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Thread: Roland JC120 Thump

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    Roland JC120 Thump

    Any easy ways to lower turn on Thump on an early JC120?

    Typical SS supplies powering up.

    Replaced main filter caps, oddly the plus rail drops to about zero as soon as you power it down, neg rail doesn't.

    Tried a couple things, a little better, just wondering if there were any tricks to knock it down a bit.

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    Senior Member nevetslab's Avatar
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    Quote Originally Posted by drewl View Post
    Any easy ways to lower turn on Thump on an early JC120?

    Typical SS supplies powering up.

    Replaced main filter caps, oddly the plus rail drops to about zero as soon as you power it down, neg rail doesn't.

    Tried a couple things, a little better, just wondering if there were any tricks to knock it down a bit.
    I bet R.G. has a circuit using MosFET's to slowly ramp up the supplies to nominal value. Getting them to fall symmetrically would no doubt be another circuit, unless we can find WHAT IS pulling the positive side down quick and do something on the neg side. The cheap way out is relay-couple the speakers with a delay-turn-on & fast-off circuit (absence of ACV always does that trick well on the support circuit).

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    Logic is an organized way of going wrong with confidence

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    search Quench Arc,

    install across pwr switch,

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    Old Timer J M Fahey's Avatar
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    A snubber across power switch contacts (say .05 x 600V polyester in series with 100 ohm 1/2 W) is great to damp turn OFF pop,but turn ON thump is different.
    Most (all?) SS amplifiers "wake up stupid" (hey, *I* do )for a second until all capacitors charge and reach stable voltage.
    I mean internal capacitors, such as bootstrap, feedback, internal decoupling, etc. , not main supply ones, which also take some time to charge.
    There is *always* some time constant involved and paradoxically, the shorter it is the larger the current peaks needed to quickly charge capacitors.
    Tube amps take 30 seconds or more to heat up, so they hide this, but SS amps are instant-on.

    There is a reason all modern chipamps incorporate Standby and Mute circuits, where an RC network delays turn on, also many (but not all) SS amplifiers, just check any modern Fender/Peavey/Marshall/Laney/Crate/Ampeg/SWR/etc. but you can incorporate a turn on delay , relay based, to your Roland amp.

    Me? I am a dyed in the wool cheap guy, so sometimes where itīs really needed, such as in amplifiers for Churches, live Theatres, conference rooms, Ashrams, meditation centers, etc. , I simply add a switch in series with speaker out , label it "standby" , and tell users:
    "start with both switches OFF,turn Power ON, wait a couple seconds, turn Standby ON ... reverse procedure to turn amp OFF so first Standby, then Power" ..... works like a charm.

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    Juan Manuel Fahey

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