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  1. #1

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    Hi there,


    I did a quick post last night here

    Many of us complained about the hiss in those Jazz Chorus amps.
    I acquired a JC-77 from 1985, I like the tone a lot and honestly it looks great.
    But for that hiss... very loud hiss coming from the amp, even at low volume.
    Also, this amp is definitely loud.


    Early sound measurements showed a Volume at 0 of around 32db while with Volume at 5 the hiss sound came up to ~52dB (no instrument plugged in!)

    So I spent a lot of time looking online at possible options.
    Eventually, I purchased the service notes and studied the circuit.

    Thanks to Payne's initial work here, it made me look into the various gain stages.

    Jump to the "Mod" section for the short version (I had to split this post into multiple due to length)



    Investigations and theories
    The circuit shows several preamp gain stages, five to be precise before it reaches the power amp section.
    Between the preamp and power amp section you have the line out jacks (see picture below)

    This was my first test, is the hiss in the preamp or the power amp. Picking up the signal at line output would answer this question.

    Yes, the hiss is also heard at the line out output. So I focused on the pre-amp section.

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-jc-77_sn_pre-amp-full-png


    You can see on the diagram above that there are a few hand written notes, including test values.
    Also, if you look at the opamp configurations, they are in "non-inverting" configuration (signal gets in the "+" input) and the "-" input has a couple of resistors and capacitors around it. Those actually help you calculate the gain.
    Taking the first gain stage opamp, called "IC-2" (one half of IC-2 actually), it has R16 and R18 connected to the "-" input.
    In this configuration, gain for this opamp is roughly calculated with the formula 1+R18/R16.
    Last edited by jazzfrog; 02-15-2021 at 04:50 PM.

  2.  

    The Jazz Guitar Chord Dictionary
     
  3. #2

    Stage 1 example:


    Armed with this knowledge, we can now check the written test values in square boxes.
    21mV at "HIGH" input and 76mV right out of the first stage opamp.
    That's a measured gain of 3.62
    Using the formula above we can also calculate the theoretical gain output
    That's 1+100/39, resulting in a theoretical gain of 3.56
    Not bad, close enough.


    Other gain stages
    Subsequent calculations leads to this quick table here

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2021-02-14-9-35-10-am-png

    Stage 3 surprise
    So looking at the comparison on stage 3, I was a bit surprised to see such a configuration.
    A big discrepancy between test measurements and theoretical value.
    Since a few components were corrected on the schematics, I thought maybe it's a mistake, so I double checked.
    R46 and R48 are correct on the circuit and the diagram. 100k and 15k respectively.

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2021-02-13-10-34-26-pm-png

    A gain of 7+ is definitely high for this circuit. What if this was introducing the hiss?
    If I bring gain back to close to unity, ie 1, what would happen?
    First the effect of eq would definitely be reduced.
    Those are passive networks so the effect is subtle anyways, so maybe no big deal.
    Also, overall volume would be reduced... that amp is so loud for me anyways, it would be ok.

    I decided to proceed, by now the amp was open on my bench so what the heck was I waiting for?


    Mod that worked

    So I decided to try that, replace R46's 100k value with 1k.
    This would bring the gain back to close to 1.

    Remember the sound volume measurements from above?

    No mod:
    - Volume @0: ~33dB
    - Volume @5: ~52dB (of pure hiss)

    After the mod:
    - Volume @0: ~33dB
    - Volume @5: ~35dB (no hiss)
    BINGO!

    Now, how did this sound with an instrument plugged in? (I am using a 335 on neck pickup)

    Definitely a drop in volume. Before the mod, I could barely turn the amp to 3 before I hit around 82dB.
    Now at 5, I was around 73dB... very manageable in the house.

    I decided to try a few other values and see how it would affect eq effectiveness.
    With a value of 22k for R46, we are back to a gain above 2 and the hiss is back at around 42dB when volume is at 5.
    Not bad at all, better for sure than 52dB.

    I am going to use this value for now and see how to works for me.
    Eventually I'll try with a 2.2k value which brings the gain at 1.15 right around the measured 1.11 value.
    Last edited by jazzfrog; 02-14-2021 at 02:38 PM.

  4. #3
    Detailed mechanical work

    ===== REMINDER/DISCLAIMER =====
    You are doing this at your own risks.
    Don't blame me if this doesn't work or your amp is dead after that.
    Also, I can not debug your amp ok?
    ====================

    Disconnect the amp from your power outlet... seriously, do it.


    Remove the amp section from the cabinet:
    1- disconnect the 4 speaker wires (take note of colors and locations, take a phone picture!)
    2- disconnect the 2 reverb tank wires (take note of colors and locations, take a phone picture!)
    3- remove the backplate (the one with the serial number), 4 screws
    4- unscrew the amp from the cabinet (4 screws on top of amp)
    5- slowly slide the amp out of the cabinet

    Access to the board:
    You need to access the bottom of the board to desolder R46.
    1- remove the inputs black nut from the front panel (it might crumble as those are pretty fragile, DON'T BLAME ME, I did lose one of mine in the process)
    this will loosen the small input board, the "HIGH" input should have a could of wires and a lock washer, untangle that and put the lock washer aside.
    2- remove all buttons (just pull them) and the chorus mode selector tip from the front panel
    3- using a 10 or 11 wrench remove all nuts+washers from the shafts of the button pots from the front panel

    Access to the back of the board:
    1- removed the 5 screws that hold the board onto the chassis
    2- now slowly move the board toward the back of the chassis, carefully making sure the pots are removed from their panel holes.
    The board is still attached to other parts, don't even think of disconnecting any of those, just work with it slowly, flipping it over.

    Once that is done you can locate R46, it's very close to the HIGH TREBLE pot, 3rd pot from the left.
    Using braid, unsolder it, replace with your 1k or 2.2k...

    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-screen-shot-2021-02-14-10-09-07-am-png


    Rebuild your amp with reverse order steps.
    Make sure all your screws nuts and washers are all put back in place
    (I use a small container to hold all pieces I remove... it must be empty when I rebuild the amp, i.e. all pieces are back in)

    Please post your experience and feedback, I could have forgotten a few steps and I can update this post to make it better.
    If this mod helps you, also, please let me know, I would be glad to contribute to the community.
    Also, feel free to repost, but keep the context so it still shows it's coming from me.
    Last edited by jazzfrog; 02-14-2021 at 02:40 PM.

  5. #4
    So after a week of running with the 22k value, and having a noticeable hiss, still not as bad as before, I decided to give a try to a lower value.

    Reminder that a value of 2.2k bring the gain for this stage to very close to 0.

    So last week end, one more gut openeing for my JC-77 and went back to the 2.2k value for resistor R46.
    This week, I practiced an was fairly happy with the tone.

    My eq settings are as follows (playing a 2013 ES-335 on neck pickup only, tone at 0):
    - Hi-Treble: 2
    - Treble: 4
    - Mid: 5
    - Bass: 5

    I am liking this and will stick to those values for now.

    Anyone has tried this?
    Does this work for you? Any feedback? Any other suggestions?

  6. #5

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    Hey jazzfrog really good logical technician type work! If it wasn’t for us Technicians the stuff Engineers designed would never work.
    (OK when I was an ET it was core memories, but still)))

  7. #6

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    I don't have but would like to have a JC but am a little scared by the hissing. I would expect the origin of noise to be semiconducors.
    a. Would newer/better opamp's have better noise specifications? If they are socket mounted it could be an easy swap.
    b. What would likely result if the diodes of the distortion section were removed from the circuit? The distortion of these amps seem not the most appreciated effect anyway. And they may in reality be the last part of the original circuit, and the the hissing has been there always, hasn't it?

  8. #7
    The hiss is due to stacked gain stages.

    I actually replaced a few ICs (M5218L) and the coupling capacitors (capacitors between gain stages to isolated from DC).
    (no socket mount, it's a SIP-8 packaging --ie 8 connectors-- hard to find direct replacements)
    That made no difference.

    However, reducing one particular gain stage, made all the difference, it's all explained above.

    This mod worked on my '85 JC-77... I am curious to hear if other people try this mod.
    Last edited by jazzfrog; 03-02-2021 at 11:40 AM.

  9. #8

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    I hope people try it and it works for them. Apart from the hissing and awful distortion the JC is a jazz icon as far as amps go

  10. #9

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    I miss my JC-120 - it hissed but it had a gorgeous tone. It's one amp I really regret selling!

  11. #10

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    Changed the resistor to 2k yesterday.

    The difference is incredible!

    I am so excited about your post. The world should know about this.

    I'm using a Fender Rhodes. Volume at 5 and no noticeable degradation in tone quality. Amp is very, very silent.

    Thank you so much!

  12. #11
    Quote Originally Posted by felipe78
    Changed the resistor to 2k yesterday.

    The difference is incredible!

    I am so excited about your post. The world should know about this.

    I'm using a Fender Rhodes. Volume at 5 and no noticeable degradation in tone quality. Amp is very, very silent.

    Thank you so much!
    Great to hear felipe78!
    Which Roland JC model/year did you try this on?

  13. #12

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    Very interesting thread. I don't have a Roland JC but every now and then I get tempted to buy the 22 Watt version. They are the best looking amps ever and you get an amazing stereo chorus.

    So Roland has been "overclocking" these amps to compete with the loudness of the big tube amps of the 70's?

    What do you mean by losing the EQ effectiveness? Does this mod make the tone controls less effective? Can you compensate the gain loss by boosting the input signal with a pedal? I mean to some extent at least without overdriving the preamp.

  14. #13

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    I owned one of these. I sold it to a music store back in 2003 for $200. I bet they sold it for $500.

  15. #14
    Quote Originally Posted by Tal_175
    Very interesting thread. I don't have a Roland JC but every now and then I get tempted to buy the 22 Watt version. They are the best looking amps ever and you get an amazing stereo chorus.

    So Roland has been "overclocking" these amps to compete with the loudness of the big tube amps of the 70's?
    The modification described here only works for the preamp schematics I've been able to put my hands on ie mostly 80s/90s JCs (models JC-120/77/55)

    The 22W model you are talking about is a more recent model.
    Also, I think it's all surface mount components now (ie tiny components) vs. through-hole in the 80s.


    Quote Originally Posted by Tal_175
    What do you mean by losing the EQ effectiveness? Does this mod make the tone controls less effective? Can you compensate the gain loss by boosting the input signal with a pedal? I mean to some extent at least without overdriving the preamp.
    What I mean by "EQ effectiveness" is that the gain stage that I found causes the hiss, is the gain stage right after the eq networks (it's a fender-style tone stack btw)
    Basically, once your signal hits the EQ and is shaped by it, it is once more amplified and that's where the hiss is coming from.
    So by reducing the gain AFTER the EQ treatment (to a less hiss-causing gain factor), I am asserting that your EQ is a bit less effective since your eq'd/colored signal is not amplified as much.
    But fear not because there are TWO more gain stages after that and they ALSO amplify your signal by another four-fold.

    I am using an EQ pedal before the amp and it's perfectly fine for my usage.

    I am curious if felipe78 is seeing/feeling a loss of gain and has to crank his amp more.
    Last edited by jazzfrog; 03-07-2021 at 06:17 PM.

  16. #15

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    Quote Originally Posted by jazzfrog
    The modification described here only works for the preamp schematics I've been able to put my hands on ie mostly 80s/90s JCs (models JC-120/77/55)

    The 22W model you are talking about is a more recent model.
    Also, I think it's all surface mount components now (ie tiny components) vs. through-hole in the 80s.

    What I mean by "EQ effectiveness" is that the gain stage that I found causes the hiss, is the gain stage right after the eq networks (it's a fender-style tone stack btw)
    Basically, once your signal hits the EQ and is shaped by it, it is once more amplified and that's where the hiss is coming from.
    So by reducing the gain AFTER the EQ treatment (to a less hiss-causing gain factor), I am asserting that your EQ is a bit less effective since your eq'd/colored signal is not amplified as much.
    But fear not because there are TWO more gain stages after that and they ALSO amplify your signal by another four-fold.

    I am using an EQ pedal before the amp and it's perfectly fine for my usage.

    I am curious if felipe78 is seeing/feeling a loss of gain and has to crank his amp more.

    I'm not felipe78, but I just did this mod on my '80s JC-77 last night and can report my findings.

    First off, thanks for this! I can't believe it's been 30-40 years of hiss on these amps with no progress on the hiss (I know I've had my JC-77 for about 15 years and almost sold it for this reason).

    It was a bit trickier to get the circuit board out than I expected, but going slowly and carefully seemed to be fine. My unit had a wire connection running near the gain pot to further back on the PCB, which had been glued to the board right near R46, so I had to be careful of that.

    I installed a 2.2K resistor, as per the above and it has made a huge difference. Some notes below:


    • Hiss: Prior to this mod, if I turned the amp on with the volume at 0 there was still a huge hiss. That is pretty much completely gone now.



    • Volume: In the past I could barely turn this amp up past 2/3 without it being too loud for my small room and too much hiss. Now I need to turn it up a bit more (3-5), but there is no additional hiss no matter how loud I go on the volume knob. I thought about temporarily wiring in a 100k pot to see where the best compromise of volume and hiss was, then switching to a resistor of the corresponding value, but I'm not sure that's worth the hassle of taking it all apart again.



    • Tone control: My controls/jacks were all very scratchy and intermittent anyway, so I gave then a cleaning while I was doing the mod and it all works great. The tone controls all still seem to be very effective, so I don't think there's a loss of control here---if so, it's minimal.


    Thanks again for this! It has reinvigorated my love of this amp (it was the first amp I bought, so I've always been attached to it).

  17. #16

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    Quote Originally Posted by Tal_175
    Very interesting thread. I don't have a Roland JC but every now and then I get tempted to buy the 22 Watt version. They are the best looking amps ever and you get an amazing stereo chorus.

    So Roland has been "overclocking" these amps to compete with the loudness of the big tube amps of the 70's?
    I have the new JC-22. It has almost no hiss. It’s also 30 watts. (Why didn’t they name it JC-30?) Only downside is, I hear it doesn’t have the original analog chorus circuit, and the reverb is not a spring tank either.

  18. #17

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    I sincerely hope someone from Roland reads these posts and hears the message

  19. #18

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    Now... I miss my JC-55 Great mod and pro research! Congrats!

  20. #19

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    Jazzfrog, I am full av admiration for your ability to diagnos the problem and impliment a solution to the above hiss problem. I adroitly balance this with a deep envy based on my complete inability to do the same. Were I able I would address the hiss I am getting from my Cube 40. I spoke with one of the few tube amp repairmen here in Stockholm, but he would have nothing to do with a transistor amp. Either my amp is getting worse or I am becoming more sensitive.

  21. #20

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    Thanks, jazzfrog, for your contribution re: Roland Jazz Chorus. I had a used JC-120 years ago and kept it for a couple of years. Who could NOT fall in love with the stereo chorus effect at moderate volume? Unfortunately, trying to minimize the hiss in my small practice room began to slowly drive me nuts. It also got pretty harsh at LOUD volume... but there still times when I’d love to hear that heavenly stereo once more. I bet it would be a great Fender Rhodes amp with the hiss mod.

    ...But does any SS amp have a right to be that HEAVY!?! Haha

  22. #21

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    For what it's worth, I needed to fix one of the reverb cables on my JC-77 anyway, so I revisited the value of r46.

    To do some measuring, I temporarily installed two long wires and clipped those to different value of pots (started with 50k, but went down to 10k for fine tuning). In the end I added a 2.4k resistor back in (not because it was better than 2.2k, but because it was close enough and right in front of me).

    I found that the noise increases pretty quickly as you go with a higher value, so staying low seemed like a good idea. The gain does increase quite a bit as you head back toward the stock value of 100k, but that comes with all the hiss.

    I thought about adding a little breakout box with a knob I could fasten to the inside of the cabinet, so it could be adjusted, but I doubt it's worth the hassle.

    I find the reverb on my amp a bit lacking and might consider an upgrade to the tank, but we'll see.

  23. #22
    Quote Originally Posted by p_wats
    For what it's worth, I needed to fix one of the reverb cables on my JC-77 anyway, so I revisited the value of r46.
    ...
    I find the reverb on my amp a bit lacking and might consider an upgrade to the tank, but we'll see.
    That's the first thing I did on my JC-77.
    The original reverb had had a broken spring during shipping.
    I ordered an Accutronics 8AB2D1A spring reverb. It's medium size (bigger than original) and sounds pretty good.

    But it doesn't fit the holes on the chassis. The original reverb is attached the to the chassis, horizontally.
    After research, I was reading that "reverb tanks should be setup vertically". I don't know much about this but it was good for me, since I could only attach the tank onto wood.
    I decided to move it to the side of the cabinet, but since the wires weren't long enough, I had to rewire them as well (all the way from the printed circuit board).

    It now looks like this:
    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-img_0359-jpgA possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-img_0358-jpg

  24. #23

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    I understand that work on tube amps for the inexperienced is dangerous due to the high voltage retained in some parts. Is this true of solid state amps as well? If not so, is there a good source to be recommended for learning basic trouble shooting and analysis for the beginner.

  25. #24

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    Quote Originally Posted by jazzfrog
    That's the first thing I did on my JC-77.
    The original reverb had had a broken spring during shipping.
    I ordered an Accutronics 8AB2D1A spring reverb. It's medium size (bigger than original) and sounds pretty good.

    But it doesn't fit the holes on the chassis. The original reverb is attached the to the chassis, horizontally.
    After research, I was reading that "reverb tanks should be setup vertically". I don't know much about this but it was good for me, since I could only attach the tank onto wood.
    I decided to move it to the side of the cabinet, but since the wires weren't long enough, I had to rewire them as well (all the way from the printed circuit board).
    Ah, interesting! I was curious about how a new tank would fit. I read that the Accutronics 8AB2D1A was the one to get for this amp. I see on the listing for that model it says:

    "Mounting: Horizontal/open side up"

    As different tanks specify different mounting orientation for optimal operation, but I assume if you're sounds good then it doesn't much matter.

  26. #25
    Quote Originally Posted by p_wats

    "Mounting: Horizontal/open side up"
    Aaaah... nice, that's a good point, I didn't see this.
    Another project then. I think it could fit at the bottom of the chassis if you make a small casing for it.
    Thanks, I now have one more thing to do on this amp.

    But we are getting off topic.

    Thank you to the several of you who tried and vetted this modification for op-amp based JC circuits.
    I have not had time yet to try anything related to this on FET (ie transistors) based JCs.
    (but I do have a JC-60 on the way)

  27. #26

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    Quote Originally Posted by jazzfrog
    Aaaah... nice, that's a good point, I didn't see this.
    Another project then. I think it could fit at the bottom of the chassis if you make a small casing for it.
    Thanks, I now have one more thing to do on this amp.

    But we are getting off topic.

    Thank you to the several of you who tried and vetted this modification for op-amp based JC circuits.
    I have not had time yet to try anything related to this on FET (ie transistors) based JCs.
    (but I do have a JC-60 on the way)
    Interesting! I was thinking I may open my JC-77 up again and install sockets instead, so I can change the resistor more easily (not sure why I didn't do that right away, instead of wiring in a temporary pot, etc.)

    I have noticed that, while it is still loud enough for my purposes, it now has less overall volume on tap than my orange Roland Cube 60 (however, cranking that little amp causes the cabinet to rumble anyway...and the reverb also sucks on that one. Ha).
    Last edited by p_wats; 03-18-2021 at 10:34 AM.