48v supply for Calrec condenser mic

[sandall]

Choke goes in series - it's a near short-circuit to DC, but high-impedance to your HF "noise". It really doesn't matter how you generate your 48-50v, provided there is enough filtering, & you keep audio & mains earths separated, but most off-the-shelf units will need a minimum load current to work correctly (as you say, the mic itself only draws a few mA), so may need a load resistor across the output.

 

Thanks for the tip about µ - in return, may I offer Alt 937 for Ohm (works in MS Word, but doesn't seem to here) - in theory Alt 234 should also work, but doesn't on my keyboard

[boatman]

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I will give the 1k resistor a go.

.

 

 

Errm, not at 48V you won't.

 

The forward voltage of an LED is about 2V and the forward current is between 5mA and 10mA, so the resistor has roughly 46V across it and let's say 7.5mA running through it. Ohm's Law tells you that you need a 6.133k resistor (6.2k is close enough) and it needs to be rated at 345mW so needs to be a minimum of a half-watt (a quarter-watt resistor will burn out).

[sandall]

Seems like high voltage coming out of the transformer at about 200volts.

 

I've just got around to checking mine (both Calrec originals with the DIN connectors rewired to DIN standard). One was 198v, the other 199v, on a 240.5v supply, so yours sounds spot-on. The mic spec sheet quotes 45 - 50v at 2.6mA - my re-cap'd one gives 52v no-load, the other (newer) one gives 54v, so I suspect I'm measuring some ripple & it needs new caps.as well.

[John M]

Just how fantastic is this forum! Many thanks to all of you for such good thought out answers as well as your patience with my persistent questions! Special thanks to journey man for taking the time to measure the voltage.It seemed unlikely that a faulty transformer could generate excess voltage but I couldnt imagine going to the trouble of putting in a transformer and then giving out almost the same volltage as was being put in. Isolation springs to mind but why not lower the voltage to a safer level at the same time? Well I am off to maplin now for some resistors - thankfully just a 10 minute drive.

[sandall]

Like, I suspect, many others I stopped lurking & started posting when I had a problem I couldn't sort out for myself, so it's good to be able to occasionally return the favour (it's also a chance to show off any knowledge you've gained over the years !!)

[John M]

OK - here is a final closure for this saga

The original power supply - I decided to completely remove this and replace with a switched mode power supply.

The switched mode power supply was very noisy so I put a 680µF capacitor across the output and a 4.7µH coil in series with the +48v rail. Still noisy. Spoke to the manufacturer and was advised that its OK to connect the mains earth to the 0v rail. This has cured the problem but not 100% perfect as there is still a slight background noise if I turn the gain up to a very high level.

 

As the original power supply was now out of the case and much easier to follow the (simple) diagram it was immediately apparent that I had identified the wrong diode as being the Zener! Fortunately I found someone on ebay selling the exact required diode (about £5 with postage), changed that and now its running very nicely.

 

Took the new module out of the power supply box and reinstalled the old one - all working fine - at last. Phew! That was the longest 5 minute job ever.

[sandall]

If at first.................................

[sunray]

Sorry to bring this back to life...

Reminded by the current similar Senny thread

I was given a 'faulty' Calrec 654 XLR about 30 years ago. (Finding out something about the donor much later it wouldn't surprise if the mic is hot.)

It came with a BT 9H/... type transformer - designed for 600Ω or so circuits with low resistance coils (capable of 10's of mA DC control signals), 50V 1A psu and M & F XLRs wired:

At the time I did plug it in to confirm it didn't work but knowing nothing about the brand had no idea it was incorrect.

Before I spend any time on it what opinions are there it may now be destroyed (as I've read on several forums)?

Edited February 23, 2022 by sunray

[sandall]

I've got genuine Calrec & home-made copy PSUs for 600-series mics. I'll look out the circuit later.

[sandall]

This is Calrec's original PSU for the DIN version (Pin 1 Signal, Pin 2 +50v, Pin 3 Earth) & my copy.

[sunray]

That is brilliant, thank you.

Now I have no excuse to not dig it out and have another go.

[sandall]

See other thread for pin connections

[sandall]

Just found this -

Edited February 25, 2022 by sandall

[sunray]

35 minutes ago, sandall said:

Just found this -

Ohhhh thanks, hopefully that is the putting to bed the confusion I've seen in other places. All I have to do now is purchase a round tuit.