KM84 circuit experimenting in progress ...

[gary o]

Have taken capsule away & yes the circuit is acting like a mic picks up low freq if i tap the case .....if I shout at it I can hear my voice........Is this because I have no board the thin leads of the components picking up vibrations .... or am I goin mad .......

 

[MagnetoSound]

Sounds like a problem on the capsule side of the FET ...

Check coupling cap C1, all capsule connections, check polarising path, maybe try another capsule.

Could still be some kind of contamination in the high impedance area.

Is the preamp still cranked? What effect do you hear if the gain is 'normal'?

 

[HellfireStudios]

Are you certain you have a full 48V phantom supply. These mics, like the vintage U-87, don't have DC-to-DC converters for their polarization voltage so every bit of phantom power counts.

-James-

 

[gary o]

Have tried two capsules Oktava MK101 (same as MK219/319) and Violet Vin 67 both low level still .........Im using my RME Babyface USB interface as mic pre for the KM84 I had the pre input set to 42db that brings the mush whit noise shooshy sound up when recorded on my DAW Sonar in this case its about 30% of the track is audio if you know what I mean..... to get roughly same level using my Oktava MK012 with LD Lomo capsule I have to take the mic pre down to 12db......does that make sense how I explained it ....

going to try another FET next I think ....

Thanks for your thoughts guys ....... its annoying but Im kinda enjoying the learning process

 

[gary o]

Oops sorry James yes phatom power 48VDC from the RME Babyface pre amp.....

Just changed FET no difference in level really

I have 45V node of R5 R6  so Im a volt down ....

23.5V at the zener .5 volt down ....

21.11v at R4 R7 C5 node

10V on drain

 

[MagnetoSound]

Volts at R1/R8/C10?

Are R1, R2, C1, C4 known to be healthy?

 

[poctop]

Oops sorry James yes phatom power 48VDC from the RME Babyface pre amp.....

Just changed FET no difference in level really

I have 45V node of R5 R6  so Im a volt down ....

23.5V at the zener .5 volt down ....

21.11v at R4 R7 C5 node

10V on drain

All that looks good to me , just remember that those vintage FET design as an U87a will output about 10db less than the modern KM184 version ,
just wanted to point that out ,  if you dont have a scope you can still do the hearing method you can actualy clearly hear when the FET open up ,
and just go a little past the bias point ,
Just my 2 cents,
D

 

[gary o]

Hi Dan I was just suspecting R2  when I saw your post as far as I know all are healthy but ohm meter cant read 1gig I was just trying to work out what resistance to put in parallell to measure the one gigs ......I quickly desoldered R2 & it makes no difference in or out .... ?? only have a 68meg laying around brand new from packet but the doesnt effect the circuit ....... I have tried 2 C1 but they are old so will try another.....

recap I still think R2 healthy & R1 & C4 used them in other mics

 

[poctop]

Make sure the polarisation voltage is in the ball park also,
Hope this helps,
Dan,

 

[gary o]

Thanks poctop I have the 10v on the drain & R3 works out to about 12K ......Just to see n hear I added & took away resistance somewhere between 5v & 16V I had audio....must be something badly wrong as I say the 1 gig R2 does nothing at the moment ....

thanks guys

 

[poctop]

Did you short R11 and R12 to not get another 10db drop and what is the source voltage and idle current trough R3 ? do you get that pol voltage before R1
D,
what and how is your traffo configured  ?

 

[gary o]

How do I do that poctop, thanks

Magneto R1 R8 C10 node is 44.2V

thanks

 

[poctop]

How do I do that poctop, thanks

Magneto R1 R8 C10 node is 44.2V

thanks

do what ,  measure voltage drop across R10 and calculate the idle current , Make sure R11 and R12 are shorted cause it will pad the signal again , 
just to see,  the 44.2 is perfect ,

 

[gary o]

Ouch my brain hurts..... but its good thanks ...... I have no R11 R12 so I should have highest OP right ? so pol volts should be 46V but mine is 44.2V the node Maneto asked about R1 R8 C10...

Dont know hoew to measure idle current R3 do I connect Ma meter+in series

thanks

 

[gary o]

JUst putting volt meter across R 9 or R10 0428V is that what I need to measure thanks

 

[poctop]

Waht is the source voltage on the FET,  what is the problem at this point in time now , just low output ? and could you quantify the low output thing ,
Best,
Dan, '

the source voltage will let us know the idle current trough R3,

 

[gary o]

Thanks proctop so I measure voltage at FET source ......... very low OP and mush white noise being brought up because pre turned right up

thanks

 

[poctop]

what is the traffo , did you check the traffo winding ? the wiring of te traffo , if you remove the capsule what happens

 

[gary o]

Remove capsule the circuit it self becomes microphonic ........tried several transformers using a OEP as used like in gyraf G7 .....I have mic pre set a 42db for the KM84 & level is same as RB500 ribbon mic at only 24db

thanks

 

[gary o]

Voltage on Source leg of FET reads 3.027V