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Old 10th October 2010, 05:51 PM   #1
cbj591 is offline cbj591  United States
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Default 12AU7 Phono Stage

Good Day to All,

Are there any good schematics for a phono stage that uses only the 12AU7?

Thanks in advance.

Cheers

Brian
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Old 10th October 2010, 07:43 PM   #2
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Probably not since 12au7 is highly unsuitable for the task. Medium mu and low gm does is not what you need for low noise and few amplification stages.

/Olof
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Old 10th October 2010, 08:56 PM   #3
DF96 is offline DF96  England
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I can't think of any stage in a phono preamp where a 12AU7 would be an appropriate choice, except maybe a low gain output stage. Even there there a better valves.
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Old 10th October 2010, 09:11 PM   #4
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Quote:
Originally Posted by DF96 View Post
I can't think of any stage in a phono preamp where a 12AU7 would be an appropriate choice, except maybe a low gain output stage. Even there there a better valves.

The 'U7 makes an OK buffering cathode follower at the phono preamp's O/P and its utility for this job ends there.

-------------------------------------------------------------------------------------

Brian,

Olof summed the situation up very well. The 12AU7 is unsuited to the gain, noise, and linearity requirements of phono section voltage amplification. Pick another type! Obvious contenders include the 12AX7 and the 6922.
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Old 10th October 2010, 09:31 PM   #5
DF96 is offline DF96  England
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I would not use it for a cathode follower - gm too small, so high distortion and high output impedance.
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Old 10th October 2010, 09:48 PM   #6
Salas is offline Salas  Greece
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Quote:
Originally Posted by cbj591 View Post
Good Day to All,

Are there any good schematics for a phono stage that uses only the 12AU7?

Thanks in advance.

Cheers

Brian
I have made one 10 years ago which is good if you like 12AU7 and you can handle 18dB gain. Has 5R4GY or analogous rectifier, MKP CLC and a Mosfet Maida reg for PSU. Pictured with Russian NOS rectifier and Tangsol NOS 12AU7s.
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Old 10th October 2010, 09:55 PM   #7
Salas is offline Salas  Greece
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Phono stage you said! I posted a line stage! Too blind! Anyway, 12AU7 is not suitable for phono stage.
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Old 10th October 2010, 11:39 PM   #8
JohanB is offline JohanB  Sweden
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Hi,

I'm quite new to this forum, but not new to audio designs. Have been professional since the 1960's.

The following thoughts IMO about this:

A good 12AU7/ECC82 has a very, very low input-, flicker- and shot- noise. Specially the down-rated? and low priced Philips-Sylvania, during the 80's manufactured 6189's, that seems to have a very good and stable cathode emission coating. Sucsessfully used in some famous capacitor microphones!!

But the amplification factor is far to low for a passive RIAA network to follow, where you lose around 20dB in gain at 1 KHz. So the termal noise in the equalizing network will be very dominating and audiable.

The little higher distorsion of this tube is no problem at these signal levels, but it's still only suitable for the output stage. Preferebly in a 2-tube SRPP or an u-stage, where the distorsion is minimized by the light load on the voltage amplifier tube and the output impedance is lowered in the cathode follower section.

For the input stage (in a passive tye RIAA) are only tubes with a u (amplification factor) of 60 to 100 suitable, to overcome the termal noise in the EQ-network. 12AX7/ 7025/ ECC83/ E83CC/ 5751/ 6SL7 & similar clones are the choice.

Remember now, when we easily can use clean DC for heater voltage, the special & expensive low hum tubes, with special shielding and spiraled heater wires, has no advantage to ordinary ones.

Has anybody tried to use the the very low priced 12AT7/ECC81 as the input tube?
They should have a comparably low microphonic sensitivity, as they were used as FM-receiver oscillator in radio receivers long ago. Distortion is quite high in this tube, but still, at these signal levels absolutely low enough.

JohanB

Last edited by JohanB; 10th October 2010 at 11:50 PM.
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Old 11th October 2010, 05:04 AM   #9
45 is offline 45  Italy
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Quote:
Originally Posted by DF96 View Post
I would not use it for a cathode follower - gm too small, so high distortion and high output impedance.
It works OK for that application. At the end of the day its gm is not so lower than that of a 6SN7 and you typically get around 400 ohms Zout. Pretty good for driving 99% of line preamps. Distortion is low if you bias it "properly" and the signal level is quite low in a phono preamp.

45
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Old 11th October 2010, 05:12 AM   #10
45 is offline 45  Italy
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Quote:
Originally Posted by JohanB View Post
Has anybody tried to use the the very low priced 12AT7/ECC81 as the input tube?
They should have a comparably low microphonic sensitivity, as they were used as FM-receiver oscillator in radio receivers long ago. Distortion is quite high in this tube, but still, at these signal levels absolutely low enough.

JohanB
The real ECC81 is much better than that described in data sheets. Actually this is true for many "poor" tubes! You can get several volts with very low distortion if it doesn't have to drive heavy loads....
However I have no experience with this valve in a phono preamp - or better with many valves in general - as I switched to SS phono preamps a long time ago....

45
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