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Old 13th September 2002, 12:16 AM   #1
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Default Comments on a preamp design

The attached schematic is a preamp designed by a Japanese and raved in Asian audiophile as the best tube preamp.

Any experience and comments on this design?
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Old 13th September 2002, 08:15 AM   #2
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Hello MPEG7,

It seems to me that for a line stage you're going to end up with a lot of gain.
Anyway the first two stages use 1/2 12AX7 in a classic anode loaded cascade configuration.
It is than directly coupled to the last stage which looks like a White cathode follower.
This topology is not widespread for some obscure reason.
It has low Zout and high voltage swing.In a preamp of mine I use the same configuration but I use a 12BH7A as the last stage and a B+ of 310 VDC.(For lower Zout and higher voltage swing.)
Also note that here a NFB loop is used (25K) between output and input.
I'd use a wel regulated PSU for each stage,one of 250VDC and another of 280VDC for each channel.
Be aware also of the value of the coupling cap at the output which is in error.I'd try something like 3mF/400VDC as a starting point.Also I'd use two caps here in series and take the feedback loop from their junction.

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Old 13th September 2002, 04:15 PM   #3
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Just out of curiosity,

I do not have the exact formula in front of me, but I would think that the gain of that preamp is set by the 25K/2K for a voltage gain of roughly 12, no?

Open loop is another story.

Also, I would think that with all that distortion reduction to begin with, what with the first two stages cancelling out (I read an article somewhere, I think glass Audio or something about two stage cancellation of distortion) and that what looks like a SRPP, but at any rate that last stage which is also low distortion, one wouldn't even need the NFB.

I may try it out myself to see how it sounds.

Gabe
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Old 13th September 2002, 04:33 PM   #4
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Hi Gabe,

From the top of my hat,you're absolutely right.
On the distortion front that NFB global loop won't make that much of a difference.
To me this is just introduced to reduce the gain and by the same token linearize it even more.(It'll still be a time shift though.)
The whole circuit is so simililar to a phono preamp I once designed back in 1988 that it is rather hard to believe.
Naturally introducing RIAA networks in this circuit reduced the gain by 10 dB (approx.) in my design.(One pole in NFB + 2 passive.)
BTW you're dead on regarding the THD cancellation technique,if memory serves me correctly it was discussed in GA in 1998 in relation to a preamp running tubes at low B+.

Rgds,
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Old 16th September 2002, 12:17 PM   #5
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Thank you,

fdegrove:
Why do you like to put the feedback in between two caps?

Gabe:
If you try this soon, please report back how would like the sound.


It is said that this design was born in late 70's when the tube was a passe so it did not get popular. And it was raved this design need a PSU with a RC filter to bring a warm sound. What do you think?
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Old 16th September 2002, 12:56 PM   #6
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Mpeg7,

The split cap suggestion I proposed is to insure no DC can be present at the output.
Leaving the stage as it is would put a small voltage at the otput.
As far as the PSU goes I suggest a well regulated PSU preferably
LC or tube regulated.
Also I would use a tube rectifier.
One for each B+ per channel.
Any idea where you came across this circuit?

Rgds,
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Old 16th September 2002, 09:14 PM   #7
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1.5v is a small voltage? well, it certainly ain't lethal...
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Old 16th September 2002, 09:33 PM   #8
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Hello,

Sure,but it won't hurt to remove it either ?
Any SS amp seeing won't like that for sure?
Using the "split cap" technique will remove it entirely and at the same time assure it can't upset anything it gets connected to.
Anyone ever fiddled with this circuit?
I find it rather odd that it is presented as a line stage date from the seventies.
What for did they need a two stage preamp with that much gain?
Also I feel a 12BH7A or such for the last stage would have been a far better choice all ecomical reasons put aside?
It combines both lower output impedance and far better output swing.
Comments welcomed.

Rgds,
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Old 17th September 2002, 09:50 AM   #9
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This Japanese circuit was originally from a Chinese website and said being very popular with Chinese DIYers. (It seems every tube fans there know it and some modified versions are being produced by Chinese HiFi companies. However it said that this circuit was never used in any commercial products in Japan as when it was designed in late 70's, tube was being replaced by SS everywhere.)

I am new to this subject however actively looking for a preamp to build.

fdegrove:
Why do you think this circuit should go with a well regulated PSU. From which part of the circuit you believe that would be a good choice?
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Old 17th September 2002, 10:46 AM   #10
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Hi Mpeg7,

Why do you think this circuit should go with a well regulated PSU. From which part of the circuit you believe that would be a good choice?

I'm not sure I understand you correctly,still it is always good policy
to endow any audio gear worthy of construction with a well regulated PSU.
Some circuit topologies are more sensitive to B+ stability then others and there are many recipes to go about it.
All of which will have a sonic impact on the circuit they're used for.
In this circuit in particular you will have noticed the direct coupling between the second halve of the 12AX7 and the upper halve of the 12AU7.If the B+ here is not stable all working points of the stage will shift along with it.It is also this stage that will have the highest voltage swing at it's output hence the importance to keep it rock steady.
This of course does not imply that one should neglect any other stages for PSU stability.I'm just pointing to the weakest link here.


Rgds,
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