Inexepensive SDC on blowout 33€ (Avantone CK1/MXL 603)

[RuudNL]

Today I received my SubZero microphones.
First thing I noticed, is that the FET bias isn't optimal. (It is set with two fixed resistors.)
To optimize distortion and max. SPL, I removed the two bias resistors (470K and 82 K, on the center of the PCB, in parallel with the source resistor).
After you have removed them, there is just enough space for a small cermet trimpot. (I used 1 M.ohm.)*
With the correct setting, THD is around 0.02%!

By the way: I found a way to put back the knobs on the slide switches.
First you slide in the PCB. The knobs are inserted from the outside!
Put the slideswitch 'halfway' and put in one end of the slideswitch knob in such a way that it goes a bit under the surface of the body tube. Match the opening in the knob with the pin of the switch and press and slide the knob in the direction where you first inserted the knob.  It will now click onto the switch and operate 'as before'. 

* EDIT: In both my microphones, the value of the 470 K resistor was too high to bias the FET correctly.
The easiest way to correct this, is to replace the 470 K resistor with a 500 K. trimpot.
This makes it easier to adjust the bias correctly than using a 1 M. trimpot to replace both resistors.

 

[jarvis]

Hey Ruud,
this sounds promising, thanks for your findings. I'll try it out.

Cheers
Jannis

 

[kingkorg]

Today I received my SubZero microphones.
First thing I noticed, is that the FET bias isn't optimal. (It is set with two fixed resistors.)
To optimize distortion and max. SPL, I removed the two bias resistors (470K and 820 K, on the center of the PCB, in parallel with the source resistor).
After you have removed them, there is just enough space for a small cermet trimpot. (I used 1 M.ohm.)*
With the correct setting, THD is around 0.02%!

By the way: I found a way to put back the knobs on the slide switches.
First you slide in the PCB. The knobs are inserted from the outside!
Put the slideswitch 'halfway' and put in one end of the slideswitch knob in such a way that it goes a bit under the surface of the body tube. Match the opening in the knob with the pin of the switch and press and slide the knob in the direction where you first inserted the knob.  It will now click onto the switch and operate 'as before'. 

* EDIT: In both my microphones, the value of the 470 K resistor was too high to bias the FET correctly.
The easiest way to correct this, is to replace the 470 K resistor with a 500 K. trimpot.
This makes it easier to adjust the bias correctly than using a 1 M. trimpot to replace both resistors.

Awesome Ruud, thank you! Probably dumb question, but when 470k is replaced with pot, only two legs of the pot are used? I am still trying to wrap my head around biasing.

Can you give us some voltages or value of pot resistance when bias is corectly set?

 

[Khron]

Awesome Ruud, thank you! Probably dumb question, but when 470k is replaced with pot, only two legs of the pot are used? I am still trying to wrap my head around biasing.

http://sound.whsites.net/pots-f6.gif

"The second form of connection is a variable resistor. Not usable as a volume control, but still extensively used for other applications. It is common (and preferable) to join two of the leads together - the wiper, and one end or the other. Why join the wiper to one end? Doing so ensures that the pot won't become an open-circuit if (when) it wears or becomes contaminated with dust. By joining the wiper to one end, there will always be the full pot resistance in circuit, and this can prevent circuit malfunction in some applications."

Quoted from right over here.

 

[RuudNL]

Probably dumb question, but when 470k is replaced with pot, only two legs of the pot are used?

Yes, only the wiper and one end of the resistive track is used. It replaces the 470K resistor.
You could indeed connect the wiper and 'the other end' of the resistive track together (to prevent an open circuit in case of a bad wiper contact), but in that case the bias would be so far off that it wouldn't really serve any purpose!
At first I used a 1M. trimpot to replace both resistors (470K and 82K), but the adjustment was more critical than when replacing only the 470K. with a 500K. trimpot.
In my case the voltage on the drain was ~7 V, but that will be different from FET to FET I suppose.
I noticed also that the voltage over the FET stage differs from microphone to microphone, making it impossible to specify the 'ideal' bias point. In my microphones it was between 7 and 8 volts anyway, measured at the drain.
I connect a low distortion generator through a 470 pF capacitor to the input (250-300 mV) and check the output with a distortion analyzer. There is a very sharp 'dip' in distortion when varying the FET bias.

 

[kingkorg]

Thanks Ruud, thanks Khron for huge amount of awesomeness!

UPDATE:
Biasing made huge diference. I used one of these as snare bottom mic last week, and it was saturating like crazy, very obvious. Not so much on toms, but snare was too much for it. On acc. guitar it was unnoticeable.

Drain voltage for me was also around 7vdc. And 270Kohm value did the trick. I used pot to find value, and replaced with fixed resistor. It is cleanest of all moded mics i have curently.

 

[RuudNL]

I suppose they didn't even made an effort to bias the FET correctly...
All microphones contain 82K/470K for biasing.
But since FET characteristics differ so much, you really must be lucky to find a correctly biased one!
In my microphone  a correct bias produced a value of around 7 V. at the drain.
Can your microphone handle the SPL of a snare? (I can get 800 mV out of the microphone without significant distortion,)

 

[kingkorg]

Works fine with pad as snare bottom mic, some 15-20cm away pointing towards snare wires.

 

[e.oelberg]

looks like 3u capsules fit on these bodies

 

[kingkorg]

Another one, even cheaper, but SMD! Not so mod friendly, but very useful to have.

Here is an audio example, rough mix outtake. I had this one (Takstar) on floor tom, modded SubZeros on HiTom, snare bottom, and alongside sm57 on snare top.

Download for wav, preview is mp3:
https://app.box.com/s/gsc4me8idzzwj7owkywvhxu68nwi5g3z

Takstar:

https://www.aliexpress.com/item/Free-shipping-Takstar-cm-60-professional-recording-microphone-portable-condenser-microphone-for-musical-instrument-TV-and/32813417910.html?spm=a2g0s.9042311.0.0.KitYNh

Capsules are interchangeable. The one coming with this one is slightly different. It is cardioid counter to SubZero's hypercardioid. It has less handling noise and better rear rejection. On axis response is similar. This one has a bit more hi end roll off after 10k. It is about 5db less sensitive.  Smaler backplate holes. Slightly different backplate design.

I use these as main tom mics. Sometimes as second snare/snare bottom in non critical situations. I can hardly think of cheaper, better sounding microphones that can be used alongside anything else if used correctly. 

 

[RuudNL]

I recently had this one on the bench. Not bad at all. (All through-hole components.)
https://www.bax-shop.nl/kleinmembraan-condensator-microfoon/dap-cm-1-pencil-condensator-instrumentmicrofoon
If you buy 2 of them, they are 34 Euro. (68 Euro/pair).
They come with two capsules: cardioid and omni.
Even the FET bias was correct!