The Jazz Guitar Chord Dictionary
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  1. #51

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    Or, if one is only concerned with the hiss between phrases, or when not playing; Speaker-level gating.
    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2022-01-03-10-12-59-pm-png

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  3. #52

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    Has anyone managed to build similar type of mod to older JC-120s?

    I have a JC-120 79-82 era and hiss is bit annoying in home use.
    In my case the hiss is always on even when volume is all the way down. The level doesn't raise when I turn up the volume.

    Amp is totally fine in higher volumes on stage etc.

    I have the fourth edition of the amp (with chorus trimmer on the back panel) and here you can find schematics. (Ap 114 board)

  4. #53

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    Quote Originally Posted by jazzfrog
    You are welcome, this was a fun experiment, yet a never ending one.

    Speaking of never ending. I am now having some time to look into the transistor based JCs, and taking that on as my Winter 2021 project.

    There is only one JC for which the mod above applies, and it's the JC-120H, the head only that Roland released around 1984.
    I have one, I applied this mod and it works great. That head being a 120W head, the loss of gain is actually very well compensated by the power it outputs.

    There are many versions of JCs as the design changed and evolved over the years (both due to part changes and improvements)
    Which one is yours?
    Check here for more info:
    The ultimate JC-120 thread
    Hi, first post on this forum because I've stumbled across this thread.
    I have just acquired a JC-120H head and would like to apply this mod. However, looking at the schematics, they are a little different from the JC-77. The R46 resistor has a value of 270K instead of 100K.

    Roland JC-77, JC-120H Jazz Chorus (12-84) Schematic.pdf 9365 Audio Schematics and Service Manuals files magnet Collections

    Do you have an idea value the resistor to replace it in the JC-120H would have to have to get a similar gain reduction as in your original mod?

    Thanks!

  5. #54
    Quote Originally Posted by koolebra
    Hi, first post on this forum because I've stumbled across this thread.
    I have just acquired a JC-120H head and would like to apply this mod. However, looking at the schematics, they are a little different from the JC-77. The R46 resistor has a value of 270K instead of 100K.

    Roland JC-77, JC-120H Jazz Chorus (12-84) Schematic.pdf 9365 Audio Schematics and Service Manuals files magnet Collections

    Do you have an idea value the resistor to replace it in the JC-120H would have to have to get a similar gain reduction as in your original mod?

    Thanks!
    If you read my post you'll see that the formula is 1+R46/48
    In my case, it's 1+100/15... in your case it's 1+270/22... same ratio no?

    So there you can now figure out what works for you so that 1+270/22 is not 13+ but closer to 1-something...

  6. #55

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    Hello,

    I have also the transistor version. I would like to know if the same verry good mod is possible.

    Sorry but, i'm french and my english is poor... but thanks a lot for all.

  7. #56

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    Hey all, just joined this forum specifically to ask this question.

    Does anyone know if this mod will work on the JC-40's? Are the schematics too different to know without trying? I can't imagine it'd be too different.

    This hiss is very frustrating at times when trying to record at lower volumes. My room is small and square, so the louder I make any recorded signal, the (exponentially) worse the phase problems are.

    And I wouldn't want to try this myself (I only know how to solder) - I'd try to pay my friend to help me with it, unless any of you live nearby (Nashville area) and would be willing to provide me this service!

  8. #57
    All,

    Because this mod is for op-amp based JCs, it only applies to:
    • JC-55
    • JC-77
    • and the JC-120H head


    No update nor progress for transitor-based JCs.
    Last edited by jazzfrog; 10-22-2023 at 03:10 PM. Reason: typo

  9. #58

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    Quote Originally Posted by jazzfrog
    All,

    Because this mod is for op-amp based JCs, it only applies to:
    • JC-55
    • JC-77
    • and the JC-120H head


    No update nor progress for transitor-based JCs.
    Hey! I'm looking to do this mod on a JC-55..however it looks like maybe according to this there is only one gain stage after the EQ stage? I am very inexperienced in electronics and would be looking to do this mod with a tech friend of mine. I took a couple classes a few years ago but I haven't retained much of what little I learned.

    here is screen shot from the JC 55 schematic I found at this website: SYNTHESIZER SERVICE MANUALS - FREE DOWNLOAD



    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2023-12-06-12-42-58-am-png

    I would appreciate any insight you may have. I know a big part of your work here involved checking the measured values against the theoretical values. Are you able to come up with any ideas/theory that I could check with my friend from looking at the 55 schematic?

    Thanks in advance! It's really cool what you've done here..makes me wish I had sprung for a 77 instead of the 55...but I love the 55 so much, I hope there is a way to do the mod.

    -Chris

  10. #59
    Hi there,



    Do you hear the hiss? Is it a problem for you?
    Do you have a way to measure the amount of hiss coming out? Audio?

    So indeed, I assumed too much, the tone and gain stages on the JC-55 are slightly different.
    If you don't mind the back and forth, I'd like to work with you and figure this out (let's use direct messaging and we can post our findings, cleanly, once we are done)

    At first sight it looks like the last gain stage, before VR-2 Volume is where to focus our attention.
    The IC with R14 at the input and R13 as feedback is where things should be played with.

    Rf/Rin is roughly the gain for an inverter configuration (signal is applied to "-" input).
    So our gain formula is R13/R14 = 1.5M/150k = 10

    I wouldn't go super drastic at first by changing R13 from 1.5M to like 150k directly (ie gain of 1) but maybe try 1M (gain ~6.6) then 500k (gain ~3.3). One of those should help with hiss... hopefully you can "scientifically" measure the hiss amount (I used my Apple Watch db measure... semi-scientific)


    Here is the PCB payout and I highlighted the location for R13
    Attached Images Attached Images A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2023-12-06-8-04-33-am-jpg 

  11. #60

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    I think I have found the definitive solution to my Jazz Choir's hiss:
    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-fsss-jpg

  12. #61

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    just curious why bother? It's not like the amp has a great sound for jazz, rock, fusion or country. Nowadays with excellent SS options from quilter, it hardly seems worth the effort.

  13. #62
    Quote Originally Posted by jzucker
    just curious why bother? It's not like the amp has a great sound for jazz, rock, fusion or country. Nowadays with excellent SS options from quilter, it hardly seems worth the effort.
    First I think "great sound" is all relative. My JC-77 has an awesome jazz sound with the guitars I use for jazz.
    I'd go even as far as saying that my JC-77, as it is currently, sounds better than my '67 Vibrolux Reverb... and it's with MY guitars and their pickups.
    I don't need to justify it nor convert people... it works for me.

    Second, what is not "worth the effort" for one could be just fun for another? or just a challenge to be tackled?
    Again, I have no need to convert anyone to use a JC, I just put my findings out there for whomever wants to give it a try.

    Third, looks.
    Like cars nowadays, there are very few current models that look good TO ME.
    I find the JC series to have a look that I enjoy.


    Finally, all this is a matter of taste, and that is very personal.
    Thank you for that question, it made me think a little.
    Happy Holidays!
    Last edited by jazzfrog; 12-07-2023 at 11:45 PM.

  14. #63

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    Hi!

    Thanks for the reply!

    So using a simple dB meter on my iPhone and with nothing plugged into the amp I measure:

    • 32dB ambient noise in the room with the amp OFF
    • 39dB with the amp ON and volume set at ZERO
    • 59dB amp ON volume set at FIVE.


    SIDE NOTE: When the chorus is OFF there is no hiss at zero, only a low mid frequency hum, but as you turn the volume up the hiss becomes audible.
    With the chorus ON hiss is present at zero. Was this the case for other people's JC amps? I know that in Juno Synthesizers the chips in the chorus sections can become very noisy...That does not explain the increase of hiss when turning the amp up with the chorus OFF, however.

    I will try and get together with my tech friend and do some experimenting with lower values on R13. I'm considering installing a switch to be able to revert to the factory values at will for situations where I might want a bit more gain. The JC 55 is pretty small and in a live setting I could see myself missing some of the gain, whereas in my studio I find the hiss a nuisance on recordings and want to minimize it as much as possible.

    I'll keep you posted on how it goes! Thanks again for your insight. Very helpful!

  15. #64

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    I just bought a used JC-77 after considering the JC-40 for a while due to the stereo inputs. In looking for information on how to convert the 77 inputs to stereo I came across this post and found it interesting.

    My plan was to replace the R7 100k resistor on the low input with a R6 33k to balance the two inputs out but in reading about this mod I am curious to try my hand at performing this mod as well. This particular model doesn't have a great amount of hiss in my opinion but I am particularly interested in the gain reduction as this amp is incredibly loud!

    Just curious to know if anyone has done any work on modding their inputs- it looks like a relatively straightforward swap.

    Thanks to jazzfrog for the OP, your work looks solid. I would appreciate your expertise on this input mod if you see this and have the time to respond.

  16. #65

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    I have three different JC amps. When it comes to hiss, it only seems really bad, when I have my bright switch on. Instead of using the bright switch, when necessary I eq the front end with a pedal.

    If I want a ss amp that has more attack and extends the highs some, more than other ss amps, the JC is a great choice.

    It can do a lot of things really well.

    I think it takes certain fuzz pedals and makes them sound huge. With octave fuzz pedals the upper octave and intermodulation when hitting two notes, is more pronounced. Juicey env filters can also benefit. The chorus has a depth that would take two different amps to achieve. On my 70s JC the chorus has that old boss sound.

    When it comes to jazz it works well also. I would say if I am using some time based fx (delay, verb, or certain types of modulation), the JC seems to work a lot better then sending them into the front end of any other amp that I own, or have used.

    (I do like how sending a hotter delay into a front end of a tube amp will smear it. Sometimes I like to do that with time based modulation also. That is a different approach for a different type of sound.

    The JC amps have a useful place, that I have not really heard much in other amps. If someone likes what it does, that is a preference. When I need what it does well, I grab it.

  17. #66
    Quote Originally Posted by roneranger
    I just bought a used JC-77 after considering the JC-40 for a while due to the stereo inputs. In looking for information on how to convert the 77 inputs to stereo I came across this post and found it interesting.
    True stereo means you have 2 different input paths to 2 different power paths and to 2 different output paths (ie speakers)
    The JC-40 indeed has 2 separate inputs with their own gain stages and 2 separate outputs to speakers

    Quote Originally Posted by roneranger
    My plan was to replace the R7 100k resistor on the low input with a R6 33k to balance the two inputs out but in reading about this mod I am curious to try my hand at performing this mod as well.
    Doing what this will only make both input identical in gain, but as you can see on the schematic below, both input are going to 1 point only (the "2")... they will be merged
    So youare going to endup with 2 inputs to 1 output... no stereo for sure.

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2024-02-18-7-55-39-pm-png

    For some form of stereo using the JC-77 you could use the stereo line out of the JC-77 as an input.
    Get your L/R signal out of some preamp and connect it there... a Strymon delay or reverb pedal can be setup as a line out pedal.
    You'll need the output of a preamp because:
    1. you'll need a line output (ie higher voltage than guitar input)
    2. you'll need a way to control volume (as you can see on the power section below, there are NO volume button)

    I highlighted in blue/orange the 2 independent paths... I've never tried, so do it at your own risk, but it should work!

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2024-02-18-8-03-34-pm-png

    Quote Originally Posted by roneranger
    This particular model doesn't have a great amount of hiss in my opinion but I am particularly interested in the gain reduction as this amp is incredibly loud!
    Excellent

    Quote Originally Posted by roneranger
    Just curious to know if anyone has done any work on modding their inputs- it looks like a relatively straightforward swap.

    Thanks to jazzfrog for the OP, your work looks solid. I would appreciate your expertise on this input mod if you see this and have the time to respond.
    For stereo fun, I'd say get a 2nd JC-77 and something like this to manage the phase between your 2 amps and you are good to go.
    To my ears, to (some of) my guitars and my playing, those old amps sound perfect, are so under rated...

  18. #67

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    Yea I realized in looking at the diagram further that the two inputs sum to a mono signal anyway but I figured it was worth trying out either way.

    Opened the unit today and swapped the high/low input resistors out for paired 33k's as planned. The two signals still aren't completely balanced but they are quite close and my pedal rig sounds great through it. Opted to hold off on the R47 swap after I did some testing with my pedal rig plugged into the amp. Certainly going to consider the mod in the future but again, I wasn't noticing an excessive amount of hiss with the amp so it will likely be something I will consider in the future or once I do some recording tests.

    So for anyone looking to convert their 77 from high/low to stereo inputs, it can be done!
    Last edited by roneranger; 02-19-2024 at 04:28 AM. Reason: added response to other poster

  19. #68

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    So has anyone figured out the equivalent mod for the JC-120? This amp is much more widely used than a JC-77.

    Various schematics are available (1979, 1982, and 1984 schematics here: Need schematic diag Roland JC120 - Music Electronics Forum)

    Can someone with knowledge look at those and figure out what the equivalent part to change out would be? It doesn't appear that its R46 on these schematics, but you can see that there are various other spots with 100k. There is a guy on youtube and some have posted that has a JC-120 mod, but his info is kind of shoddy, not well explained, not demonstrated. For one thing he advised in his post to 'clip R10 resistor' for channel 1 and that's it for that channel. Well, I did that, and it did absolutely nothing. So I don't really trust his info.

  20. #69

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    Further info found here:
    Roland JC120 | HUNT AMPLIFICATION | Guitar Amp Repair | Phoenix, Arizona

    This tech says:
    "This project was 100% about reducing hiss, which is inherent in these amps. To rule out noise sources one by one, I replaced all the electrolytic capacitors, then all the preamp JFETs (after measuring for Idss and sorting from 100's of other RF JFETS), then all the preamp BJTs. This reduced the noise considerably, especially in the normal channel.

    After replacing almost all the preamp resistors with mil-spec metal film, the VIB channel was still just a little noisy. With nothing else to try, and the obvious source of the remaining noise being the large value volume pot, I replaced it with a much lower value, and the necessary EQ compensations. Because the signal is well buffered at that point in the preamp, there was no tone change - and the hiss all but disappeared!"

    So it would appear that changing the 'large value volume pot' on the VIB channel to a lower value was the smoking gun here. Could it have been something that simple this whole time? What would be a good value for this pot vs. stock?