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Old 6th January 2013, 04:53 PM   #851
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The Paradise has an input offset trim that reduces DC into the cartridge to minuscule levels. BJTs have a noise advantage under 1kHz because the 1/f corner is much lower then in J-Fets. The 2SJ74 is obsolete so we decided on a BJT input. We could have made a single ended Fet RIAA though or a differential input. Both options give higher noise though then parallel symmetric.
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Old 6th January 2013, 05:01 PM   #852
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The maximum current that can flow into the cartridge, even without offset trim, is the idle divided by the hfe. The DC ohmic resistance of the cartridge plays a role too. This was discussed in detail before. That can neither destroy nor demagnetize the cartridge.
I have a quite valuable Lyra Titan i and it sounds like on the first day although i abused it with much higher current during my transimpedance input experiments.
We have a Paradise R2 playing since a long time in many systems and on several shows with different cartridges and people are reporting very good results. That Paradise did not have the offset trim. We included it later in the R3 version.
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Old 6th January 2013, 05:30 PM   #853
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Thanks for the detailed explanation Joachim.
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Old 6th January 2013, 07:14 PM   #854
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High hfe in the input stage lowers initial offset. You could also match Ube from NPN to PNP. In the mirrors a hfe of 400 is more then enough.
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Old 6th January 2013, 08:08 PM   #855
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Censored comments about scare mongherers Censored
Built mine R2 with poor matching of transistors and ofset is steady at 0.0+-5 mV (0.05 )
with MC one use cartridge loading say 100 Homs and using formed capacitors helps again
So just get over it.
Ofsett is worth keeping in mind but do not panic over it.

Forming the caps is easy Just get a 317 to suply the nominal voltagge and live the lot there while the boards get populated

Last edited by Bksabath; 6th January 2013 at 08:10 PM.
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Old 6th January 2013, 08:17 PM   #856
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What are "formed capacitors"? What is a "317"?
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Old 6th January 2013, 08:24 PM   #857
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Sandro, is saying that new capacitors take a time to come up to spec and that until that time they will fluctuate and DC will drift.

317 means a simple LM317 regulated power supply.
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Old 6th January 2013, 08:35 PM   #858
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Quote:
Originally Posted by hugenh View Post
Hi,
could one of those in the know (Joachim perhaps) please sum up the latest consensus on optimal hFe?

should i really use the same high hfe throughout the whole board? in the assempby guide input and three other sections are circled in. joachim stated someplace that 400 is enough for the input, but i canot find references to the other sections. there are over a thousand posts in the relevant threads, i`m lost.
Quote:
Originally Posted by Joachim Gerhard View Post
High hfe in the input stage lowers initial offset. You could also match Ube from NPN to PNP. In the mirrors a hfe of 400 is more then enough.
ok, thank you joachim. just to confirm: hfe around 500 in the input stage is desirable? any tradeoffs for high hfe there (hbarske stated in one post that hfs >500 provokes instability). in the current mirrors, what is the highest acceptable hfs (have none <480)??

i know, it all depends on a lot of other things, but is there something like a recomendation of optimal hfs-range, provided all pairs are tightly matched?

Quote:
Originally Posted by Joachim Gerhard View Post
You could also match Ube from NPN to PNP.
Ube is between .745-.752 for all devices npn and pnp. thats less than 1%, no need to bother with a closer match?

Last edited by hugenh; 6th January 2013 at 08:39 PM.
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Old 6th January 2013, 09:49 PM   #859
Waly is offline Waly  United Kingdom
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Originally Posted by analog_sa View Post
If i put the wrong wand in the arm a 2k cart may prematurely go to Paradise.
Fearmonger indeed. Not a chance in hell to kill a 2k cartridge with the paradise input bias current. The combination of matched devices and the input bias nulling can bring the cartridge current to well under 1uA (worst case).

In fact, if you connect a 2k cartridge to this preamp, you deserve the "horrible hiss". The noise will indeed be horrible because of the input current noise (wich doesn't cancel as the bias, being from uncorrelated sources). I can estimate the input noise current to about 5pA/rtHz which with your 2k cartridge will give and extra 10nV/rtHz, to add to the 2k impedance noise of 6nV/rtHz plus the 1-2nV/rtHz voltage noise of the preamp. Total is 12nV/rtHz, which is indeed a "horrible hiss" even for a MM 2k cartridge. A decent JFET input opamp like the OPA827 has only 3nV/rtHz typical.

The Paradise should be used only with low impedance MC cartridges. I have to agree, the HF stability of the Paradise is indeed questionable. A small cap (100p or so) connected across the input may cure some stubborn oscillations, a MC cartridge won't care about the cap.
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Old 6th January 2013, 09:55 PM   #860
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Hi Wally

I guess analog_sa was writing about a 2keuro cart not 2kohm impedance.

Now your comment about 100p on the input is quite good... would you please elaborate on why to use it ?
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