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Old 6th October 2004, 06:07 AM   #21
1audio is offline 1audio  United States
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Jox knows what he is speaking about. He has had to make them work. If he can supply a circuit it will work. I believe it will be pretty simple.

I can't help with the Behrenger cold but if you post the connector pinout we can give advise on a suitable supply.
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Old 6th October 2004, 07:29 AM   #22
Jennice is offline Jennice  Denmark
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If it's a pre-polarized mic capsule, you'll need a 5 mA current source with a voltage of 24V with nothing connected to it.
If your pre-amplifier has a BNC connector, it's a DeltaTron supply you need (which is what I described above).
Your pre-amp output should bias itself to half the supply voltage (roughly), with the signal as AC "on top" of the DC. That's why you need a capacitor.
However, remember that the pre-amp is basically an impedance lowering device, with unity gain. You'll need to make the gain stage yourself, but a decent op-amp will do the trick.

Jennice
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Old 6th October 2004, 10:03 PM   #23
TheoM is offline TheoM  United States
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Default signal conditioner

I've not used a signal conditioner such as the one on ebay you recommended.

http://cgi.ebay.com/ws/eBayISAPI.dll...e=STRK:MEWA:IT

Its going in an hour. Based on what we know, will it help the noise problem? Will it provide the right current, and some gain? thanks. Ted.
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Old 6th October 2004, 10:32 PM   #24
1audio is offline 1audio  United States
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It will work. It should fix the hum problem if the problem is from the interface. I have the manual and can help with any details. If it doesn't work for you I'll buy it from you (so I have a pair and can look at vibration propagating through a speaker cabinet).
-Demian
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Old 6th October 2004, 11:39 PM   #25
TheoM is offline TheoM  United States
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Default Qu

Yeah. I use spectra plus - and do a lot of things like that - esp for tweeking crossovers. Several years ago I used contact mics to measure propegation of sound through wood for a guitar maker with some very interesting results. (Good transmission through the wood leads to phase cancellation - dead spots - in the guitar. ). Alas, leo fender made guitars sound better by making them, in some respects, worse.

I would induce white noise into the wood via a headphone playing white noise, and then record the results with the contact mic in different places. I developed tables for engineering music tracks such that we can make any guitar sound like almost any other guitar using eq. silly, I know - but that's why I want this mic to work. ted.
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Old 7th October 2004, 07:06 AM   #26
Jennice is offline Jennice  Denmark
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TheoM,

You'll probably want to make your mic.-supply as a current source with 27 - 28 Vdc as voltage, and 5 - 10 mA constant current.
The mic-preamp you have (if it's a tube thingy with the mic capsule in one end and the BNC in the other end), is not designed to have gain.

Consider it to be an impedance changing circuit. Some megaohms to the capsule, some hundred ohms to the BNC side.

However, it is very critical that you filter out ripple in this supply. Any ripple will have severe influence on the output signal, especially since any existing ripple is introduced before a gain stage. You might want to consider using 3 x 9V batteries in series to feed the current source. Current demand isn't high, so you'll have quite a few hours of operation from such 3 cells.

As for the gain stage, there's no rocket science in it - just use low noise op-amps.

Jennice
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Old 16th December 2011, 12:28 PM   #27
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Quote:
Originally Posted by Ouroboros View Post
Which mic is it? I used to use a B&K 4133 mic for measurement work, and this was a true unpolarised capacitor mic that needed a 200V polarisation voltage. (From a little PSU that used 'D' cells).

If yours only needs a standard phantom feed then this is quite easy. Have a look on the web for the application notes on the SSM2017 (or the replacement SSM2019) from Analog Devices and this shows the schematic of a pre-amp with phantom feed to the mic.

If yours needs a higher voltage it's quite easy to arrange.
Hello

I have myself few B&K 4145 mics capsules, unpolarised capacitor mic that needed a 200V polarisation voltage, but I don't have a power supply/preamps.

Would you have or know any schematics of transistors power supply/preamps for my 4145 ?

Thank

Bye

Gaetan
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Old 16th December 2011, 12:59 PM   #28
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Normally the mic capsule screws on to a cylindrical pre-amp body. This pre-amp is powered from the dc supply, while providing both the dc feed to the capsule (through a resistor chain of hundreds of Megohms) and crucially, a very high input impedance buffer stage for the capacitor microphone. The buffer stage is not easy to design or make yourself, and the pre-amps (from B&K or other manufacturers) are not cheap.
Getting the 200V dc is simple, and a low-cost flyback supply is easy enough to design, but you still need to design the buffer amp with an input impedance of 1G Ohm or so. The mic capsule is only about 20pF, so you can understand how high the Z(in) has to be to get a useful LF response.
(One reason why vacuum tube capacitor mic pre-amps continued in use for so long).
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Old 16th December 2011, 09:32 PM   #29
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Quote:
Originally Posted by Ouroboros View Post
Normally the mic capsule screws on to a cylindrical pre-amp body. This pre-amp is powered from the dc supply, while providing both the dc feed to the capsule (through a resistor chain of hundreds of Megohms) and crucially, a very high input impedance buffer stage for the capacitor microphone. The buffer stage is not easy to design or make yourself, and the pre-amps (from B&K or other manufacturers) are not cheap.
Getting the 200V dc is simple, and a low-cost flyback supply is easy enough to design, but you still need to design the buffer amp with an input impedance of 1G Ohm or so. The mic capsule is only about 20pF, so you can understand how high the Z(in) has to be to get a useful LF response.
(One reason why vacuum tube capacitor mic pre-amps continued in use for so long).
Hello

It would need a fet input buffer or an fet input opamp buffer.

I've found this circuit on the web, just need to change voltages, what do you think of it, does it need any improvements ?

Thank

Bye

Gaetan
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Old 19th December 2011, 05:25 AM   #30
1audio is offline 1audio  United States
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I'm not sure it will work for the B&K mikes. The cap leakage needs to be really low. You need a biasing method for the transistor or it will probably shut itself off. The real ones use things like teflon standoffs for the critical connections and the input cap is much smaller.

If you are interested I'll scan the HP circuit and post it. Its the simplest and one of the best I have tried. You will need some 1G resistors or similar.
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