MickMad

Ultra Pimped Alice - my take on a classic DIY design
« on: February 24, 2020, 03:20:16 PM »
Hey there,

I'm new to the group and I would like to contribute something I've been working on and off in the last couple of weeks.

I randomly came across the BM-700/800 cheap chinese SDC mods using the Alice circuit and I really liked the idea as well as the result, getting a microphone worth around 400€ for less than 100€ is no joke; but I didn't like that altough most mods described how to change the high pass frequency (to tame the proximity effect) and some even implemented a low pass (to tame some shrill in the high frequencies), or how to add a pad, none of them made these 3 fundamental features easily accessible; some designs allow switches to be mounted inside the mic body, but that requires to physically open the mic everytime to use them, or to salvage a donor body that already had switches inside of it, which cannot be easily had for the super competitive price of the BM-700 / BM-800 / NW-700 / NW-800 chinese mic.

So, I took the best designs I found online (my main source was Homero Leal's Pimped Alice one, you can find it at https://hlpimpedalice.wordpress.com/ ) and decided to implement these three functions using three switches that can sligthly poke out of the donor body; it only requires to drill three holes on the body. I made two boards, one with all the components and a second one that hosts the three slide switches; the main board has 2 slots and soldering pads that end on the edge of the slots, and the slave board has some "fingers" that can be soldered to the pads on the main board. It's like a NES cartridge where there's plated contacts that stick into a connector, only this time there's no connector, you solder them.

You can find a rendering attached to this post.

I just ordered all the parts needed so I can't vouch for the actual functionality of the mic just yet, but I triple checked all the dimensions, the schematic (using Homero's one as well as the original Schoeps CMC5 one), the values of the components (with LTSpice), quality and footprints (the footprint where chosen after the component was picked on Mouser). I made sure to add an insulated standoff for the high impedance point of the JFET, too.

I'll come back on this post whenever I receive everything, I'll assemble three mics (two for me, one for a friend) and will test them all.

Cheers,

Mick

Edit: I would like to thank Homero Leal for giving me some pretty useful information about this design, he was really helpful :)
« Last Edit: February 24, 2020, 06:26:43 PM by MickMad »


Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #1 on: February 24, 2020, 07:09:32 PM »
Why not do something like Khron did and have it be multipattern? Since Chinese capsules are the same price single and dual diaphragm, it won't cost much more. Of course then you have to build the DC converter (or go with Cardioid/Omni  without fig8), I think Khron has the schematics on his blog.

If you're going with a K67 capsule or a Chinese "CK12" capsule, why not just make the high pass permanent like Neumann did? Unless you're using the better Chinese capsules (797, 3U Audio, apparently Maiku) the high end might be way too bright if not attenuated (or use a K47 capsule or 3U's K67 capsule variant for a flat circuit).

You could always cut out switch holes and make a switch PCB for the mic. That's what Khron did in his.
« Last Edit: February 25, 2020, 12:24:10 AM by Icantthinkofaname »

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #2 on: February 25, 2020, 02:08:11 AM »
Why not do something like Khron did and have it be multipattern? Since Chinese capsules are the same price single and dual diaphragm, it won't cost much more. Of course then you have to build the DC converter (or go with Cardioid/Omni  without fig8), I think Khron has the schematics on his blog.

If you're going with a K67 capsule or a Chinese "CK12" capsule, why not just make the high pass permanent like Neumann did? Unless you're using the better Chinese capsules (797, 3U Audio, apparently Maiku) the high end might be way too bright if not attenuated (or use a K47 capsule or 3U's K67 capsule variant for a flat circuit).

You could always cut out switch holes and make a switch PCB for the mic. That's what Khron did in his.

I intend to use the TSB-2555B capsule for this build, and I forgot to mention that I planned this for a single polar pattern mic. As in Homero's design, there's a bit of DC derived from the 48V phantom that can be used to drive one big capsule, but as you mention a DC-DC converter would be needed for a more general approach.

Technically this board can be revised to make the low pass fixed and to replace the last switch with a polar pattern switch, but I should investigate this further.

I like these ideas and I would too like to experiment with bigger capsules and multipattern mics, I will do this after I try this batch.

Yes, the low pass could be permanent, just jumper the pads that should go to its switch.

Edit: missing last part about fixing the low pass
« Last Edit: February 25, 2020, 06:14:19 PM by MickMad »

RuudNL

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #3 on: February 25, 2020, 03:21:15 AM »
Am I mistaken, or do I see a standard Schoeps circuit?
There is a solution for every problem!

http://www.vansteenisaudio.nl

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #4 on: February 25, 2020, 07:47:29 AM »
Am I mistaken, or do I see a standard Schoeps circuit?

You're not mistaken :) as I mentioned in the first post, it is indeed a classic Schoeps design with added switches for LP, HP and -10 dB pad, and with a couple of tweaked values, but unlike the Schoeps design there's no step-up regulator to bias big capsules since I don't have the need for that right now.  I should probably call this "yet another Schoeps" or YAS for short  ;D but I think the "Ultra Pimped Alice" is better, since it shows that it's the Alice one, that got "pimped" by Homero Leal and then "ultra pimped" by me with the switches board.

You can find the schematic attached to this post. If you're interested I also have the LTSpice simulation, with and without capsule data; I used some parametric filters to mimic the frequency curve of the capsule as shown on the datasheet, it was useful to check that the LP and HP filters were working properly.

RuudNL

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #5 on: February 25, 2020, 02:04:21 PM »
No problems with the design, but i doubt if the two/three capacitors in parallel will make any (audible) difference.
Well, it doesn't do any harm and it might feel good...   ;)
Personally I never noticed any difference, as long as you use good quality electrolytic capacitors.

Thanks for adding the schematic. The link in the first post felt a bit like a puzzle ride...
There is a solution for every problem!

http://www.vansteenisaudio.nl

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #6 on: February 25, 2020, 03:35:28 PM »
No problems with the design, but i doubt if the two/three capacitors in parallel will make any (audible) difference.
Well, it doesn't do any harm and it might feel good...   ;)
Personally I never noticed any difference, as long as you use good quality electrolytic capacitors.

Thanks for adding the schematic. The link in the first post felt a bit like a puzzle ride...

You're welcome :)
The big parallel caps are inserted in the circuit with the switch to form a high pass, it's a subtle one but it should be enough to tame low frequency rumble.

You can check the simulated frequency response in the attachment: top is HP off, second one is HP on, third one is HP + LP on.

I avoided inserting the pad because it just reduces by 10dB overall.

Edit: the screenshots are with the simulated capsule frequency response enabled.

RuudNL

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #7 on: February 26, 2020, 02:42:22 AM »
I meant the 10nF//1uF//100uF and the 47uF//0.1uF.
There is a solution for every problem!

http://www.vansteenisaudio.nl

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #8 on: February 26, 2020, 02:57:19 AM »
I meant the 10nF//1uF//100uF and the 47uF//0.1uF.

Oh I see; most sources say that there's an audible reduction in noise, this is in fact one of the mods that you can do to a standard chinese PCB to improve its performance, so I'd rather make room for the extra caps than regret not having them later :)

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #9 on: March 21, 2020, 08:40:51 PM »
Quick update:

I had to wait a long time for the capsules to arrive due to the Coronavirus situation here in Italy, but they arrived eventually.

I had a small issue on the PCBs, where the slots to fit the switches PCB were a tad too tight, I found something around the house that allowed me to file them just barely enough to fit it tho.

I went up and soldered all the components on the PCBs today, I also tried to glue the capsule to the 3D printed stand with silicone, others suggested using E6000 glue but apparently the silicone I had laying around is kinda the same thing. Everything is fine except for two 47uF resistors that are a tad too high and therefore they touch the inside of the part of the body that slides off when you unmount it; tomorrow I'll have to rework them to give me some slack on the pins and bend them inside a little (I don't have any other 47uF cap atm).  Other than that, it looks nice, you can see in the attached photo :)

Finally, I'll solder two 22nF caps on the XLR connector and the capsule wire, then I'll test it.

I don't have access to a drill, so I can't make the holes on the body to fit the switches actuators but I can try this with an open body at least.  I don't even have a decent XLR cable, so I'll have to make one with what I have laying around :D

Stay tuned for the audio tests!


Khron

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #10 on: March 22, 2020, 01:50:10 AM »
All part of the design process ;) That's why i strived to keep the electrolytics ("even" normal 11mm tall ones) as close to the middle of the board as possible. Even without any 3D rendering ;D

Oh, and i chose not to go with pointlessly massive and/or voltage-overrated poly capacitors...

https://groupdiy.com/index.php?topic=61845

Everything is fine except for two 47uF resistors [ahem, capacitors ::)] that are a tad too high and therefore they touch the inside of the part of the body that slides off when you unmount it; tomorrow I'll have to rework them to give me some slack on the pins and bend them inside a little (I don't have any other 47uF cap atm). 
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #11 on: March 22, 2020, 07:11:20 AM »
All part of the design process ;) That's why i strived to keep the electrolytics ("even" normal 11mm tall ones) as close to the middle of the board as possible. Even without any 3D rendering ;D

Oh, and i chose not to go with pointlessly massive and/or voltage-overrated poly capacitors...

https://groupdiy.com/index.php?topic=61845

Yeah my mistake; I went and actually checked the height of every component before buying but I guess these caps slipped my check, since I also just noticed that they're the wrong voltage rating for where they are in the circuit... one of the two 47 uF caps sees 35V but I bought them with a 35V voltage rating, which is a nono for me as I always apply some derating.

Everything is fine except for two 47uF resistors [...]

This is why I shouldn't post on forums at 4 AM  ;D

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #12 on: March 22, 2020, 02:02:57 PM »
So, I ended up cutting the switches levers (they're plastic) because I don't have access to a drill to make holes in the body, also because I found that the high-pass filter doesn't do much for the proximity effect (maybe I put the cutoff frequency too low for it to do something useful)  and I like the high-pass always inserted, it darkens it just enough.

You can find an audio test here:

https://soundcloud.com/mickmad/microphone-test

All in all, for the final price I paid for this mic I must say it doesn't sound bad at all.

When this coronavirus thing is over, I'll go to a friend of mine and compare it in a better environment to a Neumann U87a he has; we'll put the two mics on stands and record them together thru his Apogee Duet pres so that an A/B test would yield relevant results.

Mick

RuudNL

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #13 on: March 24, 2020, 02:02:26 PM »
"This track is not found"...
There is a solution for every problem!

http://www.vansteenisaudio.nl

MickMad

Re: Ultra Pimped Alice - my take on a classic DIY design
« Reply #14 on: March 24, 2020, 06:55:36 PM »
"This track is not found"...

Damn, the track was private but everybody with the link should have had access to it anyway... Oh well, I made it public now :)


 

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