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Old 5th September 2011, 06:45 PM   #71
EUVL is offline EUVL  Europe
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> See my MPP thread in 5 minutes.

Fixed +/-18V rails ?


Patrick
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Old 5th September 2011, 06:58 PM   #72
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No, only one 36V floating supply. Sure, the plus-minus notation is wrong.
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Old 6th September 2011, 05:51 AM   #73
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Very clever Joachim.

Essentially a SEN IV frontend (CEN also possible) but with cascode.
Then uses 2 cascoded current mirror to reflect the signal current directly into the R_iv (or RIAA filter network).
The current mirrors are hanging on the rails and hence no connection to Gnd.
They essentially do the same job as the "coupling" caps.
So the extra complication is justified.

The output buffer is probably necessary for RIAA.

If I am not wrong, one can also do current mirror with FETs, with all the matching headaches.


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Old 6th September 2011, 07:46 AM   #74
Calvin is offline Calvin  Germany
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Hi,

Quote:
I know you can supply V2, V3 from V1.
For the 4th time
V2 and V3 are independant and optional supplies. They are not generated from the floating supply V1. Instead V1 might be generated from V2 and V3 via a switcher-IC like the LT1533.
As the sim showed even variations of 5% in hfe didn´t increase THD considerably. The BCs are specced with a maximum deviation of 0.9 and a Vbe-difference of 2mV. Taken from a batch the variations are generally lower. Until a test setup prooves different I take the sim for believable.
Working in the current domain noise voltage figures are of minor importance and there is no need for dedicated low voltage noise devices.
Simulation with the 2N4401/4403, which are well known as performing very lownoise in phono MC-stages, showed higher noise figures.
Running the BCs in a current range of 2-10mA results in low noise figures, high bandwidth and good THD-behaviour (a nice falling spectrum with increasing order of K). From simulation results one would not be able to blind judge between JFET and bipolar.
And You´re right Patrick...this should not become a contest.
I just wanted to show an alternative solution for those who may have difficulties in sourcing or matching JFETs. But the cons You named mostly don´t apply or do apply for the JFET circuit also. The only worth to mention differences between JFET and bipolar circuit are the need for matching JFETs and the additional parts to bias the bipolars.

Joachim is basically right about the functioning of the buffer with the floating supply. But he´s right only half. The problem is, that the signal current divides between the I/V-stage and the Buffer-stage!
This has a serious drawback. The output voltage now depends on the value of the load impedance at the Buffer-output again and takes on the value of Isignal(dac) x Riv||Rload.
The buffer basically looses its main functionality, which is to isolate the IV-stage and to make the parameters of the IV-stage independant of the following load impedance. It even looses the function to supply higher currents than Isignal(dac)-I(Riv) to the load. Even though V1 is a voltage source (constant voltage, unlimited current supply), the buffered stages output current is limited to approximately the DAC-signal current.
This are the reasons why the Buffer-stage should have its own independant power supplies.
"The Starless and Bible Black MC RIAA" reminds me of a topology named Satri which also worked with current mirrors. The doubled CMs are used in integrated circuits, but need very careful matching when built with discrete parts or oscillation occurs.


jauu
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Last edited by Calvin; 6th September 2011 at 07:53 AM.
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Old 6th September 2011, 11:47 AM   #75
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You are right Calvin. The Buffer needs a separate supply i think.
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Old 6th September 2011, 11:50 AM   #76
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The Satri would be approximately Post 62 with current mirrors on top and bottom. Jürgen Ultee uses something like that in the Van den Hul Grail.
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Old 6th September 2011, 12:08 PM   #77
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Euvl, yes with self conducting Fets a mirror is possible too. See Tietze-Schenk or Willy Sansen. As you know matching is not so easy then so you see MosFet mirrors in integrated circuits mostly.
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Old 6th September 2011, 12:10 PM   #78
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Made a correction to my thread about what Calvin has found out.
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Old 6th September 2011, 02:57 PM   #79
EUVL is offline EUVL  Europe
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If I understand correctly. Joachim's circuit uses the current mirror to fold the signal current back to R_iv. This is, in a way, similar to this circuit I posted a while ago using a MOSFET folded cascode (the idea came from John Cur"l's blowtorch thread) :

Zen I/V Converter

The nice thing about the current mirror is that it hangs on the rails, and has no Gnd connection.

I have a feeling that it might even work with fixed rails. But I need to check a few things first beforehand.

If it works in Sims, we'll most likely give it a try, with a CEN frontend.




Patrick
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Old 6th September 2011, 03:15 PM   #80
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Yes, a folded cascode works too. Instead of the drain resistors one can use a mirror as constant current source or a constant current source. That way you can get a tremendoes Gain-Bandwidth product. I have your SEN circuit ( slightly modified ) working as a pre-pre.
It sounds good and it is quiet. Yes, the CEN is something to try too.
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