Jumping into my first recap job on an old audio board I’ve had in the garage for a couple years. I’m trying to take the time to understand what I’m doing as best as I can but need some help.
From my reading I think I have a decent understanding of the function of coupling caps and how they work. Much of what I read about old pro-sumer audio boards is that their coupling caps were commonly too low a value by today’s standards and therefore cause the board to lack bass response. Calculating the values to use as a substitute is where things go over my head. I’ve found the formula I can use C=1/2(3.14*f*R) but that assumes I know (or can figure out) the impedance of the following circuit. I’m doing good to know where one circuit ends and another begins much less what the impedance is!
Are there go-to values or rules-of-thumb that someone with more experience can share and stop me from pulling out my hair? I read a suggestion about my same board: “these values are not bad, but because there are so many in cascade, I might double them all.” I can easily do that but it seems rather uncalculated and haphazard.
If I don’t want to filter out any audible low frequencies couldn’t I just choose a value that will set the cutoff frequency at well below 20hz and not stress the exactness of the value? I guess thats the attitude of the advice above. What's the downside of choosing too high a value? I’ve read a bit about phase shifting but my eyes started to cross. Is that related?
From my reading I think I have a decent understanding of the function of coupling caps and how they work. Much of what I read about old pro-sumer audio boards is that their coupling caps were commonly too low a value by today’s standards and therefore cause the board to lack bass response. Calculating the values to use as a substitute is where things go over my head. I’ve found the formula I can use C=1/2(3.14*f*R) but that assumes I know (or can figure out) the impedance of the following circuit. I’m doing good to know where one circuit ends and another begins much less what the impedance is!
Are there go-to values or rules-of-thumb that someone with more experience can share and stop me from pulling out my hair? I read a suggestion about my same board: “these values are not bad, but because there are so many in cascade, I might double them all.” I can easily do that but it seems rather uncalculated and haphazard.
If I don’t want to filter out any audible low frequencies couldn’t I just choose a value that will set the cutoff frequency at well below 20hz and not stress the exactness of the value? I guess thats the attitude of the advice above. What's the downside of choosing too high a value? I’ve read a bit about phase shifting but my eyes started to cross. Is that related?
This is all very circuit specific. You may end up with a condition know as motorboating which is a very low frequency oscillation at the output to the next stage. It will be much safer to use existing values, the performance of today's capacitors over ones from even 10-15 years ago will give an upgrade in itself. A bit more information about the specific item your working on may get a response from someone who knows the item or has worked on one.
No. The coupling cap value depends on the LF corner frequency you want and the input resistance of the next stage. Both are matters of circuit design/analysis. If in doubt, use the value already present or something near it.appleburger said:
Old circuits may have rolled off extreme LF. They probably had good reason to do this, as there is little point in amplifying signals which few speakers can reproduce and few people can hear, yet producing intermodulation which people can hear.
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