dbelousov

Strange servo
« on: April 03, 2021, 10:07:46 AM »
There is a schematic. Instead of using resistors in the 100k+ range and small caps, it uses small resistors and "quasi-bipolar" lytics. C115 is still here, but isn't it redundant in this case? IC101 is TL072, so it's not bipolar.

I have a temptation to replace R149 with something like 300k, R144/R146 with links, the lytics with a 0.47-0.68 film cap, and short C115, so it will be a "conventional" servo. There is enough space for a 10uF mylar cap on the board, but my limited knowledge of audio amp design doesn't provide me any reason why I should use such a part instead of making it a "normal servo" with a smaller cap. There will be enough space even for PP in this case for the smoothest LF roll-off possible. I'm making a full recap of this unit now anyway, so why not?
Dmitry


JohnRoberts

Re: Strange servo
« Reply #1 on: April 03, 2021, 01:41:16 PM »
There is a schematic. Instead of using resistors in the 100k+ range and small caps, it uses small resistors and "quasi-bipolar" lytics. C115 is still here, but isn't it redundant in this case? IC101 is TL072, so it's not bipolar.

I have a temptation to replace R149 with something like 300k, R144/R146 with links, the lytics with a 0.47-0.68 film cap, and short C115, so it will be a "conventional" servo. There is enough space for a 10uF mylar cap on the board, but my limited knowledge of audio amp design doesn't provide me any reason why I should use such a part instead of making it a "normal servo" with a smaller cap. There will be enough space even for PP in this case for the smoothest LF roll-off possible. I'm making a full recap of this unit now anyway, so why not?
Yup, unclear what they are trying to accomplish other than being able to say there is a "servo" in it.

There are enough electrolytic capacitors elsewhere in the audio path that I would lower my expectations. This is also ASSuming that the caps are the weakest link in that audio path to improve.

It will be an interesting mental exercise. A TL072 can certainly support higher impedances, for a proper servo using a film cap, while not with great DC precision.

JR 
Cancel the "cancel culture", do not participate in mob hatred.

dbelousov

Re: Strange servo
« Reply #2 on: April 03, 2021, 04:05:28 PM »
Thank you.

Searching for its service manual, I stumbled across schematics for the current iteration of this series, Servo 200/300/600. There is no servo at all. Literally. It's still called "Servo", though, but without servos.

The only possible reason I could imagine is that its designers aimed to make it work with any op-amp available in their closet. TL072 shortages or whatever :) I don't mind replacing them. I have a stash of the OPA1656, modern TI FET input uber op-amps, so I'll give them a try, they are quite precise parts.

Dmitry


 

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