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Thread: Line out circuit help

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    Supporting Member Randall's Avatar
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    Line out circuit help

    I could use some help understanding how this particular line out circuit works on a Magnatone amp. (upper right, page two). I see it comes directly from the speaker out, so how is it reduced to a line level signal with just a 470 ohm resisitor? And I see the 47 ohm to ground, so if that is shunting much of the signal, wouldn't it drag down the speaker out signal as well?

    Mono Twilighter Schematic rev 2 20150617.pdf

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    It's weird, because it WAS working fine.....

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    Supporting Member eschertron's Avatar
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    It's shunting 470 + 47 Ohms to ground, so not much of a load. And yes, It's pretty basic. With a cap to drain off "some" high end, very little speaker emulation - a speaker emulator like in a SansAmp has 3 or 4 filters for a steeper slope.

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    Supporting Member Jazz P Bass's Avatar
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    Agree. The divider is not that much of a load.

    Looks like a divide by 10 for the line out.

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    Lifetime Member Enzo's Avatar
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    Classic voltage divider. 470 ohm over 47 ohms would work about the same as 1k over 100 ohms or 4.7k over 470 ohms.

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    Old Timer J M Fahey's Avatar
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    Whatīs that power amp output .... 20W into 4 ohms?
    So speaker voltage is around 9V RMS?

    Attenuate that 10X as mentioned above and you end up with 900mV RMS.
    Is that "Line level" enough for you?

    1nd 1uF + 47 ohms means a cutoff frequency of 3300 Hz at mere 6dB/octave.
    Very little, very weak, but better than nothing

    FWIW a Guitar speaker drops 24dB/oct , and simulators mentioned above do that.

    To avoid waking up my spiderweb covered calculator I use this useful page:

    http://www.sengpielaudio.com/calculator-RCpad.htm

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    Juan Manuel Fahey

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Randall View Post
    I see it comes directly from the speaker out, so how is it reduced to a line level signal with just a 470 ohm resisitor?
    Enzo and Juan pegged it. It's a voltage divider. Remember that the speaker output is of considerably lower voltage, but higher current than the voltage amplifiers in the signal chain. And then...

    Quote Originally Posted by Randall View Post
    And I see the 47 ohm to ground, so if that is shunting much of the signal, wouldn't it drag down the speaker out signal as well?
    Nope. With a speaker load measuring less than 20 ohms a parallel load of 470 ohms (the series resistor in the divider that is parallel to the speaker) is peanuts. For this reason the current through that particular circuit is insignificant. And as a result the line out circuit acts as a voltage dependent circuit parallel to the speaker, a current dependent circuit. The circuit relies on the input being fed as a high impedance. This prevents any significant current draw and keeps it a voltage relevant circuit. If the impedance of a fed input from this circuit were of a low impedance it would cause significant problems for the line out, but still have nearly no affect on the speaker output because the parallel load is still on the order of over 50:1 for an 8 ohm speaker load and over 100:1 for a 4 ohm speaker load.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Whatīs that power amp output .... 20W into 4 ohms?
    So speaker voltage is around 9V RMS?

    Attenuate that 10X as mentioned above and you end up with 900mV RMS.
    Is that "Line level" enough for you?

    1nd 1uF + 47 ohms means a cutoff frequency of 3300 Hz at mere 6dB/octave.
    Very little, very weak, but better than nothing

    FWIW a Guitar speaker drops 24dB/oct , and simulators mentioned above do that.

    To avoid waking up my spiderweb covered calculator I use this useful page:

    http://www.sengpielaudio.com/calculator-RCpad.htm
    Holy sh*t, Juan. Look at all these audio calculations here
    http://www.sengpielaudio.com/Calculations03.htm
    That site's great! I bookmarked that page. Thanks for that

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