What is this microphone (OP-AMP) amplifier circuit doing?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 15, 2021, 11:41:12 pm
content: Attached is a schematic that I drew up (hopefully error free?) of a 1/4inch microphone amplifier PCB that I found in my spare parts bin, possibly from an old reciever.What exactly is happening near the signal input, with the 100k resistor, paralleled 50v electrolytic, and then a ceramic + blue 56k resistor, with a connection to ground? I simply don't understand what it can be doing, and I tried researching it.Secondly, near the output on the schematic, there is two potentiometers (on the PCB it's actually 1 potentiometer "module?", but with 6 legs (looking very similar to [this image](https://ibb.co/mqtGQHQ)). What exactly is this doing? I assume it's something to do with logarithm, but am not sure.Thanks

author: fourfathom
author_rank: Super Contributor
timestamp: April 16, 2021, 01:07:53 am
content: When a schematic doesn't make sense, try redrawing it. Like this:

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 02:52:23 am
content: It doesn't make sense to me because I have no idea what the circuit is doing, not because of the way it's drawn...

author: Dubbie
author_rank: Unknown
timestamp: April 16, 2021, 04:02:51 am
content: FourFathoms point is that sometimes redrawing can be very helpful, as your brain can recognize patterns a lot easier when they are presented in a familiar form. There are conventions to drawing schematics, and consciously or not, you are used to seeing things drawn a certain way.

author: magic
author_rank: Super Contributor
timestamp: April 16, 2021, 06:45:44 am
content: Not sure what purpose C1 would serve as drawn

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 16, 2021, 07:14:01 am
content: Quote from: LooseJunkHater on April 15, 2021, 11:41:12 pmmaybe it's a microphone amplifier100k resistor on input offers DC path to you microphonecapacitor 2.2uF transits the signal from mic to amp (it's like a 'short' regarding AC signal)the capacitor parallel with 56k resistor cuts off high frequencies (parasitic ones), it's a low-pass filteron amp output you got 2 'identical' outputs to simulate maybe stereophony from a single microphonethe potentiometer must be BALANCE, to equilibrate L/R channelsregards, pierre

author: fourfathom
author_rank: Super Contributor
timestamp: April 16, 2021, 07:30:09 am
content: Quote from: magic on April 16, 2021, 06:45:44 amJust another way to attenuate high-frequency noise. It's got to be a small capacitor, and it forces high-frequency signals to be common-mode. Don't recall seeing that configuration before...

author: gcewing
author_rank: Regular Contributor
timestamp: April 16, 2021, 07:44:34 am
content: Here's my take on it:R4 and R1 form a voltage divider, but with a ratio of 0.99 it won't have much effect on the input. Maybe R1 is just there so that the left end of C3 isn't left floating at DC.C3 and R2 form a high-pass filter, with a cutoff frequency of about 0.05Hz if I've done the maths right.The op-amp is configured as a non-inverting amplifier with a gain determined by R3, C2 and R5. At DC, C2 is an open circuit, so it functions as a voltage follower with unity gain. At higher frequencies, the gain rises. At 50Hz it will be about 100, and it will approach a maximum of about 200 somewhere above 500Hz. This curve is probably chosen to compensate for the characteristics of the microphone.You haven't shown values for C1 and C4; I'm assuming they're something small. At very high frequencies, probably well above audio, C4 will shunt the input to ground and C1 will short the inputs of the op-amp together, reducing its gain to zero. This will be to prevent the circuit from becoming unstable and oscillating at high frequency.I'm guessing the ganged pot is a panning control, so you can set where the mic sound appears to come from between the left and right channels. If that's the case, it will be wired so that the two pots move in opposite ways, i.e. when one wiper moves towards the op-amp output, the other moves away from it.

author: nali
author_rank: Frequent Contributor
timestamp: April 16, 2021, 08:07:24 am
content: I'd double-check the placement of C1, it'd make more sense if it was across R3 to provide some HF rolloff

author: magic
author_rank: Super Contributor
timestamp: April 16, 2021, 03:30:03 pm
content: Quote from: fourfathom on April 16, 2021, 07:30:09 amBecause it's a little silly.I guess you're right that it kills signal gain above the C1·R5 corner, but opamp noise gain skyrockets in its place...Maybe C1 was meant in parallel with R3.

author: fcb
author_rank: Super Contributor
timestamp: April 16, 2021, 04:21:02 pm
content: Quote from: magic on April 16, 2021, 03:30:03 pmLooks pretty normal, seen this sort of thing in other mic amps.R4 reduces peak current that can be dumped into U1 - sort of ESD protection, also acts with C4 to form an LPF.R1 keeps C3 from floating, if C3 was to float to high and you then plug in a dynamic mic, you could easily dump C3 into it a futz it.C3 is ac coupling capacitor, it also acts with R2 to form an HPF, although the as someone else pointed out, the roll-off is at <0.1Hz - so un-useful for audio.C4 forms an LPF with R4, but probably main purpose is to reduce susceptability to RF.R2 ties U1+ to 0V and is part of C3 filter.C1 rolls off HF gain with R3 - probably chosen experimentally by the original designer when he needed to substitute his fingertip for production.C2 reduces gain to unity for DC - commonly done to avoid multiplying input DC offset of opamp, forms main HPF.R3/R5 determine gain in band of interest.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 06:49:40 pm
content: Attached is an image of the drawn PCB layout.There are unlabelled capacitors, and that's because I am unsure about their values.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 07:01:06 pm
content: Attached is 2x images of the actual PCB. The pot + 1/4inch mic input are desoldered.The 6-pin desoldered device is the pot, while the 9-pin desoldered device is the 1/4inch mic input.

author: tooki
author_rank: Super Contributor
timestamp: April 16, 2021, 08:42:46 pm
content: Quote from: LooseJunkHater on April 16, 2021, 06:49:40 pmCould you please just draw normal schematics using standard symbols in normal electronics schematic software? (Try EasyEDA if you don’t know where to begin.) While these you’re doing have a certain aesthetic appeal, they’re really hard to follow, and could really use rearranging to make them clearer. (For instance, the group of resistors and cap that swings down near the potentiometers for no reason at all.) Edit: OK, I see what you did there, basically tracing the actual PCB traces. Not at all obvious because the drawing is rotated 90˚ vs. the photos.But you’ve omitted practically all values, even ones you knew in the original schematic in the first post. As for the caps you are unsure of, write their markings, we can likely figure it out.I have no clue what your 1/4" jack is doing. I assume you mean it's got some switching depending on whether it's plugged in, but your, um, creative drawing doesn't make any sense.

author: tooki
author_rank: Super Contributor
timestamp: April 16, 2021, 09:49:36 pm
content: Here's a quick, normal schematic of what you posted as the PCB layout drawing, just with no attempt at the jack switching.

author: tooki
author_rank: Super Contributor
timestamp: April 16, 2021, 09:52:02 pm
content: Quote from: LooseJunkHater on April 15, 2021, 11:41:12 pmFYI, that's called a "dual-gang potentiometer", or more colloquially, a "stereo potentiometer".

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 10:45:00 pm
content: Quote from: tooki on April 16, 2021, 08:42:46 pmSorry yeah I'm dumb, accidentally posted photos that were at the wrong angle...Regarding the 1/4" jack, yeah I didn't know how to word it. You're correct in saying it has internal switching depending on whether it's plugged in, hence the two PCB board images. I showed where things connect when a mic is plugged in, and when one isn't.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 10:46:51 pm
content: Quote from: tooki on April 16, 2021, 09:49:36 pmThat image is beautiful. I really need to learn how to use EasyEDA.Also I'm super nooby at drawing schematics, as may be obvious. The images post in this thread are only like my third attempt at ever reverse engineering and drawing PCB traces + schematics, so I'm learning

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 10:47:37 pm
content: Quote from: tooki on April 16, 2021, 09:52:02 pmAha! That's what it's called! I didn't know that, thank you!

author: tooki
author_rank: Super Contributor
timestamp: April 18, 2021, 02:43:37 pm
content: Quote from: LooseJunkHater on April 16, 2021, 10:45:00 pmBut the one when not plugged in is not clear at all.Quote from: LooseJunkHater on April 16, 2021, 10:46:51 pmSorry, I was not trying to be overly harsh! Sorry if I was.Regardless of whether you draw schematics in EDA software, general graphics software, or with pen and paper, the most important thing IMHO is to stick to conventions: use standard symbols, standard orientations (e.g. V+ at the top, ground in middle, V- at bottom; signal flows generally from left to right; etc), avoid 4-way junctions, and so on. This makes it much, much easier for the reader to know your intent.(Out of curiosity, what software did you do your drawings in?)

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 18, 2021, 05:55:55 pm
content: Quote from: tooki on April 18, 2021, 02:43:37 pmI usehttps://draw.io. It's a pretty versatile website (and has a FOSS Windows Application, if you don't want to use the web) which allows you to make diagrams of all sorts of things, not just electrical engineering relatedAlso, I didn't think you were being overly harsh!