Fisher 400 rumble circuit?

Discussion in 'Fisher' started by mp53, Feb 5, 2014.

  1. mp53

    mp53 Member

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    Can anyone point to where the changes are to be made in deleting this circuit? MP
     

     

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  2. HiFiHarv

    HiFiHarv Active Member

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    I'd like to know this too as I have been wanting to make this change in a 400C preamp. I truly think the rumble circuit compromises the phono section.
     
  3. IMTubeHead

    IMTubeHead OsteoFisheritis

    What is a rumble circuit?? That's a new one for me:stupid: My first thought for an answer would be...Right in front of the rumble seat!!:D {{JK}}

    John
     
  4. audiodon

    audiodon AK Subscriber Subscriber

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    From a few decades later.
    It's a high pass filter so low frequencies don't pass from the turntable to the speakers. High pass in this case is "above frequencies where you'll hear rumble". It attenuates the low-lows. I'm not sure of the exact frequency or attenuation slope, but LF rolloff (high pass) probably starts above 60hz.
    Viewed from the perspective of rolling off, the rate of attenuation increases as the frequency decreases.
    Viewed from the perspective of a bandpass filter, it's a high pass filter with the highs passing starting quite low.
    It's kind of like having to turn the temperature down to increase (turn up) the level of air conditioning - when you use that sort of thing.

    I'd like to know too.
     
  5. sony6060

    sony6060 Super Member

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    The RIAA and the coupling cap values affect the band pass of the phono circuit. I see no reason to change the values. I have upgraded a few Fisher phono stages and all report the change in performance is substantial.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Rumble happens at very low frequencies, typically less than 10 Hz, and occurs due to either transfer of table motor noise, or low frequency acoustic feedback from the speakers. It can even happen due to resonances in the tone arm as well.

    The problem with virtually all rumble filters included in vacuum tube equipment back in the day, is that the slope of their roll off was usually at best 12 db per octave, and often less, meaning that the roll off invariably started happening well up into the lower portions of the audio spectrum. For record changers and the like, this was really quite necessary with some installations, but with high quality tables and installations, non-defeatable rumble circuits hurt performance. However, rumble is a real power robber, so keeping it at bay is important when it presents itself.

    For reference, I use an active three stage unity gain filter of my own design, that has a center frequency of 15 Hz. That means that at 15 Hz, response is down 3 db. One octave below that however (7.5 Hz), response is down a whopping 18 db, so you can see how sharp the drop off is with this design. I include this filter in the tape monitor loop of the preamps I use. When I see the woofers going nuts with rumble when the filter is not engaged, they instantly stop when the filter is engaged. However, even this sharp of an active filter still produces a 1 db roll off at 20 Hz, so you can see that the filters most equipment included really intruded into the lower audio realms.

    In the 400, two stages are used to help contain rumble. The first stage is the .022 coupling cap on the output of the phono preamp. This can be increased in value two or three times, with really .047 being a very usable value in this position.

    The second stage is the 1000 pF at the grid of the line amp. This can be safely shorted out to help flatten response significantly. This was done as part of my Improving the Fisher 400 thread, and will help the response of all signals applied to a stock unit, except those applied to the Tape Monitor input.

    In the 400C, the active rumble switch circuitry is basically removed from the circuit in the lowest (20 Hz) setting, with the .022 uF phono preamp output coupling cap again providing the principle roll off at that setting. Also again, you can safely double or triple the value of that cap for improved LF response -- but be aware, that will then upset the corner frequencies as labeled on the three position rumble switch as well.

    I hope this helps!

    Dave
     

     

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  7. sony6060

    sony6060 Super Member

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    I use downstream of the phono stage a .047uF capacitor to replace that 1000pF cap. I never though about shorting out the 1000pF. I'll play with that. Thanks Dave.

    Edit- I assume sources plugged into the aux input have a blocking capacitor in-line as to not affect the 12AX7 tube grid bias. I never seen a source such as a CD player without output coupling capacitors.
     
    Last edited: Feb 7, 2014
  8. HiFiHarv

    HiFiHarv Active Member

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    So, Dave G, as far as the 400C pre is concerned, are you saying that if I can locate and change the .022 phono out coupling caps (C-16 and C-38, i think, is that correct?) to .047 then that will eliminate the undefeatable 20hz roll off setting? Am I correct now that the 20 hz setting is only caused by this cap value in the circuit, or will there still be some amount of roll off? Curious too about at what frequencies this would leave the other settings on the switch. Though I probaby would never use it anyway.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    It will simply move the roll off to below 20 Hz. Understand that you will never -- nor should you want to -- be able to eliminate all roll off in practical vacuum tube amplifier designs. The fact that most of the circuits are R/C coupled means there will be roll off at some point -- which is a good thing. You just don't want to to occur at an unnecessarily high point in the audio range, but still in fact want there to be enough roll off to prevent gross rumble issues.

    Dave
     
  10. mp53

    mp53 Member

    Messages:
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    Well then here's my choices of what I have on hand: I have a RTX .022 in that phono circuit now, I can add a .022 NOS PIO, use a .047 Wima MKP 10[I have Wima's in the preamp sections also],a .047 Russian 40-y or go with Euro- Foil .047.In fact I have enough Euro-Foils to populate the whole amp. MP
     
  11. monkboughtlunch

    monkboughtlunch Super Member

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    Interesting. At what point in Hz does a stock 400 receiver begin rolling off the phono low frequencies?
     

     

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