Preamp circuit for electret mic to mini DVR (5v)

Thread Starter

John Austin

Joined Jul 30, 2017
17
I am trying to create an audio preamp circuit for getting a clean audio signal from an electret microphone to a small mini DVR that runs on 5v DC. The mic and circuit will have a 9v or 12v supply. I have been working on this nightmare for over two weeks and could really use some specific input. The Eagle schematic (.pdf) is attached. Can anyone verify that this circuit appears to be correct? I bought some OPA2134 op amps by mistake and that is why you will see the second, non functioning symbol in the diagram. I am not sure how to treat those pins. I am also not sure specifically where to run my grounds. My DVR is a small 5v type for drones. If you google images "Readytosky DVR" you will see the pinout. There is a black 'audio in GND' and a white 'audio in +' on the DVR. I want the DVR to record soft voice and to be on the sensitive side to pick up animal sounds around a feeder at night.
Can anyone help me by looking over my circuit and tell me specifically what I may need to change or modify to get this issue behind me?
Thank you in advance ;)
 

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#12

Joined Nov 30, 2010
18,223
You need a capacitor between the output and the DVR to avoid sending +6VDC to the DVR.
The size of the capacitor depends on the input impedance of the DVR and the lowest frequency you want, like this:
Xc = 1/(2Pi F C) where Xc = the input impedance of the audio in pin.

You should probably connect pin 5 to your center voltage point (+6v) and a direct wire from pin 7 to pin 6.
That way the amplifier is in the center of the operating range and it thinks its job is to hold the inverting input at 6 volts.
Stupid job, but it gives the spare something to do.

I would use a pot for R12 so it's adjustable.
You did not name the mic, so I can't look it up.
 

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Thread Starter

John Austin

Joined Jul 30, 2017
17
I had purchased some pre wired electrets on ebay and when I compared their sound to one of the scavenged DGU605AA mics when connected directly to mic IN on the PC, the difference was night & day so I abandoned the ebay mics and will be buying some of the DGU's.
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
#12,
Here is the latest version of my schematic... I am still waiting on the delivery of my op amp IC's. In the meantime, I set up the mic on the breadboard, supplied by 5v with a 6.8k resistor, sending 1.5v to the mic. I connected the ground to the DVR's ground and signal to the DVR's audio in. I got a very weak result, but could tell that a signal was being sent and recorded. That being said, I assume that once I set up my pre amp circuit with the OPA2134 opamp as you have outlined, I should expect a fair/good audio recording on the DVR? By the way, audioguru has given me a lot of input also on another forum and with both of your help, I am confident that I will have a quality circuit and very nice audio.
Thanks to you and audioguru!! Those IC's can't get here soon enough ;)

upload_2017-7-31_11-0-3.png
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
By the way, the .22uf capacitor on the output is just a placeholder. I do not understand the math to size that, but can study it if need be.
 

jjw

Joined Dec 24, 2013
722
I searched for the specs of the DG6050AA and seems that it is a micro speaker?
It might work as a mic, but is the dc supply for it needed?

Your circuit amplifies now also the 6V dc bias and the opamp output is stuck at +12V. Put a capacitor in series with R14 to gnd.
I would change R14 to about 2k, series C ~ 2uF depending of the needed low frequency response and R15 big enough for the gain.
 

#12

Joined Nov 30, 2010
18,223
Your circuit amplifies now also the 6V dc bias and the opamp output is stuck at +12V. Put a capacitor in series with R14 to gnd.
OMG! How did I miss that? I was so careful to check the Vos X gain and forgot the DC bias.:oops:
I would change R14 to about 2k, series C ~ 2uF depending of the needed low frequency response and R15 big enough for the gain.
I agree. No need to use a super high impedance op-amp and waste it with a low impedance feedback loop.
For a gain of 100, 2.2K and a 250K feedback pot or 10K and a 1 meg feedback pot. And, oh by the way, an op-amp with a gain of 100 needs a metal box to live in or it will pick up all sorts of electrical noise.
By the way, the .22uf capacitor on the output is just a placeholder. I do not understand the math to size that, but can study it if need be.
If you can't find the input impedance of the DVR, just assume it's 10,000 and you already guessed the right capacitor for 72 Hz. If you hook this up and only hear bird chirps, the capacitor is too small.:D But that op-amp will drive a 600 ohm load, so that is not part of the problem.
Thanks to you and audioguru!!
I don't understand why AG didn't fix all this. He's about 10x as good as I am.

ps, move pin 5 to the other end of R10 so it only has to deal with DC.
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
I believe I have comments from audioguru and #12 incorporated now........... should a gain of 100 be enough, or will it just take some testing?
upload_2017-8-1_7-47-20.png
 

#12

Joined Nov 30, 2010
18,223
should a gain of 100 be enough,
That much gain (on a microphone) is unheard of in my experience doing sound and lights for live performance.:eek:
But I understand that woodland creatures do not speak directly into the microphone.:rolleyes::D
You are just going to have to try it and find out.
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
#12,
I'm open to any and all suggestions and recommendations you may be able to offer to get this circuit working.
Thank you.
This is what I have right now. Audioguru has told me it won't work. However, I do not know, or at least understand, what it will take to make it work as I am a structural engineer. I do not know electronics, but am trying to understand and learn. I do know that I will need more gain than usual as I am trying to record very quiet voice and 'woodland creatures' (actually hogs at a corn feeder at night)
upload_2017-8-2_10-13-57.png
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
I have the circuit working on the breadboard now. I connected a 3.5mm jack socket and pair of earbuds (L&R) to the signal coming out of pin#1 on the opamp and GND connects to the same GND as others. After some minor adjustments, the audio coming out of this circuit is very sensitive and clean as was hoped for. I am very happy with that result. However, when I connect the 'audio in' from the DVR (powered by 5v via buck converter), instead of earbuds, I get a lot of noise recorded by the DVR. I can hear my voice mixed in with the noise, so I am getting a very dirty signal to the DVR, but need to know what needs to be adjusted to clean it up and hopefully record audio like what I was hearing on the earbuds. Thanks for any help.


upload_2017-8-3_11-58-40.png
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
I corrected a few items that were labeled wrong. I also changed R15 to 10k. Voice is coming through better, but still getting steady electrical noise, not a buzz or hummm.

upload_2017-8-3_15-34-15.png
 
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jjw

Joined Dec 24, 2013
722
The buck converter might be the noise source.
How much current the dvr needs from 5V?
Could you use a linear 5V regulator?

You could make a test from some other audio source.
For example from phone / tablet headphone output.
 
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jjw

Joined Dec 24, 2013
722
You could try to filter the mic supply: replace R12 with two series connected 10k resistors and take a 10uF capasitor to gnd from their connection point.
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
jjw,
You're right. I don't know how much current (milliamps?) the DVR needs, but will see if I can find out. I might be able to use a linear regulator. Would that be a matter of stepping the 12v down to 5v with resistor(s)?

earlier... I tried several things to get rid of the noise.... I decided to hook up a 9v battery to supply the DVR (stepped down to 5v) separately. I got perfect audio recorded to the DVR !! It sounds really clean with zero noise. Is there anything I can do to supply my DVR's buck converter with the 12v supply and get rid of the noise?
I would like to avoid having to add a separate power supply for the DVR. Is there a way to filter out this noise?
 

Thread Starter

John Austin

Joined Jul 30, 2017
17
jjw,

Yes, I was setting up the circuit and got perfect audio using just earbuds. The noise started when I connected the buck converter and DVR.
I need to filter that noise somehow.... thanks for your help by the way

The audio I got on the earbuds was exactly what I'm looking for. Very sensitive, crisp and clear with zero noise. Audioguru and #12 helped me get to this point and I've almost got great audio going to the DVR....

I found one thing online that seemed to indicate that the DVR was drawing 800ma.
 
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Thread Starter

John Austin

Joined Jul 30, 2017
17
I did some searching on linear regulators and I am all for going that direction if I can eliminate adding a separate battery/power supply.
Which IC would you suggest would be the best to use in the regulator circuit? I will go ahead and order some.
Or, can I get it done with just using 2N3904 transistor(s)?
 
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