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Thread: Compressor or Expander?

  1. #1
    Senior Member
    Join Date
    Apr 2007

    Compressor or Expander?

    This circuit looks more like an expander than a limitter or compressor, but the 1M pot is for limitter sensitivity.

    With no signal on the input, it appears V2 is close to saturation.

    With a strong enough signal on the input, and with the wiper on the 1M limitter sensitivity pot turned away from the grounded terminal, it appears C2 provides grid leak bias to V2 to reduce the current through V2.

    When this happens, it appears the voltage drop accross the 510 ohm resistor decreases which cause the voltage on the screen grids of the 8417 PA tubes to increase.

    This increases the gain of the PA. That makes the circuit an expander, but it is supposed to be a limitter or compressor.

    Any ideas how this circuit might work as a limitter or compressor instead of an expander?

    Data Sheet for V1 and V2:
    Attached Thumbnails Attached Thumbnails mic_amp.jpg  

  2. #2
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Glasgow, Scotland
    It would work as a compressor if V2 were normally biased near cutoff. Then positive signal peaks would cause it to conduct, discharging the 40uF cap and lowering the PA screen voltage. But negative peaks would have no effect, since the tube was practically off anyway. So basically V2 would be a half-wave rectifier with gain.

    I conclude that the circuit won't work unless V2 is biased to pull only a little current at idle. It also needs to be a very strong high-gain tube to make much of an impression on that 510 ohm screen resistor. You'd need something like an EL84: the high-current section of the 7247 would hardly make a dent in it. Even so, you would probably want to increase the screen resistor too. And/or commit heresy by using a MOSFET ;-)

    In short, I don't believe the circuit will work at all, as drawn.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  3. #3
    Senior Member
    Join Date
    Apr 2007
    I've traced a more complete schematic.

    It appears that when the operator closes the mic switch, S1, to use the microphone, the normally closed contact of the relay opens to lift the ground from pin 2 of V1.

    This allows the signal from the mic to be amplified, but it also appears a large noize spike is generated in the process.

    During the noize spike, C3 apparently allows the -12.6v to zap pin 3 of V2 causing a huge current spike through the 510 ohm resistor to lower the gain of the PA and prevent the noize spike from being heard.

    I am not so sure about the following, but it seems that when the operarator opens the mic switch, a noize spike is also generated, and the signal from V1 zaps pin 2 of V2 to prevent the noize spike from being heard also.

    The circuit definitely does not appear to limit or compress the microphone signal while the mic is in use, so it does not appear to be a compressor or limitter in the traditional sense.

    The idle bias seems a little heavy, it appears to drive the tube at its maximum wattage, 3 watts. (295v x 9.7ma = 2.9w)

    Sorry for not having a more complete schematic to begin with.
    Attached Thumbnails Attached Thumbnails micamp.jpg  

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