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Old 21st October 2007, 07:00 AM   #1
neoific is offline neoific  United States
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Default First DIY job: Dual Tube Recording Preamp

Foreword
Ryan Turner is a highschool student from Memphis, TN. Having recently gained interest in sound engineering, he has assembled a home project studio. He is a typical guy, although somewhat technical and a computer programmer (PHP). Ryan lacks preamps in his studio, he is stuck using the gain on his mixer; no tube preamps are very limiting.

Goal
To create a dual-channel recording quality tube preamp for general purpose usage. Also, to learn something along the way and share this knowledge with others.

Information[list=1][*]http://store.tubedepot.com/contact.html - Call these guys for the tubes and higher quality audio components; local and reliable (Friend referred me to them).[*]http://www.jameco.com/ - Use them for other less important components; keep getting their 3 of their damn catalogs in the mail every time they make a new one.[*]http://www.nmr.mgh.harvard.edu/~reese/RC10/rc10.pdf - Read this about tube info; its from Harvard, its gotta be great.[/list=1]

Lacking Information[list=1][*]Which type of tube should I design for, assuming I want a general purpose, warm dual-channel preamp?[*]How do I convert balanced audio into unbalanced for the tube and then back to balanced?[*]Should I even bother doing my own design or should I steal someone else's?[*]What other subjects should I read up on besides tubes and then general electronics?[/list=1]

I have this formatted because I hope to turn this thread into a project thread as well as a tutorial upon completion. So please give me some guidance!
Regards,
Ryan Turner
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Old 21st October 2007, 11:45 AM   #2
keefaz is offline keefaz  Europe
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Have a look at:
http://www.gyraf.dk/gy_pd/g9/g9pd.htm
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Old 21st October 2007, 12:31 PM   #3
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Ryan,

Welcome! It's nice to see a youngster taking up the cudgel.

Quote:
How do I convert balanced audio into unbalanced for the tube and then back to balanced?
You don't convert. Instead, you use a differential gain block, which has both balanced I/P and balanced O/P.

Quote:
Which type of tube should I design for, assuming I want a general purpose, warm dual-channel preamp?
IMO, you should use a type with modest gain. The 6SN7 is REVERED and certainly would be a reasonable choice. JJ's ECC99 is another possibility. The '99 has the advantage of a very low plate resistance, which makes it a GOOD load driver.

Quote:
Should I even bother doing my own design or should I steal someone else's?
There's very little new under the sun regarding tubes in particular and electronics in general. It's more a matter of combining things in a "new" way. ALL of us "borrow" from circuits we've perused.

Quote:
What other subjects should I read up on besides tubes and then general electronics?
Physics and Chemistry, as those sciences impact directly on electronics. While we are on the subject of "boning up" on electronics, run a Google search for NEETS. NEETS is US Navy training material, which will be of considerable benefit. Also, visit a local library and study the 2nd edition of James Brophy's Basic Electronics for Scientists. FWIW, Brophy's work was 1 of the texts this "graybeard" used in college.
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Old 21st October 2007, 01:52 PM   #4
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Don't know if this is a legitimate concern or not, but I always worry about damaging the inputs of solid state equipment. You never know how much protection they've put in. One tremendous advantage of tube circuits is headroom. The downside is that gives them the ability to pass large transients that the solid state designer might not have anticipated. Thus, IMO it's wise to find out the maximum acceptable *peak* voltages for your equipment, and provide some limiting to protect it.
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Old 21st October 2007, 02:28 PM   #5
neoific is offline neoific  United States
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Thank you all for your replies!

I've decided that the design that makes most sense to me are from Jensen, specifically http://www.jensen-transformers.com/as/as054.pdf

However, I'm a little confused about the whole "See AS016" box. Does that part connect to the "See other schematics" in AS016?

I don't know if thats the one I want to build for sure, but I do plan to purchase the parts and build it on a breadboard to see how it sounds.
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Old 21st October 2007, 03:16 PM   #6
neoific is offline neoific  United States
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Sorry for the double post, but also on the schematic it doesnt list voltages for the capacitors; what should I do for that?
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Old 21st October 2007, 03:27 PM   #7
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Quote:
Originally posted by neoific
Thank you all for your replies!

I've decided that the design that makes most sense to me are from Jensen, specifically http://www.jensen-transformers.com/as/as054.pdf

However, I'm a little confused about the whole "See AS016" box. Does that part connect to the "See other schematics" in AS016?

I don't know if thats the one I want to build for sure, but I do plan to purchase the parts and build it on a breadboard to see how it sounds.

Ryan,

The circuit you linked is a "mike" preamp. Is that what you want?

Assuming a "mike" preamp is the object of your desire, I strongly suggest you find a different circuit. I find it hard to select a worse candidate for a cascode than the 12AX7. The transconductance (gm) of the 12AX7 is LOW. High gm triodes are appropriate at the bottom of a cascode "totem pole". Also, while the quality of Jensen trafos is excellent, they are costly. CineMag makes GOOD "iron" too and sells at a lower price than Jensen.

If you want to use a cascode gain stage, the 6922 is a good choice. The EF86 audio pentode is a fine alternative to a cascode and many good circuits employing it can be found.

The 12AU7 Jensen shows is not particularly linear. The 6SN7, 6FQ7/6CG7, and 12BH7 are (IMO) better choices.
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Old 21st October 2007, 03:43 PM   #8
neoific is offline neoific  United States
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Quote:
Originally posted by Eli Duttman



Ryan,

The circuit you linked is a "mike" preamp. Is that what you want?

Assuming a "mike" preamp is the object of your desire, I strongly suggest you find a different circuit. I find it hard to select a worse candidate for a cascode than the 12AX7. The transconductance (gm) of the 12AX7 is LOW. High gm triodes are appropriate at the bottom of a cascode "totem pole". Also, while the quality of Jensen trafos is excellent, they are costly. CineMag makes GOOD "iron" too and sells at a lower price than Jensen.

If you want to use a cascode gain stage, the 6922 is a good choice. The EF86 audio pentode is a fine alternative to a cascode and many good circuits employing it can be found.

The 12AU7 Jensen shows is not particularly linear. The 6SN7, 6FQ7/6CG7, and 12BH7 are (IMO) better choices.

Yes, the preamp I want to build would be for my studio's mics.

Thanks for the advice, but honestly it means nothing to me; I don't understand much of what you're saying and without a schematic I would be lost.
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Old 21st October 2007, 04:35 PM   #9
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I second the 6922, it's an excellent triode for preamps. You might also take a look at Lundahl transformers, they make some excellent input and output transformers for these sorts of things.
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Old 21st October 2007, 06:39 PM   #10
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Quote:
Thanks for the advice, but honestly it means nothing to me; I don't understand much of what you're saying and without a schematic I would be lost.

Ryan,

You asked for it! Altec microphone preamp here. TFK microphone preamp here. Those links and MANY more can be found on John Atwood's site.

"Hey Mister, how do I get to Carnegie Hall?" Practice, practice, practice! Dude, it's time to study, study, study.
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