DIY build of the Pioneer C-21 preamplifier - Page 3 - diyAudio
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Old 28th December 2011, 06:45 AM   #21
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Originally Posted by rjm View Post
James, your post reads like a classified add, but I agree it would be simpler and cheaper (in the long run) to buy one rather than build a copy.

The electrolytics in a 40 year old component are of questionable status, despite your checks. Let's face it they must be on their way out by now. That's not what's holding me back though; it's just a matter of purpose - I'm more interested in the circuit than the preamplifier per se.
When you said it, I reread my post and it does seem a bit like classified. It's just that my enthusiasm got carried about C-21 but trust me, I got nothing for sale and C-21 is going to stay with me for a very long time. I like to keep such samples as a gauge for my audio journey.

Recapping the C-21 is real simple so I may just do that one day soon.

Since this is a DIY section, I am all in for building a C-21 clone but I got curious why you want to build this particular pre thus the question.
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Old 28th December 2011, 08:38 AM   #22
Previously: Kuei Yang Wang
 
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Hi,

As you are in Japan, last year or so MJ did a review on a Preamp by Phasetech (their solid state offering).

It uses a very interesting open loop line stage with what you may call a Fet-Diamond frontend, notable for it's use of the still available and easy to get 2SK246 and 2SJ103. It would be easy to arrange this circuit run both with and without looped feedback.

Kimura San's headphone amplifier incidentally may also be used without any feedback with minor re-adjustment.

Past that, no need to teach chinese (mainland or otherwise) KopyMao's new tricks...

Ciao T
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Old 28th December 2011, 12:20 PM   #23
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Richard Murdey
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Probably this one, or this slightly cheaper variant.

Nice find.

re the 47 labs amp, yes I see, I think: the feedback resistor could be grounded to cut the feedback loop back to the previous stage and the whole thing should still basically hang together.
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Old 28th December 2011, 12:26 PM   #24
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Quote:
Originally Posted by megasat16 View Post
I am all in for building a C-21 clone but I got curious why you want to build this particular pre thus the question.
I got the impression from looking at the schematic that the designer had considerable skill and the circuit was carefully optimized for sound quality. PRR disagrees, but that was anyway my feeling after looking through a dozen or so similar documents. The other option was the Classe DR-4, but I considered that a bit too complicated.
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Old 28th December 2011, 02:50 PM   #25
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Hi,

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Originally Posted by rjm View Post
We try. Given that what most people do bores me to death due to a total lack of innovation I have to cast the net wide for things that are interresting.

Quote:
Originally Posted by rjm View Post
re the 47 labs amp, yes I see, I think: the feedback resistor could be grounded to cut the feedback loop back to the previous stage and the whole thing should still basically hang together.
Essentially, the resistor to ground (part of the FB Loop) sets the transconductance of the circuit (it is voltage in - current out).

A second resistor then can set the gain because with a known transconductance we can set the gain simply with a resistor from the output to ground.

I would personally replace the diamond transistor with a pair of J-Fet's (1pc N-Channel, 1pc P-Channel), the mentioned 2SK246 and 2SJ203 would be a good choice.

The beauty is that while these circuits CAN use looped feedback, they do not have to and they can even be made switchable between the two alternatives if desired.

For line outputs a direct output is okay if the design is sound (pun intended), for 'can's I would add a nice open loop Fet-Diamond buffer...

Ciao T
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Old 28th December 2011, 02:54 PM   #26
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Hi,

Quote:
Originally Posted by rjm View Post
I got the impression from looking at the schematic that the designer had considerable skill and the circuit was carefully optimized for sound quality. PRR disagrees, but that was anyway my feeling after looking through a dozen or so similar documents. The other option was the Classe DR-4, but I considered that a bit too complicated.
Looking through a wide range of circuits I find the C-21 very generic in terms of circuit, with much attention paid to and money spend on the passive components (not necessarily a bad choice TBH), but the gain circuit can really be improved upon, a lot actually...

In my view the design of the Phase Tech CA3 instead is one that evidences considerable skill in circuit design and optimisation out of 10 I'd give it 9, with the pioneer scoring maybe 3...

Ciao T
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Old 4th January 2012, 12:24 PM   #27
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Hey up, back after New Year...

Agree that the C-21 circuit is "generic". I just thought it was a well-thought-out implementation.

So turning to the 2SJ103/2SK246 jfets, I suppose with access to complementary pairs any bipolar-based preamplifier circuit can be re-jigged to use jfets, including the LH002 "diamond buffer", variations thereof, or the current-feedback-amplifier "diamond buffer + current mirror" circuit of 47 labs.

Since Phase Tech don't give too much info on their web page and I don't have easy access to the MJ article, I'm left in the dark about the circuit. Looks from the photo to have about a dozen TO-92 transistors per channel, and the blurb touts it as a "full symmetrical non-feedback circuit" which sounds rather like the 0247 circuit with the feedback connection cut... though "full symmetrical" suggests a doubling somewhere dealing with balanced input signals.

This might possibly be related.

The search continues.

Other than being more careful to match components and remember that voltage offsets will be less stable, is there any "gochas" in using jfets over bipolars that I'm missing?
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Old 4th January 2012, 02:26 PM   #28
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Hi,

Quote:
Originally Posted by rjm View Post
Since Phase Tech don't give too much info on their web page and I don't have easy access to the MJ article, I'm left in the dark about the circuit. Looks from the photo to have about a dozen TO-92 transistors per channel, and the blurb touts it as a "full symmetrical non-feedback circuit" which sounds rather like the 0247 circuit with the feedback connection cut... though "full symmetrical" suggests a doubling somewhere dealing with balanced input signals.
Richard. You are in Kyoto and you cannot get MJ Backissues? How do you think us kawaii gaijin who rarely visit Japan are doing!?

I get them when I'm in Hong Kong... Some of Shops there usually have a years worth or two in stock for crazy people like me... I can see if I scan the schematic when I have time, sadly my scanner is sheet feed, not flatbad, so it will take time.

The Symmetric Circuit in this case is meant rail symmetric.

It uses the K246/J103, then a enhanced wilson current mirror (IIRC) with gain into a load resistor and then a buffer. It's actually very similar to the circuit you find in Ayre gear as well, but CH has squirrelled away massive numbers of K170/J74 (I have more modest numbers, but already pair-matched) so he uses them.

Quote:
Originally Posted by rjm View Post
Other than being more careful to match components and remember that voltage offsets will be less stable, is there any "gochas" in using jfets over bipolars that I'm missing?
No, by using BJT's you get some extra gotch's though...

My favourite J-Fet mod of all times is to replace the input transistor of Naim Line Stages with a BF244 (or 245, whichever has the right pinout) and then remove anything between J-Fet and Pot. Quite a revelation actually...

Ciao T
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Old 4th January 2012, 04:05 PM   #29
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My best bet would probably be to find someone who has them already at home. Current issue isn't a problem, but I wouldn't know where I'd score a back-issue...

Like many Japanese I have abandoned the bookstore for the internet and they have abandoned us : only about half as many in Kyoto now as there were when I came, and it was the best ones that went first.

I digress.

Funny you mention Ayre, I read a rave Stereophile review many years ago that I still remember. Its always been in the back of my mind, wondering how the jFETs were configured in that equipment.

I'm gonna go search the Japanese DIY scene for circuit ideas, see if anything pops up. Googling around English didn't come up with anything I hadn't seen before.
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Old 4th January 2012, 06:31 PM   #30
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Hi,

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My best bet would probably be to find someone who has them already at home. Current issue isn't a problem, but I wouldn't know where I'd score a back-issue...
Call/Fax the publisher, they will probably have warehouses full...

Seibundo Shinkosha Publishing Co., Ltd.

They do seem to accept on-line ordering:

??????/???????

You might also call Phase Tech, they may have copies of the reviews available...

Quote:
Originally Posted by rjm View Post
I'm gonna go search the Japanese DIY scene for circuit ideas, see if anything pops up. Googling around English didn't come up with anything I hadn't seen before.
For reference, the service manuals of the top of the line Luxman, Pioneer, Sony, Nikko et al Gear can be quite inspirational.

Though as said, really innovative things come (unfortunatly) invariably from fairly small companies.

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