[Elecraft] K3 Harmonic Distortion

[Jack Smith]

David:

Even a rather poor transformer improves significantly when driven by a 
"zero ohm" source such as an op amp. The typical modem line transformers 
that are spec'd at 0.5% THD, for example, come in one to two orders of 
magnitude better when driven by a low impedance source. In fact, it's at 
the point where my lowest distortion audio generator (HP 200CD) isn't  
good enough to measure a $4 transformer when driven by an op amp. I'm 
generator limited at -63 dB at the moment.  I hope to improve that 
measurement floor in the next week or so.

Driving the same transformer with a 600 ohm source Z brings you back to 
the 0.5% THD range.

It's all quite interesting and reminds me again that "it's not what you 
don't know that's the problem; rather it's what you think you know that 
bites you in the backside."

Jack


David Cutter wrote:
> I haven't done the sort of testing that you must do for your 
> applications, but I've used the OEP8000 transformers in several 
> applications with good results for speech only.  I've no idea of its 
> high frequency response.  They are tiny in comparison.
> http://www.oep.co.uk/admin/UploadDocuments/OEP8000.pdf
>
> David
> G3UNA
>
>>
>> I've ordered a couple of TTC-108 transformers today and will run some 
>> experiments with them over  the next week or so. I've looked at 
>> similar transformers developed for modem and telephone line coupling, 
>> but not the TTC-108 specifically, when trying to find a cheaper 
>> output transformer to use with the Softrock Lite receivers. There, 
>> the emphasis was more on high frequency response, up to 100 KHz, but 
>> distortion was also a major issue. The solution of choice in my 
>> opinion for the Softrock receivers turns out to be a MILSPEC Triad 
>> transformer, SP-70, with excellent frequency response and quite low 
>> distortion. In single lot prices, an SD-70 costs about four times as 
>> much as a TTC-108.
>>
>> Jack
>>
>>
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