# Confused about op amps and wiring for parabolic mic

I consider myself a fairly smart guy (20+ yrs web programmer, Computer Science degree), but in the past couple years, I've started to delve in the realm of electronics design and I'm constantly getting frustrated by lack of faster progress. (I'm YouTube/internet taught which is a lot of the problem).

I'm trying to build a parabolic mic/listening device because I work in cat rescue and there's been countless times when I would be trying to listen for stranded baby kittens or a cat stuck under someone's house or in a shed or garage and having an amplifier would make things so much more effective.

I have a scattered understanding on how op amps work and their function in amplifying a mic source. I follow tutorials and schematics on the web, but nothing ever works. No sound with the exception of hearing a power off pop in one design.

I've found that even though there is a plethora of tutorials out there, most either assume the reader knows certain items and they're skipped or others simply forget to mention some bits that without them renders the project undoable.

Maybe I'm just frustrated to find myself having to learn baby steps again.

Components for my project (as I understand them):

• 2 electret mics (housed in my case in a metal megaphone bell)
• 2 potentiometers (one for each earpiece)
• earbud set with 1/8" stereo plug (also has a mic piece on the wire. I've seen some tutorials that made use of this but didn't explain how)
• on/off slide switch (have other types as well)
• lots of different op amps (salvaged) but unfortunately not the most common ones (741, lm386 etc). Many of them are listed as comparators, buffers, motor drivers etc but these should function just like any op amp for volume control yes?
• I've determined which of my op amps are specifically for audio applications or are listed as Op Amp specifically: LM924N, BA5412, BA328 BA3308, 4560D, 2060D, LM380N, 2115D, 4580D are just a few.
• I'm trying to focus on dual or quad opamps so I can run each channel separately; one channel through each opamp of the IC.
• Various resistors and caps as needed
• Want to run this with a single 9V battery

My questions and struggles on this project are:

(I apologise for so many questions, but for most of this, there's just lots of conflicting information out there, it seems)

1. Powering an opamp: do I have to have two separate power sources, one for VCC+ and another for VCC-? If not, do I have to include a voltage divider and create a virtual ground (as shown: https://youtu.be/MtccB9K09ck)? Is this required for an op amp to operate or can I simply connect VCC+ to positive on 9V battery and connect VCC- to the negative side of the same battery?
2. When, where and what size resistors and caps: I get the need for resistors (including the pots) to control the feedback to the opamp and the gain and I roughly understand caps are used to smooth out or clean up the overall signal, but of the tutorials I've seen, it seems every one of them has resistors, caps and pots all over the place in different locations of the circuits, some including random new components and locations (random to me, but I'm sure they have functions) or different locations of common items (like different locations for the pots for instance). Are these differences just simply design preferences/individual attentions to details or quality? What components/locations are absolutely necessary for this type of circuit to work (ie bare bones, to heck with quality etc)?
3. Inverting vs. non-inverting: Will either configuration work for my purposes here? Ie for a volume controller amplifying faint sounds? Is one better suited than the other?
4. Powering the mics: I assume the positive wire from each microphone goes to the Sin for each opamp and the negative wire connects to negative on the 9V battery. Is this correct?
5. Powering the earbuds/headphones: I have the output of one opamp going to one side of headphones (ie connected to red wire) and the output of the other opamp going to the other side of the headphones (connected to black wire). Is this correct? No other connections needed?
6. Testing salvaged caps of higher values: I have a RadioShack multimeter (cat no. 22-812) to test my caps. It has capacitance testing and has Auto ranging or manual ranging. Set to "Auto", all my smaller caps (10mF or lower) test fine, but anything bigger (100mF or above) always shows "OL" no matter how long I wait. I understand OL could mean cap is bad or is out of range. I've never had problems testing anything else with this tester and I would hope "Auto" range wouldn't have any problems with bigger capacitors. How long should I wait for the tester to charge one of the larger capacitors? I'm including this question because I'm wondering if bad caps might be messing up this circuit.

I apologise again for such a long question. I try to normally find out this type of info on my own and let others pester you guys for answers. But I'm getting tired of being stumped and I really do want to understand this stuff. I recognize the potential and fun to be had with this type of work. I also want to save more kittens.

UPDATE: (response + more questions for analogsystemsrf)

Here is my attempt based on your instructions:

I'm using IC JRC 4580DD. It's a dual op amp +-2V - +-16V, input bias current is 100nA typ, 500nA max.

Questions:

• You mention 2 op amps. Is this one for each channel? You mentioned them at first but only referenced one op amp in the instructions.
• I'm confused on the MIC_OUTPUT (junction between mic and 10k) going to between the two 220k but also going directly to the full 9V through the 10k (at top of pic). Is this correct?
• I wasn't sure if there is supposed to be a 10k res with the 0.1uF between Aout and ground.
• Regarding speaker hookup: if what you posted is for one channel, does Aout go to one side (left channel, say red wire) and then Bout go to the other (right channel, black wire). How does ground fit into this?

I greatly appreciate the help. I hope my rudimentary artwork is clear enough.

• maybe useful info ... google shotgun microphone Jul 10, 2020 at 5:46
• Start with a written description of a functioning circuit with diagrams rather than trying to piece things together from vague videos.
– JRE
Jul 10, 2020 at 11:17
• "Learning from Youtube" is a waste of time. The time you spent watching a video and trying figure out if the video maker has even the slightest idea how things work would be better spent skimming written articles, and looking quickly to see if there's a circuit diagram, a description of its function, a list of parts, and a troubleshooting section. If anything on that list is missing, don't waste time on it - move on to the next description. Keep looking until you find a good, complete description and only then try to work from it.
– JRE
Jul 10, 2020 at 11:23
• Eh. Nope. Not even close. Two glaring problems: You have a 100uF capacitor in series with your battery. Capacitors don't pass DC, so your circuit gets no power because of that capacitor. You have a 1M resistor between Aout and your earphone. If your circuit were getting power, none of it would make it to the earphone through that huge honkin' resistor.
– JRE
Aug 13, 2020 at 5:45
• Did you take a look at the project I linked to? It has a proper circuit, and it explains what each part of the circuit does.
– JRE
Aug 13, 2020 at 5:46

Let us design a channel.

We'll use 2 opamps, operating on 0/+9v.

• We'll make the first decision, to use only 1 battery. This simplifies our work. Pick ONE of your favorite 9V battery. We will generate VDD/2 as needed, perhaps for each opamp. Thus the opamps will have 9 volts to work with. And this simplifies the Power Switching --- only ONE switch, a SPST switch, is needed. [ With +-9, you'd have needed DPST switch.]

• Some opamps will not work well with only 9v.

• Whatever opamp you pick, pick a CMOS opamp, so the input bias current becomes trivially small. The other choice --- bipolar --- requires a significant input current for the first couple transistors to operate

• Have a thick central Ground wire, not so thick you can't solder to it, but not some skinny #32 gauge wire. Just something we can hang 10 components on, without risk of breaking it. You can make it shaped in a "U" or even in a complete loop; we ain't gonna worry about GroundLoops here.

• The electret microphone has pins/wires. One to Ground, and one to +9v THRU a 10Kohm resistor (that is brown-black-orange resistor). At the junction of the mic and the 10Kohm, that is our mic_output. The resistor is needed to provide power to the tiny amplifier/transistor inside the electret mike. Without that resistor/+9v, you'll get nothing. The resistor provides the LOADLINE for the transistor. We use 10Kohm, because that provides some current (as much as 9v/10,000 = 900 microAmps or depending on the transistor --- maybe a FET --- down at 500uA or 200uA; just depends on the transistor; and even 900uA will not rapidly drain your battery)

• Next you need a DC_blocking capacitor between the electret_mike MIC_OUTPUT and the input to your opamp gain stage(s). Without this cap, the opamps will be thrown all the way to one side (ground or +9v) and you won't hear any kittens. How big a cap? this is sort of important, because big caps (big in nanoFarads or microFarads) determine the bass content of what you hear. For 160Hertz rolloff, you need 2 * PI faster radians/second as the time_constant, which is exactly 1 millisecond. Since 1UF and 1Kohm gives 1 millisecond, we know have a landmark on picking that cap. Perhaps better is 0.1uF (ceramic is OK here) and 10Kohm, but we don't know yet.

• ohhh about that volume control --- You could use a 20K ohm or 50K ohm pot (3 pins, the center? being the wiper. Or even 100K ohm. Just install the pot between the Ground wire and the VOUT of the electret_mike. Attach the wiper (ensure this is the wiper, that is varies, using your DVM) to one end of that DC_blocking capacitor that connects to the opamp.

• Now for the first (and final?) opamp gain stage. We will use non-inverting circuit (why not). We need TWO RC circuits for this gain stage.

• opamp input RC: two resistors, in series between Ground and +9v. The midpoint ties to opamp Vin+ pin (+ polarity input). Use 220K ohm (red-red-yellow). This biases the opamp to work at half the battery voltage, allowing several volts of output swing for the opamp, and plenty drive for earphones. And that capacitor from volume_control to gain? that cap connects to the midpoint, along with opamp Vin+ pin.

• opamp feedback RC: two resistors and 1 capacitor. From opamp output pin to Vin-, install 1MegOhm resistor (brown, black, green). From opamp Vin- pin to GROUND wire, install a series RC --- 10K ohm and 0.1uF. Order of R and C does not matter.

• now YOU need to figure out how to install the headphones. And a DC_blocking capacitor may be needed.

• Bypass Capacitor between 0/+9 to provide extra surge currents at high frequencies (those treble tones). Use 100uF. Wire this as Battery, switch, bypass_Cap+rest_of_circuitry.

• wow. thank you for such an in-depth answer. I'll work on it and reply if/when I get stumped again. Thanks! Jul 11, 2020 at 23:05
• I've added an update with additional questions if you feel inclined :) Aug 13, 2020 at 2:08
• A diagram would certainly flesh out this answer Aug 13, 2020 at 14:46