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Old 10th May 2011, 08:42 AM   #1
PMA is offline PMA  Europe
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Default New preamplifier kits with JFET or BJT input stage

I am preparing a new preamplier kit. The main reason to do so is a global lack of 2SJ74 Toshiba JFET devices, that are getting very hard to buy and genuine parts are almost unobtainable.

So I decided to offer a kit with single differential input. The input stage uses 2SK170BL devices, that are still in production and quite easily obtainable. It is also possible to use conventional BJT devices like BC550 in the input stage of this preamplifier, without any PCB modifications.

The new preamplifier will be named PRE4 and it is based on my Dispre 2–JFET preamplifier. The main difference is that the PRE4 uses only single differential input biased by constant current source, rather than complementary differential topology used in the Dispre 2-JFET.

It will be possible to use PRE4 as a headphone amplifier, without any components changes needed.

Output stage will be again based on a diamond buffer topology with idle current set between 25 and 40mA.

We shall again get very good parameters with wide frequency response, very low distortion and ultimate stability. Slew rate will be a bit reduced, to some 30V/us, as a result of a single differential pair topology.

A functional sample has already been tested, and final PCB is in production now. After performing tests on the final PCB I assume to release PCBs in some 3 – 4 weeks from now.
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Old 18th May 2011, 05:30 PM   #2
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The evaluation PCB has been produced:
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File Type: jpg PRE4.jpg (151.3 KB, 745 views)
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Old 19th May 2011, 04:32 PM   #3
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The sample has been tested.
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File Type: jpg PRE_4.JPG (124.9 KB, 691 views)
File Type: png PRE4_final_sq.PNG (21.0 KB, 685 views)
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Old 22nd May 2011, 09:32 AM   #4
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Hereby the THD FFT measurement. This is more or less the limit of my measurement method, so the preamp THD is lower. Please note total absence of the power supply related spectral components.
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File Type: png PRE4_THD.PNG (74.3 KB, 622 views)
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Old 22nd May 2011, 09:36 AM   #5
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a very professional work, as always.

regards
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Old 24th May 2011, 09:38 AM   #6
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Thanks for your kind words. The boards are already in production
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Old 18th July 2011, 12:07 PM   #7
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How the use of single differential input affect on sound, compared to dispre jfet preamp?
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Old 18th July 2011, 01:57 PM   #8
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Surprisingly, they sound quite similar. Maybe Dispre 2-JFET is more on the mellow side and PRE4 little bit more emphatic. In any case, I prefer the single differential JFET PRE4 to older complementary-differential bipolar Dispre 2 without hesitation. Regarding real danger of 2SJ74 numerous fakes, I assume the PRE4 to be a good choice.
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Old 18th July 2011, 09:58 PM   #9
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Thanks for quick reply.


Is the power supply pcb module the same like that for dispre jfet?
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Old 20th July 2011, 10:01 PM   #10
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Yes, it is the same power supply. And PRE4 PCB has same dimensions and mounting holes as Dispre 2 and Dispre 2-JFET. Also the position of the Alps pot is the same.
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