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Thread: Roland JC120 Schematics Required

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    Roland JC120 Schematics Required

    Hi, does anybody have the circuit diagrams for a 3rd edition JC120. Serial no 534785.
    I would like to add an effects send, return inserts

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    Thankyou

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    Old Timer J M Fahey's Avatar
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    Just curious about a Fet gain block I found in the above schematics between preamps(s) and Power Amp.

    I guess itīs some kind of limiter broadly equivalent to "VOX limiter" , I bet it preclips signal, not sure if in a symmetric or unsymmetric way , can any of our friends so skilled simulate it?

    Maybe it still "does nothing" with shown 200mV input, but expect it visibly clips with, say, 1V RMS or more, and output level is set by VR5 , but would love confirmation.

    Click image for larger version. 

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    TIA

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Just curious about a Fet gain block I found in the above schematics between preamps(s) and Power Amp.

    I guess itīs some kind of limiter broadly equivalent to "VOX limiter" , I bet it preclips signal, not sure if in a symmetric or unsymmetric way , can any of our friends so skilled simulate it?

    Maybe it still "does nothing" with shown 200mV input, but expect it visibly clips with, say, 1V RMS or more, and output level is set by VR5 , but would love confirmation.
    Input is 200mV RMS
    VR5 does set the output level.

    Click image for larger version. 

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    Click image for larger version. 

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    Last edited by Dave H; 01-18-2019 at 04:16 PM.

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    Correct. It is indeed a soft limiter they employed in some revisions of the JC-series amps (including JC-120) and in some bass amps of the same era (BA-series, can’t remember?)
    Open loop differential amp has kinda lowish input sensitivity for linear output but it clips very smoothly and softly (even BJT ones). I think this kind of circuit enjoyed a long history in triangle-to-sinewave-converter circuits before it was even discovered by guitar amp designers. Roland service manuals instruct to adjust the trimmer so that the circuit produces symmetric clipping. The circuit largely prevents overdriving the power amp to hard clipping. Largely, because some revisions of JC-120's employ "current feedback" and the transient spikes - generated by edges of "harder" clipping - the arrangement produces when receiving clipped input signal will overdrive the power amp to clipping. As is typical to solely "pre-limiting" arrangements to prevent power amp clipping.

    Unfortunately, given variation of JC-120 revisions you likely need to find the exact schematics for them to be of any use. You likely are well aware of that already.

    A little bit of OT, but the much-critiqued JC-120 distortion circuit also varies quite a lot during the “early era” of JC-120 production. Sometimes its overdriven FETs, sometimes feedback diode clipping, particularly nothing consistent throughout early revisions. IMO, Roland GA-120 / 60 amps, from the time period of Roland introducing also the earliest JC-series amps) have great distortion tone - I mean Great - and the circuitry isn’t too much different from that employed in some JC-series amp revisions. In all other regards Roland also has pretty impressive track record in developing all sorts of distortion effects. …and later on this “tube modeling” -thing, whether analog or digital. (The two largely go hand-to-hand). So, I wonder how they blew this one.

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    Old Timer J M Fahey's Avatar
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    Thanks for both of you.

    As of Roland on one side making groundbreaking and killer sounding distortion pedals and on the other side being so goofy inconsistent on JC series amps (and some of the early Cubes) my guess is they didnīt really care about that, such amps being intended for **clean** playing, so "some kind of distortion" was added as an afterthought, so, "who cares?" attitude.

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    Juan Manuel Fahey

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