Rebuilding Calrec Small Diaphragm Condenser

I bought this Calrec mic (a CB7C) along with a faulty Beyer M201 for £25. I can replace the capsule on the Beyer, but I thought I might get some fun out of the Calrec.

So, here it is:





Appears to be originally battery-powered with a DC-DC converter and unbalanced output.

What I'd like to do is put in a new circuit for 48V. I was thinking of either getting a board from another SDC and putting this in (maybe someone has an old board they replaced?), or making something on stripboard. My worries with strip board are the impedance between the tracks which could be an issue at the capsule end. If I do make my own circuit, I'd prefer transformer-balanced out as I have some small Beyer 1:5 and 1:10s which I can use.

Any thoughts?

Roddy

KM84 type circuit with the 10:1 might be fun

What battery does the Calrec use? Maybe if the microphone works you can use a output transformer with the stock circuit and make a circuit to drop the 48 phantom down, this will depend on current draw.

Edit found this at Saturn Sound
http://www.saturn-sound.com/images/original%20drawing%20-%20fc1200t%20circuit.gif

and this
http://www.saturn-sound.com/Curio's/calrec,_some_old_favorites.htm#1000

Thanks, Gus.

The mic is far from working! I'm not sure what batteries it used, but that schematic you posted was a good find.

I think it may be the same as a CB700, although it has CB7C printed on it.

Unfortunately, the PCB is in bad condition - there are lifted tracks and extra wires on the solder side added plus some other bodges. Also the case for the coil at the DC/DC converter is broken.

I think it may be easier for me to just add a new circuit. I like the suggestion of the KM84 circuit. It may be a struggle getting it to work on such a small piece of stripboard though. I was impressed by your version on a larger perfboard though. I wouldn't even mind dropping in a ready-made Schoeps-type circuit if I can find one either to be honest.

[quote author="rodabod"]I wouldn't even mind dropping in a ready-made Schoeps-type circuit if I can find one either to be honest.[/quote]

thats not very diy :wink:
you could even try making a small tube condensor with the beyerdynamic transformer. need to fine an output cap which is small enough...


click on pic for a big one

...or pop the xfo in the PSU, just like the Royer Mod and lots of other tube mics.

All good!

Roddy

I take it this mic just screws apart because of the battery power.

I wish I could get the 2 knackered Calrec SDC's that I have apart. I got told that they Aradited the threads, so you destroy the mic opening it>

[quote author="Rob Flinn"]
I wish I could get the 2 knackered Calrec SDC's that I have apart. I got told that they Aradited the threads, so you destroy the mic opening it>[/quote]

I'm not sure, Rob. I know they used epoxy over the PCBs so that they were sealed, but I never heard of them using Araldite on the threads. If they are not working, then I'd still give it a go. Maybe with some wrenches and rubber to grip. Preferably on the more solid sections as the body probably bends easily. You could try asking Hebden Sound.

Max - that tube mic looks pretty tasty. An RFT MV*** I assume? I could maybe try something with that prototyping board you use. Does anyone know what that stuff is called?

Hi Rob,

I don't know those mics but Araldite / epoxy will soften greatly with heat.

Is there any way you can apply a heat gun to the threads without risking cooking the capsules excessively?

Good luck!

Roddy,

I think it is this:

Farnell product number 1172146

http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1172146

Hope that helps.

Stewart

actually it´s branded as neumann MKD and was a (quite noisy transistor) measurement microphone, the capsule is a 1" mk102 (today gefell sells the mk102.1 , it´s successor)
yeah thats normal stripboard, didnt you breadboard a chip-preamp before?
just note, that the high-Z parts are floating, means the grid-1nFcap-capsule connections are not connected to the board but floating in the air.

-max

[quote author="ioaudio"]didnt you breadboard a chip-preamp before?
just note, that the high-Z parts are floating, means the grid-1nFcap-capsule connections are not connected to the board but floating in the air.
[/quote]

Yeah, I've used stripboard for lots of stuff. The difference is that your stuff doesn't have any tracks - I'd need to learn how to use it. Also, I'd need to figure out how to make these floating connections.

I'm thinking something KM84-ish might be nice. I already have some valve stuff in thr pipeline, and haven't made any solid state mics before...

rob flinn,
i just had an idea:
you might try winding resistance wire around the part of the mic where the thread is located (isolated from the housing) and heat it up this way.

-max

roddy,

maybe this picture helps for the highz part.
the 1nF cap is mechanically mounted to the board, the connections are floating and do not touch the board
you could alternatively use isolated standoffs.

Neat!

What transformer is it, rolled your own?


Regards,

Anders

yep, homegrown