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Why specifically a polyester capacitor in the feedback of this H-bridge driver?
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Icchan:
I'm refurbishing/repairing older Yamaha DM1000 mixing console and while I was at it, I noticed that they've used specifically a Polyester (or as they say in the Japanese repair manual: Mylar) capacitors for the dc-blocking in the feedback loop of the operational amplifiers that are driving the H_bridge of the DC_motors of the moving faders.

That somehow caught my eye and I can't come up with any specific reason why they'd choose relatively expensive and large polyester capacitor instead of basic ceramic?

The service manual can be found here: https://www.manualslib.com/manual/896121/Yamaha-Dm1000.html

And on the PDF page 266 you can see the motorized fader driving circuits.

Here's a snip from one of them:

So I was thinking, if I ask from the community, this might be a very good learning opportunity :)

So why do you think they chose specifically a polyester capacitor for this location?
KE5FX:
Given that it's audio equipment, they may have wanted to avoid acoustic noise from a ceramic capacitor. 
Gyro:
47nF isn't big for a film capacitor, and polyester is the cheapest of the film types. Maybe they wanted closer tolerance than a ceramic, or maybe just habit - an audio company would be used to fitting film capacitors in opamp feedback loops so they may have just done it by default. It may even be that 47n polyester was a company stock item and 47n ceramic wasn't.
TimFox:
Assuming these are all TH (leaded) parts, I doubt the polyester/Mylar capacitor was more expensive than an X7R ceramic at the time of manufacture.
Even though this is not in the signal path for the audio, the polyester capacitor will not suffer from the voltage co-efficient of X7R (or worse) ceramic dielectrics.
A quick look for current-production 0.047 uF leaded parts in Mouser (at quantity 1) shows PE film at roughly $0.32 ea and X7R MLCC leaded at $0.35 to $0.40.
Icchan:

--- Quote from: Gyro on September 28, 2023, 09:11:12 pm ---...maybe just habit... maybe 47n polyester was a company stock item and 47n ceramic wasn't....

--- End quote ---

As boring of an explanation might be, it could be true :D
I was just wondering exactly because it's not in audio path what so ever and since these are SMD-parts, these are somewhat large.. though all the boards are HUGE in this device anyway so there might not be any reason to skimp on size.

It was just so specifically stated in the schematics, that these are polyester, it felt weird.
We'll see if anyone comes up with some insight as to why or is the most boring explanation the correct one :)
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