sailor wrote:Jim, I never said 12 HZ. was the -3db frequency. I took the number you came up with 5.89hz and moved it up one octave and rounded it up to 12 Hz. The exact answer is 11.78 HZ a .2 cap would be 5.89 for flat response.
Thank you for clarifying - it wasn't clear from your prior post (or I simply misunderstood your statement), so I wanted to clarify it in mine.
Also - a single C-R section rolls off the signal at a rate of -6db per octave, so if the "flat" frequency - the beginning of the rolloff - was 12 Hz, then the signal would be down 6db
(not 3db) at 6 Hz, 6 Hz being one octave below 12Hz. If the rolloff started at 12Hz the -3db
point would be about 8.5 Hz.
Why make this more difficult than it needs to be.
If I may... why be so defensive? I was only trying to help by sharing some useful info. I'm sorry if you felt I was calling you out for some reason - that was certainly not my intent. I just wanted to share some things I hoped would be interesting and worthwhile with all the forum readers.
But it IS
more complex than it first appears. For instance, what if the original designer used a smaller cap value to limit the low frequency energy to an output trafo that can't handle a lot of deep bass? Or in an amp with more than two stages, maybe the amp won't be stable unless the two poles (another term for corner frequency) are staggered a good ways apart in their -3db frequencies? One rule of amp design is that the farther the multiple poles are apart the more stable (all else being equal) the amp. Often that requires some compromise to get adequate spacing to ensure stability.
A cap blocks AC the same way if it is in the circuit of an amp or in a speaker.
I think you meant DC, not AC, since a cap will pass an AC signal but block DC.
In any case, as long as the cap is part of an identical network you've pretty much got it. There are some differences due to the inductive nature of the load seen by the C-R circuit in a speaker crossover network, but for our purposes here we can ignore that for now.
No one would ever say I need my tweeter to roll off at a certain frequency which say requires a 4 mfd cap but I think it would sound better if I changed it to a 8 MFD cap.
Actually that sort of thing happens all the time - not only the frequency but the rate of the rolloff has to be carefully considered when you work with a tweeter. Remember, a 1st order (single C-R section) high pass filter only rolls off at 6db per octave. So the tweeter will see significant energy a full octave lower than the frequency at which the rolloff begins. The tweeter may be fine at 3K - but an octave lower (at 1.5K) it could have performance and/or durability problems. There are also phase issues involved.
I think most people, including myself up until a year ago, have installed larger capacitors than necessary because most people don't know what happens when you increase the MFD value of a cap. After all bigger must be better.
There was a time I did too!
The site above makes it simple to understand and easy to figure for everyone.
Yes, the site calculator does the math for you very well. But the recommendation for determining the low frequency corner (1/10 the lowest frequency of interest) might well cause trouble in a variety of ways. So use the calculator, but I would recommend you not use the "1/10th" recommendation.
I also use calculators on the internet to build crossovers, to figure Ohms law and a dozen other kinds of electronic problems.
That's great, the web is FULL of useful tools like that. But use the results carefully to get the best results.
Incidentally, if you want to read a NICE
piece explaining blocking distortion, amp designer Randall Aiken has a great page on it here:
Blocking Distortion Explained