A few people contacted me regarding active x-over filters for use with 811B horn.Tested Sallen Key 2nd order 1.2KHz LP & HP filters for distortion at various Hz's with ARTA spectrum analyzer.
Used a 100K load to simulate the input Z of a typical tube amp.
At measurements were taken at -3dB level.
For HP filter :
@ 1.2KHz THD = .15% THD+N = .16%
@ 2.4KHz THD = .12% THD+N = .16%
@ 4.8KHz THD = .099% THD+N = .10%
@ 9.6KHz THD = .096% THD+N = .099%
@ 19.2KHz THD = N/A THD+N = .0017%
For LP filter:
@ 1.2KHz THD = .098% THD+N = .1%
@ 600Hz THD = .098% THD+N = .099%
@ 300Hz THD = .082% THD+N = .084%
@ 150Hz THD = .071% THD+N = .071%
@ 75Hz THD = .055% THD+N = .057%
@ 37.5Hz THD = .051% THD+N = .051%
The only harmonics I could see were 2nd order that were down about 80dB from the fundamental.
Edit: I added a simple bipolar power supply for the filter.
Battery supply runs down too fast to be useful for long term listening.
The prototype circuit on breadboard for evaluation.
Edits: 08/10/14
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Topic - active filter : 2nd order LP HP OPA227 distortion % ARTA ... - deathtube 667 11:27:44 08/09/14 (0)