sizing up some mic capsules for use in a DIY circuit

[0dbfs]

Hi,

I have a couple doner mic's that I would like to turn into something and learn from.

My goal is to ascertain what I can about the existing capsules, and build an amplifier circuit around the existing capsule in the existing body that works.

The mic's are front address condensers and the existing circuit has been modified in a way that makes the mic's un-useable for me at this point in time.

I plan to research the various fet circuits and build in a simple 48V fet circuit for these mic's. I really want to keep the initial cost down and learn a bit and may plan for some higher grade trafo's down the line. I have some different trafo's around that I might be able to use for these.

I plan to keep the capsules, use the bodies, and replace the amplifier.

The mic's in question (un-named for now) use a "stretch-rubber" type of capsule mount system which appears to be common for several of the mic's they sell. They also use a very thick PCB instead of internal support rails which attaches to the capsule mount system. The body tube slides over the PCB and screws into the XLR-base section.

I don't have any reason to believe that the existing capsules are anything "special" or particularly uncommon.

I am wondering what test methods I might be able to use to determine the ideal operating conditions of the capsules (I can't find any specific tech information about these capsules). ie; optimal or acceptable voltages, response characteristics, etc. Are condenser capsules generally "ok" within a certain range of operational parameters. ie; loading impedances, polarization voltages, etc..

Thanks in advance,
jonathan

 

[rodabod]

They are Rode mics then? If so, the earlier models were 797 capsules which are nice with a little top-end rolled off. Based around the K67. Not sure about their later caps, but wouldn't be surprised if they were similar.

500M to 1G and 60-80V are probably good ballpark figures for throwing a circuit together.

 

[0dbfs]

Hey Roddy,

Thanks. Yes they are rode mic's. Most likely 8 or so years old so I guess that would fall into the earlier category.

The information I have found so far shows that the mic (broadcaster) uses a Rode manufactured HF2 capsule. Not sure if this older mic would have that in it or not. Apparently the NTK and NT1000 are also reported to use the HF2. It's listed as 26mm/1". Not sure if that would be a rebadged 797's K67 or not.

I'm wondering if something like a km-84 style amp would work well with this type of capsule? It doesn't look like an 84 is set up for the 60-80V range at the capsule though.

Thanks,
jonathan

 

[rodabod]

Maybe take a look at the capsules yourself or post a photo.

If they are 797 caps (or similar), I'd recommend at leat trying a low-pass filter on them unless you want something really bright. A transformer output will generally help soften the sound in my experience, so you may wish to also take that route.

I've not tried the KM84 circuit, so can't comment there.

 

[0dbfs]

Thanks. I'll compare to some other photo's and see what I can tell when I have them apart again.

Is there a non tube circuit in particular that you might recommend which is pretty straightforward in implementing different compensation filters and output transformer's?

Thanks,
j

 

[rodabod]

A "proper" circuit would be that of the U87 I suppose. I can't think of any others like it, though they may exist. It's certainly not the simplest though, at least compared to the KM84.

What I might try personally would be a little experimentation. You could maybe try a little HF NFB on the transistor stage with that KM84 circuit to reduce the treble if necessary after building it. Or, if it is happy enough to drive it, just add a passive network.