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Author Topic: Audio Filter for CW.  (Read 8803 times)
KA0HVE
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Posts: 117




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« on: July 08, 2013, 07:34:34 AM »

I put up a station 2 weeks ago.  Strictly CW.  My rig has 3 levels of filtering built in with the narrowest being 500 Hz.

I got out my old Bencher audio filter last night and hooked it up.  Amazing!

That thing seems to pluck a signal out of nowhere and make it highly usable.

I just returned to the hobby after a 20 year hiatus.  How popular are accessory audio filters these days?
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K8AXW
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« Reply #1 on: July 08, 2013, 08:52:53 AM »

For those who own the older transceivers an outboard filter is a great accessory.  As for "how popular?"  I've never seen a poll taken on that question but would suspect that anyone who has an older rig would have least considered one.

The Idiom Press SCAFF1 website provideds audio comparsons between their filter and no filter.  As you said, "amazing!"
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KR4TH
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« Reply #2 on: July 09, 2013, 07:50:10 AM »

I purchased recently an autek audio filter on ebay for my FT 757, what a difference.  It makes an old receiver very usable.  I turn off the preamp, and reduce the rf gain to half and the filer does its job.  I use it on cw only and it is arm chair copy on many weak signals that would strain to copy without the filter.
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W1JKA
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« Reply #3 on: July 09, 2013, 08:10:32 AM »

I use an old Autek with my old FRG 7700 rcvr, on cw I prefer it to DSP on my IC 7200.The cheap Hypermite audio filters are often used by QRP'ers on the small mono band QRP rigs.
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K8AXW
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« Reply #4 on: July 09, 2013, 09:37:56 AM »

JKA:  The Hi-permite filter you suggested jumped out at me.  Have you or does any other viewer here tried this little unit and how do you like/dislike it?  I would like to try one of these, especially considering they cost only $20 including shipping! 

The SCAF-1 is pretty expensive although it seems to do a good job.  (As per their videos)

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W1JKA
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« Reply #5 on: July 09, 2013, 10:39:00 AM »

Re:KA8XW

I have a Hi-per-mite for outboard use with my MFJ Cubs and one inside my SWL+30 to also enhance the audio amplifier circuit. Both these rigs have sharp tuning so I  find the main advantage is a very noticeable decrease in background/white/or other QRN noise which enables me to hear/work other stations I would have missed without the filter.I also built the NEScaf filter(also relatively cheap) to occasionally use outboard with my K-1.Although the K1 has 3 very nice filter options in it I find the NEScaf does the same as the Hi-Per-Mite cutting down noise/qrn from seasonal propagation or antenna noise from dew, ice snow conditions. For the price the Hi-Per-Mite is a very nice unit that actually works for the small mono band qrp rigs plus fun and easy to build.
« Last Edit: July 09, 2013, 11:36:44 AM by W1JKA » Logged
K0OD
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« Reply #6 on: July 09, 2013, 11:53:17 AM »

I use an old Timewave DSP-9 audio filter with my OHR100A QRP radio. The Timewave is rated at 100Hz at the narrowest with an excellent shape factor. The bare OHR-100a is many times wider. You can find such filters cheaply on Ebay. Voice ops tend to buy them for their claimed noise reduction benefits and then find out they're more trouble than they're worth.

Years ago 500 Hz was the defacto standard for CW selectivity. I use the 250 Hz setting for most tuning with my Flex-5000, and 100Hz in contests on crowded bands like 40.

Hipermites have a good reputation. Fun simple kits. I'm not sure NEScafs are still available from that QRP club that put them out.

But you asked how popular they are nowadays: Probably less popular than 10-20 years ago when the outboard DSP units were the rage.   
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N3HEE
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« Reply #7 on: July 10, 2013, 05:48:27 AM »

I use a passive audio filter by W3NQN on my older gear.  Amazing filter!  Fun kit to build.  http://w3nqn.com/
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W8JX
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« Reply #8 on: July 10, 2013, 08:38:41 AM »

I put up a station 2 weeks ago.  Strictly CW.  My rig has 3 levels of filtering built in with the narrowest being 500 Hz.

What rig I am curious.
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KA0HVE
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« Reply #9 on: July 10, 2013, 12:39:00 PM »

Ten-Tec Century 21.   Grin
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AD9DX
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« Reply #10 on: July 10, 2013, 05:47:18 PM »

I have a Scaf-1 and when I used my FT-2000d I loved the thing. I still have it inline, but rarely use it anymore.
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EX, KC9TRM, KB9IRZ
AD9DX
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« Reply #11 on: July 10, 2013, 05:48:43 PM »

Ten-Tec Century 21.   Grin

With that rig, using said filter would be a huge improvement
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EX, KC9TRM, KB9IRZ
KA0HVE
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Posts: 117




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« Reply #12 on: July 11, 2013, 08:45:53 AM »

On 40-meters at night I find an audio filter to be a necessity for fainter signals.  It knocks out the static and on the 90 Hz setting you hear the CW well above the noise.  But if either rig drifts!?!?!?   Grin
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KB1WSY
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« Reply #13 on: July 11, 2013, 09:20:11 AM »

I'm planning to add a CW audio filter to my "Novice" station vintage 1968. For extra fun, the technology and parts must have been available at that time in history.

I'm intrigued by the "SelectoJet" project, which was presented in a two-tube version in the 1963 ARRL Handbook and in a five-transistor version in the 1968 Handbook. There are no inductors. The majic is performed with phase shifters. Both versions let you adjust (1) the audio frequency to filter, (2) whether to operate it as a sharp amplifier or as a notch (rejection) filter and (3) the selectivity. On paper it looks ideal.

Does anyone have advice on the effectiveness of this method of filtering? Perhaps one of you built these little gadgets, yea, those many years ago?

73 de Martin, KB1WSY
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WA9FZB
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Posts: 171




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« Reply #14 on: July 12, 2013, 01:00:12 PM »

Not sure what novice rig you are setting up, but when I was a novice (1962), the best we could do was to add a Q-multiplier to any receiver having a 455 Kc (wasn't KHz yet) IF.  The Q-multipliers really worked well.  I had one that was built into my first receiver (a Knight-Kit R-100) and actually added one to a Drake 2-B.  Not many of us could afford "real" filters, but used Heathkit QF-1's were going for $15 or so.
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