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Reviews Categories | Filters, Audio: (DSP and others) | MFJ-722 Help

Reviews Summary for MFJ-722
MFJ-722 Reviews: 1 Average rating: 4.0/5 MSRP: $79.95
Description: SSB/CW Audio Filter
Product is in production.
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K5MH Rating: 4/5 Mar 18, 2004 22:06 Send this review to a friend
It's not magical but it might improve your receiver.  Time owned: 3 to 6 months
The attribute that attracted me to the MFJ-722, apart from its relatively low price, was its apparent simplicity of operation. This analog audio filter allows the operator to choose one of nine selectivity options (including filter bypass) via a single rotary switch. A tunable notch filter is also included.
My Icom IC-718 transceiver already had DSP noise reduction and automatic notch filter (ANF) capabilities, as well as an INRAD 400 Hz i-f filter. What it lacked was an i-f filter falling between the 400 Hz filter and the multipurpose 2.4 kHz filter. I hoped that adding selectable filtering to the audio output would compensate, to some degree, for the absence of additional i-f filters.
The MFJ-722 does improve reception. The primary benefit of the added audio selectivity is noise reduction. Often the MFJ filter is more effective than DSP noise reduction in combating background noise, and it is less likely to degrade the readability of the desired signal. For some band conditions, the DSP noise reduction is more effective than the MFJ filter, and sometimes both devices together are more effective than either one by itself. Most of the time, however, when operating on either CW or SSB, I use the MFJ-722 and keep the DSP noise reduction switched off.
With regard to elimination of interference from nearby CW signals, the MFJ filter provides modest improvement over the selectivity obtained with the 400 Hz i-f filter by itself. On average, at a comfortable listening level, the audible bandwidth of a CW signal is about 650 Hz with the 400 Hz i-f filter alone and 450 Hz with the i-f filter plus the audio filter in its narrowest setting. Without the 400 Hz i-f filter, the average audible bandwidth for the MFJ filter in the narrowest setting is about 750 Hz. The audible bandwidth of the same CW signals, when heard through the Icom's 2.4 kHz i-f filter without audio filtering, is about 3.1 kHz. Thus, the MFJ audio filter would provide a dramatic improvement in selectivity to an IC-718 that lacks a narrow i-f filter. On the other hand, the MFJ filter does not provide the razor-sharp selectivity than can be obtained with state-of-the-art DSP audio filters. A comparison of frequency response curves for the MFJ-722 (from the owner's manual) and the SGC audio DSP filter (QST, January 2004, pp. 70-71) indicates that, even in CW wide mode, the SGC filter is narrower than MFJ-722 at its narrowest setting. The passband of the SGC filter in CW narrow mode is 34% as wide at the -50 dB points as the narrowest passband for the MFJ filter.
In short, even though the MFJ-722 certainly will not transform an IC-718 into an IC-7800, it will provide substantial noise reduction and it will allow considerable flexibility in adjusting the passband width for different modes and band conditions. These benefits are obtained with the turn of a switch. Although the tunable notch filter may be a worthwhile feature, I use it only on SSB and only as a last-ditch measure. At other times, the notch filter is set to its maximum frequency, where it has no apparent effect on either CW or SSB reception.
It should be noted that the MFJ-722, which is housed in a partially plastic enclosure, is sensitive both to rf interference and ac hum. Even after the dc input line was bypassed, my filter picked up, faintly, a Spanish-language broadcast station. A 100-ohm resistor in series with the headphones minimized that problem. However, the device tended to emit loud, anguished squawks when the transmitter was keyed. The purchaser of an MFJ-722 should plan on installing, as I did, a transceiver-controlled DPDT relay that will bypass the filter circuitry when the transmitter is keyed. Fortunately, the MFJ-722 enclosure is spacious and the necessary modifications are not difficult to make.

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