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Old 18th January 2012, 09:01 PM   #11
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It's been awhile since I've seen a more ridiculous design. Cascading a small signal pentode into a 12AX7 (high gain triode) is going to give one helluvalot of open loop gain. That would be OK if you needed that for NFB, but this isn't an NFB design.

Secondly, this has a -20bdv attenuator up front. Why include all that open loop gain, only to throw it away? That makes no sense at all.

Finally, driving the output directly from the 12AX7 plates is just asking for trouble. Long runs of cable, an amp with a smallish Zi, are going to load the 12AX7 right into distortion and nonlinearity. Triodes like light plate loading, and the output isn't buffered.

Take a pass on this one.
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Old 18th January 2012, 10:36 PM   #12
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I agree with the most said before from the other guys , and something more you include all that gain without any negative feedback , and you attenuated it in the input , but you still have high level of noise due to large amplification ( EF86 plus 12AX7 ) and due to partition noise of the EF86 in pentode mode amplifier .
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Old 19th January 2012, 04:27 AM   #13
Edwind is offline Edwind  Indonesia
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I'll try it from simplest one by mod this design by using NFB.
Is a buffered output necessary if input impedance of the power amp around 20Kohms with interconnect cable about 1meter (3-4 feet)?
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Old 19th January 2012, 04:49 AM   #14
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Quote:
Originally Posted by Edwind View Post
I'll try it from simplest one by mod this design by using NFB.
Is a buffered output necessary if input impedance of the power amp around 20Kohms with interconnect cable about 1meter (3-4 feet)?
20K, alone, is a near vertical loadline for a 12AX7, and is way to small. Add in cable capacitance, and it'll roll over and die. Find a better design.
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Old 19th January 2012, 10:16 AM   #15
DF96 is offline DF96  England
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For a short cable the low input impedance of the power amp will rescue the frequency response, but this is not the way to do audio!

Instead of fumbling around trying to patch up this design, why not try to understand all the things which are wrong with it and then start again, with your new knowledge?
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Old 19th January 2012, 11:22 AM   #16
Edwind is offline Edwind  Indonesia
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By the way, does anyone know Jadis JP200 preamp?
If know, what do you think about that?
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Old 19th January 2012, 12:48 PM   #17
DF96 is offline DF96  England
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I just Googled the JP200. Three 12AX7 per channel (parallel triode pairs), with heavy NFB to get a voltage gain of 11. Looks like it was designed by someone more used to using BJTs, or designed a phono preamp then changed his mind.
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Old 19th January 2012, 01:37 PM   #18
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Hey Edwind,
Have you been looking at the Chinese boards at eBay for this ancient, nogood preamp. In that case, look no more....

Actually, feedback seems to be in the ballpark of 50dB!! Normally a tube preamp need 0dB.

Last edited by revintage; 19th January 2012 at 01:46 PM.
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Old 20th January 2012, 03:37 AM   #19
Edwind is offline Edwind  Indonesia
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So what a great linestage preamps?
Maybe I can learn from it's design by build it.
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Old 20th January 2012, 05:39 AM   #20
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You got a good solution from me. Try it and it will take you far. And yes, you will learn too.

Think it will take 2 hours max to rebuild. Please report Monday.
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