Isn't the equation 1/2*pi*r*c? If I do the math on my headphones at 27 ohms (a little worse than your 32 ohm example) I get:
1/(2*3.14159*(8.2+27)*100e-6) = 45.2145Hz The 8.2 ohms is actually in the K3. If I do it again for your 32 ohm example I get 39Hz. With your 220uF cap that moves it down to ~18Hz for a pair of 32ohm cans. Personally I think the more robust connection is a good point to keep and I'm ok with the 45Hz output. I tried running things through my headphone amp which has an 18KOhm input impedance (which should give a -3dB point of 0.883Hz even with the 10uF caps) and I wasn't hearing stuff down too low. I'd have to dump it into a spectrum analyzer to see how low it really goes and I'm not so sure I have too many sound cards that are all that clean down there. Guess I'd have to go looking... ~Brett -----Original Message----- From: Paul Christensen [mailto:[email protected]] Sent: Monday, July 06, 2009 8:23 AM To: Brett Howard Cc: [email protected] Subject: Re: [Elecraft] K3 Audio Low Pass Filter > I've been informed Elecraft is already replacing those caps with 100uF > caps in the current production units. 100 uF is the maximum size compatible with the existing PCB pads for C9/C13 on the main DSP Board. Moving up beyond 100 uF presents a problem. Even at 100 uF, I believe the SMT caps are now rated at close to 10 WVDC. There's just no more scaling back on the SMT cap's voltage rating in order to accommodate a fit with a higher C value above 100 uF. Why would anyone want a higher value than 100 uF at C9/C13? Consider a set of modern, medium-Z headphones with a Z of 32 ohms at 100 Hz. The -3dB turnover point calculates to roughly 150 Hz. Anyone wanting to take full advantage of the lowest vocal fundamental (e.g., AM or ESSB modes) wants that point down to ~ 70 Hz. So, a cap value of 220 uF would be preferred. I went to 330 uF in a radial style to allow extra margin for a wide range of headphone vintages and types. To make most use of the 100 uF coupling caps, I would ensure that my headphones are of the newer, medium Z type and not the older style with 4-8 ohm drivers. My guess is that scaling the caps beyond 100 uF is only of interest to a small minority of ops. The majority will probably be happy with the current choice of 100 uF. Going from 10 uF in the prior design to 100 uF is already a significant step in the right direction. Paul, W9AC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
