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Old 3rd February 2007, 10:23 AM   #1
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Default Looking for a Balanced Mic Preamp w 12AX7

Hi all ...
i m tryin to build a balanced mic pre amp for a Behringer B1 Condenser type mic , it needs a phantom supply that i ll build separately ,
this preamp will use in a home studio as the mian Vocals/Instruments Preamp , so it so important for me that the pre be noiseless .. and one other thing :P i only have one 12ax7 for that :P ...... and it s almost impossible to find an input/output transformer here, so , do i have a choice ??? can i build a pre without any in/out transformer and only with one 12AX7 ?
any circuit ?any idea ??
thanks so much ...
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Old 3rd February 2007, 10:50 AM   #2
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You did say balanced and only a single 12AX7. Build a differential gain block around the 'X7. Set the grid leak resistor value consistent with the mic's loading requirements.

12AX7s exhibit a high O/P impedance and POOR drive capability. So, buffering is an absolute MUST. What's in your parts "stash"? Do you have a 12AU7 or some FETs?
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Old 3rd February 2007, 07:24 PM   #3
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hi , i wish i had or i could find some more tubes , but there s nothin here , i only have one more 6av6 .. but i think i can find almost any FET , is bf245 ok ?
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Old 3rd February 2007, 10:53 PM   #4
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Look for the ZVN0545A. As stated previously, the 12AX7 has POOR drive capability. The ZVN0545A has low capacitances, which make it easy to drive. Read MOSFET Follies. You will DC couple a FET source follower to each 'X7 anode in the difference gain block.

While considerable better than the 'X7 in the drive dept, the ZVN0545A is not a "world beater". If you plan on driving a nasty downstream load, cascade followers and use the little FET to drive a big FET.
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Old 7th February 2007, 11:01 AM   #5
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hi , thnx for replies ,
i m not an expert in electronic ,, do u think i can find a schematic ?
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Old 6th September 2012, 09:35 PM   #6
el_fela is offline el_fela  Argentina
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This sounds interesting to me... I'd like to get an schematic as well.
I've built a guitar preamp based on fender schematics using two 220V/12V transformers and a 12v voltage ermm, how do you call it? stabilizr? those 3 pinned ICs... well, it sounds really good, and I think it could be easily adapted for microphone use.
Im not that good at this so I would need an schematic and some hints as well... I understand that for the circuit to be balanced we would have to use the tube in a PI configuration, right? in order to get a 180 phase signal... If anyone's interested i could share the schematic I mentioned earlier.

Felipe
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Old 7th September 2012, 11:37 AM   #7
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Quote:
Originally Posted by Ahmad_tbp View Post
hi , thnx for replies ,
i m not an expert in electronic ,, do u think i can find a schematic ?
You can try my mic pre design which uses a 12AX7 at the input. The basic circuit you need is here:

http://www.ianbell.ukfsn.org/EzTubeM...rdCCTsht2.jpeg

and further details of the design and PCBs can be found here:

http://www.ianbell.ukfsn.org/EzTubeMixer/


Cheers

Ian
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