[Elecraft] K3 Audio Low Pass Filter
Joe Subich, W4TV
lists at subich.com
Mon Jul 6 15:57:54 EDT 2009
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:w9ac at arrl.net]
> Sent: Monday, July 06, 2009 3:02 PM
> To: lists at subich.com; 'Brett Howard'
> Cc: Elecraft at mailman.qth.net
> 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: elecraft-bounces at mailman.qth.net
> >> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul
> >> Christensen
> >> Sent: Monday, July 06, 2009 11:23 AM
> >> To: Brett Howard
> >> Cc: Elecraft at mailman.qth.net
> >> 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
> >>
> >>
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