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Fisher 400: did it include a built in rumble filter that is always "on"?

Discussion in 'Fisher' started by monkboughtlunch, Jul 5, 2018.

  1. monkboughtlunch

    monkboughtlunch Super Member

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    The 500b/c and 800b/c models all included a "low filter" to help eliminate rumble in idler drive turntables. As the 400 model eliminated several front panel functions to achieve a lower price point, I am wondering if the 400 retained a low filter that was permanently "wired on" (with no front panel on / off switch) when selecting Phono input modes. Or did the 400 eliminate the low filter in the circuit completely?
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 400 receiver also includes a permanent LF filter, that can be removed if desired. It can also be removed on the 500B/800B models as well, but should never be removed on the 500C/800C units. Doing so will make for lazy auto switching between FM Mono and FM Stereo MPX functions due to the large transient created by the diode switching matrix when triggering between the two modes.

    Dave
     
  3. monkboughtlunch

    monkboughtlunch Super Member

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    Thanks Dave. So on the 500 and 800 models, does moving the low filter slider switch to the "off" position not deactivate the LF filter? Is there still a permanent LF filter in place on the 500 and 800 when set to "off"?

    Does the permanent LF on the 400 provide the same effect on rumble reduction from an idler turntable that a 500 or 800 model with the LF slider set to "on" would do?
     
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Yes, in both the 500 and 800 receivers, there is a permanent LF filter that is permanently on, and then a separate switchable LF filter that is activated when turned on by the front panel switch. When turned off, the switchable filter is out of the circuit. The permanent LF filter has the same effect between all three receiver lines.

    Dave
     
  5. 62caddy

    62caddy Trust but verify Subscriber

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    Can such a filter be installed in other equipment without affecting LF response? The 50 Hz filter of the MX113 takes too much of a bite out of the low end but a very low pass filter would be extremely helpful when playing slightly warped LPs - perhaps at 15 Hz. Even if the 50 Hz filter could be modified to filter @ 15 Hz, it would be a great asset for playing records.
     
  6. thornev

    thornev Well-Known Member

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    Dave... Which if any of the 500 mods defeats the permanent LF filter? Thorne
     

     

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  7. monkboughtlunch

    monkboughtlunch Super Member

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    Thanks Dave. Do you know the filter frequency Hz cutoffs Fisher used for:

    a.) Permanent "always on" low frequency filter
    b.) Front panel switchable low frequency filter (500B, 800B, 500C, 800C)
     
  8. thornev

    thornev Well-Known Member

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    Monk - The 500-C manual says the low filter SWITCH (not the permanent filter) is -3 dB at 60 cps which means it's meant to remove hum caused by wall socket AC hum grounding problems. And then it also says 12dB per octave below 50 cps which means it's a shelf filter as opposed to a peak filter. FYI, the high frequency filter is -3 dB at 5k and 12 dB per octave above 6k which I think is a pretty drastic filter since we can hear 6k+ effects. Thorne

    PS - The manual also says the OVERALL frequency response goes down to 25 Hz +- 1.5dB. So I would guess the low end is because of the permanent LF filter. Since the switchable filter kicks in at 60Hz, that would seem to indicate that it overrides (or at least supersedes) the permanenet LF filter.
     
    monkboughtlunch likes this.
  9. monkboughtlunch

    monkboughtlunch Super Member

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    Interesting. So was the Low Frequency panel slider switch essentially to eliminate turntable rumble from idler type drive systems? Or was the permanent LF filter designed to address idler rumble?
     
  10. thornev

    thornev Well-Known Member

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    On design questions I defer to Dave G as he understands Fisher design better than anybody. Thorne
     
  11. audmod01

    audmod01 Super Member

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    62caddy;

    I think you meant a high pass filter with a very low frequency cut-off knee. If you pass only 15Hz and below ( a low pass filter ) you will only have the sub-audible rumble from ripples and warped surfaces of LP records being passed to the rest of the audio circuits. It will be interesting to learn from Dave what the corner frequency of the built-in permanent high-pass filter is on these models.

    Low frequency excursions have to be particularly snubbed in console applications where the possibility of audio feedback can easily be enough to bounce a stylus out of the groove while playing records. Tuning across the FM band rapidly causes very large excursions of low frequency energy to be fed to the speakers in a system if the audio is not muted (or volume reduced) while changing stations. It is possible to blow or bottom the voice coil in woofers if the energy is too high (think drunk college students at the controls).

    Joe
     
    Last edited: Jul 7, 2018

     

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  12. fred soop

    fred soop Super Member

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    The phono preamp should really always have both high and low filters but beyond the audio range. This circuit has a low filter that is -3.0 dB at 20 Hz (could be a bit less cut but probably not audible) and drops very steep such that any rumble or turntable mechanical frequencies are well below the point of doing any harm. The high filter starts at 50 kHz and is mainly to keep unwanted frequencies (well above anything audible) out of the later stages. It is possible for supersonic frequencies to actually damage transistor power amplifiers, especially the older ones using slower transistors and quasi complimentary circuitry.

    Low filter .....
    - 0.3 dB @ 30 Hz
    - 3.0 dB @ 20 Hz
    -18.6 dB @ 10 Hz
    -36.0 dB @ 5 Hz

    High filter .....
    0 dB @ 20 kHz

    - 3.0 dB @ 50 kHz
    -12.6 dB @ 100 kHz
    -24.9 dB @ 200 kHz

    [​IMG]
     
  13. 62caddy

    62caddy Trust but verify Subscriber

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    Thanks for correcting my misnomer. Obviously not looking to eliminate all frequencies above 15 Hz. I know other McIntosh models were intentionally designed cut off all frequencies below 20Hz and above 20kHz. Almost all LPs I own display warpage to some extent which is visible on the scope display as well as well in woofer movement.

    Warpage is usually most pronounced toward the outer edges of the record, decreasing as the stylus travels inward. Hopefully @dcgillespie will have some ideas on the subject.
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The permanent LF filter was always cited in the literature of the day as being present to minimize any signal generated from record warp, and the thump generated as an FM tuner was tuned in and out of a carrier signal. The permanent filter and the switchable filter actually work together in the first line amplifier stage (whether the switchable filter is on or off) to form a two pole filter that is much sharper (12 db/ octave) than what a single pole filter produces (6 db/octave). The output of the LF filter stage is taken between the two poles, so that when the switchable filter portion is turned off (more realistically, effectively made benign in the circuit), the permanent portion still operates as a 12 db/octave filter, but primarily as a garbage screen (only effective below 20 Hz) to deal with the warp and thump issues mentioned above.

    When the switchable portion is turned on, then the effective filtering moves well up into the audible range, to deal with potential console acoustic feedback, noisy changer mechanics, boomy speakers, etc.

    Of note, it appears that Fisher first used a Rumble Filter that was part of the RIAA preamp circuit in their earliest stereo models (think original X-101ST), but dropped it in favor of the scheme noted above in later units (X-202 and beyond). Interestingly however, the President preamp (400CX-2 for consoles) revives the old RIAA rumble filter circuit -- as a permanently activate filter -- used in addition to the later LF filter feature mentioned above, so it gets a double whammy filter. This may have been to reign in some wicked console feedback due to the better LF capability of the President's power amplifiers.

    Dave
     
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