Considerations for powered vs non powered microphone?

[vini_i]

I want to build a microphone mixer that provides plug in power for non balanced unpowered microphones. (such a device seems to be non existent) The microphones i would like to mix are Rode Video Mic Go and Rode  smartLav+, which both require plug in power. Besides just mixing, having volume control of the mics would be nice.

The circuit I came up with is the following. (any thoughts or comments?)



What i'm not sure about is whether i need to make any special considerations if i want to use a self powered microphone with this mixer in the future?

[DaJMasta]

I'd take a look at some mixer circuits and see how they do their thing, there are a number of good designs around and some deal entirely with single ended (unbalanced) signals.  This one is going to need a bit of a redesign:
The 7805 seems to be backwards 
The opamp is being run as a single supply, but the input signals have no DC biasing, even though you do have the virtual ground on the noninverting input
The biasing voltage for your mics may be ok.... but I'd probably use a 3.3v regulator.  At least the Rode is spec'd down to 2.5V, and while many electrets like this can go up to 10V or so, I'd stick towards the low end if you don't know for sure
There's no gain, which if you're not plugging into a mic level input, could be an issue
It's probably worth having a cap on either side of the voltage regulator and on the noninverting input to stabilize things a bit.  The divider to give you the virtual ground on the noninverting input should also probably use higher value resistors, 100k on either side would probably work just fine.
Looks like the TL081 also specs a minimum 10v input voltage between the two rails, so I'm not sure it will perform or act as advertised below that.  There are a good number of lower voltage requirement audio opamps and many will be specified for single supply use too.

It's also worth noting that at least for the Rhode, there's actually two pins used for signal output.  It's not a stereo mic, so this is either two components of a balanced signal, or it's just two lines fused together.  If it's a balanced signal, you may get lower noise by handling it as a balanced input, and this is easy to do with a single opamp stage if you don't need variable gain in that first stage.

[Zero999]

I'd take a look at some mixer circuits and see how they do their thing, there are a number of good designs around and some deal entirely with single ended (unbalanced) signals.  This one is going to need a bit of a redesign:
The 7805 seems to be backwards 
The opamp is being run as a single supply, but the input signals have no DC biasing, even though you do have the virtual ground on the noninverting input
The biasing voltage for your mics may be ok.... but I'd probably use a 3.3v regulator.  At least the Rode is spec'd down to 2.5V, and while many electrets like this can go up to 10V or so, I'd stick towards the low end if you don't know for sure
There's no gain, which if you're not plugging into a mic level input, could be an issue
It's probably worth having a cap on either side of the voltage regulator and on the noninverting input to stabilize things a bit.  The divider to give you the virtual ground on the noninverting input should also probably use higher value resistors, 100k on either side would probably work just fine.
Looks like the TL081 also specs a minimum 10v input voltage between the two rails, so I'm not sure it will perform or act as advertised below that.  There are a good number of lower voltage requirement audio opamps and many will be specified for single supply use too.

It's also worth noting that at least for the Rhode, there's actually two pins used for signal output.  It's not a stereo mic, so this is either two components of a balanced signal, or it's just two lines fused together.  If it's a balanced signal, you may get lower noise by handling it as a balanced input, and this is easy to do with a single opamp stage if you don't need variable gain in that first stage.

I agree with the above but question whether it's necessary to power the mic from a regulator? Normally the mic will bias itself at a voltage determine by its internal JFET. The voltage on the other side of the series resistor doesn't make any difference, so long as the current is limited to a safe value for the mic.

The main reason for powering the mic from a regulator, is to reduce effect of ripple and noise on the power supply but if it's run off a battery, this shouldn't be necessary.

[Audioguru]

I agree that input capacitors should be added so that the opamp will be correctly biased. With the inverting inputs grounded by the volume controls the output voltage of the opamp will be as high as it can go.

The output level from a microphone is very low but the opamp has a gain of only 1 like a piece of wire.
If the opamp is non-inverting the it can have a high input impedance so it does not load down the output from the mics and it can have plenty of gain.

A TL081 opamp is for  "general purpose". A TL071 is a TL081 selected for low noise (an audio opamp).

A 9V battery quickly drops to 7.2V when the 7805 voltage regulator begins to fail. A low dropout regulator should be used. A low voltage audio opamp should be used.

[vini_i]

MOD NOTE:
The large pixel size images caused a page rendering error, they have been removed.

The posted schematic was thrown together. I have fixed most of the issues. Here is the full updated schematic.

The 7805 seems to be backwards

That was an oversight and is fixed.

The opamp is being run as a single supply, but the input signals have no DC biasing, even though you do have the virtual ground on the noninverting input

Correct me if i'm wrong, by applying half battery voltage to the non inverting input will bias the OpAmp at half battery voltage. With capacitors at the input only the AC component makes it into the OpAmp and with a capacitor at the output the DC bias is blocked and only AC makes it out.

The biasing voltage for your mics may be ok.... but I'd probably use a 3.3v regulator.  At least the Rode is spec'd down to 2.5V, and while many electrets like this can go up to 10V or so, I'd stick towards the low end if you don't know for sure

This was already fixed with a MIC5205-3.3YM5-TR 3.3v voltage regulator in a low noise configuration.

There's no gain, which if you're not plugging into a mic level input, could be an issue

I figured i'll try unity first and if need be add some gain.

It's probably worth having a cap on either side of the voltage regulator and on the noninverting input to stabilize things a bit.  The divider to give you the virtual ground on the noninverting input should also probably use higher value resistors, 100k on either side would probably work just fine.

Great idea, i added the capacitor to the OpAmp bias. The regulator got some caps as well.

Looks like the TL081 also specs a minimum 10v input voltage between the two rails, so I'm not sure it will perform or act as advertised below that.  There are a good number of lower voltage requirement audio opamps and many will be specified for single supply use too.

The TL081 was just a place holder. The OpAmp I was planning to use is the OPA2134. But now that i'm looking at it i'm not sure if it can run from a single supply. Any thought?

[Zero999]

Looks like the TL081 also specs a minimum 10v input voltage between the two rails, so I'm not sure it will perform or act as advertised below that.  There are a good number of lower voltage requirement audio opamps and many will be specified for single supply use too.

The TL081 was just a place holder. The OpAmp I was planning to use is the OPA2134. But now that i'm looking at it i'm not sure if it can run from a single supply. Any thought?

It should be fine

Wide Supply Range:
±2.5 V to ±18 V

http://www.ti.com/lit/ds/symlink/opa134.pdf

[Zero999]

Correct me if i'm wrong, by applying half battery voltage to the non inverting input will bias the OpAmp at half battery voltage. With capacitors at the input only the AC component makes it into the OpAmp and with a capacitor at the output the DC bias is blocked and only AC makes it out.

The AC coupling capacitors should be after the volume controls, otherwise the DC bias will shift when the volume is adjusted.

BTW sorry, I had to double post. I've had problems with  viewing this thread. I wanted to edit it my previous post but couldn't so had to create a new one.

[vini_i]

The AC coupling capacitors should be after the volume controls, otherwise the DC bias will shift when the volume is adjusted.

Would placing capacitors before and after the potentiometer be ok?

[Zero999]

The AC coupling capacitors should be after the volume controls, otherwise the DC bias will shift when the volume is adjusted.

Would placing capacitors before and after the potentiometer be ok?

Yes. That would be better but you'll need to double the capacitance, as they would be in series.

[EEVblog]

I want to build a microphone mixer that provides plug in power for non balanced unpowered microphones. (such a device seems to be non existent) The microphones i would like to mix are Rode Video Mic Go and Rode  smartLav+, which both require plug in power. Besides just mixing, having volume control of the mics would be nice.

The circuit I came up with is the following. (any thoughts or comments?)

What i'm not sure about is whether i need to make any special considerations if i want to use a self powered microphone with this mixer in the future?

The AC coupling capacitors should be after the volume controls, otherwise the DC bias will shift when the volume is adjusted.

Would placing capacitors before and after the potentiometer be ok?

Yes. That would be better but you'll need to double the capacitance, as they would be in series.

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