KM84 circuit...low gain.

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
G

Guest

Guest
I have built a KM84 circuit inside a MXL2003. It sounds great, but it takes about 60db of gain on my preamp to get good levels. It's quite useable, but I'd love it if I could get about 10 more DB out of it. All the voltages seem right... any thoughts? Oh, and I am using a LL1636 wired 10/1. Thanks.
 
Thanks for the response Jacob... I used a 2n3819 as indicated in the schematic on your website, any idea what the optimum voltage should be, or what to look for in the specs to tell me? I am definetly no expert on J-fets... Thanks!
 
I got 10.5v at the drain and almost 3v at the source... change j-fets perhaps? and what voltage should the capsule be getting for polarisation? Thanks again.
 
Capsule polarization should be about 45 volts but you can't measure it at the capsule - you need to measure it before the 1 gigohm resistor, and assume it's right at the capsule, and it should be unless you've got something leaking like a fingerprint or that sort of thing. One other thing is the value of the feedback capacitor - it's shown as 4pF, make sure it's not 470pF or something like that! I made that mistake once (or twice). Very low gain if you do that.
 
You want the source R to be 3.9K and the drain V to be about 10V. This is the price you pay to get a good operating point with a simple 1 FET circuit.

FWIW Tim H's schematic has a error the 5.6K should be a 56K I asked Tim to keep the error to see if anyone spotted it.


Zebra I changed the microphones transformer in the pictures I sent you with the backbone constuction microphone. You think anyone might want to see the finished microphone?
 
[quote author="Gus"] I changed the microphones transformer in the pictures I sent you with the backbone constuction microphone. You think anyone might want to see the finished microphone?[/quote]

I certainly would. Does it have a capsule now? Send the pix and I'll upload them. Funny about the 56K!
 
Thanks guys for all the help, you all rock!! :razz: I got the fet biased correctly now... But it was that darned feedback cap that was killing me, thanks dale! One question though... if the fet isnt biasing correctly, does that make it operate in a less linear fashion?
 
Fets, bias and phantom. What I think neumann designers had in mind with the km84.

First the FET is a high gain one IIRC x60. The circuit runs off phantom. The circuit does not use a DC to DC converter for the capsule. X60 is to much gain without feedback. The transformer is 7:1 I believe in the KM84.

Note the use of a 24V zener to a 10K to the 47K in the drain leg, with the drain at 10V and the "top" of the 47K at about 20V the current is about .21ma 10V / 47K. .21ma is low current so the 48V phantom does not drop alot at the microphone circuit allowing a voltage in the mid 40s to still be advailbe for the capsule. Now from the Neumann book "Microphones" the circuit gain should be the capsule cap in pf / feedback cap that in parallel with the FETs open loop gain, stock km84 34pf and 4pf. The fet being used at a low current and the use of feedback controls the gain so the signal level is not to great for the transformer.

The circuit is kind of like the KM54 circuit if you look closely the AC701 has feedback from the plate to capsule. From what I have read people like KM54s that often don't seem to like microphones circuits with feedback.

I like this circuit because it is clean and allows you to hear the capsule/grill more, it is a good test circuit IMO.
 
Wow, Thanks for the explanation of the circuit Gus.. I'll have to read that a few times. But I did get the feeling that the circuit is doing little to alter the sound of the capsule. A very natural and balanced sound... I'll have to build more in the future :grin:
 

Latest posts

Back
Top