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Old 17th May 2012, 01:46 PM   #23151
1audio is online now 1audio  United States
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My ac calibrator (Optimation from the early '70s) uses a 2watt Vishay metal foil feedback resistor in the 10v amp just to meet specs. It's not new knowledge that thermals can have a big impact on linearity. Especially when you are trying for less that 100 ppm. accuracy.
I was just thinking that adding heat would reduce the impact of the ac component.


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Old 17th May 2012, 01:58 PM   #23152
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I was just thinking that adding heat would reduce the impact of the ac component
so you want to put a temp servo around the feedback R ? - maybe "constant power" feedforward to feedback R substrate

what do the thermal time constans do to limit performance? - could it be cheaper than even bulk metal foil?
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Old 17th May 2012, 04:11 PM   #23153
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More a DC bias that preheats the device so the AC is a smaller percentage.
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Old 17th May 2012, 04:19 PM   #23154
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Problem is that the "smaller percentage" is REALLY small since it's in relation to absolute temperature. If you were going to do it, you could take the same approach as the LM399, basically a miniature Easy Bake Oven.
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Old 17th May 2012, 04:26 PM   #23155
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I think most of the time, unless the device temperature coefficient itself is a strong function of temperature, the notion that higher quiescent temperatures reduce the effects of signal-induced self-heating is a misconception, albeit intuitively plausible.

If one is working at very low temperatures where carrier freezeout starts to occur (say below 100K in conventionally-doped silicon) then it is a very different story. For example 77K (liquid nitrogen, hence very convenient as such things go) is a very poor temperature for operation of silicon JFETs.


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Old 17th May 2012, 04:30 PM   #23156
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Originally Posted by SY View Post
Problem is that the "smaller percentage" is REALLY small since it's in relation to absolute temperature. If you were going to do it, you could take the same approach as the LM399, basically a miniature Easy Bake Oven.
Now for temperature stabilization against ambient changes, the 199 was a nice realization, albeit rather consumptive of power. And the buried zener was pretty quiet as zeners go.

But if the voltage reference portion were itself subject to significant dissipation changes I don't know how well the temp servo would follow them.
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Old 17th May 2012, 06:11 PM   #23157
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Originally Posted by hags View Post
batteries then ?
The $10K that Mr Sutherland charges for a twin case phono stage with low part count is more mind-boggling, imo.

+20 years ago, Avantgarde in Switzerland came out with an add-on battery PS for the MAS (Metaxas) preamp they distributed in Europe.
Bumped up the price of the pre considerably, and base retail wasn't exactly cheap to begin with. *

PS design has evolved in 20 years time, and batteries by no means deliver clean DC.
For a phono stage that has to stay under $1k retail, batteries may be a/the cheaper solution.

(* couple of years later, Mr Gassmann of Avantgarde, came out with an all-battery phono/pre/power amp combo under his own brand name. The powersupply of the power amp carried an automatic charging circuit, plus 8 Sonnenschein batteries of a size used for motorcycles. Total cost ~45K Swiss Francs in the early '90s. Afaig, Mr Gassmann still has the hots for batteries, his latest XA preamp, in a truly beautiful cast-alloy case, runs on can juice also)
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Old 17th May 2012, 06:15 PM   #23158
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I still have seen no compelling evidence that this matters at all not even a little. That is the WAVAC remains a large (pink?) elephant in the room.
"If you build it they will come..."

But has SY compared a -120 db amp to a -140 db one?
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Old 17th May 2012, 06:18 PM   #23159
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Originally Posted by bcarso View Post
Now for temperature stabilization against ambient changes, the 199 was a nice realization, albeit rather consumptive of power. And the buried zener was pretty quiet as zeners go.

But if the voltage reference portion were itself subject to significant dissipation changes I don't know how well the temp servo would follow them.
Feed-forward temperature stabilization: 2 layers of resistive conductor, and opposite signal on the 2'nd layer. Or 2 bifilar wndings, 2 wires, noninductive. No, 3 wires: 3'rd to sense average temperature.

Reminds me famous joke about modern engineer, light bulb, and 8 technicians.
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Old 17th May 2012, 11:00 PM   #23160
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Would the heater signal get more complicated since it has to decrease as the amplifier output goes positive or negative?

Is it likely that any such methods significantly change the phase of the feedback within the cross-over (at least for resistors close to 20K)?

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