Electret mics again...

[Dr.EM]

I've managed to get hold of some decent electret capsules now, so I intend to proceed with making a pair of phantom powered mics with them. I have found these 2 schematics on the web, which are sutbly different :? . Note the polarity of the zener diode. I think that the bottom one is correct myself, but I want to be sure of it, I don't really know how it works though. These also claim to have a balanced output, is that true? I can't see how they would myself.

At maplin they sell microphone cable, but its not stocked in thier stores, which defeats the point of using them to erradicate the postage charge. I found this on rapid, its more length than I need, but its the only one at a reasonable cost for this, i'm just wondering whether its suitable, it does look ok, if rather thin. I intend 3-4m length per mic if that helps, not rough handling, home use.

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Thankd for any advice.

 

[Nigel Goodwin]

I've managed to get hold of some decent electret capsules now, so I intend to proceed with making a pair of phantom powered mics with them. I have found these 2 schematics on the web, which are sutbly different :? . Note the polarity of the zener diode. I think that the bottom one is correct myself, but I want to be sure of it, I don't really know how it works though. These also claim to have a balanced output, is that true? I can't see how they would myself.

The circuits look exactly the same to me?.

They also look to be balanced as well, notice that the mike insert is connected across the hot and cold XLR pins, there's no ground connection to the insert.

All a balanced mike consists of is just a mike insert, with either end of the coil connected to the hot and cold, with no connection to ground. The ground connection in the XLR just connects to the metal body of the mike to provide screening.

At maplin they sell microphone cable, but its not stocked in thier stores, which defeats the point of using them to erradicate the postage charge. I found this on rapid, its more length than I need, but its the only one at a reasonable cost for this, i'm just wondering whether its suitable, it does look ok, if rather thin. I intend 3-4m length per mic if that helps, not rough handling, home use.

**broken link removed**

What connectors are you intending on the cable?, you can buy 6m XLR microphone leads for under £3 each from http://www.cpc.co.uk, and for an order over £30 postage is free. I've got all my leads from there!.

 

[Dr.EM]

Thanks for replying. On the first schematic, the "+" of the 12v zener is facing up. On the other, it is the other way round. Unless i'm reading that top one wrong, and the "+" represents the "line" end of the diode. What is a suitable wattage for the zener by the way, 500mw, or 1.3w, shouldn't think it any more than that. 48v phantom power i'll be using.

The balancing explaination clears that up, thanks

I will be using XLRs yeah, those are great prices for leads, but i'd only need 2 of them, so the £4.99 postage/handling charge seems excessive on that.

 

[Nigel Goodwin]

Thanks for replying. On the first schematic, the "+" of the 12v zener is facing up. On the other, it is the other way round. Unless i'm reading that top one wrong, and the "+" represents the "line" end of the diode.

You're reading it wrong, the "+" represents the cathode.

What is a suitable wattage for the zener by the way, 500mw, or 1.3w, shouldn't think it any more than that. 48v phantom power i'll be using.

It's VERY simple to work out the maximum dissipation in the zener, it's fed off a 48V supply via two 22K resistors in parallel (giving 11K). As it's a 12V zener the 11K has 36V dropped across it, so you can work out the current from 11K and 36V. Having got the current, you know the zener voltage is 12V, multiply those together to get about 50mW - this is dissipation in both the mike insert and the zener (most in the zener).

So a 300mW zener would be fine!.

The balancing explaination clears that up, thanks

I will be using XLRs yeah, those are great prices for leads, but i'd only need 2 of them, so the £4.99 postage/handling charge seems excessive on that.

Order some more, or find something else to make £30, they have some quite reasonably priced gear (plus some VERY over priced stuff as well!).

 

[Dr.EM]

I would do, but the chances are if I order a cable from rapid too, I can get free postage on that order, effectively saving nearly£9 of postage charge between the two places. I really hate paying postage :lol:

Theres no serious problem with using that rather thin cable is there? It might not be physically very durable, but electrically its ok isn't it, I don't know how to interpret those capacitance figures. I hear foil screen is good for mics, so thats one thing good about it.

 

[Nigel Goodwin]

I would do, but the chances are if I order a cable from rapid too, I can get free postage on that order, effectively saving nearly£9 of postage charge between the two places. I really hate paying postage :lol:

Theres no serious problem with using that rather thin cable is there? It might not be physically very durable, but electrically its ok isn't it, I don't know how to interpret those capacitance figures. I hear foil screen is good for mics, so thats one thing good about it.

It should be fine for fixed installations, but is probably a bit fragile for a lead which gets moved about much.

 

[Dr.EM]

Should be alright for the fairly light studio duties I intend then. Thanks for all your help

EDIT: found rapid sell those ready made assemblies for a similar price, so going with them.

 

[Dr.EM]

Ok, made the first one (am making a pair) works fine. Not too good at long ranges (almost echoey), bit of an unusual frequency curve (bit too much 200hz region and 6k) but overall, decent sound. Speech and vocals are very clear if you are fairly close to the mic, not flattering, but quite natural. I think this should work well on acoustic guitar; and as drum overheads when paired later. Fairly low noise, shielded totally in a metal tube, balanced output. Wouldn't be suitable for boom mic type use though, how ever do they get such good sound when they are located so distant?