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Why specifically a polyester capacitor in the feedback of this H-bridge driver?

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TimFox:

--- Quote from: MathWizard on September 29, 2023, 06:49:52 pm ---I have lots of sub-1uF green mylar caps, the kind rated over 100V. Next time I build an oscillator, I'll have to compare them to ceramic cap's.

I hardly have any variety of cap types, and most are just cheap ebay ones.

--- End quote ---

Polyester/Mylar capacitors will compare well against X7R and other Class-II dielectrics, but are inferior to NP0/C0G Class-I dielectrics.
However, 0.1 to 1 uF C0Gs are not to be found:  Kyocera quotes a range of  0.5 pF to 0.1 uF for surface-mount C0G devices.
Polypropylene film capacitors compare well to NP0/C0G and are available well past 1 uF (although large).

floobydust:

--- Quote from: Icchan on September 28, 2023, 08:54:44 pm ---[...] So why do you think they chose specifically a polyester capacitor for this location?

--- End quote ---

Many times it's just because the part is already used on the board somewhere else and it's easier to use it again. Fewer unique parts on a BoM, the better.

TimFox:
I remember addenda and updates to -hp- manuals where it was obvious that a decree had come down to reduce the number of discrete resistor values required in a BOM, to simplify production.

tom66:
I second/third the statement about microphonic capacitors.  If I'm reading the circuit right you have FT800 possibly acting as a muting transistor and two channels summed in via R809/810.  So we can ignore the FET for now, the gain of the circuit is (with R827 @ 160k) around 80x. This would make it particularly sensitive to microphonic effects. Any current injected into the virtual ground via the capacitor would need to be offset by the amplifier causing a large output voltage.  The film cap eliminates this risk, and it's a mixing console, so manufacturers are going to want to consider this as sensitive ears might notice effects like that.

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