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

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gary o

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http://www.recordingmag.com/resources/resourceDetail/315.html

I just made a 012 using the changes recomended in the above article.....its work sounding nice better than my Oktava shop bought one more detail in the high end....

heres my questions is the bias points for the transistors critical ??l, I thought I read it was.....Am I trying to get 20V & 6V as marked on the schematic ?

my phantom power is different from my pre a mixer I have and a cake walk pre I have.......so it doesnt work out to 20V and 6V anyway

I was going to add 2 pots to adjust bias voltage for the 2 transistors......So then I measure the voltage in my shop bought MK102 its way off the schematic.......or is that because the schematic is for the modded version ?


any thought very welcome ,thanks for reading.



 
What will help is a number of voltage readings
measure the voltages at the bias string nodes
39K,82K,36K

The bias string sets the operating points for the PNP and JFET.



 
Thanks Gus I will do this .......I was gonna replace those 3 resistors with a 100k pot a 47k pot and a 12k resistor so I can adjust the the bias points for the transistors......is this an ok idea......as you gathered I dont totally understand what Im doing....Im just trying to get the best out of this circuit & learn too.......

Rave rave rave just gotta rave a little about the Octava MK012 with the MK101 head (Im told capsule of the 219 & 319) I think it s sounds great as is & the home made electronics even better even if bias points not set up perfect !!!!! I built scratch DIY version so I could A & B the stock 012 and the modded or DIYed....

I have a Violet lollipop Vin 67 capsule cost 450pounds its meant to attach to the voioet Globe mic body & with that sound like a U67.....I have used a real U67 sadly so I dont no but to me the MK101 capsule sounds better.....I have tried with M jolly single layer grill .....I dont think it sounds any better really (would have to add pop filter for sure anyway) I also tried the capsule without the resonater front resonator disc ....I prefer it with disc on myself, didnt sound bad without.....me hi fi like i spose prob good good on accoustic guitar.....

I can only compare my 012 to my Violet Vin67 on a AKG C28A (C12 electronics) or a home made Ela251 mic both have T14 transformers and 6072 tubes but am astonished to my ears the 012 electronics on either capsule Mk101 or Vin67 sounds way better....

Im gotta make another DIY 012 soon cos so easy n cheap to make.....

going to measure voltages and play with the transistor biasing now.......
 
Had a bit of trouble posting in this thread......I think it was a pound sign charactor.......strange.....

Anyway while Im here can anyone recomend a FET other than the 2SK170BL to try in the this circuit......will others work and do good job ?

I was reading Octava mods mod of the MXL V67G saying that the 2SK170BL should be replaced in that mic with a FET used in the U87 ?

anyone know of this super FET ?
 
This might be a good time to go through the procedure for setting the FETs bias. I think that my understanding is not as I would like it and the MK012 is common enough to be a great testbed/learning tool for us all.

Regards, Ethan
 
Im going to bread board another 012 circuit to try & add trim pots....I would like to try a 2N3819 (thanks magneto) too & to try different capacitors.....il need help with the transistor biasing tho

thanks
 
I agree that the MK101 is great. I liked it better in stock form also, too bad it's not so easy to put the grille back together.

Here Jim Williams recommends an other FET, J305 http://www.tomshardware.co.uk/forum/44947-32-jfet-swap-microphone
 
Wow thanks for info guys as I said I may need some help with the number for the baising of different FETs....

Pasarski so you removed the inner mesh then ?.......I left mine stock after bodging some tea strainers as a 1 layer grill to me there wasnt much in it so I left it... im sure yrs doesnt sound worse..... did you remove the baffles front & and back......again I lked mine stock, I think its easy to make a mic sound different but not nessercaraly better.....

thanks guys.
 
I would leave the stock resistor values.  The PNP does not matter much.  Try different Jfets and measure what happens, then measure the jfet specs.

The voltages can/will give help understand what is happening

20VDC (-6VDC +VBEdrop)  20-6.6= 13.4VDC across the 7.5K resistor 13.4VDC/7.5K = 1.78ma  that is the source/drain current of the source follower jfet. 

13K x (.5 X1.78ma)= 11.57VDC, 26VDC + 11.57=37.57VDC (close enough to 40VDC)  I am not using the bias string current and base current for the PNP.

So select the Jfet or move the "tap" node of the 39K and 84K of the two top resistors 123K total

Lets do this 36K +39K +84K = 159K
26VDC/159K =.164ma
36K X .164ma = 5.904VDC close to 6
84K x .164 ma = 13.77
39K x .164 ma = 6.39

all add up to 26.064VDC

5.904VDC + 13.77VDC = 19.67VDC
19.67VDC(gate voltage) -20VDC (source voltage) = -.326VDC

VDS =6VDC
VGS = -.326VDC
Id =1.78ma
 
Doing the math on the bias voltage divider is instructive. Let me pose a few more questions.

Idss - Given that the operating Id is selected to be 1.78 mA (and why was that value selected?). this implies that the 2SK170 should be the BL variant as specified. (Idss selection - GR 2.6-6.5 mA, BL 6.0-12 mA, V 10-20 mA) Using  a GR would put you too close to saturation and the distortion that will follow. You could use a V selection I assume. What problems in using a V? (cost, other effects?)

Vgs - Vgs sets Id for any specific device. Then to set the bias correctly Vg would be adjusted (via the 39k to 84K divider) to change Vgs to the point where Id was 1.78 mA. Is this what we are talking about where it specifies adjusting the bias on a device basis?

This appears to be the ""mechanics" of it. I must be missing almost everything.

Higher order questions: Why select Id as 1.78 mA? Will changing Id change the gain (and should we adjust it to match 2 mics)? What about noise and clipping?

Thanks!

--Ethan

 
Thanks for your thoughts guys.....I have to admit the maths goes over my head Im afraid, most of it that is keep it coming tho maybe will sink in ....& others learning too

Ive built this thing in a biscuit tin & have been swapping components & unscientifically listening to any changes singing Rick Neslons Poor litttle fool into my DAW....

I replaced the string 39k 84k 36k with 100k pot & a 47k pot & a 12K fixed resistor........159K, so now i should be able to adjust the voltage into the Gate of my 2SK170BL & to the Base of my 2N5087G.....

Please tell me if im talkin crap & doing silly things by the way....

Im using a mackie mixer with its phantom power.....I have 22.7 V at junction of the 2 x 13Ks ....schematic says 26V here ....should be ok
tho yes??

Heres where I need most help what are the voltages i need to set to set the transistors for optimum sound & where do I measure from please....

This is what im getting now having not even turned the pots......  22.9V top of 100K pot (as I said not 26V as in schematic)

wiper of 100K & 1 side of 1G R2 is.....19.9 V

wiper of 47K pot base Q2 junction 4.76V...

I dont no if im measuring the right places, Iv connected my meter between ground and the above nodes....

Thanks for help....

 
Tried 2G resistors vs 1G resistors..... to my none golden ears sound the same in non scientific tests..
 
Yes I removed two layers of mesh and the resonator discs. That made the mics sound a little duller and lifeless to my ears. I recently put the discs back to the other mic and tried to put one fine layer of mesh there also which was not easy and didn't succeed perfectly. But the Mic sounds better now. Next thing I'll try is one layer mesh with resonators. I wish I hadn't touched the grilles in the first place. I think they have put some thought in designing the grille and it nicely complements the capsule that can sound a little boring other wise.
 
I would pay attention to the voltage drop across the 7.5K.  JFET and PNP are in series.

Lets say the microphone is "seeing" a 1K load.  1V/1K = 1ma
 
> Given that the operating Id is selected to be 1.78 mA (and why was that value selected?)

That is NOT the designer's "selected current", it is what Gus computes from the facts already presented. Full analysis is tedious: I know because I did a partial workup before I saw Gus' math. My answer is 1.88mA, meaning Gus and I neglected different 5% negligibles. This may be more accuracy than Octavia's parts-pickers had.

The real "selection" is apparently that it obeys the old 2mA limit on Phantom draw. Allowing 10% for biasing that leaves 1.8mA for hard work.

But the recent raise to 10mA limit does no good, because we need "high" voltage to polarize the capsule. 2Ma in 3K4 is 6.8V drop from 48V un-loaded Phantom supply, leaving just over 40V for capsule polarization. That's OK and we don't want much less, so 2mA is an upper limit. OTOH, good JFET+BJT action into loads near 1K suggests more than 1mA for hard work. Hence almost-not-quite 2mA.

> my phantom power is different from my pre a mixer I have and a cake walk pre I have

With nothing plugged in, do you get +48V on the holes?

For this mike, you really should.

> is the bias points for the transistors critical ??

The more critical voltage is the +40V at the XLR pins. This is capsule polarization. Sensitivity will vary directly as this voltage varies. Significantly lower voltage, like 25V, will increase THD on loud sounds.

The transistors will self-bias with anything that makes the capsule happy and with almost any transistors.

See attached image. The voltage across the 84K is applied (with small shift) to the 7.5K. Transistor action levers the 159K/84K against the 7.5K, so the transistors together act-like 14K. The 159K divider makes this about 13K. The 2*13K power combining resistors plus the 2*6.8K Phantom resistors work out to 9.8K. So the JFET Drain should sit near 13K/(13K+9.8K) or 0.57 times the unloaded Phantom supply, or +27V. If JFET Vgs is about 0.7V more than BJT's Vbe then it works out to 26V.

This system IS "self-biasing". The 0.6V-0.7V and 0.5V-2V device voltage tolerances are SWAMPED by the ~~14V voltage between them.

The PNP only needs to have hFE over 50: from 50 to infinity makes less than 10% difference in current and this current is NOT critical except that we'd like to find over 30V at the "40V" point so the capsule gets polarized good.

The JFET nominally should have Vgs=1.3V at 1.9mA. This means Idss must be well over 2mA and Vth should not be large. Most jellybean JFETs will be fine. 2N3819 seems a good pick to me. 2SK170-BL is not an all-purpose device. It has large gate area, therefore large capacitance and leakage. It will work; it may not work any better (unless you believe it does).

> replace those 3 resistors with a 100k pot a 47k pot and a.......

NO. Leave them alone. They are correct.

As long as the JFET will pass 2mA, it should work. If you get a high Idss and high-Vth FET (source sits several volts higher than the 39K-84K junction), use a different FET. If you can't, then the voltage across the 7.5K will be large, current will be large, drop in 6K8 Phantom resistors is large, "+40V) will be somewhat lower. I can't find a reasonable JFET which won't bias acceptably.

The other mods.... don't make technical sense. But I respect Dorsey. And reading on, I realized that these mikes were assembled at low cost with odd parts. Changing a good 680Meg resistor to 1Gig will NOT change the top-end. However if Octavia was using bad parts (a good high-R resistor is not common nor cheap) then any good part may improve sound in unexpected ways.

It's a extra-clever circuit. Aside from being quasi-balanced (irrelevant in small studio work) I see no problem with the topology or the nominal values. If it was assembled with scrap parts, and you are in a position to invest, put good parts in. Certainly crappy electrolytics should be replaced before rot sets in. If Q1 is suspected of being east-europe reject stock, use a good new asian JFET. NPO or Silver-Mica cap at C1 is cheap. The 10nFd before R1 should be any *good* film-cap. C1 should not suck. C3 C4 should be good and should be reasonably well matched for best hum rejection.

Note that the JFET source is bootstrapped. Its source-induced THD is far under 0.05%.

There is another FET distortion: nonlinear Cgd. At high levels with large-gate FETs, this may dominate.

The emitter follower THD is tenths-percent near clipping and lower for all normal levels.

But excess "modding" just invites mistakes and accidents.

> 22.9V .... not 26V

You have very nearly 40V at the "40V" point, right? Then you are fine. Take out the trouble-prone pots and enjoy.

> I removed two layers of mesh and the resonator discs. That made the mics sound a little duller..... I think they have put some thought in

Yes. Over-diaphragm resonators are standard tricks of the mike-designer trade. They can bump a droopy top-end. They are not free; the fact they are used is a hint that they may be useful. The clearance tolerances may be microscopic.... getting it back just-right may not happen.
 

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Wow! Thank you PRR for taking the time to compose this. Good thread...not much mic discussion here lately.
 
Yes big thankyou to PRR & Gus for technical info blowing my mind really.....Im so glad others are getting something out of this, wouldnt want your efforts going to waste as sadly for me a lot goes over my head, tho I can keep reading info while doing my experiments & things slowly sinking in...

As I say I have this kinda breadboarded on my desk at the moment.....my goal is to build a best version on this circuit without size restrictions of original mic & so on...I have one cap thats bigger than original mic! am keeping original mic stock for now for reference, i think its easy to make mics sound different but not always better & can loose sight of the sound sometimes, I think my mic has many of the improvements already look & sounds good to me......however.....

I would like to try the J305 FET after the good reports I read.....should tha work with the current bias setup ? that was the reason I installed the 2 pots you see...

Once Im happy with the best sound I can get for me il will make a case for it to live in.....

thanks for yr contributions chaps

 
I built this circuit some time ago traced from my 012s.  I built it to fit in a larger body for a 32mm etc capsule.
This is a nice transformerless circuit.  Low parts count and clever like PRR posted.

Parts I used
Two 1 gig from MXL microphones
170 to try it from a MXL microphone, had one in sight on the bench when I built this, it is a SF type circuit.  Try what you have depending on how a JFET is used specs can matter more or less.
2n3906 picked because I could not find a higher Hfe PNP in my parts
Metal film 7.5K because I had one, carbon films for the others
2 Pan 100uf 50VDC HF output caps again because that what I had(not that critical IMO)
2uf polar film (Pan V's) could use a 10uf tant etc(this matter a bit)
Two .01 pan V's output filters, If I was to build it again maybe COGs
1,000pf polystyrene input, select for your taste (this selection is were you will hear any if at all sound change IMO, because of the voltage difference across the plates)
I had this in body I machined it sounded OK with different capsules.  I took it apart some time ago, if I find the board I will measure the voltages

This circuit with a M7,K47,CT12 might be a fun build.  I might build it in a Skylar body with a k47 like with a small change to the circuit.
 
Thanks to PRR and Gus from me too! Dorsey article has bothered me because it explains the circuit in so few words. Finally I understand something about it and why this circuit is said to be clever.
 
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