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Old 3rd December 2012, 09:51 PM   #7901
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Frans, a PM send.
You get an older bottle anyway.
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Old 4th December 2012, 02:18 AM   #7902
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Quote:
Originally Posted by Joachim Gerhard View Post
The IF3901 is like a pink elephant or a unicorn if you wish.
Can not wait until you get some Stefano.
The CSS has to provide THE EXACT SAME CURRENT THEN THE INPUT STAGE PROVIDES AT IDDLE, ALL THE TIME. If that does not happen the DC voltage at the output node is not EXACTLY at 1/2 voltage of the supply. That condition gives the MAXIMUM DYNAMIC RANGE ( Tietze- Schenk ).
I am also thinking that with one monolithic i.e. one pair of IF there should be plenty of gain, very low noise more thermal stability due to the monolotic.

I was also thinking why wouldn't we use Jfet for the current source?
In this instance using the same paralled IF monolithic?
Is it a stupid idea?
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Old 4th December 2012, 02:39 AM   #7903
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I thought that making the CSS also a J-Fet BJT cascode could give it a similar drift so that the drift cancels. Problem is that this will cut the output impedance in half. We loose 6dB of gain that way. We could of cause then make the RIAA resistive components two time higher to restore the gain. If that is a good idea i can not say.
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Old 4th December 2012, 02:42 AM   #7904
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Of cause we could make the top J-Fet BJT cascode higher in impedance but that may introduce drift gain.
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Old 4th December 2012, 02:45 AM   #7905
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Sorry, i did not find the time to find my old solution. I think it was a decoupled LM317 CCS on top of a BJT cascode. It was parallel symmetric though so some of the drift cancels.
That is then a kind of Paradise without the mirrors. So you understand part of the history why the Paradise saw the light of day.
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Old 4th December 2012, 02:53 AM   #7906
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here is the idea I was describing before.
It onlu gives 46dB gain which is clearly not enough.

The problem however is:

1) the noise simulation shows 0.87nV/rtHz for only 46dB which puts it into the noisy side
2) the topology is INVERTING and I don't think there is much it can be done here. I would have expected that since output is sourced from a common source

Another thing that needs to be noted and that I just thought of is that output of the first stage can't be half of the rail, this wouldn't lead to maximum headroom, but has to be rail-Vcascade-2V(for saturation) which clearly shows that for good headroom capability you need high voltage rail which migh be a problem for regular JFets or at least operating to their maximum derating.
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Old 4th December 2012, 02:56 AM   #7907
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You can put a BJT as cascode on top of the j-Fet current source.
Phase inversion is no problem. Revers the cartridge leads, they are not balanced but floating.

Last edited by Joachim Gerhard; 4th December 2012 at 02:59 AM.
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Old 4th December 2012, 02:59 AM   #7908
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When you use BF862 instead of 2SK170 the gain goes up and the noise goes down.
One way to raise the gain is of cause to double the values of the RIAA resistors and half the values of the caps. You can still use your teflons, just put them in series.
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Old 4th December 2012, 03:01 AM   #7909
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Yes, when i talked about best dynamic range at 1/2 supply voltage i forgot the saturation headroom. Good catch.
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Old 4th December 2012, 03:03 AM   #7910
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If you do not have BF862 models simply use 8 x 2SK170. That of cause raises the input capacitance and that can create an oscillator at very high frequencies. That can be avoided with 0.1uH coils in each gate.
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