Considerations for powered vs non powered microphone? - Page 1
Forum section: electronics / projects

author: vini_i
author_rank: Regular Contributor
timestamp: January 15, 2017, 05:49:06 pm
content: 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?

author: DaJMasta
author_rank: Super Contributor
timestamp: January 16, 2017, 06:15:00 pm
content: 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 backwardsThe 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 inputThe 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 sureThere's no gain, which if you're not plugging into a mic level input, could be an issueIt'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.

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2017, 06:55:45 pm
content: Quote from: DaJMasta on January 16, 2017, 06:15:00 pmI 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.

author: Audioguru
author_rank: Super Contributor
timestamp: January 16, 2017, 10:15:05 pm
content: 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.

author: vini_i
author_rank: Regular Contributor
timestamp: January 16, 2017, 10:57:06 pm
content: 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.QuoteThat was an oversight and is fixed.QuoteCorrect 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.QuoteThis was already fixed with a MIC5205-3.3YM5-TR 3.3v voltage regulator in a low noise configuration.QuoteI figured i'll try unity first and if need be add some gain.QuoteGreat idea, i added the capacitor to the OpAmp bias. The regulator got some caps as well.QuoteThe 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?

author: Zero999
author_rank: Super Contributor
timestamp: January 16, 2017, 11:55:27 pm
content: Quote from: vini_i on January 16, 2017, 10:57:06 pmIt should be fineQuotehttp://www.ti.com/lit/ds/symlink/opa134.pdf

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2017, 12:00:44 am
content: QuoteThe 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.

author: vini_i
author_rank: Regular Contributor
timestamp: January 17, 2017, 02:38:34 am
content: QuoteWould placing capacitors before and after the potentiometer be ok?

author: Zero999
author_rank: Super Contributor
timestamp: January 17, 2017, 05:18:35 pm
content: Quote from: vini_i on January 17, 2017, 02:38:34 amYes. That would be better but you'll need to double the capacitance, as they would be in series.

author: EEVblog
author_rank: Unknown
timestamp: January 19, 2017, 10:18:51 pm
content: Quote from: vini_i on January 15, 2017, 05:49:06 pmQuote from: Hero999 on January 17, 2017, 05:18:35 pmTest