48v supply for Calrec condenser mic

[boatman]

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The OP's voltage of 5 or 6v suggests ....

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Or the 51V zener has been replaced with a 5.1V zener by mistake!

Edited March 3, 2017 by boatman

[sandall]

Or the 51V zener has been replaced with a 5.1V zener by mistake!

 

That's an interesting thought......

 

 

[Jivemaster]

RS Stock No. 654-7678

 

If you have a continued use for PP3 batteries, I'd try the mic on five or six batteries. If it works then I'd go through the PSU board and replace components completely. Electrolytic caps die with age, other components can fail or get erroneously changed. If the finished mic has value then £10 of components may be good value.

[Robin Pratt]

If you care about originality, you can still get dual 32uF caps in a single can.... at a price!

 

http://modulusamplification.com/Capacitor-can-32uf32uf-500v-ma-cap-P1425412.aspx

[Jivemaster]

Having seen Boatman's post (51v Zener vs 5.1v zener) and searched RS for a 51v zener and been presented with mixed 51 and 5point1 zeners, that would be my early component change. As the electrolytic cap could be at end of life that's another obvious change. Sadly Maplin don't offer a 51v zener so buying five from RS could be the easy answer.

[boatman]

51V zener is 18 pence + VAT from CPC! By the time you've added your other bits and pieces you should be over the £5 minimum charge for free delivery of online orders.

[John M]

I changed both capacitors and put them the wrong way round... high voltage reversed with low voltage. Smell of burning, turned off and realised the problem. Took them both out and they seem to measure OK, put them back in the right position but still low voltage output - it could only now be the diodes as the resistors are ok. Seems like high voltage coming out of the transformer at about 200volts. Another problem - the physical size of the 450v capacitor now blocks the Jack socket and now that I have cut the legs there is no movement - probably take the capacitor out and solder an extension on!

 

I have done quite a lot of searching on the web for the zener BZX61C51v which is a 51v 1.3w (dissipation) but not really very sure about what other parameters are applicable. Does anyone know if there is a straight equivalent. I thought I found some on the Farnell web site but they come from USA and charge about £15 for delivery.

 

I hooked up a new Vigortronix 48 v power supply to the mic. Smoke came out of the new power supply LED resistor - disconnected the LED and measured voltage @ 48v good! Called Vigortronics and they advised that for 48volts I need to change the LED resistor to a higher value as its only suitable for lower voltage units.

 

Not too sure about how best to connect earth to the microphone. On one hand I feel like perhaps the safest option is to connect the mains earth to the cable screen but this is not done with the original power supply. So - I cut the mic cable,

connected the +ve (red) to the new power supply (PS) and disconnected it from old PS

connected the screen to the -ve (new) PS, to the mic and to the old PS box (to give connection to the jack socket output) and

left the signal lead connected to the mic and the old PS box so that the jack socket could be used to plug into my (bass guitar) amplifier.

Turned the amp and the PS on - loads of clicks and hum. Maybe the mic?

So the saga continues. Any help with the zener and earthing arrangements will be appreciated.

 

I changed both capacitors and put them the wrong way round... high voltage reversed with low voltage. Smell of burning, turned off and realised the problem. Took them both out and they seem to measure OK, put them back in the right position but still low voltage output - it could only now be the diodes as the resistors are ok. Seems like high voltage coming out of the transformer at about 200volts. Another problem - the physical size of the 450v capacitor now blocks the Jack socket and now that I have cut the legs there is no movement - probably take the capacitor out and solder an extension on!

 

I have done quite a lot of searching on the web for the zener BZX61C51v which is a 51v 1.3w (dissipation) but not really very sure about what other parameters are applicable. Does anyone know if there is a straight equivalent. I thought I found some on the Farnell web site but they come from USA and charge about £15 for delivery.

 

I hooked up a new Vigortronix 48 v power supply to the mic. Smoke came out of the new power supply LED resistor - disconnected the LED and measured voltage @ 48v good! Called Vigortronics and they advised that for 48volts I need to change the LED resistor to a higher value as its only suitable for lower voltage units.

 

Not too sure about how best to connect earth to the microphone. On one hand I feel like perhaps the safest option is to connect the mains earth to the cable screen but this is not done with the original power supply. So - I cut the mic cable,

connected the +ve (red) to the new power supply (PS) and disconnected it from old PS

connected the screen to the -ve (new) PS, to the mic and to the old PS box (to give connection to the jack socket output) and

left the signal lead connected to the mic and the old PS box so that the jack socket could be used to plug into my (bass guitar) amplifier.

Turned the amp and the PS on - loads of clicks and hum. Maybe the mic?

So the saga continues. Any help with the zener and earthing arrangements will be appreciated.

 

Maybe that suggestion of using multiple 9v batteries would be a great way of discovering whether the mic is faulty or not.

Edited March 7, 2017 by John M

[sandall]

Ignoring the rest of the saga.....

 

If you are worried about an unearthed metal case (as you have now), why not connect the case to mains earth & connect signal earth to the case via a low-value resistor (say 10R - 47R, 1/2W)?

[Jivemaster]

High volts/power zener diodes ceased to be common parts 20 years ago, they may be findable or maybee not. Try looking out a suitable regulator chip, but lots stop around 36v. If you are going to build a new PCB then put 10 x 5v zener diodes in series -may dissipate the heat better too.

 

If you want to worry about lack of mains earth continuity then put the transformer inside a plastic project box, inside the metal case which is wired to audio earth somewhere.

 

Do you have good reason to think that the microphones are good? It would be a nuisance to find that you've made a new psu only to find that the capsule unit is un-serviceable.

[boatman]

According to the CPC web site, the 51V zener I mentioned in my previous post has a stock level of 21.

[John M]

I had a delivery from CPC today including some 51v zeners, another capacitor and other odds and ends.

 

The output from the transformer seems very high at about 200volts - is that usual - and if so does that mean the zener needs to be higher rated than normal. I measured across the existing zener and reading 200v so that seems to be blown as I understand it should show 51v. Please correct me if I am wrong!

 

I tried the new 48 vigortronix power supply again and still quite noisy. I think perhaps its underpowered at 100mw. I will measure the current and see.

 

Guess what - I found another "project" in my loft. The great british spring reverb unit. Its built into a drainpipe and apparently quite sought after. It also needs a power supply. This time 20v at about 200ma. There are quite a few laptop power supplies that are rated at anything from 2 to 6 amps which seems a bit excessive so I have ordered a smaller (and more expensive) one from Amazon. This time I tried first with batteries and it works fine!

[sandall]

The output from the transformer seems very high at about 200volts

Not sure about 200V, but I remember it was quite high. There is a big voltage drop across the first (2W) resistor. I'll check mine when I'm back in civilisation.

[John M]

I connected up 5 x 9v batteries all new. Voltage was 49v. I tried 6 batteries but showing about 58v which seemed a bit excessive. Mic works fine with no interference.

Current to the microphone was 2.5mA so my new power supply should be well within spec at 100mA.

 

Getting a bit fed up with the old supply now as it seems to be never ending problems. Going to contact the new power supply manufacturer (vigortronix.com) and see if they recommend anything so subdue the noise.

This is the module I used (48v one)

http://www.vigortronix.com/5WattACDCPCBPowerModule.aspx

[richardash1981]

Going to contact the new power supply manufacturer (vigortronix.com) and see if they recommend anything so subdue the noise.

This is the module I used (48v one)

http://www.vigortron...owerModule.aspx

 

A switch mode power module like that is always going to put quite a lot of noise onto it's output rail. With an unbalanced arrangement like this, any noise from the power supply is likely to end up in the microphone signal (which is tiny) and cause issues. 50mV ripple may be "low ripple" to them, but is a lot more than the zener regulator in the original gives! I would always expect to need additional filtering between the supply and the microphone, and to be careful to make sure that there isn't noise injection via the ground connections. The latter shouldn't be a big problem because it's an isolated power supply.

 

As far as filtering the 48V DC line goes, I would use something like a 1k series resistor and 47u or 100u capacitor (63V working, like C2 on the schematic) to clean up the rail. The voltage drop will be only a few volts at the current you measured, but the ripple reduction will be huge, giving you a nice clean DC for the microphone. A 1/4W resistor will be fine (unless you short the output!). When building up, make sure that the -ve connection goes from the microphone to the capacitor, then back from the capacitor to the new power supply pin 4. This avoids coupling noise through common wiring resistances. If you want to run two microphones, give them separate DC filters from the common PSU.

[John M]

Richard! You are spot on thats exactly what happened - a whinny tone and odd ticking in the background. I touched the mains earth onto the -ve rail and that did stop most of the noise - but I guess its not a great idea to leave it this way with the possibility of earth loops and maybe safety implications?

 

I have connected a 100µF/100v capacitor across the output and in a hopeful effort put some silver foil over the module and connected that to the mains earth - but its still noisy. The pcb originally had an LED with a resistor across the output and the resistor started smoking- A call to Vigortronix confirmed that when using a 48v model a higher resistance needs to be installed. I disconnected the resistor and havent got around to putting a new one in (it cant be seen when inside the box)

 

I will give the 1k resistor a go.

 

The -ve from the microphone comes via the output jack socket to the circuit board DC output (I used the vigortronix pcb board) and then the circuit hits the -ve of the 100µF capacitor and then onto pin 4 of the module. The mic/DC power lead routes into the box next to the mains input but doesnt touch it

 

I contacted Vigortronix and they also suggested using the capacitor and a 5µH choke - I managed to get a 4.7 but not sure if it goes in parallel or series with the supply.

 

In your experience does the physical proximity of a switch mode module to the signal wire have any effect or is it all electrical interference?

 

And...yet another pain. The screws for mounting the new circuit board block the lid from sliding into place - hopefully I can countersink the screw holes and then use coutersunk screws as opposed to the existing round heads.

 

I did toy with going back to the original circuit and building one from scratch - maybe even splash out on a full wave rectifier! Problem is I suspect the old transformer is faulty as the secondary is giving out about 225 volts. There does not seem to be much available for off the shelf transformers - most are 12v, 5v etc.What would be the ideal quoted output voltage to allow for 48v (or 51v) DC output? Would it be practical to use a lower voltage transformer and then use what I guess is a voltage multiplier to raise the DC voltage? This seems to be how most mixers provide 48 volts from 18v transformers.

 

By the way - Not sure if its general knowledge but in order to get µ just hold down the Alt key to the left of the spacebar and type 230 on the numeric key pad - it doesnt work with the numbers across the top of the keyboard though and you need numlock on.

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