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Old 23rd October 2010, 11:37 PM   #1
cbj591 is offline cbj591  United States
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Default 3 - 12AU7 Preamp. Is there such an animal?

Good evening everyone,

Question - just wondering, is there a schematic out there for a three 12AU7 pre-amp?

My chassis looks a little bare on the tube side compared to the toroidal on the PSU side. I've got the tubes, just didn't know is there was a decent design. Right now I had planned to build the Fore-Watt, but that extra tube would balance the aesthetics out.

I didnt want to just mount the 3rd tube without using it.

As always, thanks for the input.

Cheers
Brian
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Old 23rd October 2010, 11:57 PM   #2
DougL is offline DougL  United States
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Brian,
I am from the form follows function school of design, so I can't really fathom your request.
But I can ask your goals. 50's romantic? Ultramodern low distortion?
How much gain do you want / need?
Do you "believe" in feedback?
What kind of loads do you want to drive?
What max output do you need?
Do you "believe" in followers?
Do you "believe" in transistor CSS?
Do you like transformers / autoformers / chokes in the signal path?

If 4 tubes would be even better, look at Aikido.

HTH

Doug
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Old 24th October 2010, 12:16 AM   #3
Zen Mod is offline Zen Mod  Serbia
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Doug - you just throw ( at least) a year of reading at poor greenhorn .........

but - there is a hope for him , considering his avatar .........

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Old 24th October 2010, 12:18 AM   #4
cbj591 is offline cbj591  United States
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low distortion, I don't need a ton of gain.
as far as feedback. i dont really care. which ever sounds better.

I dont really have experience with design, so i dont know if i really believe in followers and transistor CSS.

I just look through the designs and look at peoples reviews and decide to build from there. I love audio and got into tubes because i wanted to see what all raves were about.

I think i do it more because love the build process, but dont want to build a piece of junk.

so, i will just rework how i was going to mount the tubes and maybe i'll add a decorative tube protector to balance the look.

cheers
Brian
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Old 24th October 2010, 01:06 AM   #5
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Brian,

Do you need any gain, whatsoever? If your signal source(s) are 100% digital, a buffered volume control is most likely best. A (sic) single 12AU7 wired as 2X cathode followers, along with constant current sink (CCS) loading, will work quite well for that job.

Need a bit of gain? Pick a different tube, such as the 12B4. The 12AU7 is not especially linear and (μ = 20) yields a fair amount of gain.
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Old 24th October 2010, 01:18 AM   #6
cbj591 is offline cbj591  United States
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From what I am seeing, I do not need any gain then.
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Old 24th October 2010, 02:29 AM   #7
cbj591 is offline cbj591  United States
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Quote:
Originally Posted by cbj591 View Post
From what I am seeing, I do not need any gain then.
I take that back. I am using the pre-out section on a Krell KAV-400xi. Per the manual, it has a gain of 36.1db.

So right now, im not sure what i need. I figure I will just build the 2 tube Fore-watt or similar.

Should that be adequate?

Thanks
Brian
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Old 24th October 2010, 02:32 AM   #8
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Quote:
Originally Posted by cbj591 View Post
Right now I had planned to build the Fore-Watt, but that extra tube would balance the aesthetics out.

I didnt want to just mount the 3rd tube without using it.
You could build it with a reg. tube (Foreplay III) or with a rectifier tube (Hag. Clarinet).

Clarinet has 15dB gain.

jeff
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Old 24th October 2010, 02:41 AM   #9
DHT112A is offline DHT112A  United States
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There's one in here

Audio Reality
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Old 25th October 2010, 04:48 PM   #10
DougL is offline DougL  United States
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Quote:
Originally Posted by Zen Mod View Post


Doug - you just throw ( at least) a year of reading at poor greenhorn .........

but - there is a hope for him , considering his avatar .........

It's just I really dislike making assumptions.
The preamp he has chosen will probably drive a tube amp with normal sensitivity just fine.
A Firstwatt F4 that needs 20vpp from the preamp for full output, not so much. A typical SS amp with a 10K input Z, not so much.
If he is just using a CD, this preamp will give him about 20db of "empty" gain, which will result in higher noise and distortion.
Also people have many preconceptions about what they want to build.
I certainly do, LOL

Brian,

Enjoy your project, and good luck.

Doug
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