Building a Tube Mic Pre

[Chocolatemotor]

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!

 

[Eli Duttman]

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

 

[Frank Berry]

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.

 

[fdegrove]

Hi,

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

Cheers,

 

[EC8010]

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.

 

[Frank Berry]

"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.

 

[fdegrove]

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

 

[Frank Berry]

"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.

 

[Eli Duttman]

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

MUCH more than I care to admit!

 

[fdegrove]

Hi,

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.

Another Frank who tends to agree with fdegrove most of the time.

Let's agree to disagree....

Cheers,

 

[Frank Berry]

You're the one that brought up the output transformer without mentioning an input transformer.

So ... do you only design output transformers?

Sorry Frank. I'm just being nasty. I know full well what you were saying. I respect you for your ballz.

 

[Da5id4Vz]

You might want to check out this link:

http://recordist.com/ampex/apxschem.html

Scroll down to the MR-70 record/Play amplifier. There are also drawing s for 50 and 60 dB mic pre mods for theses things.

Its a highly regarded amplifier for R-R machines and I understand that one of the reasons its been becoming increasingly scares is its popularity as a mic Pre.

-Dave

 

[electronaut]

ChocolateMotor,

I agree that building a tube mic preamp is a bold endeavor, especially if you've never built a tube amp before, let alone a preamp. I started down that path myself and a couple of years went by before I had a solidly reliable unit.

Of course, there are easier paths -- if you can find a kit, that would be a great way to start. Definitely check out the Gyraf project. Also, for a copy of just about every mic preamp schematic known to human kind, you can go to Pat's Tube Schematics

And there's also a version of the Altec 1566 that I think you can buy in kit form.

Lastly, a guy named Greg Norman who works for Steve Albini's Electrical Audio studios here in Chicago modified the classic Ampex 350 electronics into mic preamps. The report is available here.

Hope that helps. Good luck!!

E. Lectreau

 

[EC8010]

Hello electronaut. I looked at "Pat's Tube Schematics", and although he beats his drum hard, I didn't see anything that made me think "gosh, that's clever, I'll have to try that". I'll probably be rained on for saying it, but most of the "engineers" in the music industry would be flummoxed by Kirchoff or Thevenin, let alone anything more complex.

Once Chocolatemotor gives some details of what he needs, we might be able to move on from mere speculation.

 

[electronaut]

Hello electronaut. I looked at "Pat's Tube Schematics", and although he beats his drum hard, I didn't see anything that made me think "gosh, that's clever, I'll have to try that". I'll probably be rained on for saying it, but most of the "engineers" in the music industry would be flummoxed by Kirchoff or Thevenin, let alone anything more complex.

Once Chocolatemotor gives some details of what he needs, we might be able to move on from mere speculation.

Well, I referred ChocolateMotor to Pat's Tube Schematics not because of how good or bad of an engineer Pat might be, but because he might be a good resource for schematics, especially if ChocolateMotor has a favorite preamp he might want to start with or something.

You are right about Pat beating his drum hard -- I guess I usually ignore those things and didn't take much notice, frankly.

 

[Da5id4Vz]

I think I can count on one hand the number of recording engineers I know of who are also Electrical Engineers. Perhaps on one finger (no not that one) because I don’t think George Massenburg finished his degree. Tom Scholtz however may be the only MIT grad with more than one platinum album.

Likewise here at DIY we see some who are great EE and knowledgeable about audio as well as practitioners who also do a fine job.

I guess Im not to sure I understand what your getting at.

I posted this link:
http://recordist.com/ampex/apxschem.html
because it has virtually every schematic for Ampex tape rec/play amps.
I think the MR-70 was the tube RC amp for the AG-350.

 

[thoriated]

The MR 70 was the last vacuum tube model audio tape recorder Ampex made, and used nuvistors.

 

[EC8010]

I guess I'm not too sure I understand what you're getting at.

I don't have a problem with studio operators preferring one piece of kit over another. Microphones are certainly different, and you need to select the right tool for the job. What I do have a problem with is when they start thinking they can fiddle with the insides of electronics and make design choices without having any knowledge of the engineering involved. Those older designs were carefully thought through by chaps with slide rules. Admittedly, their designs can often be improved by substituting modern components, but only if you understand what the original designers were doing in the first place. Thus, I find a lot of modern non-engneering-based tub-thumping rather insulting.

EC8010: Grumpy dyed-in-the-wool engineer who has listened at both ends of the chain.

 

[Da5id4Vz]

Well Said.
Back in the old days when I was still in "the business" I found that moderate success was often built on image and attitude. The truly talented could make a living without such grand posturing.

Tub Thumping! Boy doesn’t that say it well.

I suppose for my part I should go back and read the mans article to see what he's all about. After all I have declared myself an MR-70 fan.

-Dave

 

[EC8010]

The MR 70 was the last vacuum tube model audio tape recorder Ampex made, and used nuvistors.

Yes, I noticed that (6CW7). And as they're so small, it just cries out to be hung from the back of the microphone - allowing sensible signals to travel to the desk.

Will we ever hear from Chocolatemotor?