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"My" first design headamp, DC-servo question
"My" first design headamp, DC-servo question
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Old 30th December 2015, 06:44 PM   #131
Mooly is offline Mooly  United Kingdom
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"My" first design headamp, DC-servo question
Oh, definitely. Cheapo jobs that self destruct or that don't do the job are no use whatsoever.

Mine is a rather ancient Philips from around 1992.

Philips.JPG
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Old 31st December 2015, 10:08 PM   #132
Mayday is offline Mayday  Sweden
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"My" first design headamp, DC-servo question
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Originally Posted by Mooly View Post
Oh, definitely. Cheapo jobs that self destruct or that don't do the job are no use whatsoever.

Mine is a rather ancient Philips from around 1992.

Attachment 522105
That's quality for you
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Old 24th January 2016, 03:35 PM   #133
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"My" first design headamp, DC-servo question
Well I've decided to try AD797's as well.

I will re-work the diamond buffer board to fit 100R trimpots on two of the four CCS's and swap the output resistors for parallel carbon film 3R3.

I'm nearly done, enough for testing with DMM anyway, with the AD797 board.
Lytics are Panasonic FC 4.7uF, they're parallelled with 100nF 0805's.
I've added 22uH in series with the powersupply as that atleast made a rather big difference in simulations.
Click the image to open in full size.
Click the image to open in full size.
It looks messy, but it'll look better once the flux etc is cleaned off
Click the image to open in full size.
Click the image to open in full size.

I've got a few more wires to solder in and then it is ready for some extensive testing with the DMM making sure that there are no shorts etc before I try it with the diamond buffer.

I hope I've gotten the PS decoupling close enough, and good enough, to prevent the AD797's from oscillating.
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Old 24th January 2016, 05:07 PM   #134
Mooly is offline Mooly  United Kingdom
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"My" first design headamp, DC-servo question
You might get away with it, although its definitely one to use the scope on.
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Old 24th January 2016, 06:36 PM   #135
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"My" first design headamp, DC-servo question
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You might get away with it, although its definitely one to use the scope on.
Well, I don't have one unfortunately.
All parts are datasheet values, except for the added inductor...one on each rail and op amp.
All parts fitted on a 4*6cm board.
I have 15pf in series with 100R across the FR(300R).
200R to GND, 150R input resistor and 82R between op amps and diamond buffer.
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Old 25th January 2016, 06:48 AM   #136
Mooly is offline Mooly  United Kingdom
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"My" first design headamp, DC-servo question
When dealing with wide bandwidth parts its the layout as well as component values that plays a big part. For example wiring and PCB runs start to become inductive at high frequency and can actually form 'oscillators' in combination with the active devices on the PCB.

Its not easy to explain... a short piece of wire is anything but a short circuit at high frequency.
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Old 25th January 2016, 07:19 AM   #137
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"My" first design headamp, DC-servo question
Quote:
Originally Posted by Mooly View Post
When dealing with wide bandwidth parts its the layout as well as component values that plays a big part. For example wiring and PCB runs start to become inductive at high frequency and can actually form 'oscillators' in combination with the active devices on the PCB.

Its not easy to explain... a short piece of wire is anything but a short circuit at high frequency.
I get it =)
I'll try it, should it not work I'll try some other opamps (changing required parts accordingly).

Hmm, maybe swap signal wiring(perf board "traces") for thicker Cu wire might be a good idea? Used the roll closest (0.2mm) as I use that to easily create "traces" on perf board but I have 1mm thick wire as well.
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