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Old 8th July 2013, 05:41 PM   #131
lsheng is offline lsheng  United States
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Join Date: Feb 2010
Adding such dc servo loop to a otherwise flat-frequency-response amplifier is effectivly changing amplifier to a high pass filter, the corner frequency is K/(2*Pi*R5*C4) as in #4 schematic, where K is amplification from servo opamp output to main Amp output. In this case, K is roughly 0.5. so the corner frequency is 0.15Hz, change R5 to 100k, this corner is moved to 1.5Hz, to a 20Hz music frequency lower limit, it is going to be hard to tell the corner difference between 0.15Hz to 1.5Hz, unless other conponent has different value so your corner is not 0.15Hz to start with.

To a music signal, I do not believe 0.15Hz or lower corner frequency can make an audable difference, but lower corner frequency will make the whole amplifier slower to settle at power up.

Originally Posted by nonsqtr View Post
Hi all, I found this thread, which contains some servo information at the level of detail I probably need. (I explained my servo issue in the other thread, I'm trying to make a 2-stage preamp work, and I'm getting what appears to be some servo-related oscillation or "maximum range exceeded" or some such thing).

Let's see, I've never used SPICE but I'm good with electronics/schematics, should I be trying to simulate my circuit this way? I have 1meg/.47uF integrator, and 100k resistor in series with servo output. I noticed specifically Mr. Gootee's comments about "the resistor isn't big enough", maybe this is what's happening to me?

How can I find out? What's the best path to the goal (smooth servo operation)?
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Old 8th July 2013, 07:46 PM   #132
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Originally Posted by AndrewT View Post
a non inverting servo feeding the +IN input will drive the output to rail.
An inverting servo feeding the -IN input will drive the output to rail.

A proper selection uses a Non inverting servo to feed the -IN input.
or an inverting servo to feed the +IN input.

Think it through.
Output offset goes +ve. Non inverting servo goes +ve at both it's input and output.
That +ve is sent to the -IN and forces the amp output towards -ve, i.e corrects the +ve offset.
Jeez.... yeah, I'm not THAT much of a beginner.

Inverting servo should feed the non-inverting input, or a non-inverting servo should feed the inverting input. Easy.

And, take your pick, there should be zero difference in terms of the result, right? Either way, your output gets driven to 0 DC and stays there.

This is a much more sophisticated problem. Something is actually changing the behavior of the first stage, not just the servo but the amplifier itself. I'm thinking it's the second stage "bias adjust circuitry", and I'm about to start playing with that, so I should have some details for you by the end of the day. The thing is, when the AD is plugged into an ordinary 10k load (resistor) there's only 20 mV of output offset change when the gain pot is rotated through its range. When the AD is connected to the second stage instead, suddenly there are VOLTS of offset when the first-stage gain pot is rotated through its range. That's the first problem I need to understand, before I can select the proper component values for the servo. Yes?
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Old 9th July 2013, 07:03 AM   #133
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
could one or other servo be going into unstable resonance and thus hitting rail voltage?

Some opamps invert output when going outside the spec limits.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 12th July 2013, 11:54 PM   #134
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Originally Posted by AndrewT View Post
could one or other servo be going into unstable resonance and thus hitting rail voltage?

Some opamps invert output when going outside the spec limits.
Yes. In my case it turns out to be a little more complicated than that - I'll explain it in the other thread. (As best I can, at this point).

I believe I already linked to the John Hardy schematic, yes? If not, here it is again. http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf

Note the "bias adjust circuitry". That's the culprit, in my case.

And if you look at the 990c schematic here http://www.johnhardyco.com/pdf/990.pdf you'll see the pair of diodes across the inputs...

Anyway, so, if you're DC-coupled to the previous stage the currents can run backwards into the previous stage's servo.
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Old 12th October 2013, 09:49 AM   #135
sajti is offline sajti  Hungary
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Location: Százhalombatta, Hungary
Is anybody use ceramic capacitor in DC servo? I need 1uF, and it's available both film, and ceramic. Is there any reason to not use ceramic?

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Old 12th October 2013, 12:59 PM   #136
gootee is offline gootee  United States
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I am not an expert, but the type of ceramic cap that is available in large values (i.e not NP0 or C0G) might have issues with being microphonic, and with temperature or voltage coefficient of capacitance (capacitance might vary when temp or voltage varies). Or maybe not. But you should check. And if any of those ARE significant, then you should also check how they might affect servo operation.

Other than issues like those, polypropylene and ceramic usually both have very low dielectric absorption ("voltage memory" effect); better than most other types except teflon, I think.

Edit: WIMA had some little red polypropylene box caps that came in 1uF, that I think were roughly 1 cm x 1 cm footprint, or less.
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg

Last edited by gootee; 12th October 2013 at 01:03 PM.
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