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Old 8th July 2013, 12:28 PM   #11
EssB is offline EssB  United Kingdom
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Quote:
Originally Posted by nonsqtr View Post

The only major difference between my circuit and this one:

http://www.jensen-transformers.com/as/as083.pdf

is that mine has the servos connected to the - input instead of the +. It shouldn't matter which way you apply the corrective voltage, right? "Shouldn't", anyway.
Of course it matters! get it the wrong way round makes positive DC feedback
and the output hits a power rail.

Quote:
Here's a specific question: what is the function of the resistor that's in series with the servo output? Notice in the schematic above, the series resistor on the left is smallish while the one on the right is rather large. Your previous thread seemed to indicated it might be related to the ability of the servo to capture extreme offsets.
R9 and R3 attenuate the A3a servo output by 1500, R15 and R10 attenuate the A3b servo output by 1000. This allows the servos to work at reasonable output levels into high enough impedance and also attenuates any VLF servo noise.

Please post your exact schematic, a photo of a hand drawn one is fine as long as it's legible - JUST POST THE SCHEMATIC
We're not remote mind readers!
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Old 9th July 2013, 12:35 AM   #12
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So, upon further experimentation, what appears to be happening is the bias-adjust currents from the second stage are flowing backwards into the first stage, thereby disrupting the servo and causing the "settling at non-zero values" I was seeing, and also the variable and greatly increased offsets (from current flowing through a 10k feedback resistor and a few more ohms in the gain leg). These currents are ostensibly set at 2.2 uA, meaning 22 mV-ish across the 10k, which is definitely enough to cause problems. When the bias adjust circuitry is disabled, the offsets change "only slightly", I no longer see the 2-volt swings when I rotate the AD's gain control through its range. Still working...
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Old 12th July 2013, 11:58 PM   #13
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Well, so, I'm still seeing something I can't explain. Maybe someone can figure it out.

What was happening here, is the "bias adjust" currents were running backwards into the previous stage, thereby disrupting both the stage itself (causing offsets) and the previous stage's servo (causing it to settle somewhere "other than zero").

So, the simple fix was to put a capacitor in series between the stages.

Now, the servo issue is fixed, things settle as they should - HOWEVER - I'm still seeing some very large offsets in the first stage, which are not present when the stage is used alone with an ordinary (10k) load.

When the stage is used alone, I can rotate the gain knob through it's range and I see a total change in DC output offset of maybe 10 mV. That, I believe, is "working as it should".

However when I connect the second stage via the blocking capacitor, rotating the first stage gain knob generates offsets OVER A VOLT at the output of the first stage.

What could be happening here?
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Old 13th July 2013, 04:06 AM   #14
gootee is offline gootee  United States
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Did you try inserting, say, 10k to ground at the first stage's output, before the blocking cap? I assume that you also have some sort of R to ground (a path for DC) just after the blocking cap? Usually that last one should be adjusted to equalize the DC impedances seen by the input pins, to minimize the output offset with no servo (or something like that).
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Old 13th July 2013, 05:38 PM   #15
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Quote:
Originally Posted by gootee View Post
Did you try inserting, say, 10k to ground at the first stage's output, before the blocking cap? I assume that you also have some sort of R to ground (a path for DC) just after the blocking cap? Usually that last one should be adjusted to equalize the DC impedances seen by the input pins, to minimize the output offset with no servo (or something like that).
Yes, I have the 10k direct load, followed by the cap, followed by 1k to ground. I selected the 1k by looking at the second stage output offset without any AC input, that's the value that seemed to bring the normal offset closest to 0 thereby keeping the servo happy.

At this point, the first stage servo does correct, but it seems like it's working too hard. I don't think there should be 1-volt DC offsets, and I can't imagine where they might be coming from. Same behavior with half a dozen chips, btw.
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Old 13th July 2013, 07:23 PM   #16
gootee is offline gootee  United States
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A schematic of your exact circuit could be helpful.
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Old 14th July 2013, 11:23 PM   #17
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Quote:
Originally Posted by gootee View Post
A schematic of your exact circuit could be helpful.
I have no way of posting a schematic. I already linked to the originals, the second stage is exactly a John Hardy M-1 except that instead of an input transformer it has a 330 ohm resistor (and now a 33 uF cap in series). The first stage is exactly the same thing, except that there's no "bias adjust circuitry" and no output transformer - the output feeds the 330 ohm resistor.

Really, it's the world's simplest circuit. It's just two non-inverting amps in series, coupled by a 330 ohm resistor (and now a 33 uF cap in series). Each amp has a servo, it's very vanilla, exactly like the John Hardy version. The stages work fine and "exactly as expected" by themselves - it's only when they're (AC) coupled that I see the DC offsets in the FIRST stage. They're very dramatic too, instead of 10 mV I see a whole volt.

What this would NORMALLY tell me is there's some DC creeping into the first stage somewhere. I can't figure out where that might be happening. There's no change in the impedance of either input leg, no change in the feedback path, and no DC link to the output. Doesn't make sense.
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Old 15th July 2013, 12:01 AM   #18
gootee is offline gootee  United States
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You don't have a phone with a camera, and paper and pencil? Or MS-Paint?
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