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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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