Superlux S502 MK2 test, teardown and improvements

[Emmathom]

I have studied the circuit board a little bit. Nothing special, standard Schoeps CMC5 from 1973. But the component selection is nasty. JFET is MMBF5486, the output transistors are BC860C. All of them noisy as hell..
The oscillator circuits do not have the same frequency, they are shifted by about 23kHz. And now the first problem: After a while the beat frequency gets smaller and smaller. I saw 19kHz in the spectrum with nearly 1mV level! Maybe most of us do not detect this, but my grandchildren would...
Time to take the solder iron

Hum... thank you for these investigations... Please let us know your mods and the results !
That's incredible because even after a 2 hours recording I didn't notice any change in the sound : will investigate on my own too...
Regards

 

[ubxf]

Does the Superlux S241 have the same capsule?

 

[Emmathom]

Does the Superlux S241 have the same capsule?

Now that is "the" question I asked myself (about the sound quality - the capsule is different) with the secret hope that the answer would be yes and that therefore the S241 would be a small cardio SDC with an very good quality/price ratio...

So same for the E524/D ??? Reviews on Thomann are positive...
I guess that on stage, where you don't want to bring your $$$$ mic, a E524/D would do the job on a conga pair (for ex.)

 

[jp8]

The oscillator circuits do not have the same frequency, they are shifted by about 23kHz. And now the first problem: After a while the beat frequency gets smaller and smaller. I saw 19kHz in the spectrum with nearly 1mV level! Maybe most of us do not detect this, but my grandchildren would...
Time to take the solder iron

Both amplifier sections share the same GND, so I cannot think of a reason why two separate Upol generators would be required. What could be the rationale?

I guess what you'll be trying is to remove one oscillator and feed the Upol from one generator to both capsules?

Jan

 

[bremusound]

Maybe the intention for dedicated DCDC converters is the ability to compensate differences in capsule sensitivity.

 

[jp8]

Does the Superlux S241 have the same capsule?

I was wondering that too. I don't know about the internals, but the S241 body diameter is bigger than the 20mm of the Schoeps/S502 mics. The S241 could

Jan

 

[jp8]

Maybe the intention for dedicated DCDC converters is the ability to compensate differences in capsule sensitivity.

Good point, but on the pictures, I don't see any adjustable devices that could be attributed to such a feature.

FYI, in my SDC builds, I use a CMOS pol voltage generator and always with adjustable supply voltage, so you can adjust for gain differences and make matched pairs of mics.

Jan

 

[bremusound]

There is a pot for each Vpol generator, they go above 80V if you want… the pots are on the „other“ side of the pcb

 

[jp8]

There is a pot for each Vpol generator, they go above 80V if you want… the pots are on the „other“ side of the pcb

Ah, my bad, I only watched the first picture. The trim pots are on the second picture. Anyway, the question remains how Superlux adjusts them: for same Upol left and right (the easy way), or same audio output level using calibrated sound sources on both capsules (the better, but more expensive way).

A single oscillator with a single balancing pot would be preferred, I guess. Probably also cheaper to build.

Jan

 

[MicUlli]

Ah, my bad, I only watched the first picture. The trim pots are on the second picture. Anyway, the question remains how Superlux adjusts them: for same Upol left and right (the easy way), or same audio output level using calibrated sound sources on both capsules (the better, but more expensive way).

A single oscillator with a single balancing pot would be preferred, I guess. Probably also cheaper to build.

Jan

I fully agree. One of the oscillators should be put to death. Same pol voltage for both sides. And the 6V-lines of both channel should be connected together too. This would enable single channel operation (of course doubtful)

 

[LevinGuitar]

I've made a few mods (but find the post about them) : replaicing the coupling cap (680pF I guess)

Haven't read all thread, but if the membrane and backplate have separate connections from ground you have a way to avoid the imput cap at all (and best cap is no cap as have been told here )

 

[Emmathom]

Haven't read all thread, but if the membrane and backplate have separate connections from ground you have a way to avoid the imput cap at all (and best cap is no cap as have been told here )

Would be great so... yes the best component is the one you can do without.

I guess that after @MicUlli reports about his mods I will open mine too and do all it must be done in one shot. But I keep in mind to remove (eventually) the coupling capsule cap.

 

[mhelin]

Does the Superlux S241 have the same capsule?

S502mkII has smaller diameter, about 20mm, and thus smaller about 18mm capsule whereas the S502 original and SS241 have about 22mm dia and 20mm capsule or thereabouts. In addition the S502mkII capsule has the odd ring and propably won't fit any body of about that size (Rode NT5 or t.bone SC140).

 

[jp8]

In addition the S502mkII capsule has the odd ring and propably won't fit any body of about that size (Rode NT5 or t.bone SC140).

That ring is the 2nd capsule contact? So the backplate and membrane are isolated from the capsule body?

I don't mind 3D printing a nosecone for such an interface. I always make my own nose cones to preserve the stock PCBA as a reference. All I'd like to know are the thread dimensions and I'll take care of the internals. That's least of a problem for me. Actually the fun part of it.

Jan

 

[MicUlli]

Teardown is complete. Here is the circuit for the left channel. For simulation i added some components from phantom power supply...
The right channel marks all components with the postfix 'a', all components are identical.

 

[MicUlli]

Hi folks,
most of the measurement stuff has been done, here some results.

The first pic shows the frequency response on axis and under angles up to 180° for left and right channel. My estimation that the mic is not as loud as indicated in the datasheet is true. Sensivity is around -38,6 dBV (11,7 mV/Pa). The rear sound rejection is really remarkable good. Under 45° sound incidence the response goes up to 16 kHz without degradation. This is important for ORTF settings. For sound up to 90° incidence angle we see a 3 dB enhancement, the mic sounds "fresh" without harshness. The highpass 3 dB corner is 70 Hz (free field conditions), very well done for an SDC capsule. Another highpass corner is at 25 Hz because of the Schoeps circuit. The capsule matching is tight enough.

After adding some modifications to the electronic part some enhancements could be measured as the second pic shows. The A weighted noise levels are -116,1 dB and -117,0 dB (modified). Note the improvement at higher frequencies. The self noise could be reduced from 16,5 dBA to 15,6 dBA, perhaps not worth doing a mod
The max output level of the circuit is +1 dBV for 0,5% THD. This translates to a max. SPL of 133,4 dB, very tight to the datasheet.

The 3rd pic shows a weakness of the circuit. There are a lot of noise components above the audio band. This holds also true if only one oscillator is at work, nevertheless one oscillator must be put to death because you never know where the beat frequencies occor. The noise is gone when both oscillators are disconnected. This problem could not be solved whatever i tried. Must be the messy board layout..

I also have tried the mic capsule with my MBC5X0 board with very good results. Because of the smaller pol voltage (39 V vs 60 V) the sensitivity is 3 dB smaller, but also the electronic noise. Therefore the self noise stays at 15,6 dBA and fortunately the max. SPL goes up to 140 dB . NO MESSY noise components above the audio range. Probably i will design a board with 2x MBC5X0 circuits that can be used as a replacement, stay tuned..

If you want to improve your S502MKII i recommend the following steps:
1. MANDATORY. Put one oscillator to death by removing L2 or L2a. Connect both channels together for pol voltage with a wire between cathodes of D1 and D1a and for oscillator supply with a wire between plus of C5 and C5a. This ensures that the mic works with only one cable (L or R) connected.
2. RECOMMENDED. Desolder T1 and T1a, replace them with 2 low noise JFETs like 2SK209BL or NSVJ3910 (my favorite). Desolder T2, T2a, T3 and T3a, replace them with BC807-25 or FMMT591 (my favorite). This gives the pretty low noise floor at higher frequencies.
3. PLAYGROUND. Desolder R1 and R1a, replace them with BAV199 INF double diode (connected antiparallel). Add 2 MKT caps 0,15u/50V between wiper of R2 (R2a) and source of T1 (T1a). This lowers the noise floor at low frequencies.

After modding the circuit adjust R2 and R2a for a voltage of 1,6 V across R3 and R3a. This is a compromise between headroom and low THD at smaller levels.

My verdict: Superlux S502MKII is a very fine mic, cheap enough and a must have for all mic freaks

 

[jp8]

Great work, Miculli! Thanks for sharing.

Jan

 

[MicUlli]

After calculating some technical and commercial aspects i am going to consider the design of pcbs for the Superlux S502MKII based on my MicUlli-design (MBC5X0), see https://groupdiy.com/threads/a-very-versatile-input-stage-for-sdc-mics.80172/

Requirements:
1. Board shall have the same size as the original
2. Using surface mount components, only electrolytic caps are discrete
3. Optional DCDC converter for -20V (will lead to 59V pol voltage and a higher sensivity)
4. Board shall also be usable for the S502 1st version (with the bigger capsules), but limited to 39V pol voltage
Advantages:
1. Better SNR (as already proven)
2. Better max. SPL (as already proven)
3. Much smaller THD
4. No spurious noise artefacts above the audible range

I need at least 2 boards, some of you want 4 boards (thanks for that ), but it makes no sense to order components for a batch smaller than 15 units. So let me know (drop a PM) whether you are interested in some of these "S502MK1+MK2 repair kits"

So long MicUlli

 

[Emmathom]

Hi @MicUlli I'm in for 1 board, maybe 2 in case of I'll buy a MkII
Will PM you and thanx for your effort ! Nice, man

 

[RuudNL]

Will it be only the PCB, or a PCB with the needed (SMD) components?
If it is the last case, I am interested in two sets.