dB re nominal current.
So say current noise << 8nA p-p.
TC = tempco ?
Then say 0.01%/°C.
Patrick
So say current noise << 8nA p-p.
TC = tempco ?
Then say 0.01%/°C.
Patrick
> parallel a bunch of JFET CSS's running at their zero tempco current until you hit 8mA target?
Thought so too, but for most of the FETs we use, like 2SK170, 2SK369, ... the zero tempco Id is not to be found in datasheet.
Anyone knows by any chance ?
Patrick
Thought so too, but for most of the FETs we use, like 2SK170, 2SK369, ... the zero tempco Id is not to be found in datasheet.
Anyone knows by any chance ?
Patrick
These are pretty interesting drivers and are indeed more like art. Like most full range drivers they have some specific qualities that are interesting but not my cup of tea.
And even if it were (like for BF245), it would be subject to part-to-part variation, so I guess you'd need to measure and select for it.
The shot noise of 8mA is ~50pA/rt-Hz in 20k BW this is 7nA rms or 50nA p-p or so. I don't see how except with a big V across a big R. 8V and 1k has only 4pA/rt-Hz, can't you take your first idea and just servo the source voltage of a FET to put 8V aross a resistor tied to a minus rail? FET and servo can be < 4nV of the resistor.
^
Assuming a reference and an (integrated) opamp for the servo, which references and opamps would qualify?
Assuming a reference and an (integrated) opamp for the servo, which references and opamps would qualify?
You need low noise, LOW Gm parts like the new LSK489, low capacitance, low noise, AND LOW TRANSCONDUCTANCE.
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OK, I have one. Out of curiosity. Have you ever tried driving complex loads, like 600 ohm line transformers, with your amps - i.e. JC-2, Vendetta or similar? Any pitfalls?
Thanks!
The TA type design is actually better for driving difficult loads compared to the OP Amp. This is because the driver stage senses the complex load and compensates accordingly. So long as you start with enough peak current and a very high slew rate, then the JC-2 line stage can drive just about anything.
I built a version of the Vendetta Line stage into a line driver for the Grateful Dead, in the mid 80's, and they used it for years, and it beat any IC put against it.
I built a version of the Vendetta Line stage into a line driver for the Grateful Dead, in the mid 80's, and they used it for years, and it beat any IC put against it.
What, is this like on demand video? I might have some ideas for Patrick but they will have to wait till tomorrow. Though I can say that the 0 TC point of JFET's has very little practicle use, it varies all over the place and is not really 0.
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I bet, it did 🙂 Thanks for the info, John! I will give a similar circuit a try.
By the way, have you seen this site?
Internet Archive Forums: the wall and how was it wired
I didn't realize until reading this that GD were after the 'clean sound'.
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Scott,
I was trying to avoid a servo, and I know it is not a easy one.
Else I wouldn't put it up here for discussion. 🙂
So very much looking forward to your ideas.
Patrick
PS I wonder how AD did it in the AD1865 -- also with servo ?
I was trying to avoid a servo, and I know it is not a easy one.
Else I wouldn't put it up here for discussion. 🙂
So very much looking forward to your ideas.
Patrick
PS I wonder how AD did it in the AD1865 -- also with servo ?
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No, virtually all are done with bandgap but you need exact area ratios for that. I need to qualify my other comment the zero TC point of JFET's is not of much use for precise circuits, it works well for getting close +- a few % sort of use.
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Here this should work, you don't need much of a servo. Even 1/2 of a diff pair gives 20 per volt of collector drop, here ~250.
EDIT - 6.2V is the nominal zero TC point in the zener equation. You could also stick any two (same) pnp's in one of your little heatsinks and trim the 24.8K for 0 across the bases
EDIT - 6.2V is the nominal zero TC point in the zener equation. You could also stick any two (same) pnp's in one of your little heatsinks and trim the 24.8K for 0 across the bases
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