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Old 15th April 2004, 11:09 PM   #1
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Default Building a Tube Mic Pre

I have a small amount of knowledge of electronics, but I am extremely interested in building a tube Mic Pre. Does any have plans/ schematics in order to do this?
I would most most likely benefit from a simple plan to start.
Thanks!
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Old 17th April 2004, 02:31 AM   #2
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For a mic with LOW O/P, the circuitry used in a phono stage, minus the equalization network, will work. Moving magnet phono cartridges have O/Ps of about 5 mV.

Here's a URL for a pentode (EF86) based circuit that makes a nice mic preamp.

http://www.svetlana.com/graphics/TB/no.23fig1.jpg
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Old 17th April 2004, 02:51 PM   #3
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Please give me more information.
Balanced or unbalanced input?
Balanced or unbalanced output?

I designed a tube microphone preamp for a local CBS/Infinity radio station a few years ago.
I used a single 6922 tube in an SRPP configuration.
The preamp sounded better than the squalid state preamp that was in their mixing console.
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Old 17th April 2004, 02:53 PM   #4
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Hi,

Input is usually unbalanced,output is balanced using an impedance xformer.

Cheers,
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Old 17th April 2004, 03:06 PM   #5
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Sorry, Frank, but studio microphones are all balanced devices, and condenser microphones may need phantom power (+48V from 3k4 source impedance) connected between the centre tap of the input transformer and ground.

To be honest, I wouldn't have said that a microphone amplifier is an ideal first valve project. As has already been noted, the problems and solutions of RIAA preamplifiers and microphone amplifiers are very similar. They're both quite hard to do well. Of course, if the amplifier is to be matched to a single (known) microphone, to be used in a single (known) way, such as a presenter's microphone, life might be a lot easier. A universal microphone amplifier that can cope with inputs ranging from a ribbon microphone being used in a Blumlein pair to a large capsule condenser microphone in front of a saxophone is much harder.
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Old 17th April 2004, 03:11 PM   #6
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"studio microphones are all balanced devices, and condenser microphones may need phantom power (+48V from 3k4 source impedance)"

Correct.
I also provided a balanced 48 volt phantom voltage in my design.
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Old 17th April 2004, 05:46 PM   #7
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Hi,

Sorry. I should have been more explicit...

What I meant was that most mike preamps I've seen have single ended active circuitry.

So most of the time_ I'm aware of_ you have impedance xformer balanced in, to SE out, amplifying stage, SE into xformer and balanced out again.

Cheers,

GYRAF
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Old 17th April 2004, 06:12 PM   #8
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"Input is usually unbalanced,output is balanced using an impedance xformer."

Frank,

Don't try to explain yourself out of that statement ... Just say "I was wrong."

Want to ask how many times I've had to admit the same?

Another Frank who tends to agree with fdegrove most of the time.
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Old 17th April 2004, 06:57 PM   #9
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>>Want to ask how many times I've had to admit the same?<<

MUCH more than I care to admit!
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Old 17th April 2004, 07:05 PM   #10
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Hi,

Quote:
Don't try to explain yourself out of that statement ... Just say "I was wrong."
Only when I actually am wrong....

We're talking about mic pre design not microphone design, so unless you need to design the input trannies yourself, the question at hand was: do you need a balanced or single ended design.

The only thing "wrong" was the way I put it, I suppose. Hence the clarification.

Quote:
Another Frank who tends to agree with fdegrove most of the time.
Let's agree to disagree....

Cheers,
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