Sunday, December 20, 2009

A lot of noise about noise

A very long discussion has been raging on the Elecraft reflector in the last few days over whether the K3 receiver is noisy. This discussion has remained surprisingly well-mannered although there have been one or two instances of someone complaining about someone else making criticisms of the K3 that they can't personally hear or see.

Paul, WW4PT has suggested that the problem is with people who won't use the RF Gain control. While I dare say that may be the answer for some of the people who complained that their K3s were noisy, I found it hard to say if that was the only issue being experienced. Some complained of the K3 being tiring or fatiguing to listen to for a long period. Others suggested it may be due to high frequency artifacts produced by the DSP. (Incidentally, why do people use the word "artifacts" in this way? I thought artifacts were objects made by ancient cultures.)

One benefit of the discussion is that it got a lot of people to experiment to try to put a finger on the problem. I always found the K3 audio to be a bit lacking in bass compared to other receivers, and actually preferred listening to SSB on my FT-817ND. After experimentation I found that the problem was partly due to the external speakers and headphones I have been using.

Early K3s used too small values of coupling capacitors in the audio output stages resulting in the loss of a lot of bass. Modifications have been published for the line output and speaker output, and I have carried out those mods on my K3, but the headphone output requires a modification to the DSP board that is deemed too difficult for DIY. The only solution is to return the DSP board for exchange, which Elecraft offers for $69, but by the time you add in international shipping, taxes and tax collection fees for those of us in the UK that will come to more like £69, which is quite a lot to spend to remedy what was arguably a fault in the original K3 design.

Fortunately there is a cheaper solution: use high impedance headphones. And it just so happened that I had a suitable pair of Sennheiser headphones already, which I bought to use with a Sony Walkman CD player. Even with these headphones I still prefer a "warmer" sounding audio, but the K3's receive equalization settings provide all the audio tailoring I need.

During the course of my experiments I used Spectran to analyze the audio output of the K3. What I found was interesting. The K3 has a text-book filter response with steep sides and a flat top, and virtually no noise (more than 100dB down) outside the filter passband. (See the screen grab above.) By contrast, the FT-817 frequency response falls away more gradually, containing considerable energy even at frequencies of 10KHz or more. Don W3FPR did some similar tests with his FT-817, his K2 and some Yaesu radios and got similar results. The absence of unwanted high frequency energy in the K3 output makes the receiver sound clearer, but it also allows you to hear noise more clearly, where other receivers produce a soothing mush. This might also be a reason why some people claim the K3 sounds "noisy".

After making some equalization changes, swapping to the high impedance headphones and getting rid of the external speakers my K3 sounds great. I just need to find some small external speakers that have a good bass response when used with the K3.
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