DIY Micrphone

Woah! Made the beautiful build even more beautiful. Fantastic work. I really must pull my finger out and get the new pre-amps out to the PCB house.

@marcdraco Make sure the pre amps can fit in this enclosure. Lol

Stunning reconstruction! That's got to be the best by-the-letter project build I've seen. Well done! 😀 Mind if I Tweet it?

@diyperks Thank you Matt, that's really good to hear! Go right on ahead. Appreciated!

@midnight_vanisher That was a major consideration. The origmain board is within a mm or so of Matt's vero layout but I can get a lot more in the same space using surface mounted parts, including high/low pass filters, a headphone amp and a new volume control which is less twitchty that the THAT, and more power filtering.

I'm just re-doing the head unit with a three-wire design to retain compatibility with Matt's head unit. I've done three versions now using different FETs and lower noise design. On paper that is. I should have the first batches of the rev3 boards back in a couple of weeks.

The current batch have an extra power in but I figured this batch would be better using a sort of "phantom" power but that required a couple of unusual tricks and a different output stage - whic h is this. It only sends a single phase but the impedance is balanced so we still get excellent noise rejection.

This reduces the sensitivity by 6dB but these capsules are crazy sensitive anyway.

@marcdraco Excited to hear of this V2. Curious how it'll all look after making this board especially hearing all this spaceman language.

The main thread has some of my ideas rendered in 3D. I've actually done a small batch of "Cerberus" heads which was my original design using  pair of discrete FETs (LSK170). Performance is excellent - I've just received a THAT1512 over the weekend so I might be able to rig a preamp to compare to Matt's original design.

It's broadly similar to what Matt did (well, it is NOW) to retain backward compatibilty but I've had a brainfart over the weekend and decided I was stuck in a rut over the head. So I've designed no less than three completely new ones, all using different tricks and technologies. The simplest one (and the one least likely to work as is) is a simplified version of the OPA-Alice designed to operate at split 15V supplies.

The other two are both discrete FET designs using eitheir LSK389s (dual monolithic, super-low noise JFET)  or a full-realised SMD version based on an original layout I did for the discrete LSK170s.

These are all experimental at the present time but I'll drop the full board designs when I know they work. I've published a lot of my earlier ideas and while most work well enough, none have really matched what I managed with the separate supply.

I'll also write the whole thing up so that every last volt and milliamp is explained so everyone call follow along - without all the blasted maths. I love electronics, but I hate maths (I'm rubbish at it) so I'm using building blocks that are reliable and easy to describe.

There are several more ways of doing this but these are sufficiently different as to give us all a better idea of what works well and (more important) what doesn't. It's been quite the journey over the last few months - sitting on my hands waiting for the prototypes to get back from China and then working out if I could improve anything (Ii could, I did).

Right now we're up agains the limit of what's possible with an electret such as the JLI2555 capsule. Even Matt's original far outperforms what that capsule is capable of in terms of noise floor. Getting beyond this (and again, I already have in the lab) means an entirely different design using a 48V supply.

Not difficut but not what Matt imagined for a desk mic. I'm thinking something more like the slightly weird look of the Russian-made Sputnik mics - with Matt's head mounted atop a tube which contains the rest of the main electronics.

This should be right up your street since you have a lathe of course. I'm limited to a 3D printer and I can't afford to run it given the price of power in Europe now... Besides, there's the screening issue too.

But here's a rendering of the final (prototype) preamp with the improved power, headphone amp and all the trimmings that you can't do with through-hole parts.

Here's a close up of a batch of the heads. The problem with this design is two-fold. Firstly, I'd use a pair of diodes in place of a 1G load resistor (which, it transpires isn't needed for an electret!) and also the pad spacing for the LSK170s is so tight, they are very hard to solder. The diodes can be removed with a soldering iron making the board noticably quieter and I have a few of these left available at cost which is about £3 a piece plus shipping which is large letter in the UK/Europe if anyone really wants to get the jump. But be aware that this design *requires* external power which Matt's board doesn't supply. This was my mistake since I built these "blind" without reference to Matt's original design.

My bad, it was Soyuz, not Sputnik - (nice segue from your spaceman comment). 😉

Regardless my feelings for current troubles, there's no doubt these designs are at worst unusual and at best, artistically beautiful. The large metal tube contains all the electronics and the capsule (it's dual diagphram I believe in this one) is mounted in the head. I believe it's a 34mm capsule too so that will give you an idea of scale. There are some videos on YouTube that show the ladies and gents making these and it's really quite awe-inspiring. As an electronic-dude, it was weird to see people still using "wire wrapping" in the 21st century but I can see the reasoning.

@marcdraco Reminds me of a the Neat Microphone, King Bee 2. Very affective design for the capsules and the casing.

@marcdraco That's very pleasing to hear when the results come out even more promising. People are going to be stoked! It sounds like its capable of doing the trick. The last one almost worked in certain variations, but its all getting it down to a T. We might have to build this next. Lol  😉

I  finally worked out how to (unreliably) join several boards in KiCAD. I'm unsure if this is a bug in the 3D side or the plotter, but as is, I can get four different ideas onto a single board that can be chopped up with a hacksaw. 

Only one of these will be for the Perks microphone (the best one) but the others are experimental. I'll have to wait on Matt to let me know which way he wants to go. The one with the chip socket ("Kryten") is designed to be upgradeable as better ICs become available. It requires just a dual BiFET or MOS op amp (TI have an extensive range) to work. 

Power is always the issue with these things given the limited amount of current that phantom systems are designed to deliver. P48 delivers about 11-15 mA at 48V for example. My version of Matt's pre-amp should give about 15mA at +/- 10V. It's hard to pin it down as the voltage drops off significantly as the circuit draws more power, but most modern ICs of this type have a fairly low consumption.

The Rev. 3 "Cerberus" (bottom left) has been modified to deliver a more balanced output but still with paralleled LSK170s for greatly reduced noise. This one also delivers a small amount of pre-amplification so it's the most sensitive of the bunch.

The "Hannah" (top left) is the real beauty though, delivering noise performance on a par with the best of the bunch or better. This version uses the LSK389 dual monolithic FET but doesn't require the extra circuitry the discrete FETs (Cerberus) do.

The one I'm really playing with is top right, codename "Hydra". That has an array of four low-noise FETs again in a paralleled arrangement. The main advantage of that one though is there's nothing to solder in close quarters. 

They all fit right on the back of the JLI capsule and clamp/solder to the ground pins for excellent screening although a Faraday cage is still required since the circuitry is slightly exposed.

I have (experimentally) designed one that should self-screen but I don't see that one as having any real world use and the performance is only "m'eh" since it's only able to use a single FET due to space restrictions.

In the main thread I've posted the final (re) designs of the above. I literally had them uploaded to JLC PCB and about to go into production when I realise there was a fatal error in the design. (My fault for relying on memory rather than double-checking in SPICE).

Well, I say "fatal" it wasn't fatal but it was sloppy on my part, so I've spent the last couple of days cleaning up that mess and added some completely new designs to the quad, while simplifying Hannah a little. The one with the Op Amp allows for a fairly large swathe of off-the-peg dual amps to be used. They have to be FET input and low power (i.e. about 5 mA quiescent per amp, or 10 mA for the whole package) and capable of running at up to +/-15V, although that drops off sharply as the circuit demands more current.

The other two new designs use 2SK208s which are ostensibly cheaper versions of the LSk170s only they are available from my board shop and can be fitted by them which saves everyone the hassle of that.

I've got them on a rush order since I've been on this project for several months and people probably think I'll never do anything, but that's the thing with development like this, the turnaround time is measured in many weeks so I'm often sitting on my hands or tinkering while I wait to see what works and what doesn't.

Then it's back to the drawing board, calculator and SPICE simulator. 🙂

They've gotten dramatically better as I've been able to test out different designs but waiting for the board shop is tedious.

Assuming Matt's OK with this, I'll open source the final designs (complete with silkscreen logos) so everyone can buy their own, pre-made at JLC or a board shop of their own choice. On paper at least, these are the most performant I've made. A lot depends on the +/-15V power which is something I won't claim to have much experience of. It's a bit like having P48 but with a lot less room to move. The only advantage (and bizarrely, I missed this one which was right under my nose) is that you have a negative supply which reduces component count since the main circuit can be DC coupled.