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Old 7th April 2011, 02:09 PM   #2161
Salas is offline Salas  Greece
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Going to put 45mA Mosfet output stage on same rails with telescopic cascode input stage also having its common base bias just resistor sourced? Too much interaction IMHO. Needs some rails insulation scheme.
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Old 7th April 2011, 02:41 PM   #2162
MiiB is offline MiiB  Denmark
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For sure inserting CCS's in the two stages is the natural next step...for modeling the intial simple schematic resistors work well (could also insert a CCS symbol)
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Old 7th April 2011, 02:42 PM   #2163
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Yes, this can be bettered but i think the concept basically works just fine. First i had extrem simplicity in mind and now we see the limitations.
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Old 7th April 2011, 02:49 PM   #2164
Salas is offline Salas  Greece
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Could make enough do with just Cmults on the front rails...
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Old 7th April 2011, 03:18 PM   #2165
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I whould like to filter the bias to the input cascodes at least.
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Old 7th April 2011, 10:27 PM   #2166
RCruz is offline RCruz  Switzerland
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Old 7th April 2011, 10:34 PM   #2167
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I think running the pre-pre on plus-minus 60V is kind of crazy too but as you where.
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Old 8th April 2011, 02:29 PM   #2168
MiiB is offline MiiB  Denmark
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Why is that crasy..??? I do this on an IV-stage, must say the i sounds guite wounder full...ohh...I remember one LA-audio MC head-amp...That thing was quite wonderful......
a

The pre pre could be regulated some down to further reduce noise, as the front J-fets only sees the app 10V under the cascode... (like the front of the pearl 2)

Anyway My MAC...quit on me...so boards will start when i get the Poor (The mother of bad words) thing back....!!!!

Last edited by MiiB; 8th April 2011 at 02:36 PM.
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Old 8th April 2011, 02:54 PM   #2169
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I had the LA-Audio pre-prre and never got it to work without incredible microphony. I rebuild it with Jürgen Ultee. We used the Arthus Loesch circuit of a Fet-Tube cascode and that workes fine until today. I still have it. I will make a photo.
As to the Pre-Pre can we agree on plus-minus 24V ? That is really decent my friend. plus-minus 48V for the second stage whould be enough too because 60V DC is not really safe any more when you get your fingers on it ( OK, just ). Running the input stage on less voltage whould allow us to filter the PSU down from the second stage supply. I think Salas is on the mark that current draw in the second stage should not influence the delicate Pre-Pre. You already agreed that filtering down to the Pre-Pre is ok. I have a schematic ready in my head. I will post it later today and i also have an idea for a alternative transimpdance input that circumvents the unlinear input capacitance of the Fets and should be even faster at the expense of loading the cartridge somewhat. It will work well though with typical low impedance movings coils like the Lyras, Shelter, Zyx, My Sonic Lab, Transfiguration etc.
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Old 8th April 2011, 05:23 PM   #2170
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Lets get serious. Here is a high PSSR version of the input stage. No Leds, no elcaps, no conventional RC filtering. Bias is done with an adjustable micropower voltage reference. It has an extremely low dynamic impedance of 1 Ohm, runs on less then 0.1mA, has a tempco of 150PPM. The way i have connected it gives even lower noise. The 100kOhm resistor gives enough exess current to run the bases of the cascodes. Hfe of the cascode transistors should be around 200 or more. I use 2SA1015 and 2SC1815. i have degenerated the Fets a bit less and have doubled them. In MiiBs design the 20Ohm resistors that are AC wise in parallel worsen the SN of a ca.5 Ohm cart like the Lyras already by 4.5dB. Now we have 4 x 10 Ohm in parallel, giving 2.5 Ohm so we win 3dB. The doubling of the Fets also gives an SN advantage of 3dB and doubles Gm. I think another P-Channel has to disappear from the market but it is worthwhile. I can also tell you why plus-minus 28V is optimal. Doing much more then 5V over the Fets increases noise because of raising gate current and heating, doing more then 10V over the bipolars comes at the risk of avalanche noise. Making the collector voltage higher then 14V in this case does not give a dynamic range gain eather because to run them on half the PSU voltage is already optimal. If we keep the 60V supply for the second stage ( i whould go for plus-minus 48V if i had a say here ) we have to burn 32V. Fortunately the input stage does not draw more then 15mA so a regulator that can stand 500mW is ok. I have made a design like that, that works like a cap multiplier and a voltage regulator in one go but maybe MiiB has a better idea. I have also shown a transimpedance version. Yes, folks, that works with Fets too and i have shown an AC coupled version in Jan Diddens Linear 0. There i used high value, low voltage Polymer caps. Here, voila, is the DC coupled variety. I had that idea after Andy visited me and we had that delicious steak he paid for me. "The torture never stops" Frank Zappa.
Attached Files
File Type: pdf JFet Pre-Pre High PSSR.TSC - TINA.pdf (63.4 KB, 164 views)
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