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Simplistic NJFET RIAA
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Old Yesterday, 10:56 PM   #17981
LATB is offline LATB  United States
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I have a question regarding stability of the bias in UFSP, as measured across R4. I had problems to find a reproducible precise setting for one of the channels, and decided to run a longer-term monitoring over a few hours -- see below.

This is from taking measurements every 5sec for the past 10h (using a little Arduino), and as one can see the bias is wandering around by around half a Volt or so.

I don't think this is related to temperature, so I was kind of surprised.

The other channel is much more stable (second plot).

I was wondering, should I be worried about this or does this have no impact on performance?
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File Type: png fc-vbias.png (62.2 KB, 79 views)
File Type: png fc-otherChannel.png (55.4 KB, 83 views)
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Old Yesterday, 11:17 PM   #17982
Salas is offline Salas  Greece
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Simplistic NJFET RIAA
Hi, it is related to localized temperature and it's normal for this kind of fets in this design. Just blow air with your mouth on Q1 Q2 and you will see a V jump.

Such high yfs low vgs (off) fets are inherently self running away especially at the high Id low Rs values necessary in lmc/mc mode to minimize noise contributions. I had tried active constant current sourcing instead of drain resistors to harness them but noise went unacceptably high.

If you select a lower sensitivity mode like hmc/mm it gets tighter because of the higher source pin resistor's value bringing more local feedback stabilization. A luxury not affordable in my lmc/mc modes noise budget.

In conclusion, that's how it works don't worry. Actually it works better in your graphs than in some past builds.
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Old Yesterday, 11:33 PM   #17983
LATB is offline LATB  United States
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Excellent, thanks Salas!
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Old Yesterday, 11:44 PM   #17984
Salas is offline Salas  Greece
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T.P adjustment works here for DC level centering. If you will measure voltage between drain and source of your Q1 Q2 it should be about 8V DC. Maybe 7.8V or something. Not important. We just set T.P for half that i.e at about 4V.

Imagine a car speeding on an 8 lane empty motorway steering not dead on but about the center line. Because bit idiosyncratic. Each lane is a Volt. Left and right to the car's course it is all free space for AC signal swing. Which is few hundred mV tops in this case. What you basically want is not let it drift and crash at any side of the motorway. Nor to let it come close enough to any one side for clipping the signal peak it carries. But some mid-line wobble is no problem when on such a wide empty motorway.
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Old Today, 01:17 AM   #17985
alexkosha is online now alexkosha  United States
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I did not experience with such swing in my Ult build. I monitored TPs over 2-3 days (about once in hour) and 4V BIAS was very steady with closed lid. Any internal movement or lid removal caused change, but that behave was expected. My box is taller and probably less temp-conductive due to thick wood side-boards.... It is quite steady with internal temp equilibrium and I did not see changes after about 3 hours of On time. Have you shorted inputs?
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Old Today, 01:32 AM   #17986
Salas is offline Salas  Greece
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Open box is naturally more affected by small ambient room temperature changes or weak air drafts. Then the wobble graph paints a more sinister picture of that motorway trip.

Jah Wobble's Invaders Of The Heart ~ Rising Above Bedlam
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