KM84 circuit experimenting in progress ...

I have just made a point to point version of this circuit in a choc tin using a Oktava MK101 head with same capsule as MK219/319 for .....got all voltages correct so I think I have the 2N3819 biased correct Im using a OEP 1:1:6.45 transformer at moment tried couple others too...... mic doesnt sound bad but lots of back ground noise just general white noise cant work out why its so noisy Im sure it shouldnt be this noisy...... further investigation needed..... may try another capsule .....I didnt have a pair of 10M res for R5 & 6 so used 2x 12m2 shouldnt be causing my noise should it ..... as I say voltages all seem good got the 10V on the drain used a 10K trimmer + 1K on FET but needed to add another 3K so the FET would tune in to 10 Volts, but thats ok is it not ?

Thanks

Have you cleaned the boards with alcohol (carefully, if styrene caps are used in the build), and are you testing the mic with the mic body on (ground hum, EMI, and RFI can cause lots of noise)? Alcohol or too much soldering heat can create faulty styrene caps which are common sources of noise as well.

I personally haven't had any noise problems with my point-to-point KM-84's (MXL 604S donor body and capsule, Cinemag CM-2510 8:1 output x-formers). Although, I used 10M ohm resistors for R5 and R8 (I assume you meant R8 since R6 is 56k ohm). Good luck.

-James-

Thanks james sorry R 8 & 5 are larger than 10m as I said point to point there isnt a board to clean tho I will clean everything, its not in a mic case its in a choc tin but not earth or RF noise just noise floor i guess, no polystyrenes to break & I also dont have a 3.3pf cap for the feedback was leaving that out for now that shouldnt cause noise should it .....

Thanks for your thoughts

I perfboarded a km84 circuit before, and never could get rid of the noise, you have to be very careful of how you lay everything out, and make sure the capsule cap + fet don't touch the board or you have a really good stand off.

Interesting about not getting rid of the noise ..... I have no board its literally point to point but I do have a transistor socket so will solder direct now I found a good FET then clean mess with the layout ..... thanks interesting to hear others experiences.

Could you post a picture? It may help troubleshooting to know how it all goes together, and a picture says a thousand words.

-James-

Thought I had cracked the noise floor issue ......I had a 4700pf cap for C1 instead of a 470pf , I read another guy here did the same twice as well ..... I shoved in a 500pf I found expecting greatness but no ....sounds about the same the mush white noise sounds like a mic with a pad switched on and mixer turned up to compensate ..... could it be my zener, I just put a 24V zener from Maplin in there.....voltages look about right tho ......the thing is a mess more an experiment than a build its like an audition for a build, I still have to clean it up but have made many mics this way hands & crap all over them & have yet to have had a problem due to this...... my C3 is 47u on some schematics its 1.5Uf.... ? also C5 C8 C10 could they cause mush sound, I do build stuff out of whats laying around so they could be suspect....I just built M49 that had bubbly sound going on turned out to be cathode bypass cap & in my MK47 heater voltage dropper resistor was shite & caused merry bloody hell....I will try another capsule next & a clean, might be able to post a pic its not pretty tho haha.

Cheers

I have both schematics you had mentioned. I weant with the pdf version because it was a clearer read. Also, you have to look really closely but C3 is 4.7uF in one schematic and 1.5uF in the other. 47uF is too high. I used 1.5uF. Since this cap is part of the FET biasing it may be the source of your noise.

-James-

Drain resistor should be 47k. One erroneous schematic shows it as 4k7, so make sure you are working from the right schematic.

Ah thanks fellas the PDF is the schematic I have been using.... before I soldered I read all the threads about KM84 & made notes on the schematics , came across Dan pointing out that R4 is 47K so I went with that schematic then Im sure I read somewhere that C3 is 47uf looks like I forgot the decimal point tho ....big thanks the iron is warming up now ....

I was up late last night reading a large chunk of the U87 build thread about biasing the FET ..... I couldnt fully understand it but was bloody interesting wondered if same apllies to this circuit, I will read again to try to understand.

cheers

Nope 4.7uf for C3 still sounds about the same also sodered the FET instead of socket........ in the words Percy from black adder ..... BUGGER....

Biasing the KM-84's FET is almost identical (except for the part numbers). On a related note, an oscilliscope might not be a bad investment. They're great for obtaining the maximum headroom and gain in your mic builds.
Also a picture might help immensely in determining the noise issue. I know you have cleaned the "board," but did you clean the capsule connections? It is reallly the Hi-Z side (capsule to FET) of the mic that needs cleaning to minimize noise.

-James-

Hi James Havent cleaned anything as yet not got alchahol but will try to find something to clean with I dont have a board to clean its point to point..... It seems like a level problem I think maybe the FET set up wrong after reading about the U87 circuit..... I do have a little scope not sure how good it is and dont really know how to use it .....

Im gonna clean check voltages & compare level to a known mic, cant really get a pic at moment as circuit in choc tin for shielding will have to un solder it hard to explain ....

I will be back

Wow huge difference in levels between KM 84 circuit & a Oktava MK012 with lomo large diaphram head (similar capsule to K47) a lot of the noise is from just cranking up the mixer to compensate for the low output of the KM84..... I think I must have something quite wrong ..... will investigate further .....

Is the transformer oriented correctly? Are the pins on the FET wired correctly (you'll need the FET's datasheet for the pinout info)? Did you implement the -10dB pad (C9) in your circuit? If so, the pad may be shorted thus lowering your output significantly.

-James-

Transformer seem correct have tried several all worked but same sort of level low ...... I have already double checked the Pin outs via data sheet & found confusing results making me suspect maybe mine could be wrong the gate was middle leg on both data sheets so I swapped the drain & source..... result similar low sound level voltage at R4 47K still the same tho ...... weird ..... makes me think of mattadoors explaination of setting up a FET seems my voltages are right but FET just not working well, sadly I cant grasp the theory as well as others, very interesting tho ......

Thanks for great advice

It is possible you've damaged the FET along the way, it is easy to do. Have you tried swapping it out?

An oscilliscope would show you if the FET is passing a healthy sine wave. They are wonderful troubleshooting tools in general. I resisted buying one for far too long. It's a minor outlay of cash in the grand scheme of things. It also makes finding problems and FET biasing much easier.

-James-

Thanks Dan yes already killed 1 FET about time to try another ...... James I might reach for the scope not sure its a good one tho ....

Forgotto say I dont have C9 and switch for 10db ..... also no 3.3p feedback cap...

Gonna try another C1 470pf incase mine is dead .....

thanks

Heres a weird thing not sure how to discribe........... when I move the in this case lollypop head with capsule I hear quite loud handling noise & expect a higher level when i speak toward the mic but no level of voice is low .....its like the electronics become microphonic ...... does that make sense what could cause this.... I bet Im doing something really silly ....