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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 26th September 2009, 03:15 PM   #1591
Netlist is offline Netlist  Belgium
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OT stuff split to here: http://www.diyaudio.com/forums/showthread.php?t=152530
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Old 26th September 2009, 04:22 PM   #1592
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John Curl's Blowtorch preamplifier part II
synO8, the BT input stage is a V/I converter, not a voltage amplifying stage. Though voltage noise is not any superb, it is about 7nV/rtHz. Better than many opamps.

Last edited by PMA; 26th September 2009 at 04:25 PM.
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Old 26th September 2009, 04:33 PM   #1593
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Quote:
Originally Posted by PMA View Post
synO8, the BT input stage is a V/I converter, not a voltage amplifying stage. Though voltage noise is not any superb, it is about 7nV/rtHz. Better than many opamps.
What is the purpose of using low noise JFETs in the input stage. Any N/P channel JFET devices, matched at the same level, will do exactly the same. Not to mention you could raise the supply voltage to, say, +/-24V. Here goes the myth of BT being impossible to reproduce, due to lack of Toshiba devices.

Set aside 7nV/rthz is almost one order of magnitude of what a low noise opamp can do today.
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Old 26th September 2009, 04:39 PM   #1594
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John Curl's Blowtorch preamplifier part II
The supply voltage is +/-30V, IMO.
Regarding JFETs, these low Vp devices have higher transconductance than the high Vp parts, and low noise. Operated at high Id, they have very good linearity (interesting that EB operates them too low).
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Old 26th September 2009, 04:40 PM   #1595
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Syno8, please tell me which input devices that you would chose for the comp diff input stage for line amp operation?
We have used Toshiba devices, because they became the world standard, years ago, and there are not many good substitutes.
Now please, without just empty criticism, give us some part numbers that you would prefer that offer similar performance, without significant compromise.
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Old 26th September 2009, 04:47 PM   #1596
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Quote:
Originally Posted by PMA View Post
The supply voltage is +/-30V, IMO.
Regarding JFETs, these low Vp devices have higher transconductance that the high Vp parts, and low noise. Operated at high Id, they have very good linearity (interesting that EB operates them too low).
It is impossible to have a coherent discussion.

I have just mentioned that the input stage has the gain < 1 and hence doesn't have the slightest positive impact in the overall gain and noise. The JFET transconductance doesn't really matter, it does not appear in the first order approximation of the overall gain.

You feed 2SK389/2SJ109 at +/- 30V and you'll maybe see what's going to happen. 2SJ109 has Vgdmax = 30V and that includes the voltage swing.
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Old 26th September 2009, 04:51 PM   #1597
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John Curl's Blowtorch preamplifier part II
syn08, the JFETs sit at 15V defined by MOSFETs (half supply voltage). Try to look at the circuit. I have analyzed it in deep, you only criticize. Or you only do not understand discrete electronic circuits.

Last edited by PMA; 26th September 2009 at 04:58 PM.
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Old 26th September 2009, 04:56 PM   #1598
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John Curl's Blowtorch preamplifier part II
voltages
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File Type: png for_syn08.PNG (24.4 KB, 341 views)
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Old 26th September 2009, 05:06 PM   #1599
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Thanks PMA, it would seem that Syn08 has not really looked carefully at the circuit. Of course, I never run the actual devices at their max rating, too much potential for leakage, even with 1K drive impedance or so. Wavebourn did run devices close to the max rating, at least in worst case, but he was using a 1G input resistance or about 1 million times higher resistance that we would be with the the Blowtorch input stage. Where was Syn08 then, when he could have criticized something that really could go 'off the rails'?
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Old 26th September 2009, 05:07 PM   #1600
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Quote:
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voltages
Great, now apply input signal and swing the output towards the rails.
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