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Fisher x100b distortion issues

Guy Arnold

Guy Arnold

Active Member

  2. I just tested my Fisher x100b to compare it to a Fisher 175T I just finished restoring. The x100b has twice as much THD frorm 1000 to 8 khz on one channel as the other channel, about 1.3% vs .6 percent. I think I tracked it down to the section with the tone controls. The pots aren't noisy. There are some encapsulated passive filter networks in these that are connected across the two pots, one for bass, one for treble. Has anyone replace these or built them out of components for substitutes. I can't see any difference in the sine wave using the scope so I don't think it's noise. I was testing the unit after swapping out the bypass and inline caps when I made this discovery. I always suspected this unit had distortion at the high end. The THD numbers above 8 khz start to increase on both channels. I tried swapping tubes around but it did not affect the problem.
    I am using a Leader LDM-171 to make the measurements.

    Guy Arnold, Today at 1:22 PMEditReport
    #1+ QuoteReply


  3. klamsossBe willing to fail and try again.Subscriber
    Messages:
    103
    Location:
    Cincinnati, Ohio
    I can't help directly, as I dont have a distortion analyzer so have never attempted to measure THD with my own X-100-B. However I would offer: the manufacturer ratings for THD on the X-100-B (and many/most vintage tube amps) are relatively high compared with current generation solid state gear. Not sure what the mfr THD rating is for the 175T, though, so not sure how it would/should compare. Based on measurements I have seen reported on here and elsewhere, your observed THD values aren't outrageous, though.
    Someone on here should be able to give you very good guidance, especially since you seem to have identified at least the approximate location where the distortion is popping up. I recommend moving your post to the Fisher forum on here. You'll get some immediate feedback over there. Larry, Dave, B-ham and others will get you on track.

    Fisher X-100-B and FM-100-B.. Tekton Lore2.0.. Pro-Ject DebutCarbon(DC).. Schiit Bifrost4490.. Sony UBP-x800m2
    B&K ST125.2.. Schiit SYS.. Thiel CS2
 
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