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Old 2nd February 2005, 06:42 PM   #1
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Default A differential output PEARL... ...?

Hello All,

Sometimes I wish the Pearl Ono stage with a balanced output :

the circuit attacched introduces the current sourced diff pair as output stage; it is largely simplified anyway.
What do you think ... ?
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Old 2nd February 2005, 09:49 PM   #2
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There are any number of ways to do it, but yes, that will work.
Don't forget DC blocking caps at the outputs.

Grey
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Old 2nd February 2005, 10:38 PM   #3
Gaucho is offline Gaucho  Canada
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There are any number of ways to do it, but yes, that will work.
Grey, in your opinion is there another more prefered method?
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Old 3rd February 2005, 01:56 AM   #4
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Me, I'd use the one 2SK389 as a single differential instead of running the two devices in parallel--more like they did in the Ono. The problem then becomes that the transconductance of the stage drops, hence the gain drops, and you have to start looking at other things, like whether you want to increase the load resistors in order to get back some of the gain, etc. You could put a P-device after the differential in order to get some gain that way, and at the same time do some level shifting (i.e. bringing the signal back to ground). Then add followers in order to beef up the output current capability...
Err, but then Wayne's original design kinda gets lost in all the reshuffling. I'm not sure that's what you'd want to do. At some point you have to make a decision as to who is designing the thing, you or Wayne. Push it far enough, and it becomes your phono stage. Nothing wrong with that, just know it going into the project.
I like the Ono/Pearl front end, but after that I tend to think of other things I'd like to try. There's nothing wrong with the Ono/Pearl, it's just that you can't put me in a kitchen well stocked with herbs and spices without me wanting to cook something.

Grey
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Old 3rd February 2005, 09:46 PM   #5
Gaucho is offline Gaucho  Canada
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Me, I'd use the one 2SK389 as a single differential instead of running the two devices in parallel--more like they did in the Ono.
Grey, thank you for your previous post.

Why are single JFET's preferable for differential pairs? Don't matched parallel JFET's behave like a single larger device the way (I'm told) MOSFET's do? Thanks for any insight.
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Old 4th February 2005, 12:57 AM   #6
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The 2SK389 isn't a single JFET, it's a dual. This makes it ideal for a differential, in that the two JFETs (in a single case) have very, very similar characteristics and track together thermally.
Yes, running JFETs in parallel--whether they are single devices or dual--gives you behavior somewhat like a single, larger device. Whether that behavior is what you want is the question. In the Pearl, they use a dual device with the two halves running in parallel. In the Ono, the 2SK389 halves are used as a differential. In looking at the Pearl vs. the Ono, it seems to me that they "detuned" the Ono and simplified it a bit rather than start with a clean sheet of paper. It's interesting to look at the Pearl and try to second guess why particular choices were made in the design process. It's a good excuse to linger over a decent glass of flavored ethanol.

Grey
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Old 5th February 2005, 03:20 PM   #7
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Originally posted by GRollins
Me, I'd use the one 2SK389 as a single differential instead of running the two devices in parallel--more like they did in the Ono. The problem then becomes that the transconductance of the stage drops, hence the gain drops, and you have to start looking at other things, like whether you want to increase the load resistors in order to get back some of the gain, etc. You could put a P-device after the differential in order to get some gain that way, and at the same time do some level shifting (i.e. bringing the signal back to ground). Then add followers in order to beef up the output current capability...
Err, but then Wayne's original design kinda gets lost in all the reshuffling. I'm not sure that's what you'd want to do. At some point you have to make a decision as to who is designing the thing, you or Wayne. Push it far enough, and it becomes your phono stage. Nothing wrong with that, just know it going into the project.
I like the Ono/Pearl front end, but after that I tend to think of other things I'd like to try. There's nothing wrong with the Ono/Pearl, it's just that you can't put me in a kitchen well stocked with herbs and spices without me wanting to cook something.

Grey

Does running the output differential gain increase the overall enough to make up for not having the paralled transistors? I would expect to get at least as much going from single ended to differential.
DC offset always bothers me. My speakers have no crossovers and everything was dc coupled through the preamp and amp. But now I have transformer coupling in my preamp to handle small amounts to dc from sources.
I would love to redo the output stage and get rid of the coupling caps. This will require dual rail and either a differential or single ended with current source.I am totally convinced that the very best caps cannot compare to no caps in the signal path


George.
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Old 6th February 2005, 10:21 PM   #8
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Okay, so you're wanting to use the differential to cure the DC offset at the output? Were you wanting differential output or single-ended?
You're going to have to do something to level shift the output back down towards ground in order to get rid of the DC coupling caps at the output. The schematic shows 19.3V offset at the output. Sadly, that will not go away without throwing more parts at the thing.
So then the question becomes: Are you willing to keep throwing transistors into the thing just to get the signal back to ground?
It will very quickly become a question of comparing evils--is a cap more or less deleterious to the signal than the transistors you'd have to use to level shift the output...well...that's something that you'll have to decide before going any further. You're headed for a fairly major reworking of the original design.

Grey
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Old 27th December 2005, 03:50 PM   #9
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Hello all,

I'm planning to use the attached circuit as output stage of the Pearl phonostage (right after the RIAA eq.).

The circuit seems to be working OK according to the simulator. The next step would be to start testing in the real life, but before I start I would like to hear some opinions of people that might have already built something similar (or just are more experienced than me).

Thanks
Xavier
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Old 3rd January 2006, 11:23 PM   #10
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Default I did something like this ...

Well, at least I considered doing it.

In the end i went another route and used 8 single JFET's in parallell single ended mode - i.e much the same way as the original.

What clinched it in the end for me is that since the input voltage is so low, what you get out of a balanced topology is 2 inputs with half the voltage swing each. If you use the same number of transistors (or more like I did), you can keep the load resistance the same and raise the supply voltage for higher gain, and you get "more direct" noise cancellation across devices rather than having to rely on differential noise cancellation which should work in theory, but ....

If you look at say the Ono schematics, you will find that balanced conversion takes place as a later stage. Since the Ono has too low gain anyway, you really need the extra stage unless you have a high output pickup.

Most people who know me know of my preference for balanced topology. However, even I would advice against balanced input phono stage.

The main problem with a phono stage is low input level and need for extremely high gain. I agree that it is nice to consier the input balanced - it is even floating but if you get double the voltage on the gate you will get lower noise. 60-80dB gain prior to RIAA equalization is a lot, and a little noise gets amplified extensively ...

But don't take my word for it - build both and report your results back to us!

Petter
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