KM84 circuit experimenting in progress ...

[poctop]

42db would make sense for the KM84 would it ,  i think the U87 clone wich is about the same circuit as this one here is driving at that pre level ,
seems the electronics is doing the job and voltage reading is dead on ,

 

[poctop]

The ones I have put together are all consistent with regard to sound and output level. When I first put them together I just set the bias with the (10.5V?) drain voltage method. This was fine and they sounded good. I did end up recording some louder sources (horns and toms) which resulted in asymmetrical waveforms and some distortion which I had not observed with quieter sources. I then went back and revised with the scope method and they no longer do that. I didn't sort any of the fet's but I do have spares and extra's which I ordered from different sources. These do have lower output than some other mic's I have but I consider that an attribute to use in my gain staging and mic selection. For instance, I often mic up acoustic guitars with km184's and may typically use a gain setting of 21-24db on the pre. The u1621's get something in the range of 39-42db of gain on the pre for similar output levels and placement.

I haven't made "scientific" comparisons of sonics or levels. Rather use them and gather my empirical observations by working on the tracks, mixing, and taking a more zen approach to what the final results are on that particular recording/project.

All that said, I do really like having these in my kit and consider them to be high quality tools based on empirical assessments.

I did order a bunch of extra pcb's and intend on making more

Quote from 0dbfs regardind the U87 clone.
Here,  your 42db is totally making sense after all for this type of circuit.  i think people are just not used to it, single Jfet converter /transformer are nothing hot like modern mics and that is what i like about them , you can give it the color of the pre as well,  if you compare a 84 to a184 you will find that the 184 is way much hotter and the 84 remains the favorite for the overall sweetness and $$$ che-ching also.  but the noise is another story. So all in all I beleive your are dead on as for the gain. I am actually too poor to confirm that with a real 84 but i wish. So might the problem be the mic pre you are using 
Best,

 

[gary o]

Thanks poctop I thought the mic would have a low output but maybe not this low when I say 42d thats just the setting I used to compare mics would have to turn up my very quiet mic pre to get decent level for recording so I think something is still a miss...I have a Oktava MK319 here a single FET transformer circuit it has way more out put much less noise..... Im sure it must sound better than this maybe I can knock up a video demo for youtube might be fun I mus be doing something really silly & will prob feel a complete tit when I realize.... I built a M49 circuit a MGM church mic circuit & a ElaM251 circuit in the last couple a days all sound fantastic no noise lots level but I can get me head round FETs tho I did make a nice sounding Oktava 012 circuit on vero board.

Thanks for help

 

[gary o]

http://youtu.be/CwGHlB-6uFM

Dont laugh 

 

[Gus]

Are you using a A262A3E?

How is the output transformer wired/set up?  What is the ratio you have it set for?

Are you wired something like this?
Drain to B2, B4 connected to B1, B3 to ground

Output A3, A1 connected to A3, Output A2

Or do you have the outputs in parallel?

 

[gary o]

Hi Gus OEP  same as used in G7 wired in parallell..... cant remember model number 1:1 6 :45 :45 sorry cant remember numbers, tried some 10:1 as well but lower level, thanks

 

[poctop]

http://youtu.be/CwGHlB-6uFM

Dont laugh 

Awesome

 

[Rossi]

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

Nah, that sounds wrong. There are at least two versions of the RB500 ribbon, but even the high output one (too high transformer ratio) is lower output than a U87 (even the old version).

Make sure your transformer has the right orientation. Wiring as step up instead of step down is a common mistake. The KM84 transformer is 7:1, but that seems about as low as you should go. The U87 transformer is about 10.5:1, if I remember correctly.

Your source voltage seems to high. Should be about 1 to 1.5 V. You often have to go through quite some FETs to find one that produces the correct source voltage after biasing the drain voltage to about 10V. Slightly higher voltages up to say 2V will still work okay, but won't give optimal performance. 3V seems to high.

 

[gary o]

Hi Rossi thanks for your input ....... yeah Im sure this isnt right when I disconnect 1gig R2 it still works .....I initially thought if I get the FET to sit at 10V I have a good one but after reading not quite understanding the first few or so pages of http://www.groupdiy.com/index.php?topic=48030.20 a member has trouble with his build & matador helps out with some interesting stuff which will now read again as I think theres mention on how to find a good FET the lower source voltage .....

Yes my RB500 is the lower OP as well....

This is the OEP tran & how I wired it..... also tried other way round just in case ......lower still OP

http://img.photobucket.com/albums/v439/jazzy_pi/OEP.jpg

thanks

 

[poctop]

Your source voltage seems to high. Should be about 1 to 1.5 V. You often have to go through quite some FETs to find one that produces the correct source voltage after biasing the drain voltage to about 10V. Slightly higher voltages up to say 2V will still work okay, but won't give optimal performance. 3V seems to high.

i have built Numerous U87 circuit and all i was aiming for was a proper Low Idss Ffet and did not make that much diference in the 10ma range when properly biased.

assuming you have about 3V on the source then a 12K source selected resistor will bring back the idle current to about the "spec" at 0.25ma , So i do still beleive that there is nothing wrong with the FET but i am maybe wrong , wiring the traffo backward could easilly make it tough.,

Just my 2 cents,

 

[gary o]

I really apreciate everyones thoughts here everyones help here its brilliant, Im still not sure if the circuit is working right BUT have learnt sooo much from this ......

Heres a re cap of what I have tried

5 transformers wired either way round
2 capsules
about 10 FETs
Tried 4x C1s one of which was 4700pf but that didnt seem to make much differece to the sound ......
tried two R2 ...removing R2(still works should that happen)
adding more resistance or lowering resistance on the source 12K at moment
replaced R8 R5 with proper 10Ms now
changed value of c3 to 4.7U now..... had 47u no difference to sound
tried another C4

thru all the things I have tried my voltages all look right & I could always adjust R3 with trimmer to get the 10V..... but I didnt understand or even measure the source voltage, I need to re read Matadores explaination......

But the things that make me think the circuit is wrong somewhere is the fact that taking out R2 or having a 4700pf C1 dont seem to make a difference, that cant be right can it & the noise floor

 

[MagnetoSound]

You know, it seems you have changed everything out a few times now, and still it won't budge ... we may have reached the point where only good, close up pictures will tell the true story.

A fresh pair of eyes, you know ...?

 

[gary o]

Haha think your right Dan but its a bloody mess & your all gonna tell me off & then some one will out something really silly I probly done & Im gonna look a right tit haha then il get told off some more for wasting everyones time.......Pix also might not be good because its a cludge of components BUT maybe I can do an even trashier video guided tour with audio thru speakers & camera built in mic

I just tried this

Put your DMM in current measuring mode.  Take a Jfet, short gate and source together, and connect to negative terminal of a 9V battery.  Connect one DMM probe to the drain, the other to the positive 9V battery terminal.  You should be able to read the IDSS on the meter.

If your meter can't measure current, then put a 10 ohm resistor (measure it first!) between the drain and the positive battery terminal, then read the voltage drop across the resistor and divide the voltage by the resistance you measured.
[/quote

My FETS are between 00.07 ma & 00.12 ma

I put a 00.07ma one in circuit R13 down to 9K now drain 10V source read 2.1V if im measuring correct.... ground to source leg of FET..... still low output

reaching for video camera

 

[gary o]

OK video was disaster haha ended up just comparing level with SM58,,,,, but have taken a picture will post shortly ....

http://youtu.be/RcwMxtDNQTk

 

[gary o]

Grrrrrr pic wont post server error .....

hope this works instead...




Uploaded with ImageShack.us

 

[Rossi]

Well... doesn't look too clean. The super high impedance area around the FET gate and the capsule including leads and mount must be super clean. A greasy spot or a fingerprint can have a much lower resistance than a 1G resistor. That may explain why taking out R2 didn't make a difference; there's likely a leakage path that's lower resistance. You also have to check your capsule mount. I've had a rubber mount that was barely in the megaohms range, thus creating a leakage path to ground.

C2 can be anything from 470p to 1000p without too much (or any) audible difference. The resulting hi pass frequency is well below the audible range; this cap is oversized for minimum phase shift. Higher values not recommended, as a big physical size may introduce new problems. Your "magical" 500p cap actually looks pretty big to me (and not too clean), and the metal case may cause problems via unwanted contacts or parasitic capacitance. Better use a smaller size polyprop or polystyrene cap. NP0 ceramic can sound good, too.

C3 is correct at 4.7u. You can increase that value for more bass. 4.7u was probably chosen to reduce of some LF rumble; a small tranny like the KM84's can't handle too much LF.

 

[gary o]

THanks Rossi I tried two capsules in their head basketson this both of which work on other DIY mics ...... tried several 470pf ish caps but will try another now .... just cleaned comonents  again it doesnt seem to make any difference if I have my greasy fingers all over it or if its just been clean I would expect levels sound or noise to vari ..... but its excactly the same what ever I do ....

Do you think I can rule out other components as voltages seem right so concertrate on FET R1 R2 C1 I have no C2 anyway

I have another circuit I threw on bench now Oktava 012 circuit works great with same capsulesas KM84, had fingers all over it I cant get it to sound badeven when I try , same for two valve circuits I made this week M49 & MGM church mic circuit, I cant work this one out ....

Thanks

 

[Rossi]

Not much I can say. Sometimes you just can't see your own mistakes. Wait a day or two and look at it with fresh eyes.
Or just build a new version, preferably on perfboard - it makes your circuit easier to comprehend and troubleshoot.

 

[gary o]

Thats just what I said to myself thanks Rossi ....I will re visit it another time ....... well I earnt something about FETs going to fiddle with Oktava circuit

Thanks

 

[poctop]

What is Occam's Razor?


The most useful statement of the principle for scientists is
"when you have two competing theories that make exactly the same predictions, the simpler one is the better."