Insomniaclown

Re: KM84 circuit question
« Reply #20 on: July 03, 2011, 04:03:10 PM »
I made good use of adding resistors in series while I was testing FET's to get non standard values. Adding resistors in parallel works well too. I just happened to have lots of smaller values. I tried a trimmer as well, and it worked well enough, but it was just too fiddily for me.

I started off by measuring the drain voltage on each FET with a 10k source R. One thing I noticed, and I am not sure if this is true of all 2n3819's, but the higher the Drain voltage at 10K, the lower the source voltage with drain adjusted to 10V. My measurements of my 10 FET's all point to this correlation. My best FET 10.5v/2.1v @6.81k (fine tuning needed on the R) gave 19v with the 10k source R. My "worst" FET gave me just over 4v @10k source R, and over 4v @ source with drain adjusted to 10v. This gives more credence to the optimal 3.9k source R. So a quick and dirty way to sort a hundred FET's would be to pick a static source R value, measure the drain voltages, and pick the FET's with higher drain voltages. Then adjust as normal. Not sure if I have enough data to come up with a ratio.

The rotary switch plus trimmer looks like a nice macro/micro way of doing it. To tell you the truth, I really didn't mind swapping resistors in and out while adding more in series to get the bias adjusted. Turning off the phantom inbetween swaps of course. I sort of like the idea of getting a trimmer set to where the bias is right, and then just popping that trimmer right into the circuit. Then you can make more adjustments by ear if needed.

Edit: Just picked up an XR 2206 chip to build a tone generator. I'll it up to do a 1khz sine wave initially. Probably do this after the mic is constructed with a 50pf cap in place of the capsule for testing noise and the like. Then I can just plug it into a preamp with some oscilloscope software running to check out whats going on.
« Last Edit: July 04, 2011, 04:02:10 PM by Insomniaclown »
Ryan aka Rybow aka The big rybowski

"Can't sleep, clown will eat me!"- Bart Simpson


Insomniaclown

Re: KM84 circuit question
« Reply #21 on: July 08, 2011, 03:54:26 PM »
Alright. Circuit is almost ready. I have one more question. In the schematic I posted earlier, there are 2 47Ω resistors (R11, R12) that have a bridge. The schematic note says to remove the bridge for KM 83...KM 85. I am going to experiment with an omni switch, so I am wondering how crucial this bridge/resistor is to the operation of the mic.

Hopefully I'll get to fire this mic up today and see how she sounds. I did end up stuffing a 10k trimmer in the circuit, so I'll be able to vary the setting of it.
Ryan aka Rybow aka The big rybowski

"Can't sleep, clown will eat me!"- Bart Simpson

MagnetoSound

Re: KM84 circuit question
« Reply #22 on: July 08, 2011, 04:29:28 PM »
In the schematic I posted earlier, there are 2 47Ω resistors (R11, R12) that have a bridge. The schematic note says to remove the bridge for KM 83...KM 85. I am going to experiment with an omni switch, so I am wondering how crucial this bridge/resistor is to the operation of the mic.


It says to remove the bridges for KM8*p. The p is for pad. These are build-out pad resistors for the 50-ohm strapped version. Not required, and nothing to do with polar pattern.

Dan

I don't think people realize what an embarrassment of riches this place is   -  Paul Gold

Insomniaclown

Re: KM84 circuit question
« Reply #23 on: July 08, 2011, 04:35:03 PM »
AhHa! Ok, that makes sense. Thanks man! Much appreciated. I'll just leave them out then.
Ryan aka Rybow aka The big rybowski

"Can't sleep, clown will eat me!"- Bart Simpson

Insomniaclown

Re: KM84 circuit question
« Reply #24 on: July 18, 2011, 12:32:10 PM »
Booyah! Got this puppy working last night. RK 47 capsule, KM 84 circuit, Tab Funkenwerk T49 transformer. Worked on the first go. Output is nice and hot with no distortion detected. I get a very good vocal level @ 40db's of gain on my Ensemble. Noise is ridiculously low. Tested out cardioid and omni modes. Omni mode works surprisingly well! I did a test on a song that I am working on with the intention of having this vocal as a back up, but it had more presence and power than my previously recorded lead using an SM 7. This is in Omni mode too!

Nice vintage tone. Very focused but with some serious weight on it. The mid range is very nice indeed.

Little more body work to do. Have yet to test the pad switch. Anyway, great circuit! Nice and easy. I am considering doing this circuit in my other mic instead of royer modding it.
Ryan aka Rybow aka The big rybowski

"Can't sleep, clown will eat me!"- Bart Simpson

abechap024

Re: KM84 circuit question
« Reply #25 on: July 18, 2011, 12:45:44 PM »
Sweet! Pics?? :)
AC Sound - some DIY circuitboards

Insomniaclown

Re: KM84 circuit question New
« Reply #26 on: July 18, 2011, 01:09:14 PM »
Very soon. Just have to finish up the body and make it all purty like.

Edit: Just got my 6205 tubes in the mail. Guess a royer mod is inevitable.
« Last Edit: July 18, 2011, 03:19:58 PM by Insomniaclown »
Ryan aka Rybow aka The big rybowski

"Can't sleep, clown will eat me!"- Bart Simpson


 

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