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On the schematic below, capacitors C7 and C8 (highlighted in yellow) are 4uF and 2uF, respectively. What function / purpose do these caps serve? It appears to me as a high pass filter. I am trying to determine how these 2 caps might impact the sound and thus, how important or not is it to get high quality cap replacements.I am upgrading them and would like suggestions on replacement alternatives.
Thanks,
Pat
Edits: 04/09/14Follow Ups:
The circuit is obviously an older design using a floating or self balancing paraphase phase inverter. The designer chose to use a very low cutoff freq for the phase inverting triode that requires a large value coupling cap. In the old days, caps of the nec value were either leaky electrolytics or somewhat less leaky paper types. What you see in the schematic is a trick used to mitigate the leakage issues while still maintaining an effectively large value coupling cap. While the highlighted portion will act as a form of a HP filter, that effect is incidental to the primary purpose of effectively producing low leakage coupling using inherently leaky parts.
The easy refurb approach would be to replace the caps with = values but of the greatest sonic neutrality...you be the judge of neutrality. I'd be looking a high quality PP at a min.
If you're more adventurous, you can model the circuit to determine the composite response and design a circuit using only 1 cap and 1 resistor that mimics it. This is how the circuit would be implemented today with modern, low leakage caps.
While technically a high-pass filter, they are essentially a coupling capacitor driving the grid of the phase-inverter triode. C8 has a strong DC bias voltage (approx 120 V), but C7 seems to be zero-biased. It should be easy to find film capacitors of equal value with sufficient voltage rating.
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