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Old 29th April 2006, 09:44 AM   #1
Tyimo is offline Tyimo  Hungary
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Default Tube preamp with current mirror

Hi!

I would like to ask the tube experts about the attached tube preamp with current mirror:

How much gain has it?
What is the output impedance?

I would like to use it for a Mosfet power follower amp, so I need a lot of gain.....and nice sound too!

Thanks and greets:

Tyimo
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Old 29th April 2006, 11:00 AM   #2
hacknet is offline hacknet  Singapore
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i tried something like that awhile back and it seems distortion is the big problem..
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Old 29th April 2006, 12:20 PM   #3
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I don’t think you want to do that! In a current mirror like this, Q4 is configured as a diode. The plate of the 6922, which wants to see a HIGH resistance plate load for lowest distortion and highest gain, will instead see a perversely low and non-linear resistance of a few tens of ohms. Voltage gain to the plate of the 6922 will be fractional (negative dBs of gain – call it “loss”), and distortion high.
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Old 29th April 2006, 01:36 PM   #4
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On top of the previous posters' remarks is the (IMO) negative of a volume control in the O/P circuitry. Also, the I/P impedance of the supplied schematic is VERY low.

I think you can achieve your goal with a constant current source (CCS) loaded triode. CCS loading allows you to get the full mu (amplfiication factor) of the tube along with minimal distortion.

Think about a CCS loaded ECC99 section DC coupled to a power MOSFET source follower. You'd get lots of gain and bulldozer drive capability. "MOSFET Follies" shows how.
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Old 29th April 2006, 01:39 PM   #5
Tyimo is offline Tyimo  Hungary
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Thanks! Not a very good news...

Have you got something similar, simple, low voltage (ca.60V)
preamp suggestion?

I am looking for a tube line stage with a lot of gain and low Zout.

Tyimo
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Old 29th April 2006, 01:45 PM   #6
anatech is offline anatech  Canada
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Hi Tyimo,
How much drive voltage do you want out??

-Chris
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Old 29th April 2006, 01:54 PM   #7
Rocky is offline Rocky  Norway
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Quote:
Originally posted by Tyimo
Have you got something similar, simple, low voltage (ca.60V)
preamp suggestion?

Perhaps not what you are looking for, but take a look at the battery projects at the top of the left hand menu of this site: http://digilander.libero.it/paeng/projects_frame.htm. Those are very low voltage projects. Not very similar to what you suggest though..
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Old 29th April 2006, 02:33 PM   #8
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Tyimo,

The best low voltage tube I'm familiar with is the 6GM8/ECC86. 6GM8 data sheet here.

Given the 6GM8's low max. of 30 V. on the plate, I have doubts about your being able to swing the signal voltage you want.

JMO, if the B+ rail voltage is to be about 60, you will have to get part of the necessary voltage gain from a step up trafo. Plan on having more than 1 FET voltage follower in the final design.
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Old 29th April 2006, 04:27 PM   #9
Tyimo is offline Tyimo  Hungary
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Hi everybody!

Chris!
Quote:
How much drive voltage do you want out??
I need min. 7V RMS output, but 15 would be the best.
max. 1 K output impedance.


Rocky!
Quote:
http://digilander.libero.it/paeng/projects_frame.htm
Thanks for the tipp, but I know it already. I am afraid it hasn't got enough gain for my power amp. Have You got any infos about it?


Eli Duttman!
Quote:
The best low voltage tube I'm familiar with is the 6GM8/ECC86
Thanks, I kow the 6GM8, but I have some nice Siemens and Mullard E88CC tubes and I would like to use them.



Greets:

Tyimo
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Old 29th April 2006, 07:01 PM   #10
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I made that circuit back in my student days and it worked as my computer preamp. But at such low voltages, Vgk is very low, and it has limited input range. I tried it to even less than 40volts mostly for experimentation. Soundwise it is very good.
Eli, I'm surprised you dont see how this works. The tube doesn't see any load. Ideally it should be 0ohms into the mirror. The tube has a constant anode voltage. When the grid swings, the cathodecurrent swings almost as much as the tubes transconductance. At 10mA idle, the 6922 will swing almost 10mA for a change of 1volt at the grid. The mirror does just what it is called, mirrors the change in current into the load, the output resistance. and now you can have gains of much more than the tubes mu. The most awesome preamp I made with this topology was with a variable load resistance. (R4 in the circuit was a stepped attenuator).
The output impedance is totally dependent on R4, so Tyimo, hard to get gains of 15 and lower than a few kohms output impedance. But really, have you tried with more than 1kohms? I think most people are so into textbooks that they just assume it must be low. I have tried (real high output impedance) and it sounds great.
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