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Dc servo circuit for AD815 pre
Dc servo circuit for AD815 pre
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Old 1st December 2020, 02:21 AM   #11
kp93300 is offline kp93300  Malaysia
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Sorry, The corrected first sentence should read as follows

l change R2 to 100k. Now l measure 4 mv dc at the output of AD 815.
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Old 1st December 2020, 02:55 AM   #12
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by kp93300 View Post
Richard, is 4mv acceptable for the TDA amp from you ?
4mV is probably too much DC for the transformer on my monoAMP yes. But have a listen, see if you notice any change with and without a series cap.
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Old 1st December 2020, 03:11 AM   #13
bansuri is offline bansuri  Italy
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I use ADA4177-2 for DC.servos, but SO8 only. Not tooo expensive, RRout gives you more range and you can maybe use R2 100k.
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Old 1st December 2020, 03:47 AM   #14
Monte McGuire is offline Monte McGuire
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I whipped up a quick simulation, and using the following component values:

R1 = 330Ω
R2 = 15kΩ
R3 = 620Ω
R4 = R5 = 50kΩ
C1 = C2 = 100nF
OP1 = AD815
OP2 = ADA4522-1

...you get an overall circuit offset of 5.34ÁV, 3dB down points of 1.3Hz and 74MHz, and a 1kHz gain of 9.3dB. The ADA4522-1 is a fantastic servo amp - it's cleaner than a TL07x and it actually has low offset voltage, on the order of single digit microvolts, not millivolts. Because it is a chopper amp, it also has no LF popcorn noise, unlike a BiFET. The drawback is that it is not as cheap as a TL072, but it's not expensive either.

You cannot use it to best effect with high input resistors, since its input bias current is artificially high because of the switching action of the input stage, which will result in substandard offset performance. So, keep the input resistors less than 100kΩ or so and the bias induced additions to offset voltage will be negligible. The 50kΩ used here works well enough.

And, as you can see, you can use 50kΩ or even 66k5Ω with this amp and still get a low corner frequency with a 100nF capacitor. Why is that important? 100nF is the largest 3216 package C0G ceramic cap that you can buy, so you can still assemble a servo with a low corner frequency and low offset using SMD components, making the entire thing extremely small and extremely high performance. The LF point is extended using the output attenuation from the servo, which also attenuates any noise from the servo.

I have built and tested this extensively (using 100nF and 66k5Ω time constant components) with various values of force resistors to match the circuit in question, and it works perfectly. There are no traces of switching noise in the output, and because of the topology and the relative cleanliness of the ADA4522, there is no added distortion or noise whatsoever, measured to below -160dBc.

I'm using the Murata GRM31C5C2A104JA01 100nF 3216 sized capacitor for these servos and it works well. Extremely low distortion, no microphonics, small size, and low cost. This is the 100V rated version, but the GRM31C5C1H104JA01 version, rated for 50V, is indistinguishable. I would argue that these are more reliable and high performance than any film cap since they have no questionable end terminations and no possibility for microphonics.

Another benefit of the ADA4522 is that you can get it in a dual where both amps use the same chopper oscillator, preventing potential "birdies" that could happen with two unsynchronized but "close" choppers that somehow demodulate into each other. If you use twin servos for balanced circuits, the ADA4522-2 is a great choice for that reason.
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Old 1st December 2020, 04:04 AM   #15
abraxalito is online now abraxalito  United Kingdom
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Quote:
Originally Posted by Monte McGuire View Post
100nF is the largest 3216 package C0G ceramic cap that you can buy, so you can still assemble a servo with a low corner frequency and low offset using SMD components, making the entire thing extremely small and extremely high performance.
https://www.mouser.com/ProductDetail...Owyf0Xlg%3D%3D
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Old 1st December 2020, 04:24 AM   #16
Monte McGuire is offline Monte McGuire
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Aaah! Progress! 220nF was not so common 5 years ago when I last shopped for large C0G caps. Turns out, it’s now cheaper than the 100nF part too…

SMD caps just keep getting bigger and bigger, and while they can squish up when the layers get thinner, these are nowhere near that point. Thanks for the pointer!
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Old 1st December 2020, 05:30 AM   #17
bansuri is offline bansuri  Italy
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you also have to consider that there will be 10-15mV DC on the input, you do not show what is connected there.
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Old 3rd December 2020, 01:56 AM   #18
kp93300 is offline kp93300  Malaysia
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Monte.
I am going to order the parts .
Is ADA4522-2ARMZ the same characteristic but 2 amp in the package?

I will use an adapter

thanks

kp93300
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Old 5th December 2020, 07:02 AM   #19
Monte McGuire is offline Monte McGuire
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kp93300,

Yes, that is the two channel version, which is the best one to use if you need two servos sharing the same circuit. They also make a quad, but I have not needed that personally.

This is an SO-8 package, which is pretty easy to hand solder onto an adapter board with a little care. No tricky DFN or anything like that.

All the best!
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