp? I imagine that if you use a battery as source then you avoid voltage drifts (or the opamp being powered by the same source the drift would be cancelled).
I also imagine that if the voltage at the mic swang wildly this is exactly the signal that anyway we wanted to amplify, no?
Instead of referring to a particular schematic in another thread and making readers guess which one you are talking about, please paste a copy of the schematic you are talking about here so that everyone can be on the same page.

The basic problem is that if you don't remove the DC bias before sending it into the amplifier, you are multiplying both the signal and the bias and that will rail the opamp unless the bias is very small.

Say that you have a signal

Vsig = Vdc + Vac

and you want to multiply that by a gain of K (which, in this case, is roughly 31).

The LM358 can only output a voltage up to about 1.5 V less than the positive rail, which is 6 V in this case. That means that the max output is somewhere around 4.5 V. With a gain of 31, that means that the max input signal is about 145 mV. That's the bias voltage plus the peak AC voltage. So your bias voltage has to be significantly less than 145 mV, which doesn't allow you to use much bias at all.

The information that you want to amplify is contained in the AC portion of the signal. By removing the DC component between each processing block, you not only avoid having to deal with these types of gain issues, but you also allow the designer to bias each block independently according to the best biasing scheme for that block. Therefore you can bias the mic to a DC output of 3 V, but bias the opamp circuit to a DC output of 2.25 V (the mid point of it's swing) without even knowing what the DC bias of the mic happens to be.
 

Ian0

Joined Aug 7, 2020
6,294
The usual circuit for a condensor microphone is to use an inverting amplifier, bias the non-inverting input at half supply, and capacitively couple from the microphone.
259317AB-C807-4DC3-9371-D853485262E0.jpeg
The best answer to the question in the title, is that it removes an unknown offset voltage (being highly dependent on individual microphone characteristics) and replaces it by a known DC offset (being set at half the supply, by the resistors connected to the non-inverting input)
 
Last edited:

LowQCab

Joined Nov 6, 2012
2,506
In the following Circuit the Microphones are biased by the Feedback-Resistors,
simplifying the Pre-Amp.
This can be done because the Op-Amps are
running at ~5-Volts, and the Inputs and Outputs operate Rail-to-Rail.
The Bias on the Microphones is always 2.5-Volts, right where they usually like to run.
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Mic Mixer 1 FLAT .png
 

Audioguru again

Joined Oct 21, 2019
5,166
An LM324 or LM358 is never used for audio:
1) It produces crossover distortion.
2) It is very noisy (hissss).
3) It has poor audio high frequency response.
 
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