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Old 19th November 2008, 09:44 AM   #31
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Measurement wise, the Xono design must be one of the lowest noise designs on the market.
-81 dB unweighted rel. to 1mV is indeed extremly low.

Gain at 1kHz is also very high at 76dB. I use 64 dB but then I do not design for the 80uV cartridges.

Looking at the OM for the Xono is seems to use four parallelled devices.



Sigurd

Quote:
Originally posted by syn08


Sigurd,

Paralelling more JFETs is the obvious solution in an attempt to gain some extra SNR dBs. Unfortunately I have found significant stability issues when using more than four 2SK170, unless you add some gate resistors which are then, of course, pooping the whole party.

Coincidentally, the Nelson Pass Xono (which I highly regard) AFAIK uses four cascoded 2SK170 in the input stage.
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Old 19th November 2008, 10:45 AM   #32
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Sigurd,
Quote:
The attached input stage will give an SNR of about 77 dB rel 0.5mV 20-20kHZ unweighted...
...and a pitiable sound. Innocent attempts to fool physics like this have been made before, always with modest success.
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Old 7th December 2008, 12:24 PM   #33
syn08 is offline syn08  Canada
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Default Hps 2.0

Well, GPP is now re-baptized as HPS (Hybryd Phono Stage)...

HPS 1.0 had already very good performance, however I was looking forward to add a few new features and further improve the overall performance. While HPS 1.0 has under 1nV/rtHz noise (SNR is 71dB for MC and 84dB for MM) and -88dB crosstalk, I was targeting better than that. I already knew that 0.5nV/rtHz requires 4 x 2SK170 in parallel (and, for stability reasons, that's about all you can do without adding gate resistors), but what else? Having a spare 1.0 PCB, I've decided to cannibalize and use it as a "breadboard" to further experiment. The sad news is that going under 1nV/rtHz requires a completely new overall architecture... So it was time for HPS 2.0

First, I had to drop the idea of a wall wart power supply and TLE2426 virtual ground IC, even if followed by Jung super regulators. For whatever reasons, this solutions generates 60Hz harmonics which could be filtered at the output (before the Jung regulators) but unfortunately TLE2426 handles very poorly capacitive loads. It simply becomes unstable...

Secondly, the Jung regulators are pretty effective (having very good line rejection) but some degree of harmonics are still going through... The input stage (see the 1.0 schematic) had basically no PSRR so some of these harmonics appeared at the output. Nothing audible (I am talking here well under 1uV levels) but certainly annoying in measurements.

Third, moving from a wall wart requires a line transformer, so forget about hosting the power supply in the same case with the signal path... And then to add insult to injury, moving to a separate power supply requires extra wiring for sense and sense shielding, otherwise the line and load requlations are strongly affected (see Jung's article http://waltjung.org/PDFs/Improved_PN_Regs.pdf)

Fourth, the input stage could use some PSRR. Therefore, cascoding the JFETs and adding an extra local regulator would certainly help.

Fifth, improving the crosstalk to -90dB and beyond requires some power supply wiring layout changes. Keeping power supply traces, for the L/R channels, separate all along is critical. Note that in practice a -100dB crosstalk for a phono stage doesn't make much sense; a typical cartridge has a crosstalk of -20dB. So improving the original performance was more like a self challenge rather than a requirement.

And sixth, while I still don't believe in loading the cartridge with very low inpedance does anything good for the sound, I thought some input flexibility would be a nice feature; I added 4 resistors (10, 100, 499, 1k) and 4 capacitors (100pF, 220pF, 330pF, 470pF) giving 256 combinations of RC input impedances. The default impedance is still 47k/100pF. These are switchable via onboard DIP switches.

The HPS 2.0 schematic:
Click the image to open in full size.

The power supply:
Click the image to open in full size.

The signal channel:
Click the image to open in full size.

An overall picture of the implementation:
Click the image to open in full size.

Performance:

Noise:
0.44nV/rtHz @1KHz, both MC and MM
SNR: 89dB MM @ 2.5mV input, 76dB MC @ 0.5mV input

RIAA:
+/-0.15dB 20Hz-20KHz

Crosstalk:
-101dB @1KHz
-97dB @20KHz

Distortion:
Better than 0.002% 20Hz-20KHz

To me, that's about all one can wish from a phono stage. Comments are welcomed.
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Old 7th December 2008, 02:15 PM   #34
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Cute and probably not bad sounding. Power transformer is about 20 times smaller than what i use Jung regs or not this is quite audible.
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Old 7th December 2008, 02:40 PM   #35
jgedde is offline jgedde  United States
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Syn08, That board sure is handsome. Did you etch it yourself? How did you do the solder plating and have it so smooth and level?

John
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Old 7th December 2008, 03:59 PM   #36
PMA is offline PMA  Europe
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Quote:
Originally posted by analog_sa
Power transformer is about 20 times smaller than what i use Jung regs or not this is quite audible.
Superstition.
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Old 7th December 2008, 04:04 PM   #37
PMA is offline PMA  Europe
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We made a listening comparison with very expensive preamp, that uses two 100VA transformers and 164.000 µF filter caps. Anyway, that preamp was assessed as worse in a listening test. The preamp is not about transformer size that much.
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Old 7th December 2008, 04:14 PM   #38
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Default Re: Hps 2.0

Good to see that you decided to improve the design!

SNR is now so good that it is no problems to use the RIAA with medium output MC cartridges. For MM use 89 dB is not really needed, so one can wonder why not connect an MM cartridge before the JFETs.

The huge red caps in the upper right corner and in the middle, what types are those?

Ideas for improvement could be to have adjustable gain, and adjustable input resistors for MC cartridges (although doing this manually might be the best solution).

BTW,
what gains do you have,
and what are the overload margins?



Sigurd


Quote:
Originally posted by syn08
Well, GPP is now re-baptized as HPS (Hybryd Phono Stage)...

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Old 7th December 2008, 04:28 PM   #39
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Quote:
Originally posted by PMA
We made a listening comparison with very expensive preamp, that uses two 100VA transformers and 164.000 µF filter caps. Anyway, that preamp was assessed as worse in a listening test. The preamp is not about transformer size that much.

We made a similar comparison. Opposite results. Of course comparing wildly different topologies and power supplies is silly. I am only talking of transformer size, everything else kept equal. Had to redo the casing to fit a bigger unit; not something i do lightly.
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Old 7th December 2008, 04:42 PM   #40
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That input stage is vaguely reminicent of some of the Calrec Soundfield mics one of the few mic circuits I've seen using feedback all the way to the input. Can't complain about your choice of op-amps either

It's hard to morph one of these MC preamps into something for a transformerless ribbon mic circuit. James Boyk of Performance Recordings is high on them these days, one of my back burner projects.

BTW, did you really have no use for the little bit of distortion removal via the 50pf extra cap on the first amp? It's possible there is little or no difference in this circuit.
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