John Curl's Blowtorch preamplifier part II - Page 2430 - diyAudio
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Old 8th July 2012, 07:34 PM   #24291
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I first started to talk about using coupling caps between stages and in the feedback loop, in order to show an alternative approach by using servos. I have not gotten very far.
Back in the days when we thought that coupling caps were an important problem, about 1980, due to the articles by Dick Marsh and Walt Jung, along with my earlier measurements of nonlinear distortion in Tantalum and Ceramic caps. Serious audio designers had to figure out what to do? Do we use bigger and better coupling caps, or do we direct couple? And IF we direct couple, can we get away with it, without all kinds of trouble keeping the DC offsets in check, over time and temperature?
First, let me set the low limit of the value of capacitance at 1uf. This is for a number of reasons, but with bipolar inputs, you are just asking for it, if you use a smaller value. Of course, tubes and jfets COULD allow for a somewhat lower value, but then the price would increase just using these parts, so let's stick to 1uf.
What is available, today, or back 30 years ago, that might fill the bill?
Well, if you are Sony, then you might select a large value ceramic. It would be smallish, non-polar, rugged, and compatible with large scale assembly.
Some high end people might want to use a film cap, instead. Back in 1970's, I thought that 1uf Mylar coupling caps, since they did measure well, with a sine wave measurement. Darn, if somebody didn't 'hear' them and got me to remove them from one of my Xover designs. Well, what next?
Polystyrene gets really big, Teflon too, AND EXPENSIVE. Polypropylene isn't bad, it measures well, and has no obvious problems. Still, it is fairly large, and not dirt cheap, that is, if you want one from an established manufacturer. Now what is the alternative? You could use an aluminum electrolytic cap, of course you can go up much higher in capacitance, for a given case size. However, this could well be the fundamental limit on your design's lifetime. We certainly find that with test equipment. Over time, we find that aluminum electrolytic caps are often the first thing to go, even if used correctly and from a reputable manufacturer. Also, it might be noted that not just ONE capacitor is used, often you have in-out, feedback and interstage coupling caps. They kind of add up. That is why we thought that servos were a useful way to minimize the coupling cap problem, substituting an IC op amp (single or dual) and adding a couple of resistors and a 1uf Mylar cap was a useful alternative. (more later)
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Old 8th July 2012, 08:11 PM   #24292
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Here is a servo example:
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File Type: jpg servo1.jpg (185.2 KB, 191 views)
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Old 8th July 2012, 08:31 PM   #24293
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Quote:
Originally Posted by john curl View Post
.... (more later)
I am all ears !

As an aside, is there a better name for "servos" ? I always thought this name doesn't describe very well the purpose of the circuit.
I would call them "DC cancellation circuits" or maybe "DC negative feed-back loops" or just "DC loops".
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Old 8th July 2012, 08:35 PM   #24294
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how about state variable active high pass filter?
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Old 8th July 2012, 08:56 PM   #24295
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Here is another servo example, one that I prefer, today.
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Old 8th July 2012, 09:23 PM   #24296
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Quote:
Originally Posted by john curl View Post
Here is another servo example, one that I prefer, today.
Hi John,
Thanks for the schematics.
Why two IC's when one would do (connected to the (+) input)?
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Old 8th July 2012, 09:35 PM   #24297
SY is offline SY  United States
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That would require an input coupling cap, which sorta defeats the whole purpose of the servo.
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Old 8th July 2012, 10:08 PM   #24298
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Quote:
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That would require an input coupling cap, which sorta defeats the whole purpose of the servo.
Sy, how can you have a coupling cap in what is a DC amplifier? The first stage of JC's servo is an inverting low pass filter and you can do the same non-inverting, so I guess Joshua is right and save one opamp. But it might be good to maintain the second opamp as a buffer though, because stuff will be coming from the other end (feedback loop).

vac

Last edited by vacuphile; 8th July 2012 at 10:10 PM.
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Old 8th July 2012, 10:12 PM   #24299
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Quote:
Originally Posted by SY View Post
That would require an input coupling cap, which sorta defeats the whole purpose of the servo.
Thanks.
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Old 8th July 2012, 10:20 PM   #24300
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Originally Posted by vacuphile View Post
Sy, how can you have a coupling cap in what is a DC amplifier? The first stage of JC's servo is an inverting low pass filter and you can do the same non-inverting, so I guess Joshua is right and save one opamp. But it might be good to maintain the second opamp as a buffer though, because stuff will be coming from the other end (feedback loop).

vac
It's after midnight here and I'm too tired to follow AC vs. DC gain right now. One IC (without a series input cap.) may work and it may not.
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