Simplest way to connect RK-87 capsules to audio interface.

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robagn

Member
Joined
Mar 5, 2015
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5
I am looking for the simplest way to record off of these capsules. I am in possession of an audio interface which can provide phantom power, and I am under the impression that I need some sort of circuit between the capsule and this interface for it to work.

I have just assembled this schoeps/dorsey circuit: http://www.pfarrell.com/images/miccircuit.gif
but it has not worked, I am just getting a loud buzzing noise as the recording. I believe I have assembled it correctly but I have used the 2N3819 mosfet transistor instead of the 2SK170 indicated on the diagram since it was not available and I read that this was an equivalent transistor. Also, the parts of the circuit that should sit at 35V and 12 V sit at about 15V and 5V respectively, not sure what's going on there.

I am assembling this temporarily on a bread board until I get it to work btw.

I am worried I am making this more difficult than it needs to be. Is this circuit design the way to go? Is there a simpler way to record from this capsule?

Thanks for your help.
 
Microphone head amplifier circuits (and capsules) must be fully and entirely shielded in order not to hum like  yours.

Metal box and wire  mesh, all connected safely to ground.

On the positive side, your massive hum and noise points towards the circuit working (as a minimum in some way working)

Jakob E.
 
Are you sure your audio interface is delivering 48 V. phantom power? (Because all your voltages are on the low side.)
You can just measure this between pin 1 and 2 (or 3) on the XLR with nothing connected.
 
RuudNL said:
Are you sure your audio interface is delivering 48 V. phantom power? (Because all your voltages are on the low side.)
You can just measure this between pin 1 and 2 (or 3) on the XLR with nothing connected.

With nothing connected I am getting a reading of 46.8V, so a little low, I don't know if that is significant though.  When I plug it into my circuit they drop to like 32V or so.
 
gyraf said:
Microphone head amplifier circuits (and capsules) must be fully and entirely shielded in order not to hum like  yours.

Metal box and wire  mesh, all connected safely to ground.

On the positive side, your massive hum and noise points towards the circuit working (as a minimum in some way working)

Jakob E.

Well there is massive hum but snapping my fingers right by the mic and other things like that do nothing to change the hum. i.e. I can see no evidence that the capsule is capturing any sound.
 
gyraf said:
Start out by shielding until hum is gone, and take it from there. Otherwise you're in the dark.

Jakob E.

Any direction you can give me on how to go about achieving this? Would it be possible to do while just hooked up on a breadboard? I wanted to put off making anything permanent until I was sure the circuit was correct if possible.
 
I think you need to follow Ruud's approach. Disconnect the capsule, and probably the fet too, and try to get your dc voltages in the right place. If they're way too low, despite a correct phantom voltage level, then you've got a path to ground that shouldn't be there. Check the capacitors - are they inserted the right way round? Are they faulty (allowing dc current to pass)? Double check your resistor values - use a dmm to make sure you're not reading the colour codes wrongly. That kind of stuff...
 
Listen to Gyraf, the circuit board needs to be inside a metal box, any box, a biscuit tin will do ! box grounded.
The capsule needs to be inside a grounded mesh.
If you don't do this, you will get nowhere at all.
 
Could anyone link me to a decent DIY pcb kit that will work with my capsules? maybe one that includes the shielding? I just want to make sure I have the right circuit going and I need this microphone recording very soon, in 2 weeks at the absolute latest as I need it for a project.

Thanks for your help
 
robagn said:
Could anyone link me to a decent DIY pcb kit that will work with my capsules? maybe one that includes the shielding? I just want to make sure I have the right circuit going and I need this microphone recording very soon, in 2 weeks at the absolute latest as I need it for a project.
Microphone Parts do some really nice kits.

http://microphone-parts.com/microphones/

Tell them you have one of their RK-87 capsules and ask for advice.  They are very accomodating and helpful.
 

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