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Eden Electronics Dual JFET Mute Circuit

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  • Eden Electronics Dual JFET Mute Circuit

    Below is a side-by-side comparison of the dual JFET mute/power on delay circuit in the Gibson GB440 on the left vs the Eden WT-400 on the right. The rest of the power amp circuit is identical for both of these bass amps.

    Can anyone here explain why one of the 2.7K resistors was changed to 5.1K and the extra (*)100K resistor was added to the later generation Eden mute circuit?


    Click image for larger version

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  • #2
    WHo knows? Put it in the context of the rest of the circuit. MAybe it matched impedances better with whatever was going on in the preamp

    You ask about the 100k, but ignore the added thermostat. I could be wrong, but I assume whatever is going on off the left side at the MUTE line serves not only to charge the 2.2uf cap when powering up, but to also discharge it at power down to remute. Add the thermostat and what happens if the thermostat opens? I suspect the 100k is there to insure discharge of the cap.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      Eden WT-400.pdf Gibson-GB-440.pdf

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      • #4
        This is a bit cleaner and shows that the 2.7K (or 5.1K) is part of an attenuator. So they decided to knock down the signal to the power amp a bit I would think.
        Attached Files
        "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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        • #5
          Thanks Ezno and G1 for the replies. It's finally clear to me what these modifications were meant to accomplish. I did not realize that the signal could be attenuated by changing the value of these resistors. I incorrectly thought that the values were chosen only to achieve an adequate amount of muting when the JFET turned on. In a later version of this circuit , Eden kept these resistor values in the mute circuit in addition to placing a voltage divider at the power amp input. I assumed the voltage divider was solely responsible for the signal attenuation. Although after having a second look at the PM500 schematic, it's not a simple voltage divider, but rather a low pass filter. What I had previously read as a jumper is actually a capacitor (C1) of unspecified value.

          Eden PM500.pdf
          Attached Files
          Last edited by duff2; 11-17-2020, 06:09 PM.

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          • #6
            The original mute avoids preamp Op Amps thumping ON and mouse squeaking OFF when voltage rails are very low, say a couple Volts and they become crazy unstable.
            You need more than, say, 4V to turn those Fets OFF (unbiased they are a few hundred ohms or less) to let signal get into the power amp.

            They added a safety functionality by unbiasing them (so signal is derived to ground) thanks to a normally closed thermostat attached to heatsink, so when too hot it opens and Fets lose bias and become lowish ohm resistors.

            100k resistor must be, as said above, to speed bias cap discharge to a reasonable time, nobody wants firemen reaching a burning home 4 hours later.

            Series attenuation resistors are not critical, increasing them increases attenuation.

            Fets are not very good as shunts,because channel resistance is significant (check datasheet) and clearly just one was not enough, so they used two cascaded.
            Juan Manuel Fahey

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