How to decrease the signal sent to a subwoofer?

Discussion in 'Fisher' started by thornev, Apr 12, 2018.

  1. thornev

    thornev Active Member

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    Mid Hudson Valley, NY
    I forget where the thread is (found it, see below), but Dave G posted a schematic that allows one to send the left and right channel signals to a subwoofer. I've been using it for a while.

    The problem is that the signal sent is loud and I have to have the volume control on the subwoofer not even to 1. I would like to reduce significantly the signal so that I can turn the subwoofer volume up.

    I assume the way to do this is to stick some impedance in the signal circuit. There is already a 1K ohm resistor on the circuit board, one for each channel. I tried piggy-backing a 240K resistor across the 1K ohm resistor, but it had no effect on the signal level.

    I would post the schematic, but I cannot find it at the moment. It's pretty simple. It captures both channels with a 1K ohm resistor on both channels and sums the channels into a single channel so that both left and right signals are received by the subwoofer.

    Can someone tell me how I would execute this signal-reducing addition to the circuit? Thanks. Thorne

    UPDATE: There is no schematic. It was Dave G in this thread, post# 3, where he suggests the circuitry:
    http://audiokarma.org/forums/index....itch-for-center-speaker.801666/#post-11323174

    BONUS QUESTION: is there some way to control the frequency below which the subwoofer gets the source signals? For example, if I want only frequencies below 100Hz to be sent to the sub, there must be a circuit for that. A low pass filter, right? I'll look around too.
    http://www.learningaboutelectronics.com/Articles/Low-pass-filter.php
     
    Last edited: Apr 12, 2018

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thorn -- Normally, virtually any active sub woofer already includes an input low pass filter that is typically adjustable, so as to be able to set the cross over frequency for the conditions at hand. These are usually at least 12 db per octave active filters, so that the slope of the filter is rather fairly sharp. Adding any additional filtering external to the sub will really start to play with the phase shift of the reproduced LF signals unless the sub also has controls to account for this. Adding an external filter can certainly be done -- but I really wouldn't recommend it.

    As for reducing the signal to the sub, that can be done very easily, and even be made adjustable by connecting a 1K linear control between the summing point of the two resistors, and ground. Connect the summing point to the wiper (center) terminal of the control, and one of the outside terminals to ground. Use the outside terminal that causes the control to increase output with a clockwise rotation. Once that is determined, connect the other outside terminal to the wiper terminal. The control should have a linear taper, and can be of a conventional .5 watt or even .25 watt rating. Installation of the control will act to cut the signal somewhat even when turned to maximum.

    I hope this helps!

    Dave
     
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  3. KingBubba

    KingBubba "Too Much Stuff" Subscriber

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    I would just use a separate amp. It gives you great control.
     
  4. thornev

    thornev Active Member

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    Location:
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    A separate amp to replace the amp in the Marantz 2265B ? Nah.
     
  5. thornev

    thornev Active Member

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    471
    Location:
    Mid Hudson Valley, NY
    Thank you, Dave. Welcome back. Hope you had a wonderful honeymoon. I'm still waiting to take mine after 4.5 years of marriage ! :)

    I figured out that the lower the resistance, the lesser the signal which is opposite of what I would intuitively expect. I put a 10 ohm resistor in the circuit where you suggested and I had little volume with the sub turned to max. Wow. Ohm's Law at work.
     

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