On Sat, Dec 23, 2023 at 04:17 PM, Goran Finnberg wrote:

I just wonder how many mics in the USA that are still XLR pin 3 positive and has been that?

way almost forever since the XLR appeared in the USA.

?

Some older broadcast mics, maybe.

Pin 2 hot (positive) is the AES standard for XLR. seems to point at 1990 when there was a consensus forming around pin 2 hot. () I expect that any mic made since around then is pin 2 hot, but we don't have to guess if we have the mic documentation. The venerable has pin 2 positive. The oldest mic I have () is pin 2 hot in its balanced lo-z configuration.

As I mentioned, in my own simple test, the wiring given by Jules at the start of this thread was "in-phase" with one of my store-bought mics that's known to be pin 2 hot. I've not yet looked closely into the capacitance physical equations; my intuition says that when a capacitor with a given charge has its plates moved closer together, the voltage across the cap should increase (greater charge density), which causes the FET to conduct more, which reduces the DS voltage, resulting in a voltage drop at the + terminal of a 2 terminal electret.

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I’m building a few more and will do one of each and test them big time ?

Casey:

Sorry for being dense, but I wanted to seek some clarity on the SP48 polarity question. It sounds like Jules' SP48 mics are of normal conventional polarity with capsule + to pin 3, and all ~10 of mine are also wired that way and show conventional polarity... was there resolution on this question and I missed it? I don't feel comfortable when the bible of SP48 (Ricardo's PDF) disagrees with my circuits. :-)

I've found the design from Silicon Chip magazine august 2011.
I put it in files as: "Electronic Stethoscope from SC 2011-08 Aug.pdf".

Cheers,
Martin-K

--
Hard werken is niet genoeg; dat doen mieren ook.
De vraag is waarvoor je hard werkt.” – Henry David Thoureau

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The "kill" switch does not need to be on teh stethoscope. It is better put in line with the cable or even replaced by a footswich.

regarding handling noise, would a simple, low profile, push button on top of the stethoscope pick up be doable? Once the pick up is placed in the right position, press the button and hear sound.

I'm another one: UK based Emergency Medicine doctor!

I had a go at an electronic stethoscope, mainly to help overcome the considerable background noise in my Emergency Department.

I started with a cheapo ?7 stethoscope from AliExpress. I mounted a tiny omni FET-electret mounted into the stem of the diaphragm/bell housing with a thin wrap of foam padding between it and the metal for handling noise isolation. I used enamelled copper wire threaded up the tubing to a small pre-amp then Class D mini amplifier module to power headphones. No filtering.?Battery powered from 9V. It wasn't really loud enough for my liking but it wouldn't be too hard to add more output stage gain.?

Clinically, I do a lot of bedside ultrasound, and I just tend to have a look at the heart valves and lung fields where it matters. So I've not really iterated further on the design.

Any old cheapie works I suspect. In our hospital wards they buy packs of 10 for the nurses to use, with each stethoscope costing only a couple of bucks. But actually they work pretty well, especially if you're not trying to hear some really quiet heart murmur. Rather like in the pro audio world there's a bit of a law of diminishing returns, except that as a patient you'd probably like to think that your doctor is using the best tools available. I just picked up this on on eBay. I think I might have got the idea of cutting off the tubing from an article in silicon chip magazine... it was commented that the less tubing the better as I recall. So you basically sacrifice most of the tubing and the earpieces. If I find the article in silicon chip I will post a reference but I don't think the actual article is in the public domain.

This seems like a great project for me to experiment with...can any of you for here recommend a low priced stethoscope suitable for non-medical grade play time experimenting?

Sorry dont mean to derail the conversation.

Nice to meet you too, Doctor paralogic! I've never knowingly met a medical practitioner on this forum before but perhaps others have been here. As this is a mic BUILDERS forum I thought I'd better just rustle up a stethoscope! Regarding the PUI5024, it is an amazing capsule due to the amazing JFET inside it that has incredibly low noise compared to, say WM-61A or the fake one that I probably have. But what I like about the "WM-61A" that I have is (a) I have lots of them and (b) it fits in the tubing of an el cheapo single sided stethoscope. I basically soldered sheilded (unbalanced) mic cable to it. Jules is absolutely right, for this application you don't need anything more fancy than plug-in power and a direct connection. To my ears it sounds amazing, but the handling noise is a bit of a pain. I would love to know what a more expensive (Littmann or MDF etc) stethoscope would sound like. I guess with the diameter of the tubing in a Littmann, perhaps a PUI5024 would fit. Another solution, I imagine, if the mic didn't fit would be to use hot glue in some way... The trickiness here is soldering the capsule because (1) they ARE so tiny and they don't like the soldering iron to be on there too long (hold the mic in a vice), (2) getting the mic to go into the tubing, I just managed it with my fingers in the end and (3) covering and "stiffening" the soldered connection, I used very thick heatshrink tubing and a heat gun. But it works like a charm. Oh and make sure you test it before you heatshrink it!

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Is eliminating noise with electrets really as simple as adding another electret in the other direction?

No. This is a fantasy. The acoustic load of each capsule is different and there are phase issues.

As for handling noise, it might be worth trying to use a "dummy" capsule wired in parallel but facing the opposite direction with the sound blocked.

Dr. Paralogic, I think Dr. Dent has your answer.

You don't even need an Unobtainium supa WM61A capsule as most of the 6mm capsules are good to at least 2kHz.??? as recommended by Henry Spragens is probably a good start.

You can wire it for Plug-in-Power to feed most recorders with 3.5mm mike i/p or simpleP48 for P48 XLRs on the better recorders

This is a very interesting thread for me, a micbuilder and a physician (geriatrician). I have tried building a couple of electronic stethoscopes over the years. In the first attempt, I built a kit from Silicon Chip magazine, August 2011. It used a piezo transducer as the "mic", being amplified and low pass filtered? by a TL074 quad op amp. I found this to work but there was too much handling noise and room noise. The circuit contained some interesting filtering, including a "gyrator" which the EE gurus here would understand better than me.? But I think the slope of the filter curve was probably not steep enough. It seems to me that considering the apparent simplicity of an acoustic stethoscope, it does an amazing job as a low pass filter and whatever else it does to get the sound waves directly to your ears. Then years later I read a website in which a covid ward doctor was wanting to transmit the sounds of lungs from one (isolation) room to another, and came up with squeezing an (internal FET) electret capsule into the tubing of a stethoscope resulting in a sort of electronic stethoscope transducer. Then they very cleverly created an Android app that did digital signal processing to create a very steep low pass filter, which you could hear the output of on your headset to the phone. However perhaps due to limitations of sampling rate etc it was quite noisy. Maybe with a really amazing filter (digital or analog) it could have been better.? But one thing I have observed here: whilst I love the sound of my littmann cardiology III stethoscope, I did find that a cheapie $10 stethoscope chest-piece, with the tubing cut off a few cm from the bell/diaphragm assembly (the bit that sits on the chest), with a (omni electret) WM-61A? type of electret squeezed into the tubing, then wired to a plug-in-power type of input, it sounds remarkably good. I think the stethoscope diaphragm itself acts as a very effective low pass filter, without ridiculous amounts of handling noise. No need to cut into your expensive littmann stethoscope in my opinion!

By the way, I think that whilst an electronic stethoscope is hard to make better than an acoustic one, it may be useful for telemedicine applications, teaching medical students and good for amplifying heart sounds for doctors who are hard of hearing.

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