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BJT Mic Pre Inside Mic Wand

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ronv,

Man am I happy I was brought to the woodshed re: my post about your circuit. Best stupid mistake I ever made. Once I got it running in LTS its beauty became apparent. It runs on a single supply and has a Pi filter supplying the MIC bias. Clever!

The pic below shows my intended application. The R on the output of my ckts is a pot that will be in my test box/jig. I will be measuring speakers and feeding to a sound card line-in.

ECM Mic Concept.gif

I appreciate your providing the file for LTS. But not only did it not import the OPA but the ECM disappeared as well. I guess I'll have to stick to V sources.

I hope you don't mind a few questions about your circuit.

For my purposes, what fixed gain would you suggest for your circuit and what value pot would be best to least affect the FR with volume changes? You see, I cannot put a pot in the feedback loop of the OPA. Won't fit in the wand.

Given the 10k impedance of the MIC, would it make sense to take the positive side of C1 and move it to the junction of R7 and C3? That way the combined resistance of R1 and R7 will be looking at C1 for a lower f3. But I don't know about any interaction with C3 or the MIC.

Is there any magic re: the 100k values for R4 and R5? Could they be higher or lower for a lower noise contribution?

Thank you for your efforts.
 
eric,

Thanks for holding me accountable. It caused me to reinvestigate the circuit and discover how nice it is. Also thanks for clearing up the gain problem with my circuit. I could not figure it out.

BTW, I am a complete novice with LTS. 50 years ago the US Army trained me in electronics before sending me to Viet Nam. There were no OPA's then and I surely do not remember all of what little they tried to teach me. Now that I am retired, I fuss with speakers for a hobby and like to try to understand what I am getting into. However, I fear I will never become facile with LTS.

Thank you for your help.
 
ronv,



I appreciate your providing the file for LTS. But not only did it not import the OPA but the ECM disappeared as well. I guess I'll have to stick to V sources.

Come to think about it you won't have the adjustment pot either. So here is one with fixed gain that should run.


For my purposes, what fixed gain would you suggest for your circuit and what value pot would be best to least affect the FR with volume changes? You see, I cannot put a pot in the feedback loop of the OPA. Won't fit in the wand.

They make really small ones, but if you need to adjust it often back at the computer might be best. Maybe a small amplifier stage with a volume control. The input on sound cards vary a lot so just trying to pick a pot might not work.

https://www.electro-tech-online.com/custompdfs/2013/06/3362-55248.pdf

Given the 10k impedance of the MIC, would it make sense to take the positive side of C1 and move it to the junction of R7 and C3? That way the combined resistance of R1 and R7 will be looking at C1 for a lower f3. But I don't know about any interaction with C3 or the MIC.

The mic needs the resistance to develop the signal across. It wouldn't be able to drive C3.

Is there any magic re: the 100k values for R4 and R5? Could they be higher or lower for a lower noise contribution?

No real magic within reason. Lower would be better for noise, but reduces the output from the mic. 100k is kind of a compromise, but 120k would be no problem.

There are better op amps for very low noise and distortion.

https://www.electro-tech-online.com/custompdfs/2013/06/opa2134.pdf
 

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ronv,

Thanks for all your help. I think I've got it now.

Great idea to use a trimmer. Actually there should be no need for constant vol control. Once set for measurement, that should be it for a while. So I will go the trimmer route.

I have no intention to use the LT OPA in the schematic. I simply picked basic a one more or less at random from the LT list for sim purposes because I don't know how to import what I would use. And yes those Burr Brown jobs are very nice for audio.

Thanks again for your help.
 
hi oldude.

I would recommend signing up for the free YahooLTS user group. **broken link removed**

Its got loads of libs, models and example circuits,, look under the Files/Lib section.

Regards
From a VERY old dude.E
 
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ronv,

Darn, can't get rid of me yet! But we are darn close.

Before I start soldering I wanted to check with you one more time.

I took your schematic of "dudes amp" and rearranged and it and re numbered the components so it is easier for me to understand. I also substituted a better OPA and increased the caps for better line filtering and better LF response. I am using the caps because I have them on hand and they do wonders for the low end FR (especially C3 on my schematic). I hope increasing them doesn't hurt anything. R6 in the schematic below is a trimmer. And the output will go to Line In on a sound card for speaker measurement. I will not be recording anything but would like the measurements to be as accurate as possible.

Before the cap and OPA changes, the circuit was down 3 dB at 20Hz and at 20K Hz. Now it is almost straight as a string from 20-20K Hz. Indeed, with the better OPA HF goes out to 6.5 meg Hz, makes about a 3 dB rise and then starts to fall. The Q of the HF rolloff is more than 1.

I found a 10pf cap across R6 eliminates the HF peak. Since the pot is variable, so is the effect of the 10pf cap. So when R6 is down to 5K the HF -3 dB point is about 2 meg Hz and with R6 at 100k it is about 130k HZ. If R6 goes significantly below 5k, the hump comes back with a vengeance. Is this hump anything to worry about? I hope that the circuit is OK without the 10 pf cap (fewer parts is better) cause the top end seems to be self limiting anyway.

Your "dudes amp" schematic has no current limiting resistor or cap on the output. And I show it the same way. If I use it that way should I be concerned about my M-Audio Transit audio interface (outboard USB sound card)? In other words, is it safe to use as shown? If not, suggestions?

Well, that is about it except for the pictures. They are attached.

BTW, would anyone want to see the finished product?

And THANK YOU for all your help.

dudes amp sch.JPG

dudes amp FR.JPG

Thanks.
 
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Changes

The bigger caps should be fine. I don't think you need the cap between the amp and your sound card but it would not hurt anything either. It would be a good idea to add some resistance to the output of the op amp because of the capacitance of the cable. Also with the new amp you should roll off the bandwidth or it may oscillate. Without it if you look way out on the bode plot you can see 180 degree phase shift and still some gain. So I added a little cap to bring it down to 30 - 40 KHz.
Make sure to keep the output signal away from the + input and maybe add a .1 ufd. decoupling cap from the positive supply to ground right at the op amp. Hope all goes well & yes send pictures!:)
 

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Yea, I feel alot better now.

I used 10p where you used 33p. We are on the same wave length. I think the 330 Ω series R is necessary. I am glad you do too. Standard layout and decoupling practice will be followed.

I will be using the LM833N. It is an older audio OPA that while not outstanding compared to newer units is decent. And since I have a few, I will use one of them. It should do fine.

Hopefully I will be able to squeeze all the parts into my mic wand. Time will tell.

Waiting for parts now.

Thanks again.
 
Hi again ronv,

In a prior post you said,

........... if you need to adjust it often back at the computer might be best. Maybe a small amplifier stage with a volume control. The input on sound cards vary a lot so just trying to pick a pot might not work.

Could you show me what you have in mind? And what should the preamp gain be if using the small amplifier at the computer?

Is the amp acting as a buffer?

Thanks
 
I was thinking a gain of about 30 at the mic and 4 or 5 at the box, but let me play a little bit. Maybe with 2 amps we can use the - input to the first op amp and get rid of one cap.
 
I'm game. It would be nice to have a vol pot in the box and not have to screw with the wand at all. That is what I initially intended anyway.

Thanks
 
Something like this would do it I think.
 

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The transistor preamp in the first post uses an unmodified electret mic with a 2.2k resistor powering it from +12V. But then the current in the Jfet transistor inside the mic is much too high because the datasheet for the mic shows the 2.2k resistor powered from only +2V.
Change the resistor to 10k then the current in the mic will be normal when the resistor is powered from +12V. The +12V feeding the resistor must be very stable so that the preamp does not amplify its fluctuations.
The unmodified mic with a 10k resistor powering it will present a total impedance of about 2.7k ohms.

The transistor preamp has a fairly low input impedance when C2 is removed which will load down the output from the mic and reduce the output a lot.
The transistor preamp is suitable for a low impedance (600 ohms) dynamic mic.
 
Thanks guys,

ronv, why did you go inverting on the amp/buffer?

BTW, I have some video driver OPAs (AD827) that were used years ago for audio purposes. They have the capability to drive hundreds of feet of high capacitance cables. Gain bandwidth at 20k is around 60 dB. Might they be useful for the wand or the box? Of course I have the LM833Ns that should do just fine.

This is ultimately what I am striving for:

Test Box Complete.JPG

It should save my back and the mass of cables and ancillary equipment I have used before.

I am looking at your latest circuit in LTS and it is very confusing to me with the inverted second stage. But I will work on it.

Thanks. I'll be back.
 
With the inverting amp is saves the a cap so is a little better at low frequency. Since the output impedance from the preamp is pretty low it works good.
I would stick with the audio amps they will be less fussy and are made for single supply.
The only thing I would add would be a voltage regulator in front of the preamp and for the amp in the box. I think even 12 volts is pushing the mic so if you can get like an LM7809 regulator it would really be sweet and make the supply very quiet.
10K pot for the box amplifier.
 
I know I have some 5v regulators and I do have some that require a bit of design but they carry much more current I will have to look and see.

Thanks.
 
Five volts may be a little low. Nine would be better - even 8.
 
I was not going to use 5v. Just mentioned I had them. Turns out, they are 15's.

I do have some LM317s. Easy to setup just need to draw some current to get them going.

Using a regulated supply changes things. The filter for the mic supply is prob not necessary. Indeed, it might be best to use a higher supply V for the OPAs to make them comfortable and a voltage divider to set mic bias from the reg supply. So maybe those 15s might be useful after all.

What might the optimum strategy be and what do you figure the total current draw might be for the 2 OPAs and the MIC and voltage divider, if used?

I think the fixed 15V regulators I have are 100ma. I have a couple very slick potted +_15V @100ma high grade surplus linear power supplies. Those are really easy to use. Connect to AC and simply use one leg. And they are not part of the power amp supply. If I use regulators, I will pull from the power amp supply.

Thoughts?

Thanks
 
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An LM317 is supposed to use a 120 ohm resistor from its output to its ADJ pin. Then the load current is 10.4mA which is more than the 10mA minimum required.
All the circuits in the datasheet and nearly every circuit online show a 240 ohm resistor which is used for the more expensive LM117 that has a minimum load current of 5mA. The datasheet shows the LM117 and 240 ohm resistor in almost all circuits.

An opamp draws a few mA then two opamps draw about 5mA. The divider for the mic draws 2mA and the mic draws 0.5mA.
 
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