48v supply for Calrec condenser mic

[John M]

I have an old Calrec condenser mic that connects to a box that provides 48volts to power the mic and also has an attenuator (just a simple pot). Its NOT phantom powered - the 48volts use two wires and then the signal goes on the 3rd wire providing an unbalanced connection. The 48v supply is testing just 5 or 6 volts so appears to be faulty

 

Its possible to buy a cable from Saturn electronics (I think that's their name) for £50 that allows the mic to work off of a standard phantom supply, bypassing the need for the separate power supply - so thats got me thinking there must be a pretty simple way of doing the same.

 

I am thinking perhaps a capacitor (0.05micro farad?) to the unbalanced mic input to stop the 48v hitting the mic cartridge. The wire before the capacitor would go to the +ve voltage input, the earth and -ve voltage would be connected together and go to the remaining terminal. I wouldnt bother recreating the attenuator as its just another fault liability.

 

Problem might be in connecting an unbalanced mic to a balanced XLR input and of course I dont want to damage the 48v supply on my mixer.

 

Anyone with a better grasp than me with any suggestions. If all else fails I will just buy one of those Vigortronix power supply units and replace the existing one but it would be nice to do away with the power supply box if possible.

Edited February 21, 2017 by John M

[boatman]

We used to use the Calrec 600 series at Wood Norton 40+ years ago! Mike Talbot-Smith was a great fan.

 

You might find this link useful.

[John M]

We used to use the Calrec 600 series at Wood Norton 40+ years ago! Mike Talbot-Smith was a great fan.

 

You might find this link useful.

Yes - very useful. I did do a search previously but only found a post from 2008 without much info

 

 

 

 

[sandall]

I've still got 4 of them somewhere. I made a homemade PSU, cribbed from the Calrec design, with (IIRC) balanced outs for the first pair; the 2nd pair came with a genuine Calrec PSU. If you are still stuck I can drag out the PSUs & see what's inside.

[Jivemaster]

...The 48v supply is testing just 5 or 6 volts so appears to be faulty...

 

 

Is it possible to open the existing but faulty psu and repair/renovate/reverse engineer a replacement? In the same case? It's a microphone surely it will not consume more than 1 watt even at 48v. I'd look for blown electrolytics and maybe some sacrificed regulator components, or I'd just find 48v DC in a small box.

 

Some of the Maplin CCTV units are 48v and have their own wall wart so they must be findable somewhere.

[Yorkie]

I think I have a couple of 48V wall warts somewhere if they're any use

[John M]

I think I have a couple of 48V wall warts somewhere if they're any use

Thanks for all the very helpful comments. I will look up the maplin wall warts - that could be a nice simple resolution although I am tempted by the phantom power option.

 

Yorkie - how much for one or your wall warts - I hope your house isnt like mine - I usually end up buying duplicates because my wife clears up and eventually eveything ends up in no mans land (my loft) under a pile of other bits and pieces!

 

 

[Yorkie]

Sorry when I dug them out I found they were 70V. picture

[sandall]

I dragged out my Calrec-made PSU - just as well, as the larger (33/350v) capacitor had acquired what my gardening wife would describe as a bracket-fungus. A quick trip to Maplins produced a 47/450v can the same size for 99p. All is now well. Circuit Diagram (+ simplified sketches) attached (hopefully) circuit

 

The OP's voltage of 5 or 6v suggests the smaller (47/63v) capacitor may have gone short-circuit - I see I had to replace mine sometime in the past.

 

The circuit couldn't be simpler. I made up some1:1 transformers to give balanced outs, but it's easy enough to have the power unit by the mixer, as the mics will quite happily run unbalanced 20m away.

 

Edit to add - Beware, the box is connected to signal earth, NOT mains earth.

 

 

Edited February 28, 2017 by sandall

[John M]

I dragged out my Calrec-made PSU - just as well, as the larger (33/350v) capacitor had acquired what my gardening wife would describe as a bracket-fungus. A quick trip to Maplins produced a 47/450v can the same size for 99p. All is now well. Circuit Diagram (+ simplified sketches) attached (hopefully) circuit

 

The OP's voltage of 5 or 6v suggests the smaller (47/63v) capacitor may have gone short-circuit - I see I had to replace mine sometime in the past.

 

The circuit couldn't be simpler. I made up some1:1 transformers to give balanced outs, but it's easy enough to have the power unit by the mixer, as the mics will quite happily run unbalanced 20m away.

 

Edit to add - Beware, the box is connected to signal earth, NOT mains earth.

 

 

Many thanks for that. I am quite rusty with electronics and my power supply seems to be different to the diagram. I could be wrong but I have what seems to be an electrolytic capacitor (its old enough to be still labelled a condenser) which seems to have two 32 microfarad capacitors in a single cylindrical unit. There is a red blob on one of the two tags on the "top" and the tag/terminal on the bottom goes to the -ve. I can measure capacitance between the two top terminals (about 0.1 ufarad) but higher capacitance (about 2.0 ufarads) between each tag and the bottom. So unless my meter is faulty it looks like the capacitor is faulty. Can anyone confirm if its likely to be 2 capacitors in one container? The label does state 32 + 32ufarads so that does seem to answer my question. Its coloured green, manufactured by TCC, Type CE509KC, states that can is neg, TCB/YA 0852. Couldnt find the info on google. But ultimately I could just build the circuit as per your drawing. Just worth mentioning that the mic is actually marketed by Orange with a model number of CM660.

[Jivemaster]

Two capacitors in one can was very popular once, so in historic equipment it's a distinct possibility. Three pins, one is common (usu neg) also can is neg, two other pins could be colour coded but may not be now!

[sandall]

It could be 2x 32u used in parallel to make 64u or (perhaps more likely) used for C1 + C2 on the diagram. Is there another capacitor on the PCB, & is yours a single-channel unit, in which case the load current (under 3mA per mic) would be halved? What voltage rating is it? The use of a single-diode rectifier & relatively tiny capacitors look like a recipe for hum, but Calrec were canny Yorkshiremen, not fools, & it obviously worked.

[Jivemaster]

You should be able to replace the three tag capacitor with two suitably rated axial wire elecrolytics, getting them will cost more than the caps! (should still be under £5!).

 

Otherwise a small pack of PP3 batteries will give you 54v as a way of testing the mics (and perhaps 100 hours of running time).

[John M]

It could be 2x 32u used in parallel to make 64u or (perhaps more likely) used for C1 + C2 on the diagram. Is there another capacitor on the PCB, & is yours a single-channel unit, in which case the load current (under 3mA per mic) would be halved? What voltage rating is it? The use of a single-diode rectifier & relatively tiny capacitors look like a recipe for hum, but Calrec were canny Yorkshiremen, not fools, & it obviously worked.

 

Hi - No its not two capacitors in parallel, there is a resistor between the two top tags with the bottom tag going to earth.

 

the circuit is something like this - dont know how to do attachments !! The diodes seem ok

 

from transformer +ve......>|.................47ohm.........|.39k.|.........................................48v but measure 5v

Chunky diode | | |

300v 32uF === === 300v 32uF ^ cylindrical diode

| | |

From transformer -ve..............................................|........|...........................|.............-ve

 

 

 

 

 

 

 

 

 

That didnt work very well!. How on earth are pictures etc uploaded?

Anyway the diagram is

 

xfmr.....diode..>|...47R ......32uF 300v to -ve rail.....39k R.....32uF 300v to -ve rail....|< diode to -ve...48v

 

Hope that makes sense! But anyway - I will change the single capacitor for 2 x 32uF

Edited March 2, 2017 by John M

[sandall]

That didnt work very well!. How on earth are pictures etc uploaded?

Anyway the diagram is

xfmr.....diode..>|...47R ......32uF 300v to -ve rail.....39k R.....32uF 300v to -ve rail....|< diode to -ve...48v

Hope that makes sense! But anyway - I will change the single capacitor for 2 x 32uF

 

(I had to learn Postiimage)

 

Yes that sounds like the diagram I posted, except that the 2nd capacitor only needs to be 63v rating. The caps must be getting on for 40 years old, so it's hardly surprising they have died. If you are London-based you must have a Maplin within a mile or so - Radial caps; 47u 450v, JL18U @ £1.69, 47u 63v, N83KF @ 49p. Mine uses axials, which are 99p & 29p. You may need to drill out an extra ground hole or solder the negative legs together.

 

 

Edited March 3, 2017 by sandall