> You neglect the effect of the 8.2 Ohm resistor in series with > the headphone outputs.
Joe, you're right. I did not take into consideration the 8-ohm build-out resistance that Lyle just discussed. Those build-out resistors do not appear on the headphone amp schematics (page 28/52) so until today, I doubt anyone else knew they existed <g>. So, they must appear off-page along with the speaker coupling cap that also does not show up on the same schematic. Older headphones with low Z drivers would still benefit from a value above 100 uF. But with most modern headphones in the 32-ohm area, and I'll repeat...the majority of users would not benefit from higher C values above 100 uF at C9/C13. My choice of going to 330 uF was primarily driven by the fact that I didn't want to be concerned with the operating Z of a new set of headphones if tried, and I had an older DSP Board wiith 10 uF caps. Since re-work was required anyway, I settled on the 330 uF value and would still choose that value if given the chance today. Although not as critical for communications audio, the use of increasing the build-out resistance does in fact lower the damping factor of the amplifier. Not as important with newer headphones, but it can be a factor with older low-Z headphones with traditional moving coil drivers where more mass is moved. Paul, W9AC 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
