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Old 9th October 2008, 07:24 AM   #11
MRupp is offline MRupp  Germany
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Quote:
.... I have cascoded it with the 2SC3321. Do you think it is better to use the BF345 you mentioned?
There are many ways of implementing current sources, I just have these transistors and it is an easy way of cascoding though it might not be better in any other aspect ... Go read the Jung articles!

Quote:
yeah i am not sure that an higher current sense resistor would be worse now that i am thinking through actually a 1K won't change that much the input impedance of the gain stage over a 470 since there is a 4K in series the coupling cap.
I do not understand what that has got to do with the input impedance, are you sure you are not confusing something ?
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Old 9th October 2008, 07:44 AM   #12
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Please correct me if i am wrong:

the input impedance seen by Q15 is also determined by R40 (try to draw a small signal model of the system).
If this is so, what i was thinking was (I don't know if this is correct though):

the voltage gain stage Q15 has a certain Cin (lowered by cascoding with that n-jfet) : wouldn't this Cin combined with Rin of the stage determine the input cut off frequency of the stage?

If this is reasonable,actually, the result would be that: the higher R40 and the lower the cut off frequency thus causing a roll off on highs.

I am sure i am missing something because I am sure that C19 should also be taken in consideration for the cut off freq i guess an C19 is a 220uF.

Please, if i got the wrong view point, i would appreciate if you can correct it.


As for the article, i will definitely go and read it.
Thanks for the hint you gave me and the good advice.
I appreciated it.

Best,


P.S. did you find the voltage regulator on digikey? what do you think?
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Old 9th October 2008, 05:59 PM   #13
MRupp is offline MRupp  Germany
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Quote:
voltage gain stage Q15
OK, now I get it, did not realise that you were cascoding the second stage. First of all, I still do not like this type of cascode for JFETs.

Secondly, this is already a unity gain stage or "drain follower" (amplification -1) by means of feedback, so I am not sure from the top of my head how you calculate the input capacitance but my first guess is that it is already in the same ballpark as that of a cascode. Btw. I would make R41 and R51 about the same if you omit the gain setting resistors.
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Old 9th October 2008, 06:38 PM   #14
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Hi,
first of all, I did not build the ono but experimented a bit with 2SK170 in the front end of an MC-headamp.

If you already simulate the ono, you should easily see differences when trying out cascoded jfet's and, hence, different Vds-values. It seems that indeed something between 8-12Vds might be the optimum value.
I have abandoned cascoding the input jfet for a MC-headamp:
1) the cart's signal voltage is so small, so the corresponding variyng drain voltage shouldn't introduce too much distortion, 2) MC-carts are not sensible to that dimensions of input capacitance.
You need to handselect the 2sk246 for Vgs if you want them as cascodes, you might find that you won't find some that fit in well - and definitly none above 8V ;-)

Remember, I'm only talking 'bout the input stage!

Rüdiger
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Old 9th October 2008, 08:48 PM   #15
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Quote:
Originally posted by MRupp


OK, now I get it, did not realise that you were cascoding the second stage. First of all, I still do not like this type of cascode for JFETs.

Secondly, this is already a unity gain stage or "drain follower" (amplification -1) by means of feedback, so I am not sure from the top of my head how you calculate the input capacitance but my first guess is that it is already in the same ballpark as that of a cascode. Btw. I would make R41 and R51 about the same if you omit the gain setting resistors.


thank you MRupp for your answer.

i don't get it: why would you wanna make R41 and R51 the same value?

Sencond, i don't get it: how is Q15's atge on the MC a drain follower by means of feedback? I mean the stage is a common source with 4.75K feedback resistor and 220uF cap that only blocks the feedback path at frequencies around DC.


thank you very much for your attention.
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Old 9th October 2008, 08:52 PM   #16
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Quote:
Originally posted by Onvinyl
Hi,
first of all, I did not build the ono but experimented a bit with 2SK170 in the front end of an MC-headamp.

If you already simulate the ono, you should easily see differences when trying out cascoded jfet's and, hence, different Vds-values. It seems that indeed something between 8-12Vds might be the optimum value.
I have abandoned cascoding the input jfet for a MC-headamp:
1) the cart's signal voltage is so small, so the corresponding variyng drain voltage shouldn't introduce too much distortion, 2) MC-carts are not sensible to that dimensions of input capacitance.
You need to handselect the 2sk246 for Vgs if you want them as cascodes, you might find that you won't find some that fit in well - and definitly none above 8V ;-)

Remember, I'm only talking 'bout the input stage!

Rüdiger

thank you for your post.
gotcha!
A good casconding sets the VGS of the cascoded stage to about 7V or so.
I will try to see if i can cascode by tapping the voltage reference at the gate of the cascode Jfet as a voltage divider and see if i can set VDS of Q15 higher.
I will have to try tomorrow.

But thank you all for your useful posts.
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Old 9th October 2008, 10:46 PM   #17
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I have pulled out the juang's article on current source and the long article on hi end regulator and started to look at them.

I will take anyways a couple of days to read and start digesting them.

Just a curiousity: has anybody ever tried to implement the juang's super regulator and thus can report the sound of such a supply.
I will eventually evaluate to use it over the shunty, not sure yet.


With regard to the cascode of Q15 and your advice of having a VDS over 6V, this is the modification of the cascoding.
The Drain of Q15 is held at 8.25V and the source at 0.2V so VDS is 8V.

Is this cascoding properly done now?
Attached Images
File Type: jpg cascoding 2.jpg (13.9 KB, 281 views)
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Old 10th October 2008, 06:43 AM   #18
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Hi Stefano,
what is the purpose of R51/C19?
There were debates wheter a fet or a bjt as cascode device would sound better. NP once said, bjt, and Borbely votes for fet's. Maybe you can try both and report back?

Personally I think the jung regulator can make a good circuit sound even better. In fact, I liked it whereever I tried it. But, it would make a different concept and additional changes *might* be on their way. For instance, is the cap multiplier ahead of the input stage still needed then?

If I were to mod the ono, I would build the original circuit and try my modifications from there. (eventually planning the circuit board in way that would allow those mods)

Rüdiger
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Old 10th October 2008, 08:18 AM   #19
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Quote:
Originally posted by Onvinyl
I have abandoned cascoding the input jfet for a MC-headamp:
1) the cart's signal voltage is so small, so the corresponding variyng drain voltage shouldn't introduce too much distortion, 2) MC-carts are not sensible to that dimensions of input capacitance.

3) Sounds better without cascoding, at least to my ears.

4) Loses a coupling cap in a very sensitive place.

A common cathode FET (paralleled if required), powered by either Jung reg or batteries is very close to the ultimate SS MC stage soundwise. And there is no need for a monstrous coupling cap.
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Old 10th October 2008, 08:53 AM   #20
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Quote:
Originally posted by Onvinyl
Hi Stefano,
what is the purpose of R51/C19?
There were debates wheter a fet or a bjt as cascode device would sound better. NP once said, bjt, and Borbely votes for fet's. Maybe you can try both and report back?

Personally I think the jung regulator can make a good circuit sound even better. In fact, I liked it whereever I tried it. But, it would make a different concept and additional changes *might* be on their way. For instance, is the cap multiplier ahead of the input stage still needed then?

If I were to mod the ono, I would build the original circuit and try my modifications from there. (eventually planning the circuit board in way that would allow those mods)

Rüdiger

Hi Rudiger,

thank you very much for your post.
I will definitely try them both, i mean BJT and FET as a cascode and report back what sounds the best.

About Jusng super regulator it sounds interesting.
You have successful tried it hum? interesting!
I possibiliy use this concept for the regulator on the current mirror in place of the led or the MAX IC.
My plan is to get the original circuit right on a protoboard and from there go off and try once upon a time all the modifications.

I am very sorry, i haven't understood what you tried to say here:
"But, it would make a different concept and additional changes *might* be on their way. For instance, is the cap multiplier ahead of the input stage still needed then?"


One more observation:

I have run simulations for different values of the coupling caps on the MC stage, trying to substitute the 220uF caps with a 10uF first and a 30uF then.
I am not going to post this bulky posts here of course but i can report that there is a noticeable difference on the bottom end response by decreasing the values.
Therefore i guess i will go for the 220uF.
The point is that i don't like to have electrolytic caps on the signal path even though i would use a good rubycon (as far as i have read are far better than the blackgate ).

One more thing: speaking of frequency response: the simulation doesn't show a very wide band as in fact the top cut frequency is set to around 40KHz and the RIAA tolerance is 0.1dB within 20 and 20KHz.
I was wondering if the top cut freq as well as the tolerance (this of course) could be determined by the choice of proper values on the RIAA stage and if this and anyone has ever tried to redo it with a certain degree of improvement.

I have found a post on xonobal by Cheff where he designed a balance prototype of the xono with a riaa response of 0.03dB that is not pretty bad.
I would like to see if it is possible to improve the RIAA accordingly.
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