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Old 11th October 2012, 04:19 PM   #231
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You and your professor are very wise people!
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Old 15th October 2012, 05:40 PM   #232
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Hi Everybody!

MasterPiece is back.

I have been working a little bit today on it and after some thoughts I have come out with a modified topology N-CH only.
To my eyes it looks pretty good and slick both absolute and relative offset are taken in care of by the servos.
The only thing I am not sure at this point is if we can reach out the needed gain with IF3602.
Technically they have more than 20 times the gain of 2SK170.

Unfortunately I don't have a spice model for it at the moment (if anybody has a clue on how to obtain them please let me know).

Meanwhile, please feel free to comment on it. This is a start up schematic and I am sure a lot of adjustments will need to be made.
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File Type: pdf MasterPiece-R7.pdf (40.0 KB, 49 views)
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Old 15th October 2012, 06:09 PM   #233
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That looks good as a starting point. I like the N-channel solution.
The Interfet part will have enough gain. You can parallel more BF but you do not want to do that.
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Old 15th October 2012, 06:18 PM   #234
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Quote:
Originally Posted by Joachim Gerhard View Post
That looks good as a starting point. I like the N-channel solution.
The Interfet part will have enough gain. You can parallel more BF but you do not want to do that.
In the simulation it looks like I can't get more than 50 dB or the gain will collapse.
I am wondering if that is a glitch of the simulator or what.

I mean if you think that with 2 pairs of interfet we can reach out 63dB-ish that would be great.
Noise-wise they are significanlty quiter than any other fet I know of, so that should produce a very quite phono stage hopefully.

You said it is a starting point, if you have any recommendation at this point please let me know.
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Old 15th October 2012, 06:44 PM   #235
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Speaking of which I have checked a little bit online and it seems that the interfet is like a part that you get with a high minimum order.
If that is true that would make impossible for me to try this part out.
Hopefully samples can be purchased at least 10 of them.

I left a voicemail to the sales agent and see what they say.
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Old 15th October 2012, 06:47 PM   #236
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I can not explain the gain collapse at the moment. Usually more gain comes by raising Gm in the input stage or raising the values of the resistors in the RIIAA and lowering the values of the caps accordingly.
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Old 15th October 2012, 06:47 PM   #237
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Maybe Interfet has a distributor that sells less.
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Old 15th October 2012, 06:50 PM   #238
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Quote:
Originally Posted by Joachim Gerhard View Post
Maybe Interfet has a distributor that sells less.
I will check into that.
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Old 15th October 2012, 06:52 PM   #239
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When i think more about it maybe there is simply too little current in the input stage.
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Old 15th October 2012, 06:53 PM   #240
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Quote:
Originally Posted by Joachim Gerhard View Post
I can not explain the gain collapse at the moment. Usually more gain comes by raising Gm in the input stage or raising the values of the resistors in the RIIAA and lowering the values of the caps accordingly.
I am playing with paralleling more BFs which should simulate the behaviour of the IF part.
It looks like the DC offset goes out of control if I try to raise gain and I can't give an explanation to it either.

If anybody has a clue please let me know!
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