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Old 13th October 2007, 12:18 AM   #1
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Default Borbely JFET design

Hey there, I revisited most of Borbely's circuits in the last few days.

While I admire the overall technical excellence, I wonder whether the extra complexity produces better 'sound'. Undoubtedly the cascode operation yields superior performance. Does it contribute significantly to subjective improvement?

Has anybody built and compared the 'sound quality' of his versus a lesser design?

Any thoughts?

Mark.
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Old 13th October 2007, 01:36 AM   #2
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You didn't mention a specific circuit, so I can't comment on particulars, but bear in mind that it's sometimes necessary to cascode to protect a comparatively delicate device from too high a rail voltage or from excess power dissipation when running bias higher currents.

Grey
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Old 13th October 2007, 06:57 AM   #3
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Mr. Borbely uses a self biased JFET-JFET cascode which demands precise Vt selection plus precise device pair matching. Single die matched JFETs like the 2SK389BL are available for the input diff-pair, but single devices with higher Vt like the 2SK246BL must be used in the cascode, and if both of these cascode JFETs do not have the correct Vt and precisely matched parameters, distortion results. It is very difficult and expensive for DIY'ers to acquire and test JFETs to the precision required.

I use a 2SK389BL diff input, but use a LED biased MAT02 single die NPN bipolar pair for the cascode, plus a 2SJ109BL with a LED biased MAT03 single PNP bipolar pair for the complementary symmetrical cascode. The high gain and stable Vbe of the MAT0X bipolars generate low distortion over a wide input range without complicated matching.

Some of my amps directly connect the 2SK289BL and 2SJ108BL like Mr. Borbely's circuits. Some of my amps use mirrored current sources to each diff-pair with a shared JFET load for temperature tracking to match the JFET inputs.

Mr. Borbely’s phono amps look very clever, but demanding on JFET transistor selection and matching.
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Old 13th October 2007, 08:56 AM   #4
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Quote:
Originally posted by LineSource


Some of my amps directly connect the 2SK289BL and 2SJ108BL like Mr. Borbely's circuits. Some of my amps use mirrored current sources to each diff-pair with a shared JFET load for temperature tracking to match the JFET inputs.

[/B]

Hi LineSource,
would you mind to post a conceptual circuit diagram of the latter?
thanks,
Rüdiger
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Old 14th October 2007, 03:35 PM   #5
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Thanks for the inputs.

Hey Grey, Borbely first published circuits without using cascode. He added them on at a later time ("all-out" as he put it). No doubt I like the technical merits. But wonder whether it's truly necessay and how much it 'changes' the sound. Have you built jfet preamps (complimentary type design) with and without cascode? Do they sound different? Mark.
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Old 14th October 2007, 03:42 PM   #6
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Hello LineSource,

Glad to hear that you tried different combos. Cascode significantly increases component counts. Wondering whether you have compared between cascode & non-cascode jfet circuit? Do you perceive any subjective difference? Mark.
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Old 14th October 2007, 04:07 PM   #7
mlloyd1 is offline mlloyd1  United States
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to further make grey's point, for some applications that use sufficiently high rails, the choices are either to cascode or do a balanced bridge design to get sufficient voltage swing.

also, you will see in past threads that performance in some key areas (for example, noise) is improved when the the drain to source voltage is kept below (or within) a certain range.

similar to linesource's comments, i am most happy with designs that use a bipolar cascodes above the jfets.

acquiring the parts and doing the critical matching needed for jfet /jfet cascodes is more headache than i can stand at this point.

i've built some of borbely's preamp line stages and loved 'em.

i've been very happy with a borbely's servo 50 as my primary power amp. i use regulators for the front end and lots of bias on the output mosfets.

one day i'll try a jfet VAS stage and no global feedback, but i'm not quite there yet ... still studying and reading, soaking up john curl's pearls and waiting for charles hansen to drop some more ...


mlloyd1
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Old 14th October 2007, 07:09 PM   #8
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Borbely's designs are a heck of a good deal. I don't know why more Europeans don't make them as kits.
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Old 15th October 2007, 12:31 AM   #9
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Folks, thanks for the inputs again.

I located a shop which still carries 2SJ72GR, misc Jfets, and Rohm 2SB737 & D786. Very curious about Borbely's design and tempted to invest on these hard-to-find devices.

John, you use a lot of fets in your designs. This may sound silly - do they really sound that different? Like your old JC1 circuit with those J??? fets. Lost some sleep since I couldn't get them way back when I was still at university!

Mark.
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Old 15th October 2007, 12:35 AM   #10
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Get everything that you can, both bipolar and jfet.
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