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[Sticky] USB-C Microphone (official topic)

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kirby
(@kirby)
Posts: 8
Active Member
 

Flux got some weird properties 

working on the build first then the circuitry 

IMG 1549

tried multiple times to stick these damn hooks on

Matt makes solder look like a magic alloy. My brass shell seems to be solder-phobic I've tried wicking and heating both parts first but they just won't be hot enough to melt the solder.

bought some cheap helping hands gonna try again later

 
Posted : 07/06/2024 2:25 am
marcdraco
(@marcdraco)
Posts: 476
Member
 

A lot of modern solder is lead free and that stuff just doesn't flow like the old (and highly toxic) stuff. That's likely the problem. You'll need to source some SnPb stuff (60:40) as I recall. It's still available but you have to ask for it. I've found I need to raise the temperature of the new stuff which is tin (Sn) plus some other things like copper to 400 C on my iron to get the stuff to flow. It's also extremely brittle.

I should also point out that lead-based solder is highly toxic (due to the lead content). People from my era have this hanging over our heads but there is no reason anyone else should, so you must use a fume extractor when working with the stuff for your own safety and the safety of those around you.

This post was modified 7 days ago by marcdraco


Take everything I say with a pinch of salt, I might be wrong!

 
Posted : 07/06/2024 9:58 am
(@yrambler2001)
Posts: 1
New Member
 

Hello everyone, I have ordered THAT1512 from two different sellers in Aliexpress, both are the same and fakes. With them, I have the same issue and the same measurements as in this post https://forum.diyperks.com/postid/4484/

Spoiler
Orders
Screenshot 2024 06 10 at 17.24.50
Spoiler
Fakes
2024 06 10 17.18.15
2024 06 10 17.18.18
2024 06 10 17.26.23

So I have ordered a genuine one from Farnell, which looks and measures differently.

FYI:

Spoiler
Genuine
2024 06 10 17.20.01
2024 06 10 17.19.59

Comparison:

Spoiler
First comparison. Diode test mode in multimeter

Black probe on 6 pin, red probe on 3 pin

Genuine:

2024 06 10 17.19.58

Fake:

2024 06 10 17.20.06
Spoiler
Second comparison. Diode test mode in multimeter

Black probe on 8 pin, red probe on 4 pin

 Genuine:

2024 06 10 17.19.56

Fake:

2024 06 10 17.20.03
 
Posted : 10/06/2024 3:31 pm
marcdraco reacted
marcdraco
(@marcdraco)
Posts: 476
Member
 

Hate it when that happens, I really need to make an "Octopus" but that will take a little longer. The usual way is to do this with a traditional oscilloscope, the digital ones don't seem to work as well due their improved performance but what we need (those of us who do this) is a reliable and portable one. 

The difference is that an Octopus puts a varying signal through the component under test and gives very specific curves (they are also called "curve testers" if you buy a proper one) but they make this sort of debugging a breeze. 


Take everything I say with a pinch of salt, I might be wrong!

 
Posted : 10/06/2024 5:02 pm
(@f4lc0nsv)
Posts: 2
New Member
 

So, guys, I have a very serious question for you.
How can all of this be converted to a 3.5mm jack connection?
I'll say right away that I don't understand any of it; at most, I know how to desolder a capacitor and solder a new one in its place.
P.S. I have a Creative AE-5 sound card, which is why I want to make it with a jack.

 
Posted : 11/06/2024 7:07 am
marcdraco
(@marcdraco)
Posts: 476
Member
 

Posted by: @f4lc0nsv

So, guys, I have a very serious question for you.
How can all of this be converted to a 3.5mm jack connection?
I'll say right away that I don't understand any of it; at most, I know how to desolder a capacitor and solder a new one in its place.
P.S. I have a Creative AE-5 sound card, which is why I want to make it with a jack.

Welcome to our little corner.

Don't worry about your skills, everyone learns over time. My own contributions contain errors (which I'll leave here with the corrections) as I fine tune the prototypes from utter failure to pretty good. 🙂 There's a cracking example of one of my foul ups at the end of this post.

OK, so if you want to take the existing circuit and connect it to your sound card you can solder a mono (or stereo) 3.5 mm jack directly to the output from the THAT1512 (that is after the 22uF output capacitor). I've done some PCBs, current ones just on the way back, that have an auxiliary port for this precise purpose.

The key point though is this goes to the Line In port, not the "Mic in" because it's already been pre-amplified.

On Matt's original stripboard layout you just connect it to Line at the point where Matt attached the digitiser. 

Technically, you don't need the stripboard pre-amp as the AE-5 has its own pre-amp and digitiser. The down side is that distortion is a little higher than with these designs.

A typical (TRS = Tip, Ring, Sleeve) mic socket (according to the standard) should provide 5V bias and a mic input. That's it.

Tip: Signal In

Ring: Bias voltage

Sleeve: Ground

With Matt's original head inside the blue line, you wire everything as shown (with the JFET) but then wire it like this:

 

image

It doesn't get any simpler and while this is "cheating" it's the way all normal headsets are wired. Typical electret capsules have an internal JFET so it's simply a matter of adding these.

So why the extra complexity?

It's about signal quality. The "differential" design Matt uses is far better at keeping out strain mains hum; and that's not all but beyond that you need some knowledge of JFET amplifiers and I don't want to overload anyone with information.

This circuit works like this:

Current flows from the "bias" pin through a the 2k2 resistor (the value isn't critical and affects the gain but the mic input will be expecting around that sort of value - values around 1K to 5K works but is actually dependent on FETs which are by nature, not that predictable). 

Put (very) simply what you have here is a voltage divider formed from the resistor and the JFET. When the microphone is in operation the tiny movements cause a variable voltage to appear at the gate terminal which, in effect, changes the resistance in the channel.

Thanks to online simulation you can see this in action using a typical microphone JFET.

https://tinyurl.com/26dy3l66

But JFETs are weird, here's something like the classic 2N3819

https://tinyurl.com/285jfs76

See how with the same values, we can go from a fairly chunky signal with the LSK170 to just 1/10th of that with a 2N3819. Horses for courses.

We can improve the distortion quite dramatically by adding some "negative feedback" by limiting the current at the "source" terminal (the lower one in the drawing). 

https://tinyurl.com/28vbft2o

This reduces output (sensitivity) by about 50% but reduces distortion (THD) from 2% in the classic wiring to around 0.2%. Not bad for a single resistor! On the diagram above you just put the second resistor where FET2 is connected to ground.

I should note for completeness this method is used in Matt's version and in the classic Schoeps design. My own designs go further but that's beyond the scope of this discussion and I'll leave them until I've verified that everything works as it should.

I recently found an "game-over" error that I've carried through multiple prototypes that has driven me utterly round the bend, all for the sake of a missing decimal point! A case of not being able to see the wood for the trees has cost me ... a lot. 🙂 But sometimes the best lessons are the hard ones. There's a gotcha 3% THD error on the original too but I'm not going to discuss that until I've produced and tested the full V2 designs, so don't tell anyone. 

I blame it on my old brain (and the simple bone-idleness of not running the whole thing in the TINA-TI simulator. (I use several but I prefer one called LTSpice as it's much faster to draft and test a circuit: but LTSpice lacked a critical component so (to my shame) I bodged it by just putting a voltage source in and assumed... and you know what assume does? Made an ass out of me anyway. 

As a result I'll be eating out of the dog's bowl for the rest of the year. Sigh. The dog is not impressed.

Fire away if anything is unclear.

 

 


Take everything I say with a pinch of salt, I might be wrong!

 
Posted : 11/06/2024 10:29 am
(@f4lc0nsv)
Posts: 2
New Member
 

@marcdraco Thank you for that)
I will continue to try to do something. But first need to wait for all the boxes from amazon)

 
Posted : 13/06/2024 12:01 am
marcdraco
(@marcdraco)
Posts: 476
Member
 

Here's a tiny section from the new KiCAD 8.0 branch's Spice simulation.

image

and this one signal, this one little flag could have saved me a fortune in prototypes and a lot of head scratching over the last year.

In fact, this level of simulation has never been available in KiCAD previously but what's the deal? What's half a volt got to do with anything?

If you're wondering it's because that's supposed to be 2v5 (2.48 volts to be more accurate, but hey, it's close enough).

The TL431 is a fairly old chip that replaces Zenner regulators in most application with the addition of just two extra resistors to set a regulation voltage (Zenner equivalent) of anything from 2.5 to 36 volts with far better performance than an actual Zenner diode.

It's also something that I've been unable to simulate in KiCAD until recently, despite KiCAD having Spice integration for several years now. I've always used LTSPice to check my working but like KiCAD it lacked a TL431 model so I (rather foolishly 😧) used a workaround. And assumed that the calculations I'd done by hand were OK.

Well you know what assume did...

I fact, what I'd done is missed a decimal point out and make an upstream resistance an order of magnitude larger than it was supposed to be - meaning that happened rather than this:

image

The voltage at the 431's cathode is determined by the voltage at the reference pin which should be 2v5 derived from design output voltage passed through the divider. The 431 uses this reference to pull up to 100 mA of current from the supply, causing a voltage drop across a dropper resistor.

So if you have (say) 30 volts in and you need a 10 v supply, you'd arrange for 20 v to be "dropped" across that resistor. The value of that resistor isn't too critical as the device adjusts the current until the correct drop is achieved. If the dropper resistor is too large, then the 431 reaches a limit (it can't generate negative impedance) and the supply voltage drops too. This results in anomalous behaviour as seen in the first image.

I'll include this in the final Varee (capsule adaptor PCB) source so anyone can play with it but note that you'll have to source the TL431 macro yourself as I'm using one from Texas which I can't redistribute. This is an issue across much of KiCAD's simulation but the new version includes a basic Op Amp too so far more circuits can be simulated and tested before they are sent for production.

This is going to save a lot of developers (pros and hacks alike) a lot of money.


Take everything I say with a pinch of salt, I might be wrong!

 
Posted : 13/06/2024 10:38 am
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