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Books, kits and accessories for the active ham


SCAF-1 active audio low pass filter by Idiom Press

Logikit SCAF-1 - the Deceptively Simple Filter

SCAF-1 photo

Tired of noise? Noise that isn't the signal you want to copy? White noise, line noise, hiss? And off frequency stuff you don't want to hear? Do you want to modernize your old rig without spending thousands of dollars? The new SCAF-1 filter from Idiom Press makes your radio listener friendly, whether you are an SSB or CW operator.

Modern IF filtering design is the best ever. But the engineer still has to design an audio output that will accommodate the widest signal the radio is designed to copy. Typically, this is 6 kHz. for wide band AM or for FM. The audio passband must be capable of passing such signals through. The designers have accomplished this beautifully - and as a result, when using narrower band modes, off frequency QRM can be passed through with brilliant fidelity. Even if you don't want to hear it.

And then there is white noise - noise created in the IF and the audio stages that can be loud enough to drive the operator up the wall, especially when you are trying to pull in weak signals. White noise is very fatiguing for any operator tuning the bands.

Click to go to SCAF-1 audio sample page

For the CW operator, these problems are particularly galling. When everything you want to copy is below 800 Hertz, why should you suffer QRM and receiver artifacts up to 6 kHz or more off frequency ? Even key clicks from the station you are working are annoying, not to mention QRN. And line noise will drive you to watching daytime TV.

Another problem is the headphones you use. Virtually all headphones available are designed for high fidelity performance - flat to 20,000 Hertz or more - and thus willing partners in passing though wide bandwidths with all the excess baggage. If it's there they deliver it all to your ears, with superb fidelity.

Several of the latest high-end radios actually deal with these problems, by tailoring the audio passband to match the communications bandwidth. Sadly, the vast majority of modern rigs and all older radios do not.

The new SCAF-1 filter from Idiom Press makes your radio listener friendly, whether you are an SSB or CW operator. Using a modern but little known technology called SCAF filtering, the SCAF-1 is an active audio low pass filter offering user control of the filter cut-off frequency, yielding a stunning 96dB per octave roll-off of signals above the cut off frequency, and no white noise. It is the perfect partner for your crystal or mechanical filters. And, unlike most audio filters, there is absolutely NO ringing.

How do you use the SCAF-1? There are two controls - the in/out switch, and the low-pass cut off tuning control. By turning the single front knob, you set the audio low pass filter cut off frequency anywhere you want it from 450 Hertz to 3.5 kHz . And that's it. Every signal or noise above the frequency you set is rolled off at 96 dB per octave. And it is done at audio frequencies - no white noise!

graph of SCAF-1 audio frequency response

What does "roll off" mean? It means that if you set the cut-off frequency for say 450 Hertz, any signal coming through to the input of the filter is 96 dB down at 900 Hertz. Set it for 1000 Hertz, signals at 2000 Hertz are down 96 dB.

Here is a response curve. Note that the measurement equipment used to make the curve had a calibration range accuracy of 60 dB so the curves do not go to their floor.

Why doesn't this filter use DSP? DSP can offer brilliant results in signal processing, but good ones require significant computing power, and are expensive. They can be complex to adjust, and may require retuning any time the received signal changes frequency. Also, they can offer real problems on CW with processing delays that destroy the CW monitoring function. Unlike most DSP audio filters offered, the SCAF-1 filter passes signals through in real time, for perfect CW monitoring and QSK, and with no recovery pause on SSB when going from transmit to receive.

Once you start using SCAF-1 you will never want your rig to be without it. SCAF-1 does for your audio stages what crystal or mechanical filters do for your IF stages. SCAF-1 has two output jacks (not switched), and will drive headphones or a speaker.

Kit or Pre-Wired -- Your Choice!

The Idiom Press SCAF-1 is offered as a kit or is available pre-wired. Kit construction is typically a one evening project. Everything needed is supplied except solder, hand tools and a soldering iron.

SCAF-1 Specifications:
Enclosure Size: 3 15/16" wide X 1 1/2" high by 5 1/16" deep (matches Logikit CMOS4 Keyer)
Controls: - In/Out switch, Cut-off frequency control
Power requirement: 11.5 - 14.5 volts DC, 300 mA, through 2.1 mm coaxial power plug. The plug is included with the SCAF-1.
Audio input: RCA Phono jack
Amplification of inband signal: adjustable, internal control, range exceeds minus 10 dB to +5 dB.
Low pass filter cut off frequency: variable from 450 Hertz to 3.5 kHz.
Attenuation above low pass filter cutoff: 96 dB per octave

Other Uses

A SCAF-1 user has cleverly modified his SCAF-1 filter to serve as a bandwith control for his AM signal. Click here for full details.

FAQ - Frequently Asked Questions

If the above information does not answer all of your questions, please take a look at our SCAF-1 Frequently Asked Questions page. If you still have questions, feel free to contact us.

Product Reviews

We know the SCAF-1 filter is great, but you don't have to take our word for it.  See what your fellow hams are saying about the SCAF-1 on the Product Reviews Web Site:

Also look at the CQ Magazine Product Review.

Also Available: Cable Sets

The HSA Cable Set is a set of two quality cables to interface the SCAF-1 with your rig. Included is a six foot cable with a molded 2.1 mm DC power plug, and a shielded audio cable that goes from the rig to the SCAF-1.  The audio cable has a molded RCA plug on the SCAF-1 end, and a fully-shielded metal 1/4 inch phone plug to connect to the rig's audio output (either the headphone jack or the external speaker jack).  $14.95 plus shipping, order below.

The HSB Cable Set is the same as the HSA Cable Set, except that the audio cable has a fully-shielded metal 3.5 mm (1/8") phone plug to connect to the rig.  $12.95 plus shipping.

Pricing and Ordering

Order Price Description US Shipping DX Shipping
 $ 89.95    SCAF-1 Kit   $7.00   $12.00 
 $ 134.95    SCAF-1, Assembled & Tested   $7.00   $12.00 
 $ 14.95    HSA Cable Set, 1/4 inch phone   $2.00    $4.00  
 $ 12.95    HSB Cable Set, 3.5 mm phone   $2.00    $4.00  

Troubleshooting and Help

For troubleshooting hints and help for the SCAF-1 filter, and for warranty and non-warranty repairs, see KØQVF's site at

Lastly, here's a YouTube video that a customer made that shows the construction of the SCAF-1 kit, and demonstrates the filter on the air.  The video was not solicited by us in any way.  Thank you Lynn NG9D!

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