Audio filter for CW and 1Khz tone in transceivers.

[Kenneth G. Gordon]

On 24 Sep 2006 at 18:56, Michael Waldrop wrote:

> Hello everyone,
> 
> I work alot of CW using either an SB-300 or SB-303 receiver. Although
> the Heath CW filter does a good job, the audio output to either a
> speaker or headphones is a bit high in frequency for my taste. 

Mine too. And a lot of other Heathkit HW/SB rig users. That frequency 
is SUPPOSED to be 1Khz, and is pretty close to that in most cases.

I much prefer a much lower tone; around 750 Hz or so.

> The idea of using an outboard filter came about while looking for an
> alternative to the Heath filters. The cost for a replacement IF CW
> filter was $100 or more. 

Yes, although you can find used 400 Hz Heathkit filters for around $50 
on average.

> The filter is built around the popular LM-741 op amp in a high Q
> bandpass configuration  Using the High Q bandpass configuration, the
> filter is capable of providing an adjustable resonate frequency
> between 50Hz to 1KHz with an overall bandwidth of 500Hz, perfect for
> CW work. The filter runs on a single supply, a 9 volt battery, and
> draws no more than 3.5ma. I tested the filter using a dual supply but
> I could not see any difference in performance between single versus
> dual supplies. The battery supply eliminates the need for a seperate
> power supply.
> 
> A great cost savings that provides great filtering for CW work.

I would be very interested in having a copy of the schematic for that.

It sounds very much like the old MFJ/Autex CWF-1, but that used 2 ea 
747 op amps in a single-ended supply (9 volt battery) circuit. However, 
for that filter, the center frequency could be special ordered at 
practically any frequency. Nonetheless, most were sold at 750 Hz.

I have three or four of these. They commonly appear on Ebay at fairly 
low prices. I highly recommend them...for anything EXCEPT the HW/SB 
transceivers since the offset and sidetone are FIXED at 1 Khz in those 
rigs. In fact, I built one into my HW-16 and find it very useful there.

For the SB-3XX receivers, that isn't a problem, although, as someone 
else mentioned here, those receivers don't have much output at the low 
end of the audio spectrum.

There are two different ways to fix that 1Khz sidetone/offset in the 
HW/SB transceivers. One way was detailed in an article which 
appeared in 73 magazine sometime ago. I have copies of that article in 
PDF format if anyone is interested.

The second way is one I thought out sometime ago, but have not yet 
implemented. This involves changing the CW crystal, and rewiring one 
of the relays a bit

Either way uses the same filters and does not effect normal operation: it 
just lowers the offset and sidetone frequency to 750 Hz or so.

Ken Gordon W7EKB / WC2XSR

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