DIY home made P2P Oktava Mk012.......circuit questions & observations

[Gus]

Simmed the circuit with two 100 meg input resistors (R10, R7) and C10 .027uf, frequency graph looks nice.

around -3 at about 35Hz and -3 at about 20KHz with C10 as .027uf and about 17KHz with a .033uf

So for a nice vocal microphone circuit with a 67 like capsule I would use two 100meg and .033uf as a good STARTING point.  Remember it is about how it sounds in the mix.

 

[tchgtr]

Yeah Gus- I had to stare at your schemo for a while to realize that the .027 cap was the only change from the original 012 circuit. I started with a .022 cap (closest I had) and went up to a .056 (next closest I had on hand).
Don't have the gear to do a frequency plot, but I really like the mic now...tamed the top end nicely, and so far just tried it on voice and acoustic guitar, which was still clear, but not so sharp it gave me a haircut.
I might use this mic now...

 

[Gus]

.056uf sims at -3 at about 10KHz(circuit alone not the capsule and grill added EQ)

Schematic is from a LT spice sim using the part numbers that are in the download not added after

tchgtr is your build in a 2001 body with a stock capsule?  Good to read someone tried the circuit change

For more fun make C10 a fixed value like .027uf and switch in parallel caps to add to the value fixed .027uf and .0047uf OR .0068uf OR .01uf etc.  Don't switch when the microphone is connected and powered up maybe a small switch inside that you need to open up the microphone to switch.  Try polypropylene and other polys for C10

 

[tchgtr]

Yes, it's a standard 2001 body and capsule. I used the parts and PCB from the original circuit to create the 012 circuit, using the 2SK170 and a 2N3906.
Need to do some more listening, but I think this is my favorite thing to do with a 2001, other than the Royer mod. I like the suppressed high end, at least for my own voice, which never sounded too great on this mic previously.
The switched caps are a good idea. Tune the mic for each task...
This is such a cool, simple circuit, and fun to play with.

I own a pair of slightly modded MC-012s and like them very much, so I tried the circuit in an MXL 440 (a SD mic), but was not too thrilled with the results. This might be because I dismantled it's capsule a while back in a fit of curiosity, and damaged/removed the spacers, so I need to try it with an unmolested capsule, which I can borrow from the 441 that came with the set.
I thought the circuit might sound OK with another SD capsule, but maybe the Oktava capsules are too different from the MXLs.

 

[Gus]

I wonder how fast this will appear(C10 in the schematic in this thread) as a mod for the 012 circuit at other places on the web?

 

[tchgtr]

Ha! It would be a mistake to do this to an actual MC-012, so I hope people don't start doing that, but I get what you mean. Maybe I should have said that I removed a layer of headbasket and went for larger capacitor voltages for "headroom" (I miss Rodabod)...
All kidding aside, it occurred to me last night after I posted the above that this mod (with the .056 cap and a 2001 capsule) reminded me a bit of a MK-319 in stock form. A bit dark, a bit grainy, but in a warm and fuzzy way.
I certainly need a bit more time with the mic on some more sources before I'm sure that's what I'm hearing, so it's just an impression.

Think that'll fuel the fire Gus?
Have you done this in the real world? What are your impressions.

 

[Gus]

Found a 012 perfboard circuit I built some years ago I used a 219 capsule with it in a body I built,  just added a .033uf.   I need to find/build a body to install it.

I think the added lowpass cap is a mod.  The other stuff on the web is part changing.

I also think using two 100meg resistors R10, R7 instead of two 1000meg or 2000meg might make it a better vocal microphone with a china k67 like capsule.  One might want to make R10 and R7 even lower.

R6 and R5 should be matched.

If one models the circuit C8 should be 50pf to 100pf for different LD capsules and there should be a load between the outputs.

 

[Gus]

The 012 circuit might make a nice DI for guitar and bass. Adjust the input resistors and don't use the capsule voltage part.

2.2 meg one side to input cap other side to ground, 2.2 meg gate to resistive divider bias node size input cap for the input high pass

With 22meg inputs might make a good piezo DI.

 

[Matador]

Is there any gain in this circuit?  Or is the intention just to provide a lowish-impedance quasi-balanced output?

Just from visual inspection it looks like a JFET follower topology with a constant current load.

EDIT - Spelling is hard.

 

[Rybow]

Hello!

I just built up 2 of these circuits to replace the circuit in my Apex 180's. I never really use them, so I thought I'd try to change them up for fun. I have one done P2P and one done on a home etch circuit board, but I have encountered an issue with very low output from the mics. I have to turn my ensemble's gain all the way up to get an audible level from the mic. My P2P mic has very low noise even at this level of gain and uses the 2SK170BLF. My PCB version has much higher noise and I switched the 2SK170BLF for a 2N3819 just to see. Both use the 2N5087GOS as a source follower. I am wondering if it is just not a circuit you can use in an Apex 180 for some reason? Or if I mess up somewhere.

My voltages look pretty good, not quite right, but pretty good on my PCB version with 2SK170BLF.
Pin 3 40V
FET drain- 24V
7.5K-Emitter junction-10.1V
Across 84K-16.6V
Across 7.5K-16.2V
Emitter to Base-0.6V

I should also mention that my PCB mic initially was all noise no output until I switched out the 2SK170 with another 2SK170. No change in noise with the 2N3819, but the highs are rolled off. I did clean the entire board with flux remover and isopropyl paying special attention to the high impedance area. Any help would be greatly appreciated.


Thanks!

 

[pasarski]

It could be that Apex capsule needs higher polarization voltage than Oktava circuit gives. Does the original circuit have a DC-DC converter?

 

[Rybow]

It could be that Apex capsule needs higher polarization voltage than Oktava circuit gives. Does the original circuit have a DC-DC converter?

Bingo! Thanks pasarski. The orignial does have a DC to DC converter. Well, it was fun while it lasted. I guess there is no easy way to boost the polarizing voltage in the 012 circuit. Guess I'll just upgrade the current circuit and invest in some better SDC's.

If it doesn't work, I'll take a spare k67 type capsule I have and make a mic with this circuit and the 67 capsule. That should work.

 

[Rybow]

So I am going to build an LDC using this circuit. Actually, I am a couple days away from completing it, but I had one question about the schematic that Gus posted a page back. C3, C4, and C8 are 100uF, but on the schematic I used to build up my SDC's C4 and C8 are 68uF, and C3 is a 47uF. My question is, are the 100uF caps a critical component in adapting this circuit for an LDC capsule?

Thanks.

 

[tchgtr]

My guess (and it is only that) is no. The 68 uf are just there due to the original schemo for the Otava MC-012s.
BTW, I found a .033 cap to use in place of the .056 I used a low pass on my 2001 that contains this circuit (as suggested by Gus earlier), and the mic sounds much better now. Will find a .027 eventually...

 

[Gus]

Insomniaclown

The part values are what I selected 100uf is a common cap size.  The transistor numbers selected are in the stock download of LT spice.  I did that so if someone wanted to sim it they would have a schematic with parts in the download.

  The only thing different for LDs is the Low-Pass cap if needed for the capsule used.  You might need to adjust the value or omit it depending on what capsule etc you use.

Low output from a 012? even with 40VDC to the capsule the output should not be that low.

DID you handle the input cap and input resistors with your fingers?

Did you overheat the input cap?  What type of cap did you use?

tchgtr

Thanks for the feedback.  I was thinking from the sims .033uf is a good starting point for the 32mm and 2001 grill.

 

[Rybow]

Hey Gus! Thanks for all the info.

I did handle the input cap and input resistors with my fingers. I did clean connections and leads with a flux remover pen/isopropyl, but should I also use a little isopropyl to clean the casings? My P2P version is very quiet noise wise, and the output is still quite low. I used a Xicon polystyrene as the input cap. I'll also try cleaning the capsule connections.

Couple of Q's. I know the 1G/input cap/capsule/FET gate connections need to be made in the air, but is it essential that the input cap connections be soldered on the bottom? I ask because using a polystyrene, I need to mount it on top, and then run the leads underneath, which makes the leads quite long. I have some single end/double turret push pins that I could use if it's ok to just connect it all on the top.

The phantom power. I measure 40VDC at pin 3, but only 10VDC on the capsule side of the 30M resistor. I did experiment with replacing the 30M with a 10M which gave me 24VDC, and it did increase the output marginally. I did not check to see if this impacted other voltages on the board (doh!). The Apex 180 schematic shows a DC to DC converter, and a 1M resistor prior to the 1G resistor and capsule. I was going to rebuild one of them to get some voltage measurements to compare.

tchgtr- Thanks man. I have to look through my inventory and see what I got. I think I have a .033 cap to try. That sounds like a good starting point.

 

[pasarski]

How did you measure? Normal DMM won't work on high impdeance area. You have to measure before 30M.

 

[Rybow]

Thanks Pasarski. I thought it was just after the 1G that a DMM couldn't measure. Good to know. I was measuring in between the 30M and 1G to ground.

 

[Rybow]

Alright! I got one to work. I built a mic inside of a 1.5" brass tube using the 32mm 67 type capsule from my apex 435 and the 012 circuit. There is little room inside to mount the grill, so I have to build a head basket for it. I have a temp single layer grill that just slides over the mic for now. I used an orange drop for the input cap, and combined a 0.022 and 0.01 film cap to make the 0.033 roll off. Output is much better than my previous attempt. Still a bit low, so I think I need to do some more cleaning, but I am getting a decent level with a noise floor that measures (unscientifically) around -88dbfs.

I took the time to breadboard a circuit in order to check FET and transistor bias. The first 2 I put in were really close, so I went with that.

The sound. I mean, how good can a mic that has a cheap azz chinese capsule, simple russian circuit, and built into some plumbing pipe really sound? Really good! Did some quick tests on my voice and more extensive tests on acoustic guitar running through my ensemble. Sound was smooth and warm. Nice full low end as well. Good definition, and no congestion or other "weirdness". I may drop that 0.033uF cap down a bit as the mic may be too warm, but I won't mess around with that until I have the head basket secured and damping in place. Once that's done I'll try dropping those 1G resistors down.

Great circuit! Total cost was probably $50. I used mostly parts that I had lying around. Thanks Gus, pasarski, and tchgtr for all your help. I'll post pics once the mic doesn't look like a pile of crap.

 

[tchgtr]

XLNT!
It's such an easy circuit to build, and you're right, the cost is minimal. I was testing mic pre mods the other day with my 2001 with this circuit, as well as a modded Oktava 319, and my keyboardist preferred the 2001 on her voice.
I still want to experiment with SDCs and this circuit, since it sounds so good in the MC-012s, but haven't found a candidate capsule yet other than the typical MXL 603 style. I have it mounted in a MXL 440 body, so it's ready to go. Just need a fun, cheap capsule.
Will take photos in any condition...