Tube phono preamplifier by 12ax7 - diyAudio
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Old 2nd February 2012, 02:35 AM   #1
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Default Tube phono preamplifier by 12ax7

Planning to design a low cost tube phono preamplifier. After searching on the web I realized a phono preamplifier should have the following features:
1. High gain. Normally 40 to 45dB
2. A RIAA circuit is necessary
3. High input impedance is needed for piezeo phono head
4. Low input impedance is needed for magnetic phono head

I found a very simple circuit designed by 7025 (12ax7), attached below. Is ther anybody had tried this circuit? How about the performance?

THe output impedance is 220kOhm, maybe too high for some power amplifiers. I think a cathod follower should be added.
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Old 2nd February 2012, 02:54 AM   #2
Cassiel is offline Cassiel  Libya
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Quote:
After searching on the web
This circuit has been discussed here numerous times.
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Old 2nd February 2012, 04:41 AM   #3
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Oh...could you give me a link of the posts? thanks
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Old 2nd February 2012, 05:19 AM   #4
Cassiel is offline Cassiel  Libya
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RCA Phono Stage Redux
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Old 2nd February 2012, 05:22 AM   #5
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lordie - Eli will be all over this one...
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Old 2nd February 2012, 06:23 AM   #6
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Hi Siliconray,
Why do you expect this diy-community to help you design your kits? Why not hire a consultant instead as you seem to be in the commercial sector?
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Old 2nd February 2012, 09:09 AM   #7
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Thanks Cassiel. I will breadboard this circuit. I'm also thinking about Maranz-7 design. It has a cathod follower as output stage. I think a potentiometer is also necessary. But where to put it? before or after the follower?
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Old 2nd February 2012, 09:30 AM   #8
SY is offline SY  United States
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There's already about a million companies offering kits based on these not-terrific and poorly-performing designs. Why not do something different? I would agree with Lars, if you want to offer something and your design chops aren't up to it (your last post confirms that), hire someone to do it for you.
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Old 2nd February 2012, 11:45 AM   #9
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Hi SY, this one isn't a kit, just a trial project. Maybe we'll offering phono preamp kit in the future, but not this one.

I just courious about phono because many people mentioned about it at this digital time. I just want to give a try, my personal hobby project. My phono player is still on the way.

I wish I can find someone to do it, but it's not easy to find an experienced tube amp designer here.

BTW, what do you think can be 'different' from the other 1 million companies products?
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Last edited by siliconray; 2nd February 2012 at 11:55 AM.
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Old 2nd February 2012, 12:16 PM   #10
SY is offline SY  United States
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Quote:
Originally Posted by siliconray View Post

BTW, what do you think can be 'different' from the other 1 million companies products?
It would be nice to have one with good RIAA conformance, low distortion, low input capacitance, high overload margins, and fast recovery from overload. If I were you, I'd spend a few days going through the preamp chapter in Morgan Jones's "Valve Amplifiers."

Quote:
Originally Posted by Morgan Jones
When we investigated power amplifiers, we looked at some classic designs to see how the goals were achieved. There were no classic RIAA stages, they varied from mediocre to plain awful.
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