Paul, You neglect the effect of the 8.2 Ohm resistor in series with the headphone outputs. With that resistor considered, the -3 dB point with 100 uF capacitors as a function of headphone impedance is:
Z Frequency ---------------------- 4 130 Hz 8 98 Hz 16 66 Hz 32 40 Hz 64 22 Hz 200 8 Hz 600 3 Hz Although 8 Ohm phones might be "marginal" for someone wanting maximum low end response, 98 Hz is probably suitable for most communications uses. One of the reasons I have not felt the need to change the coupling capacitors in my K3 (although I will be installing 100 uF caps when I make some other changes) is that the -3dB point is 80 Hz with the 200 Ohm transducers in my Heil ProSet. There is a noticeable difference compared to the 980 Hz -3 dB point when I use an old set of Yaesu headphones with their 8 Ohm transducers. The only reason I plan to change the capacitors is to avoid problems when I grab a cheap set of headphones or the rig is used by a visitor with their own headphones. 73, ... Joe, W4TV > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Paul > Christensen > Sent: Monday, July 06, 2009 11:23 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 ______________________________________________________________ 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
