LTP as a Mixer

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Lanesplitter, Jun 5, 2008.

  1. Lanesplitter

    Lanesplitter Member

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    I am working on an amp with no NFB around the power amp. Since there's no NFB, the Long-Tailed-Pair has a grid tied to ground (thru a capacitor). Would it be a bad idea to use this normally-grounded grid as a return from a parallel FX loop? I see that Matchless uses the phase inverter as the reverb mixer.

    I'm not too concerned about signal levels. My delay pedal can return line level. I'm more concerned about the phase. I realize that the wet and dry signals will be 180 degrees out-of-phase, but with time based effects, I can't get signal cancellation, right? The 180 degrees would be like adding or subtracting a few mS to my delay time.
     
  2. jdh

    jdh Supporting Member

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    Since the LTP is ac coupled you can try this as a mixer. You are right that the phase inversion can act like an increase or decrease in time delay for echo. This inversion may be more pronounced on other processes like reverb . This circuit is a good approximation of a subtractor (or difference amp). The output will be approximately the difference of the 2 inputs times any voltage gain: (V1-V2) * Av. The result is some effects will sound hollow or thin due to the subtraction. You can use a Jensen line input transformer, JP11P-1, to invert the subtraction which gives you addition. Either way, give it a try and see what your ears think.

    Dennis
     
  3. pula58

    pula58 Silver Supporting Member

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    I Have seen reverb returned at the "other" phase inverter input. I think Vox's do this. But the phase was made correct i nthe reverb circuit
     
  4. VacuumVoodoo

    VacuumVoodoo Member

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    There's no time delay in the LTP itself since what we nonchalantly call "phase inverter" is a signal polarity inverter. Since signal from any effects working in time domain (including spring reverb) is delayed a lot more than a single period actual polarity in mixing does not really matter much. It would be a different case if you were working with a steady sine wave that does not change amplitude or frequency over time.

    If you're mixing signals from, say, two input channels with no time delay and no phase difference between them then polarity reversal of one of the signals in the mixer will have to be remedied. Otherwise total cancellation of signal components with equal frequency/amplitude/phase will occur. sometimes resulting "tin can" tone is usued as an effect in itself.
     
  5. Lanesplitter

    Lanesplitter Member

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    Great. Thanks. The amp is very sensitive to the delay pedal (DD-20) noise. Getting it out of the preamp should lower the noise.
     
  6. Lanesplitter

    Lanesplitter Member

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    It works! I don't yet have the jacks to properly install one, but I was able to wire in a pair of 1/4" plugs that hang out a hole in the chassis. I think I will put a JFET buffer on the SEND so that it can drive a long cable.
     
  7. Lanesplitter

    Lanesplitter Member

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    Right, but it will have to be a JFET. I don't have any room or filament current left for another 12AX7.
     

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