Paul, > 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>
They appear at D3 (front) and B3 (rear) of the "K3 DSP IF: Interconnects and Miscellaneous" (pg. 32/52). I found them a while ago after Lyle mentioned their existence in another e-mail. > So, they must appear off-page along with the speaker > coupling cap that also does not show up on the same > schematic. The speaker coupling capacitors are 470 uF and appear on the K3 RF Board (1 of 4) - page 3/52 connected to pins 1/2 and 3/4 of J76 KIO3 (upper right corner). > 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. That's why I will change the caps when I next get into each of my radios. I don't have a problem because I primarily use the Heil. I will probably use 100 uF, but with leaded caps I will use what I can make fit comfortably. 73, ... Joe, W4TV > -----Original Message----- > From: Paul Christensen [mailto:[email protected]] > Sent: Monday, July 06, 2009 3:02 PM > To: [email protected]; 'Brett Howard' > Cc: [email protected] > Subject: Re: [Elecraft] K3 Audio Low Pass Filter > > > > 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
