Audio: Deriving Mono from Stereo?

Discussion in 'The Projects Forum' started by Art, Feb 9, 2010.

  1. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    Hi Guys,
    I purchased a stereo valve amplifier today, and it sounds nice :)

    I also have a subwoofer enclosure with it's own active crossover circuitry and amplifier.

    I want to use the subwoofer enclosure with the valve amplifier
    (hopefully far enough away that the vibration doesn't kill the valves).
    I understand I need a mono signal for the sub that is the common of both left and right.
    It would be possible to simply connect the left and right input signals together,
    but then there is no stereo signal for the valve amp.
    Could I use 10K resistors in this circuit to do the job?

    [​IMG]

    Is there a better value to use?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    In order to use a two resistor passive summer, you must know the input impedance of the mono amplifier. The two series resistors should be about 5X or higher than the input impedance. Even at that, the summing network will reduce your stereo separation, and the amplitude loss will have to be made up by turning up the gain of the mono amp.

    The "proper" way to do this, while maintaining isolation between the right and left channels, and not losing any level, is to use an opamp "summing amplifier".
     
  3. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    I want to research and make the proper circuit to do it then.
    sine I got a valve amp and all...

    Maybe this way will suffice in the mean time though.
     
  4. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    [​IMG]

    Is it really as easy as shielding and powering that?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You might want to get rid of the higher frequencies that the subwoofer can't respond to. The link goes to a filter for a subwoofer. The autotranslation makes it a bit more interesting - http://www.electronics-lab.com/projects/audio/008/

    You could use some op amp newer than a 741. If you want audio quality, use an OPA134 instead. For the subwoofer, a TL081 or an OP07 should do well enough.
     
  6. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    THe subwoofer enclosure has it's own active crossover.
     
  7. Art

    Thread Starter Distinguished Member

    Sep 10, 2007
    785
    61
    The beast:

    [​IMG]


    Actually, I think it is probably cheap crap, but not much better or worse than the
    last PC amplifier I had. Just hope those valves last.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    In addition to the horrible translation, the schematic does not match the parts list nor the pcb layout. It is a nightmare to see that 180nF plus 220nF make the 400nF capacitor that is shown as a single capacitor on the schematic and parts list. Two 100nf capacitors are used for the 200nf capacitor.
    It uses a noisy old TL062 opamp when a much better TL072 costs less.

    The terrible translation warns about "the high honor of rhythm of elevation"
    which I think is when the project levitates.
    Tie it down.:D
     
  9. Darren Holdstock

    Active Member

    Feb 10, 2009
    262
    11
    Shiny thing! I'd go for an op-amp summing circuit too, with a nice op-amp. NE5534/NE5532s are the popular old favourites, but I'm quite fond of the TL052. A golden-eared ex-boss used to favour the TL072, but the only the SGS ones, he didn't like the ones from Texas (yes, people did double-blind listening tests on him and never caught him out). Be careful not to invert the phase though, or the subwoofers will suck chunks of bass away from the stereo speaker pair. Non-inverting summing amps could be used, or an inverting summing amp followed by another inverting stage.

    If you're making an active LP filter, a Bessel (phase-linear) response is best for audio. Use caps with a nice dielectric and keep the resistor values between 1k and 100k. There are plenty of filter applets out there, or plug some numbers into a filter cookbook equation, it shouldn't be too difficult. For a Bessel response, make ζ=0.8659.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Toshiba make many copies of American opamps. They made a copy of the TL07x opamp and made it "better" with a higher bandwidth. They all oscillated and were recalled. The "oops" letter about the problem and datasheet have been removed from their website. I wish I kept a copy.
     
  11. russpatterson

    Member

    Feb 1, 2010
    351
    16
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