700 Hz 8-Pole K3 Filter by INRAD

filter installed

5 May 2016 – The lat­est batch has al­ready sold out!  At this point there’s no tell­ing when or if there will be a next batch.  If there is, there will be at least a 14-week man­u­fac­tur­ing lead-time – from the date we com­mis­sion pro­duc­tion of the next batch – un­til your fil­ter(s) ship to you.  Please email us if you wish to be added to our waiting list for a possible next batch.

If you have a sub-re­ceiv­er or you are con­sid­er­ing get­ting one, then please note that the cen­ter fre­quen­cy de­vi­a­tion of in­di­vid­u­al man­u­fac­tur­ing lots can vary by up to ±100 Hz.  In con­trast, the var­i­a­tion across any giv­en lot is typ­i­cal­ly so small as to be in­dis­tin­guish­a­ble on a net­work an­a­lyz­er.  The point be­ing, that get­ting two fil­ters from the same man­u­fac­tur­ing lot, might be your best bet for a matched pair for di­ver­si­ty re­cep­tion!  [LEARN MORE][LEARN MORE]

“… some folks do not ap­pre­ci­ate the ad­van­tage of a mod­er­ate width fil­ter – they have been bit by the ‘nar­row­er is bet­ter’ myth ad­vo­cat­ed by many – nar­row is bet­ter for ex­treme con­test­ing, but for more ca­su­al op­er­at­ing a mid-width is the best IMHO …”

-- W3FPR


Un­less you have a pan­a­dapt­er, you’ll nev­er work sta­tions – es­pe­cial­ly weak ones – if you can’t hear them be­cause they’re out­side your pass­band!  (This is the ad­van­tage of 700 Hz, es­pe­cial­ly for ra­pid scan­ning!)  In fact, many sea­soned Ele­craft® K3 own­ers feel that us­ing the ra­dio’s 400 Hz default (NORM) CW pass­band set­ting – is kind of like “view­ing the world thru a drink­ing straw … while rid­ing a bi­cy­cle”.  That is, it would be fair­ly easy to skip right past a weak call­ing sta­tion with­out ev­er re­al­iz­ing it was there.

But even more im­por­tant­ly, pro­longed scan­ning with a 400 Hz pass­band can be te­di­ous and fa­tigu­ing.  This is why, band con­di­tions per­mit­ting, sav­vy op­er­a­tors sav­vy op­er­a­tors pre­fer a 700 Hz pass­band …

Focal Length Analogy Slideshow Presentation

Build­ing on our “drink­ing straw” analo­gy, N1LQ chal­lenged us to cre­ate a slide­show pres­en­ta­tion that would quan­ti­ta­tive­ly il­lus­trate the five INRAD CW band­width choi­ces.  To do this, he pro­posed lik­en­ing pass­band width to the fo­cal length of a cam­er­a lens; or more pre­cise­ly, to the re­sult­ing (two-di­men­sion­al) an­gu­lar field of view for a giv­en fo­cal length.

As you can see be­low – for just the three wid­est CW band­widths – his sug­ges­tion pro­duc­es a re­mark­a­bly in­tu­i­tive vi­su­al com­par­i­son, clear­ly dem­on­strat­ing the con­text each width al­lows …

Saturn thumbnails
1000 Hz 700 Hz 500 Hz
“excessive” “comfortable” “tight”

If you’re still won­der­ing “What’s so spe­cial about 700 Hz?” then please check out our com­pre­hen­sive an­swer to this most fre­quent­ly asked ques­tion.  [LEARN MORE][LEARN MORE]


Rework Eliminator™ Option Ready K2 Accessory Kits

"Option Ready" Stock K2
Assembled K2 Internal Mic Adaptor
Option Bypass Headers
Internal Mic Adaptor

Our K2 op­tion by­pass head­ers are ideal for trou­ble­shoot­ing and tin­ker­ing.  These de­vic­es sim­pli­fy the task of re­con­fig­ur­ing the Ele­craft® K2 to op­er­ate up­on re­mov­al of their as­so­ci­at­ed op­tion [read what oth­ers have said]  [place an or­der]

Con­verse­ly, they al­so make it pos­si­ble to build a K2 that’s Op­tion Ready from the out­set; when used in con­junc­tion with our re­work-free as­sem­bly method This ap­proach yields a ra­dio that’s much eas­i­er to up­grade with op­tions, and po­ten­tial­ly more re­li­a­ble[read what oth­ers have said]  [down­load doc­u­men­ta­tion]


We al­so of­fer “Du­pli­cate K2 PCB Parts” kits for build­ing a stock K2.  These parts en­a­ble you to ful­ly pop­u­late the K2’s three main cir­cuit boards right from the start.  These kits al­so have the parts you would need to add a sec­ond-hand op­tion to a “tra­di­tion­al” K2, i.e. one that isn’t “Op­tion Ready”.  [place an or­der]


Our K2 in­ter­nal mic adapt­or is an en­gi­neer­ing mar­vel marvel Check out this PCB scan or these three ul­tra-high-res­o­lu­tion PCB ren­der­ings to see for your­self. 

Once in­stalled, this clev­er de­vice can be eas­i­ly ac­cessed by re­mov­ing the K2’s left side pan­el.  And then re­wir­ing the mi­cro­phone jack or re­con­fig­ur­ing ex­ter­nal bi­as, be­comes a sim­ple mat­ter of re­arrang­ing some jump­ers jumpers

The kit fea­tures 30 mi­cro-inch gold-plat­ed con­tact sur­fac­es, which are ra­ted for a min­i­mum of 100 in­ser­tion/re­mov­al cy­cles.  This should be suf­fi­cient to en­sure years of re­li­a­ble op­er­a­tion un­der nor­mal use.

A self-ad­he­sive de­cal is in­clud­ed for handy ref­er­ence.  The com­pre­hen­sive PDF us­er’s guide pro­vides a de­tailed ex­pla­na­tion of the adap­tor’s “Sig­nal Rout­ing” and “Ex­ter­nal Bi­as” cir­cuits, and shows how to con­fig­ure them to sup­port nu­mer­ous pop­u­lar 8-pin Fos­ter mi­cro­phones.  [read what oth­ers have said]  [place an order]

Side-by-Side Comparisons
To Add All K2 Options To Change Microphones
Stock K2 Option Ready SSB K2 SSB K2 + IMA
  • Re­move the side pan­els and top cov­er.
  • Re­move the Con­trol board.
  • Re­move and dis­as­sem­ble the Front Pan­el.
  • Re­move the bot­tom cov­er and low­er-rear-pan­el/heat­sink.
  • Re­move the top cov­er.
  • Re­move the top cov­er.
  • Re­move the op­tion­al KAF2 (An­a­log Au­dio Fil­ter) or KDSP2 (Ad­vanced Dig­i­tal Fil­ter) if ei­ther is in­stalled.
  • Re­move the front pan­el sub-as­sem­bly.
  • Re­move the left side pan­el.
  • Un­sol­der/re­move two jump­er wires from the bot­tom of the Con­trol board.
  • Un­sol­der/re­move:  five jump­ers, three re­sis­tors, and two caps from the top of the RF board.
  • Un­sol­der/re­move:  one cap from the bot­tom of the RF board.
  • In­stall:  five caps; one re­sis­tor; one re­sis­tor net­work; and one tran­sis­tor – on the top of the Front Pan­el board.
  • In­stall one DIP con­nect­or on the bot­tom of the Front Pan­el board.
  • In­stall two SIP con­nect­ors on the top of the Con­trol board.
  • In­stall:  sev­en SIP con­nect­ors; four caps; two di­odes; two plugs; and one re­sis­tor – on the top of the RF board.
  • In­stall four stand­offs on the RF board.
  • Un­plug the op­tion by­pass head­ers.
  • If there’s a bi­as re­sis­tor tack sol­dered to the back of the Front Pan­el cir­cuit board, then un­sol­der it.
  • If the new con­fig­u­ra­tion re­quires a bi­as, then tack sol­der the re­quired re­sis­tor on the back of the Front Pan­el cir­cuit board (af­ter de­ter­min­ing which two pins of the mic plug to sol­der it be­tween).
  • Re­move any ex­ist­ing con­fig­u­ra­tion ap­pa­rat­us from P1 (the Mic Con­fig­u­ra­tion Head­er) on the back of the Front pan­el board.
  • Con­fig­ure P1 for the new pin-out.
  • Us­ing small nee­dle-nose pli­ers, re­arrange the jump­ers on the 3×7 jump­er block.
  • In­stall the op­tions.
  • In­stall the op­tions.
  • In­stall the bot­tom cov­er and low­er-rear-pan­el/heat­sink.
  • Re­as­sem­ble and in­stall the Front Pan­el.
  • Re­in­stall the Con­trol board.
  • Re­in­stall the side pan­els and top cov­er.
  • Re­in­stall the top cov­er.
  • Re­at­tach the front pan­el sub-as­sem­bly.
  • Re­in­stall KAF2 (An­a­log Au­dio Fil­ter) or KDSP2 (Ad­vanced Dig­i­tal Fil­ter) if ei­ther had been in­stalled.
  • Re­in­stall the top cov­er.
  • Re­in­stall the left side pan­el.
This is why our customers love our Rework Eliminator™ K2 accessories, and our alternate K2 assembly method.
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Updated 2158 UTC Thursday 5 MAY 2016
Rework Eliminator, Rework Eliminators, and unpcbs are trademarks of Ken Kaplan and Gary Hvizdak;
whereas our logo is just an alias for “unpcbs™.com”.  Copyright © 2005 – 2018.  All rights reserved.