++++++++++++++++++++++++++++++++++++++++++ Date: Thu, 21 Jun 2001 09:23:00 +0000 From: Marteinn Sverrisson Subject: [Elecraft] Simple Audio LP filter for K2 I built my K2 #2024, the SSB adapter, 2nd ant and noise blanker, no problems except I reversed one of the diodes in the SSB adapter. Have used the K2 mostly on SSB and it works well with a dynamic mic and a mic preamp ( the one on the Elecraft homepage) After doing the SSB filter mods I wanted to make a LP filter to improve the audio by removing out of band hiss. By using SPICE to model the circuit and filter I came up with the following solution: The filter is a passive LP filter which uses 2 standard 150mH chokes with (Ri=60 Ohm), 2 6.8nF caps and 2 820 Ohm resistors and does not require modification of the Control Board other than installing the sockets for the Audio filter and removing the jumpers R18 and R19. The circuit is simple, instead of the jumpers R18 and R19 a circuit consisting of 820 Ohm resistor in series with 150mH choke which is paralleled with 6.8nF capacitor is inserted for each jumper. J1-1 ---- 820 Ohm ----- 150mH ------- J2-1 | | --- 6.8nF --- J1-2 ---- 820 Ohm ----- 150mH ------- J2-2 | | --- 6.8nF --- The filter has a insertion loss of approx 3 dB and is flat to 2.2kHz. Using spectrum analysis software on my computer I found that the filter does a good job removing noise over 2.4kHz. Measured on the 10MHz band USB using atmospheric noise Freq level w.o. filter level with LP filter 1 kHz 0 dB (reference) 0 dB (reference) 2 kHz +3 dB +3 dB 2.4kHz -2 dB -5 dB 3.0kHz -35 dB -45 dB 4.0kHz -45 dB -60 dB 5.0kHz -50 dB <-65 dB (measurement noise level) The noise is reduced by 10 dB on 3 kHz and >15 dB on 4 kHz and above. 73 TF3MA http://www.raunvis.hi.is/~matti/tf3ma.html - -- Marteinn Sverrisson Internet: matti at raunvis.hi.is Science Institute http://www.raunvis.hi.is/~matti University of Iceland Phone: +354-525-4776 107 Reykjavik, Iceland Fax: +354-552-8911 +++++++++++++++++++++++++++++++++++++ Date: Thu, 21 Jun 2001 08:51:01 -0700 From: Wayne Burdick Subject: Re: [Elecraft] Simple Audio LP filter for K2; KAF2 info Hi Marteinn, Thanks for the info on your L-C filter design. Looks like a good solution for hiss reduction. A passive low-pass filter with similar characteristics is also a part of the upcoming KAF2 option for the K2. The preceding active circuitry (using low-noise op-amps) can be configured for 0 dB gain or higher with one resistor change. It also provides balanced to single-ended conversion (and vice-versa) to preserve the high common-mode noise immunity of the audio path. In addition to the LPF, the KAF2 includes a two-stage, four-poll active CW filter with a user-selectable passband and center frequency, as well as a battery-backed-up real time clock. The clock allows UTC display on the K2 LCD with a tap of the DISPLAY button. A prototype of the KAF2 is already running on my K2 and with any luck will be available later this summer. 73, Wayne N6KR Marteinn Sverrisson wrote: > After doing the SSB filter mods I wanted to make a LP filter to > improve the audio by removing out of band hiss.... +++++++++++++++++++++++++++++++++++++++++++++ Date: Thu, 5 Jul 2001 09:42:46 -0700 From: "Scott Prather" Subject: [Elecraft] Simple Audio Filter Mod for K2 Ever since I built the K2, one of my complaints with the unit has been what I would consider an excessive amount of high-frequency hiss in the receive audio. This is not unique to the K2, many of today's transceivers display the same problem. Recently, Marteinn Sverrisson posted an email message on this reflector describing a simple passive audio filter that he designed using SPICE modelling. One of the primary components in his design is a pair of 150 mH inductors. Inductors this large are hard to find, especially when they must be mounted inside a transceiver as small as the K2. The only two inductors I could find in the junk box that were even close to 150 mH were somewhat large, but I used them to build Marteinn's filter design. The filter does a very good job of "taming" the high frequency noise above 2.4 kHz, but I wanted an audio filtering solution that would be easy to install without trying to hunt down a pair of 150 mH inductors. I came up with a very simple solution that I've evaluated in the lab and used on the air over the past several days. It essentially eliminates the high-frequency hiss that's quite noticeable with headphones, especially when using narrow CW filters. It also takes the "edge" off of SSB audio. The filter has an insertion loss of 3 dB at 1000 Hz, and has the following roll-off characteristics relative to the stock K2: Freq. Response Notes 1000 Hz 0 dB Reference Frequency 1500 Hz -4 dB 2000 Hz -8 dB 2400 Hz -9 dB 3000 Hz -11 dB 3600 Hz -12 dB 4200 Hz -14 dB Unlike Marteinn's design, this filter is not even close to being flat up to 2.4 kHz. However, the 9 dB roll-off actually improves SSB audio considerably, as it removes much of the annoying high frequency components of the speech, noise, and static crashes. It also helps compensate for the adverse effects on SSB receive audio due to filter ripple. The modification consists of one component: a Radio Shack 1:1 audio isolation transformer, part # 273-1374. Instead of using this component as a coupling transformer, it's used as a pair of series inductors. It's not a particularly elegant solution, but it works well and only costs about $4.00. To install this filter, remove R18 and R19 from the control board. After clearing the holes of excess solder, connect the 273-1374 transformer leads as follows: White lead to J1 Pin 1 Red lead to J1 Pin 2 Black lead to J2 Pin 1 Yellow lead to J2 Pin 2 Using the connections noted above will assure that the phase of the two windings is correct. The transformer can be attached to the control board in a number of ways. I added a 1" standoff to the hole in the PCB between C28 and C29, and attached the transformer to this standoff with a wire tie. If the user would like to bypass the filter, just add a SPDT switch across R18 and R19 to short out the transformer when audio filtering is not required. However, I found that I prefer to have the filter in at all times. I also found that altering my receive SSB BFO settings slightly with the filter in line can improve intelligibility over the stock K2, and greatly reduce listening fatigue in the process. If anyone else tries out this modification, please let me know how well it works for you. Scott N7NB +++++++++++++++++++++++++++++++++++ Date: Thu, 05 Jul 2001 13:06:03 -0700 From: Wayne Burdick Subject: Re: [Elecraft] Simple Audio Filter Mod for K2 Scott, Thanks for your novel audio filter idea. I hope a few K2 builders will give this a try and compare results. Note that our KAF2 audio filter option (currently awainting prototype PCB fabrication) also includes a passive, balanced low-pass SSB filter, which is quite flat in the passband and rolls off quickly above 2.5 kHz. It uses two small, shielded 68 mH inductors in an elliptic configuration. There's also a DPDT in/out switch provided on the board for comparison and test purposes. Insertion loss is normally 0 dB for the LPF, but the user can alter the gain as desired with one resistor change. Other KAF2 features were described in a earlier posting (CW 2-stage tunable BPF, real-time clock). 73, Wayne N6KR ++++++++++++++++++++++++++++++++++ Date: Mon, 17 Sep 2001 19:35:05 -0400 From: John To: "'qrp-l at Lehigh.EDU'" Subject: [107184] Passive Audio Filter Message-ID: <211812926A10D4119B3200902734A6402CD8B2 at EXCH> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Back in the late 70's, there was a passive audio filter article in QST written by Frank Noble, W3MT. It was a simple inductor and capacitor in series between the receiver audio output and the headphones. It eliminated all traces of hiss and hum, and could be made more selective by putting a low-value resistor (8 to 100 Ohms) across the output (headphone side). I recently built a small version of the unit using very small components. The inductor is a 15 mH Toko coil (Digikey P/N TK4326-ND, $2.05). The capacitor is a 3.3 uF non-polarized Panasonic unit (Digikey P/N P1174-ND, $0.37). Digikey charges a $5 handling fee if you don't make the minmum of $25, and then there is shipping. But all hiss and hum from my Ten Tec Argosy disappeared with this filter when I use my sensitive Jensen headphones (which were free with rebate from Staples). I tie-wrapped the capacitor and inductor around the shell of a 3/8-inch stereo jack (the headphone connection). I originally wanted to build it into the shell of the jack or plug, but it didn't quite fit. I use a 3/4-inch stereo plug on the other end to match either the Ten Tec, OHR, or Kenwood rigs that I have. The center frequency is between 700 Hz and 750 Hz, but the filter is very broad, and SSB copy is still possible. A low-value resistor on the output (across the headphone connections) would sharpen it up and make a more pronounced peak. With a 15 mH coil, a 2 uF capacitor resonates at about 900 Hz, a 3 uF gives 750 Hz, and a 4 uF gives 600 Hz. Without the output resistor, the bandwidth is so wide that it's hard to hear the peak. But, all traces of hiss and hum are gone. Regards, John W2XS Telebyte Broadband 270 Pulaski Road Greenlawn, NY 11740-1616 Phone: 631-423-3232 x416 Toll-free: 800-835-3298 jmeade at telebytebroadband.com +++++++++++++++++++