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john65b 13th February 2011 02:38 PM

JP200 preamp?
 
1 Attachment(s)
Hey all,

It's been said that "A wise man makes his own decisions, an ignorant man follows public opinion".

I cannot help but listen (read) the many opinions that are out there for Audio. Without such opinons I would not have stumbled upon (and built clones of) the Aleph 5, Krell KSA-50, etc.

One "Money is no Object Opinion" I often look to when researching a potential component or build or is Arthur Salvatore. I don't know who he is, but kinda like what I have read. Most of you have already seen his site...

REFERENCE COMPONENTS

So one of his praises is the Jadis JP80 Preamp, and it appears the JP200 is very similar in design. As you can see (attached), it has six 12AX7 tubes. I could never find the schematic on the JP80, but the JP200 design has been published, and the PCB and kits seem ubiquitous on ebay.

Now I have always seen 12AX7 tubes in Phono preamps, where the gain is needed, but not so much in linestages. I don't know if I am "brainwashed" into believing the 12AX7 does not belong in a linestage?

Anyway, looking for some opinions on the circuit...I absolutely love my 6SN7 Aikido, but always look to someday building a JP200...

EDIT - It may be the case that the attached circuit is the Phono Stage of the preamp? This would actually make sense...

Cassiel 13th February 2011 04:50 PM

I think it's difficult to say without hearing one. Here, people who don't like the 12ax7 would say it's a bad idea and those who like that tube would say: go ahead and build it. Low current tube, not the ideal cathode follower but many line stages have been commercially built with that tube. Anyway, the JP200 cathode follower section uses 2 triodes in parallel. Looks OK to me.

Cobra2 13th February 2011 06:52 PM

What a waste...

Arne K

analog_sa 13th February 2011 08:02 PM

Quote:

Originally Posted by john65b (Post 2467157)
EDIT - It may be the case that the attached circuit is the Phono Stage of the preamp? This would actually make sense...

It would but it isn't. On the bright side though, once you build it and get thoroughly disgusted with the sound it's very easy to turn it into a passable phono stage. 20 - 30 years ago there was some justification in building this but today there is simply no excuse. Apart from Mr Salvatore's and his associates warm endorsement.

DF96 14th February 2011 10:46 AM

It looks like a valve circuit designed by someone who would rather be using transistors, but had bought a lot of 12AX7 so had to use them up instead. I estimate open loop gain around 1500, which is then reduced to 11 by feedback (43dB of feedback). It has enough gain that it could be used as an active RIAA stage!

The 12AX7 is a good valve when used for its intended purpose: high gain voltage amp in high impedance circuit. It is wholly inappropriate to use it as a CF to drive an output, as it has very poor current driving capability. 12AT7 would be much better here, with R36 reduced in value.

john65b 25th February 2011 02:28 AM

Hmm,

Also, the ARC SP-16 uses three 12AX7's in the linestage...and seems to be well received...and there is a local guy with a good price selling one...

I also had the ARC SP-6C (12AX7 and a 6922) and thought it was OK...I preferred my Audible Illusions Modulus 3A better so I sold the SP-6C...

Liked the 6SN7 Aikido over the Audible Illusions, so sold it.

Oh hell, does this ever end??

Miles Prower 25th February 2011 03:14 AM

Quote:

Originally Posted by DF96 (Post 2468084)
It looks like a valve circuit designed by someone who would rather be using transistors, but had bought a lot of 12AX7 so had to use them up instead. I estimate open loop gain around 1500, which is then reduced to 11 by feedback (43dB of feedback). It has enough gain that it could be used as an active RIAA stage!

It's pretty gawdawful all right. If I was going to design a glass op-amp, I'd use one 12AX7 as the input diff amp, then a VAS (cascode 6BQ7s, or a small signal pentode like the 6AU6, 6SJ7, 6CB6) then a Lo-Z output stage, such as a White CF.

Quote:

The 12AX7 is a good valve when used for its intended purpose: high gain voltage amp in high impedance circuit. It is wholly inappropriate to use it as a CF to drive an output, as it has very poor current driving capability. 12AT7 would be much better here, with R36 reduced in value.
I can't see using types like that as CFs either. The 12AT7 has better drive capability, and isn't all that far behind the 12AX7 for u-factor. It would still work quite well to make a SE glass op-amp.

ppp6l6gc 11th March 2011 07:23 PM

As far as the JP200 kit. I have built one modified to use 12at7s as followers. I have to say I really like it. I know most people here are critical of circuit design in theory but I usually go by sound. Mine has a very wide sound stage, not as deep as some Mac stuff I've used, but when you here things 4 feet outside the speakers I consider that pretty good. I used Auricaps at first and liked them, tried the russian caps everyones talking about and like the way they sound but they add a little back ground noise. Little odd hisses and pops. I going to let them run a little longer to see if they wear in, in not then it's back to the auris. The overall presentation is definately dependent on the quality of the tubes. I ended up using 4 mullard AX7s and 2 tele AT7s. That was when everything really bloomed. She does take about a half hour to really hit her pace. If you build the kit don't skimp on quality components, they do make a difference. I modded mine to a circuit spec'd by a gentleman from germany. It's in the forum somewhere.

tony Ma 30th November 2011 03:40 AM

Quote:

Originally Posted by ppp6l6gc (Post 2500352)
As far as the JP200 kit. I have built one modified to use 12at7s as followers. I have to say I really like it. I know most people here are critical of circuit design in theory but I usually go by sound. Mine has a very wide sound stage, not as deep as some Mac stuff I've used, but when you here things 4 feet outside the speakers I consider that pretty good. I used Auricaps at first and liked them, tried the russian caps everyones talking about and like the way they sound but they add a little back ground noise. Little odd hisses and pops. I going to let them run a little longer to see if they wear in, in not then it's back to the auris. The overall presentation is definately dependent on the quality of the tubes. I ended up using 4 mullard AX7s and 2 tele AT7s. That was when everything really bloomed. She does take about a half hour to really hit her pace. If you build the kit don't skimp on quality components, they do make a difference. I modded mine to a circuit spec'd by a gentleman from germany. It's in the forum somewhere.

Do u still remember what you change on this one?
I have one done but the sound is not balance (R louder than L) and exessive hissing

Roulston 22nd December 2011 11:21 AM

Quote:

Originally Posted by tony Ma (Post 2800167)
Do u still remember what you change on this one?
I have one done but the sound is not balance (R louder than L) and exessive hissing

I am also looking for proven mods on this kit. Nearly finshed as per the published design, but would like to prepare for getting it fixed.

Tony


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