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Old 23rd August 2007, 01:16 PM   #21
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This is becoming a quíte useful thread! Tom, I downloaded your files, they will help a lot for redesigning my servos!

This poses to a next question: what happens, if you have say three servos in a row (say, phonoamp -- lineamp -- poweramp)
when the second servos corrects an offset that is wandering as the first servo works, and so on...

Rüdiger
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Old 23rd August 2007, 03:27 PM   #22
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I have thought of writing a design note about DC-servos because they follow a certain pattern in the design process. You can have 8 versions:

Inverting or non-inverting servo

Injection in inverting or non-inverting input of the main amp

Non-inverting or inverting main amp.

I'll guess you will notice when I actually have written something in the subject.
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Old 23rd August 2007, 03:30 PM   #23
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This topic is becoming non-understandable to me. I am leaving. Decided to put some capacitors on signal path.
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Old 23rd August 2007, 03:51 PM   #24
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Quote:
Originally posted by Onvinyl
This is becoming a quíte useful thread! Tom, I downloaded your files, they will help a lot for redesigning my servos!

This poses to a next question: what happens, if you have say three servos in a row (say, phonoamp -- lineamp -- poweramp)
when the second servos corrects an offset that is wandering as the first servo works, and so on...

Rüdiger
Hi Rudiger,

In LTspice, you can hit CTRL-C (same as Edit--> Duplicate), drag a rectangle around the circuit to copy it, hit the "scale down" button a few times (magnifying glass with a "-" sign in it), or maybe do the scale-down first (either way works), place the schematic where you want it, and then left-click to leave it there. (It also works when two schematics are open, to take the copied portion to the other schematic, then select Edit-->Paste.)

So you could very quickly and easily make a setup to simulate some cascaded servo'd amps. Of course, you would probably also want to use your actual amplifier models, at some point in time.

Just "off the top of my head", I'd say that, in "steady state" at least, three identical servos "should" usually track each other OK, in a cascaded system like that. Since there's no global feedback between the stages, and _assuming_ that the DC offset doesn't get amplified to an un-correctable value or a too-high value between stages (or at least not for too long!), the servos seem like they should work as they normally would. Of course, there could be some "propagation delay", due to their time constants, but, I think there would be no real possibility of instability, and probably no "bad behavior" assuming "reasonably-low" DC offset values to begin with.

HOWEVER, applying some bad numbers: a 20 mV 1st-stage initial output offset and a total gain of 50X in stages 2 and 3 would give an initial 1V offset at the 3rd stage's output! Unacceptable!

So maybe there COULD be a major problem, with such a setup. You would definitely want to at least run some transient sims (or do some simple offset/gain calculations) to find out how much initial maximum offset could be tolerated in each stage, for example, without unacceptable behavior, since any offset at the output of the first stage could be quite a lot larger at the output of the third stage, especially before the servos had time to slew-out any initial offset.

And I don't know what your stages' initial offsets might be. So it's probably time to start thinking about having a relay on the output (of every stage, maybe?), with a very simple control circuit that would send the output to the next stage ONLY when the DC offset was below a certain threshold (and would also cut OFF the output whenever the DC output offset rose above a selected threshold), maybe even also with a short time delay before closing the relay at Power-On (regardless of offset), to give the DC offset-measuring and servo circuit enough time to measure, and get rid, of the initial offset.

:-o But NOW I might have to start wondering if in-line AC-coupling capacitors are really bad-enough to justify this additional complexity. However, given that there could also be many un-related fault conditions, for which an output-offset-controlled cutoff relay might be very beneficial, e.g. so speakers possibly wouldn't be damaged or destroyed, and given that it would also automatically prevent "speaker thump" at power-on, it actually is starting to "sound" like a good idea. I wonder if the DC offset could be averaged or integrated fast-enough for it to work well-enough, in most cases (I'm thinking of a circuit fault or failure that suddenly causes the amplifier output to slam to one of the power rails' values.)

[(Mind is wandering...) There's even a maybe-"better" (in one sense, at least) solution that would involve HAVING the AC-coupling capacitors, but using the relay to short around them, whenever the DC offset is low-enough!]

- Tom Gootee

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Old 23rd August 2007, 04:07 PM   #25
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Quote:
Originally posted by Dxvideo
This topic is becoming non-understandable to me. I am leaving. Decided to put some capacitors on signal path.
Oh! I'm sorry, Dxvideo! This is/was your thread!

I apologize for getting "carried away" while discussing "servos in general", and diverging from discussing your current need, which was why you started this thread.

I do hope that you got the information you needed, from the first part of the thread. But, if not, please feel free to email me, or post again, and I will be very glad to try to help you to get a good, working servo design, with the components that you have available.

Please note that my last post was being written when your last post was made and I didn't see yours until after I'd posted my further babblings. Again, I'm sorry.

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Old 23rd August 2007, 04:25 PM   #26
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Quote:
Originally posted by peranders
I have thought of writing a design note about DC-servos because they follow a certain pattern in the design process. You can have 8 versions:

Inverting or non-inverting servo

Injection in inverting or non-inverting input of the main amp

Non-inverting or inverting main amp.

I'll guess you will notice when I actually have written something in the subject.
Hi P-A,

That's a good idea!

I wonder if it might also be a good idea to try to formally incorporate into the DC servo schemes (or maybe at least mention) all of the various offset-correction schemes that are shown on pages 6 and 7 of AN-31 from http://www.national.com .

I hope that you'll let us know, if you do write such an Application Note.

- Tom Gootee

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Old 23rd August 2007, 04:42 PM   #27
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Quote:
Originally posted by Dxvideo
Hi,

I examined the Gootee's web site.. However I am confused.
I will use 3 x gain for preamp. So for a shortcut solution;
As Damian's application for LT1201..
I want to use Damian's circuit with OPA134 instead of LT1201 and LF411 (or 412) instead of OPA604..
Do you think this is suitable for that opamps?
I am not sure, because in Spice analyse I could run this circuit with ideal op-amp sub circuits, however with OPA134 and LF411 it doesnt look like working. I dont know the reason but thats the situation.

Another way is to use MKP type caps on signal path. Do you recommend that?

Thx
Hi Dxvideo,

What problem was there, when trying to simulate the circuit with the LF411 and OPA134?

- Tom Gootee
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Old 23rd August 2007, 05:10 PM   #28
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Andrew, et al,

AT LEAST TWO ERRORS I MADE:

#1 -------------

I posted some erroneous information, in paragraph 6 of post #19 of this "DC Servo Question..." thread:

When I gave the times for the speed of the servo's correction, I was looking at the results for a servo circuit that was different from the one I have had online, at http://www.fullnet.com/~tomg/gooteesp.htm .

In post #19 I quoted full correction after "almost 9 seconds" and I quoted 90% correction after about 3.6 seconds.

The correct numbers are: Full correction after about 6 seconds and 90% correction after about 2.14 seconds.

#2 ------------

The DC Servo circuit that I have had posted on my webpage, mentioned above, is VERY difficult to get to run, well, as configured, with Ltspice.

The simplest solution is probably to eliminate (replace by wires) the inductors in the power supply lines, L1, L2, L7, and L8, in the upper-left portion of the schematic.

It is then permissible (and probably advisable) to delete the spice-directive text line that sets gmin, i.e. ".options gmin=1e-013" can be deleted (or could be "commented-out" by adding a semicolon in column 1).

I will soon post a corrected version of the schematic, in place of the one that's there now, and will upload a new Ltspice file to the existing link.

- Tom Gootee

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Old 23rd August 2007, 07:22 PM   #29
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Quote:
Originally posted by AndrewT
that sounds like support for the philosophy that the output offset should be minimised BEFORE activating the DC servo.

The question that follows is:-
would it be better to minimise offset at completely cold and let the DC servo zero out the warm up offset?
or
set the warmed up offset to zero and hope that the first turn on offset is low enough and short enough to be tolerable.

I prefer the first alternative.
But, what if one switches on a pre-warmed amplifier?
Hi Andrew,

I definitely prefer the first alternative, too, since the initial DC offsets would seem to be the worst problem (and especially if considering a cascaded system of servo'd amplifiers), because the servo feedback loop's integrator and filter(s) need significant time to acquire the DC offset level.

Whether or not it might then be a problem, when switching-on an already-warm amplifier which had its DC offsets optimized for cold-start conditions, would depend on the magnitude of the difference between the warm offset and the cold offset. I don't have a good intuitive feel for whether the difference might be significant. But, assuming that it might be, at least in some cases, the only solution I can think of, offhand, would be to use temperature-compensation circuit techniques to make the cold and warm offsets be about the same. A single tiny thermistor, and maybe an additional resistor or two, should be all that is needed, per channel. And there are probably other good or better ways of doing it.

----

I USED TO "sort-of dislike" the idea of having to try to use a thermistor network to temperature-compensate a circuit.

But THAT was BEFORE I found the EPCOS downloadable library of Spice thermistor models, and before I learned that Spice has a steppable TEMP parameter that automagically makes all of the semiconductors and thermistors vary their behavior realistically versus temperature, and before I learned how to use LTspice to fairly-easily design a linearized thermistor network (by "measuring" a thermistor model's resistance at several temperatures with a trivial simulation), and learned how to apply it to compensate a circuit for almost no change in response over a wide temperature range.

But NOW, after doing and learning all of THAT... I no longer "sort of dislike" it. Now, I ABSOLUTELY *HATE* the idea of having to try to use a thermistor network to temperature-compensate a circuit!

----

Ha! Just joking! Sorry!

Tempco compensation with thermistors is not always easy, certainly, especially for any less-than-trivial, and maybe high-precision circuits, and with, say, more than several discrete transistors, and maybe even Gilbert Cells and current mirrors and stuff like that (although there may be much-better ways of tempco-comp, for some of those, than thermistors, of course).

But, for a monolithic (i.e. opamp or chipamp) amplifier's output's DC offset, it MIGHT be relatively easy to design a simple temperature compensation add-on for any of the common amplifier circuit topologies, I think. BUT(!), that's ASSUMING, at the very least, that the temperature variations of the parameters of the original (i.e. un-(tempco-)compensated) amplifier circuit are at least somewhat "uniform", in magnitude and, especially, in the direction they go versus the direction of the temperature change, even between different units of the same IC model. And I don't happen to know if that is always the case, or even if it's usually, or ever, the case. If it were the ("good") case, then having actual measurements of the output offset variation versus temperature (and maybe also measurements of the chip's inputs' voltages and currents, or at least the voltages and currents of the resistors around the chip), for a complete amplifier circuit (and preferably for several different "identical" sample units, and having the schematic for the amplifier circuit, would be a good starting point for the temperature-compensation designer.


ALTERNATIVELY, we could simply always HEAT the amplifier IC to the design temperature for the offset, before powering the speakers. It could even have a "standby" mode, where it was always kept at some nominal operating temperature, by having some easily-heatable component attached to it.

----

Regarding your first statement, Andrew, I just meant that maybe the amp/servo combination shouldn't be connected to the speakers (or to another amp) until after the servo had removed most of the output's DC offset, at power-on. (In a later post, I also suggested using the output's DC offset level to trigger the output relay (both on and off), rather than using a plain time-delay scheme.)

Cheers!

- Tom

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Old 24th August 2007, 08:50 AM   #30
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Hi Gootee,

With spice analyses, If I use ideal op-amp model then It gives 1Hz to 100Khz frequency response all flat... This is the application circuit;
Click the image to open in full size.
And this is the result;
Click the image to open in full size.
Then when I replace the ideal op-amps with OPA134 and LF411, the result changes like that;
..... Sorry I've noticed that I made a mistake while I was changin the components.. They give the same result.
Sorry again.
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