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Old 3rd December 2012, 05:29 PM   #7871
FdW is offline FdW  Netherlands
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Originally Posted by Joachim Gerhard View Post
Frans, your circuit does not work because there is only ca. 25mA idle in the input stage and your CCS delivers 50mA. That makes the voltage over the CCS break down.
Yes, and now it works I got the 50mA from Stefano's work (V(D7)+V(D8)+V(D9)-Vbe(Q3))/100 = +/- 50mA

With the CCS at 25mA the output voltage is (purple) about 0.002V but at 30mA (and more) it goes to 6V (or there about).

My approach to a problem like this is to first simplify it to the max (and that is what I did ).
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Last edited by FdW; 3rd December 2012 at 05:36 PM.
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Old 3rd December 2012, 05:41 PM   #7872
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Yes, and now it works I got the 50mA from Stefano's work (V(D7)+V(D8)+V(D9)-Vbe(Q3))/100 = +/- 50mA

With the CCS at 25mA the output voltage is (purple) about 0.002V but at 30mA (and more) it goes to 6V (or there about).

My approach to a problem like this is to first simplify it to the max (and that is what I did ).
And this is the output at 30mA (1mV in)
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Old 3rd December 2012, 05:45 PM   #7873
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And this is the output at 30mA (1mV in)
I'm not much of a Fet-cooker, more like a fat-cook
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Old 3rd December 2012, 06:06 PM   #7874
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Well, what is the optimum idle in the input stage ?
We want high gain and the gain of Fets goes up when the idle goes up.
50mA from 4 x BF862 gives a transconductance ( Gm ) of ca. 160mS.
Gm x R is the gain so we want high Gm and high R. The problem we then run in is DC stability and we need a high impedance node at the output of the input stage or the gain will drop again because the output impedance Rout appears in parallel to R in the formula Gm x R. You can than say G ( gain ) = ( Gm x R ) / Rout.
This high impedance comes with the problem that even a small DC shift in the input stage is transferred into a big DC shift over R, in our case the RIAA transimpedance components shunted to ground. The task is to find a low drift, low noise, high impedance CCS.
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Old 3rd December 2012, 06:15 PM   #7875
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In fact the whole input stage including the Fet-BJT cascode should not drift much.
The problem is when the voltage over the input cascode goes down the BJT will eventually saturate. Even before this happens, the less DC voltgae there is over the input cascode the less voltage there is to be modulated so we end up with thermally drifting dynamic range.
We want the dynamic range as high as possible though so that the output never saturates even with a loud input signal. With transimpedance RIAAs this is a bit relaxed because the loud treble signal on the vinyl ( ticks, pops, dirt, amplified 20 dB @ 20kHz by the RIAA ) are shunted to ground by the treble cap.
( clips ).
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Old 3rd December 2012, 06:28 PM   #7876
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Originally Posted by Joachim Gerhard View Post
In fact the whole input stage including the Fet-BJT cascode should not drift much.
The problem is when the voltage over the input cascode goes down the BJT will eventually saturate. Even before this happens, the less DC voltgae there is over the input cascode the less voltage there is to be modulated so we end up with thermally drifting dynamic range.
We want the dynamic range as high as possible though so that the output never saturates even with a loud input signal. With transimpedance RIAAs this is a bit relaxed because the loud treble signal on the vinyl ( ticks, pops, dirt, amplified 20 dB @ 20kHz by the RIAA ) are shunted to ground by the treble cap.
( clips ).
good thoughts.
Jung paper about CCS might come handy for noise measurements.

Anyhow, I am always concerned about thermal drift I find it to affect sonical performance as it changes bias over time which in terms changes slightly the sound over time.
Isn't there a way to swervo control this? I guess that given the simplicity of the circuit servo into this would be very invasive if ever.

Can we start laying on LTSpice a suitable circuit based on the basic idea?


I will try anyway later to mirror FwD's simulation and try to pass that into a "real" circuit with a basic CCS structure (which brings us back to the basic idea of JG).
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Old 3rd December 2012, 06:33 PM   #7877
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Sampler has already shown a mosfet gyrator that has low drift. If that also provides low noise is yet to look in. I will search for my design from 2 years ago. That solved the problem i think .
A servo CCS ? Why not, maybe we can get a patent on that, maybe not C:
I think Scott Wurcer came up with something like that. I think it was an opto coupler.
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Old 3rd December 2012, 06:35 PM   #7878
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Originally Posted by Joachim Gerhard View Post
Sampler has already shown a mosfet gyrator that has low drift. If that also provides low noise is yet to look in. I will search for my design from 2 years ago. That solved the problem i think .
A servo CCS ? Why not, maybe we can get a patent on that, maybe not C:
I think Scott Wurcer came up with something like that. I think it was an opto coupler.
yep that is what I was thinking about..
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Old 3rd December 2012, 06:35 PM   #7879
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How to measure Gm in Fets :Accurate measurement of jfet gm
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Old 3rd December 2012, 06:38 PM   #7880
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Originally Posted by Joachim Gerhard View Post
How to measure Gm in Fets :Accurate measurement of jfet gm
I think that a pair of interfet will do the trick anyway
Gotta wait till beginning of next year hopefully!
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