Alctron MC001 (with Mods)

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James-F

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Another Schoeps mic from Aliexpress like the MXL990 and Apex 435, goes for 66$ at the moment.
But this time it seems that the build quality is better.
Capsule is the usual Chinese 32mm K67.

I've done the usual mods (values from MXL990 schematic).
1. Removed R8 and C13, polarized the backplate directly, capsule center wire to JFET Gate.
2. Swap output Pins 2/3 to correct polarity after polarizing the backplate.
3. Zener diode changed to 9.1v, polarization voltage is exactly 60v now (was 6.8v and 43v).
4. Added bias trimmer (R13) and biased 3/4 and 1/4 of VDD at Source and Drain, stock pair was 470k+75k ended up 470k+60k so it was not far off.
5. Coupling caps C3 and C4 upped to 1uF.

Great mic after the mods, very low self noise too.




PS.
A very important note.
If you just brought the mic package from the cold post office into your warm house DON'T open it right away, wait till the package warms up to house temperature.
You all know how a cold soda can condensates a lot of moisture from the warm air..... yeah, my mistake, the entire mic was wet in seconds including the capsule, but now it is dry, working, and sounding great.

MXL 990 Schematic for reference.
 
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Khron

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You might want to consider adding a nF-range capacitor (optionally in series with a resistor) in parallel with the drain resistor, to tame the top end of the response somewhat.
 

James-F

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I like how K67 capsules sound in a flat circuit and I much prefer doing cutting and LPF in post if needed be, cutting in post has the benefit of greater control and lowers noise even further.

I've read this and keep it in mind if I ever need to tame the highs in circuit.
 

soliloqueen

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same circuitboard as the cad gxl2200. you can get this mic in a kit with a bag and shock mount for $70-80 in the right place. The only thing I don't like about these simple Schoeps circuits is that they tend to pass noise from the phantom power straight into the signal. Makes using them on interfaces with noisy phantom power difficult. Anyone have any solutions for this? Happens with both my Alctron and studio projects mics
 

Khron

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Larger value filtering capacitors? Adding some low-value ceramics (perhaps even smd) in parallel with those?

What sort of "noise" is that, though?
 

Whoops

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I like how K67 capsules sound in a flat circuit and I much prefer doing cutting and LPF in post if needed be, cutting in post has the benefit of greater control and lowers noise even further.

I'm the opposite from you, I can't stand a K67 in a flat circuit, it's just an unusable mic for me.

But just wanted to add that doing the high end roll off in the mic or in post it's not the same thing in a couple of situations. One of the most important situations is when recording vocals.
I always use a nice hardware compressor (sometimes 2 with different settings) when tracking vocals, if there's excessive high end in the mic the sound will be quite sibilant, that sibilance will just get worse after the compressor, even if you used a light compression setting.
If you tame the high end in the mic instead of post recording, that sibilance will not hit the compressor in the first place and be exaggerated as a side effect of compression.

So I really don't agree with people that advocate that post corrective EQ is the same as doing the EQ in the mic circuit.
I always preffer the mic to sound right and if possible I don't have to do any corrective EQ at all, just mixing EQ to make it sit well in the mix.

Have fun
 

James-F

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The only thing I don't like about these simple Schoeps circuits is that they tend to pass noise from the phantom power straight into the signal.
The two output transistors are emitter followers with their Source resistors being the two 6.8k in your phantom power path.
Can't do much about it unless you open your audio interface and add filtering to the 48v line before the 6.8k resistors.
Despite the very minor phantom power noise bleed that is usually caused by switching power supplies on the same outlet, I think these Schoeps mics are exceptionally quiet.

So I really don't agree with people that advocate
Nothing was advocated, it was 100% personal opinion and preference.
 
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RuudNL

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People like @RuudNL point out that, despite Mr Spragens' claims / measurements, that method adds noise.
There is a clear increase in self noise, if the resistors between the FET and the bases of the PNP transistors have a value like 10 K or so. It is not only measurable, it is also audible. If I remember correctly, forum member Rogs also mentioned this. Also the filtering (by adding capacitors) between base and emitter, will increase distortion, especially at higher frequencies. Years ago I had a discussion about this with Matt McGlynn (Microphone Parts) and he didn't deny this.
 

Whoops

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Nothing was advocated, it was 100% personal opinion and preference.

Hi mate, I was not referring to you, but to other people that “advocate” that in other threads/places.
It was a general comment and not directed to anyone specifically.

I totally respect that you like a K67 with a flat circuit
 

xeawr

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People like @RuudNL point out that, despite Mr Spragens' claims / measurements, that method adds noise.
There is a clear increase in self noise, if the resistors between the FET and the bases of the PNP transistors have a value like 10 K or so. It is not only measurable, it is also audible. If I remember correctly, forum member Rogs also mentioned this. Also the filtering (by adding capacitors) between base and emitter, will increase distortion, especially at higher frequencies. Years ago I had a discussion about this with Matt McGlynn (Microphone Parts) and he didn't deny this.
I'm not sure if there can be a difference between the theoretical and the practical increase in noise, but I'm sensitive to noise and the solution by Mr Spragens worked for me (I think the resistor was a bit lower than 10k though).
I didn't do before and after measurements, but the noise after the mod was still lower than with many tube mics. I first soldered in these push-in-connectors (don't know what they're called) - like in IC sockets, so I could quickly swap out resistors and see what I would like best.
I know this is scientifically useless and purely subjective, but I didn't even notice an increase in self-noise at all.

I absolutely love the Neumann K67 capsules, not so much the low end Chinese ones.
And although I have the possibility I rarely do any hardware compression, but I still like the sound to be as finished as possible, so I can understand why people add a LPF at the mic amp stage.

just my 5 cents. Cheers everyone!
 
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James-F

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In light of the information posted in this thread: 32mm K67 Alctron capsule
I will try to push the polarization voltage even more with a 12v zener, but for that I will need to lower R5 (6.3k in the Alctron) to lower the voltage drop across it because my JFET VDD (Point A) is currently 10.8v after biasing which is just enough for the 9.1v zener but not for a 12v zener.

I will add that the microphone as it is now after the modifications in the first post is very quiet and hotter than my Rode NT1-A, so I don't see a reason to push it further except for the sake of experimentation.
 
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