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Old 30th August 2004, 12:32 AM   #11
1audio is offline 1audio  United States
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What I would do is probably buy a decent small surplus linear supply. 20 mA at 18-24V isn't much (.5 Watt) so its tempting to use a modular supply. A wall wort type may couple too much line noise it. I would try one line this : Cincon TR15 (418-tr1524) from Mouser and isolate it with a cap (1000uF) across the connection where it enters the housing, series 100 Ohm resistors on both the plus and minus leads and another bypass cap (.1 uF at least) before you connect it to the circuit.

I am reluctant to recommend coupling caps here. Everyone has very strong opinions and they are all different. A 4.7uF to 10 uF film cap should be fine into a 10K load. You can have endless fun changing it to change the sound. Do not disconnect any of this with the audio up. It will give the pop you will remember.

Let me know if you need more advice.

-Demian
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Old 1st September 2004, 12:19 AM   #12
TheoM is offline TheoM  United States
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Default clarify and confirm

Mic

+ -
PS + 12v 20mA---l-- 100 ohm ----- l l
l l l
.1 mf filter l l
PS - ----------------l---100 ohm -----n---- l
l l
l l
10mF l
l l
Audio l l
Mic Pre + - ,g


Why the resistors? Anyway - I bought a cheap constant current supply on ebay (50 bucks), so I am assuing the above drawing is is from its leads out. Does the drawing describe what you specified? I did notice the mic has a max output voltage of 7 (seems high). Is that in the same range as typical mics for which typical mic pre's are built?

Much appreciated. Ted.
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Old 1st September 2004, 04:49 AM   #13
1audio is offline 1audio  United States
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I'm sorry but I got you confused between making a constant current supply and connecting the one you found. ($50 is a good price for it).

The resistors were a way to reduce the noise from the wall wort. You don't have that problem with the HP.

I have attached a drawing showing the connections. Let me know if its clear enough. (I will forever be confused on the XLR connections since the film industry uses the altrnative phase.)
-Demian
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File Type: jpg mike connections.jpg (23.3 KB, 134 views)
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Old 9th September 2004, 03:30 AM   #14
TheoM is offline TheoM  United States
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Default aftermath

Wired as we talked. All the voltages and current are right, no dc going through the cap. Hums like crazy, and has almost no gain. With 60db of gain from a mic pre, some signal, but very little, mostly hum - 60 cycle.

I've tried several things to get rid of it. Mic cable for the power supply (several feet), with the shield to chasis ground. no change. Shield to DC ground, no change. A cap across the power supply, little change. Going to an unbalanced input, no change. Line voltage with no ground loop, no change.

I'm at a loss at this point. Could be the mic's broken, but B&K makes pretty durable stuff. Maybe its time to ask the seller to take it back. . Maybe seperate dc+ and - cables from the power supply. ted.
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Old 9th September 2004, 08:09 AM   #15
1audio is offline 1audio  United States
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Got your note. Let me think about it. Can you detail the voltages you have measured? Replace the Mike preamp with a 600 Ohm resistor (20 mA @ 12V) and see if you still get hum. Check that the supply is working right (It could be humming all by itself) and that its operating in constant current mode- the voltage across the resistor should change as you change the current.

If the supply is in constant voltage mode then it will eat any output from the preamp.

Remove the capsule from the preamp and see if you still get hum. Turn the supply off with everything attached an see if you get hum. Make sure that the low side of the supply isn't attached to the ground pin of the power cord. (Possibly a jumper on the back of the supply).

Let me know what you find. its unlikely that the mike is dead in this way. More likely is just dead if its actually broken.

-Demian
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Old 1st October 2004, 11:26 AM   #16
jox is offline jox  Sweden
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Have you solved the problem ?
You will probably need a current regulator (like LM334) and 24 volt then it will work.

// Jox
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Old 1st October 2004, 09:55 PM   #17
TheoM is offline TheoM  United States
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Default The B&K power supply

I put the correct resistor in place of the mic and the hum stayed the same. When I connect the mic, the voltage drops at the power supply drops off significantly (to like 10V). Open circuit I was using 20V (should have been 28 I think, but I don't think that would make a difference.) There IS a strap on the back of the power supply connecting RV1 and RV2 (I don't know what those are, but I assume they are standard designations).

I was thinking I could replace the power supply with three 9 volt batteries and a few current regulating diodes to total 20Ma. .

btw the open circuit voltages are right at each point, and the cap does indeed take the dc out. It does seem possible that the ac (signal) from the mic goes back to the supply if the impedance there is lower than the input impedance of the mic preamp I'm using. It also seems possible that in addition, the power supply is creating that noise - although putting a cap to ground didn't change anything. The Power Supply is made by Power designs inc.

Anyway, thanks for the continuing help.
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Old 2nd October 2004, 08:14 AM   #18
1audio is offline 1audio  United States
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Please confirm the DC voltage on the resistor and on the mike. They should be similar if we have the right settings. And the open circuit voltage should be higher that the voltage with the loads. If you are getting a lot of hum check the following-
1) make sure that the ground pin is isolated from the supply output. (Open circuit to any pin)
2) make sure that the minus terminal on the output side is connected to the mike in neg terminal.
3) Does the hum go away when the supply is turned off?

This is a vexing problem and I'm sure it must be very frustrating to you. If you were nearby I would have you bring it over.

I saw a couple of the correct supplies for the mike on eBay. They were in the $300 range I think.

I also looked and discovered that this http://cgi.ebay.com/ws/eBayISAPI.dll...843628096&rd=1

Endevco 2775A will operate the mike preamp. I have one and more important its manual and it definately will operate the mike preamp and give you some control over its output. (i'm always learning something here!)

Or this: http://cgi.ebay.com/ws/eBayISAPI.dll...sPageName=WD1V will operate them. It has the wrong connectors but they can be changed to BNC's easily. Save the old ones, they are valuble to those working with accerometers.

Demian
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Old 4th October 2004, 07:26 AM   #19
jox is offline jox  Sweden
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Hi
Do you want to build a ICP power supply (much much sheaper) or bye one.

If you want to bye a factory manufactured I sell you to a good price about 300$.
Nordborg Acoustics AB under products/ICP power

We are also developing our own ICP power with preamp. So I can help you build one later ... as long as you are not thinking of producing it ...



//
Jox
Nordborg Acoustics AB
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Old 5th October 2004, 11:47 AM   #20
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Default dont want to hijack, but...

you guys know what your talking about so can any of you help me out with power pin diagram for a behringer shark dsp110? its a great feedback killer but i have not got a power supply and im realy confused by the pin layout / voltages. cheers for any help, steve.. ..
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