Roland JC-40 Hiss - Any way to fix this?

[ribas]

I can look into it. Do you have the schematic or service manual for it?

Hello!

I'm also really interested in investigating if there is a way to mod the JC-40 to remove the base noise hiss like you did with the JC-120 ( :handsdown: )

Here are some pictures of the circuits inside the JC-40, maybe it gives you an idea how it should be moded or maybe if it's not doable at all. (Very tiny components and complex circuit in comparison to the old JC-120 you moded)

HIGH RESOLUTION: https://mega.nz/folder/jUcHna4b#hE3tZSJLaBNUoWZ4I0KsPg

[Payne]

I am sorry, I cant help you without the schematic. Tiny components is not a problem....?

[pcjazz]

I always understood that hiss was part—perhaps most—of the JC’s charm.

[jads57]

Sell It?

[jazzfrog]

See details in this mod post here, it worked for me on a '85 JC-77.

I got rid of the hiss by swapping ONE resistor, giving up some volume (it's fine, that amp is loud) and some eq effectiveness (fine as well).

Newer JCs (JC-40 etc) use surface mount and my mod does NOT apply unfortunately.

Also, Payne's post shows transistors mods (thanks for your work Payne, it definitely helped directing and inspiring my solution).
My JC-77 uses opamps, not transistors. So again, this works on my mid-80s JC-77.

[PMC63]

I've had reduced the hiss by using the effects loop. I believe it disables the panel controls
a trade off but I feel it's worth it.

[ElRay]

Our JC-40 has developed "The Hiss", but it seems to be entirely pre-amp related. On another forum, somebody recommended to just pull the board and re-flow all the solder joints. Has anybody here tried that? Does that should like it would help, or at the very least, do no harm?

[sgosnell]

It should do no harm, but anything that can possibly go wrong, probably will. Wires can jump from the joint when the solder is melted, and other unintended consequences. Use care and all should be fine, but there are no guarantees.

[whiskey02]

The idea of buying a product and then opening it up to do/fix something the manufacturer did not do strikes me as absurd. Maybe doing something small to tweek things a bit to your exact needs is one thing but having to FIX something as it comes out of the box; No, I wouln't do that.

[lbovay]

I am sorry, I cant help you without the schematic. Tiny components is not a problem....?

I’m late to the party but this amp has great features that are still relevant today so here I go.

Just got the JC-22 and noticing the same issue, exacerbated by the treble control / bright switch. I’m gonna drop this Link here and see if someone has a genius idea to tame this hiss.

Roland JC 40 | PDF | Electrical Wiring | Printed Circuit Board

Can be downloaded as PDF with this: Scribd Downloader - Download PDF From Scribd For Free

Happy to PM the PDF as well if needed.

I wouldn’t mind losing some loudness too, I would not run it louder than 4-5 as it is. I’m gonna try the shunt resistor trick on the volume pot to get a bit more sensitivity at lower volume.

Cheers.

[Payne]

Hi, I am sorry, the schematic is simply too blurred for me to read it

[lbovay]

I sent you a PM with a link that should look better.

[Payne]

Schematic looks better and readable now.

I can see the amplifier is digital and the analogue signal is quickly converted by an DAC to be processed by an DSP - so there is not much analogue circuit.

However, it is possible to try to trace where the noise comes from, but it will need that you are a little familiar with electronics, solder iron and tools - I just can't give a precise answer from where the hiss originates, but there are something to try.

Another thing: is your amplifier grounded to earth ground at the mains connector? Typically, grounding will remove a lot of hum and interference noise

[lbovay]

Soldering skills are decent, no proper microscope at hand for smd, but should be manageable with eyeballs.

I have to replace a potentiometer as well so amp will be on the bench in a week or two once the parts arrive.

Concerning the hum I noticed that if I inject a line level signal in the front inputs I get terrible noise / hum, but this goes away once I set my pedals to instrument level (I have a nano cortex on my board so i play with that level knob). There is a slight hum at high volume as the cable from the pedalboard runs parallel to the power supply cord from that same pedalboard, but I’d like to keep it that way and can live with the hum.

Is there a way to reduce the overall loudness of the power amp section ? I’m under the feeling that cutting the power amp gain in half should lower the hiss. For now the hiss is constant and does not increase if I change the volume knob that’s why I don’t think it comes from the preamp but I could be wrong.

[Payne]

You can try to solder 100k resistor in parallel with R92 and R180 (Main Board 3/4 in input amplifier). This will lower the amplification from Av=22 to about Av=8

Alternatively also try, solder 22k in parallel with R134 and R60 in the output amplifier IC10, IC23, which will half the amplification

[lbovay]

You can try to solder 100k resistor in parallel with R92 and R180 (Main Board 3/4 in input amplifier). This will lower the amplification from Av=22 to about Av=8

Alternatively also try, solder 22k in parallel with R134 and R60 in the output amplifier IC10, IC23, which will half the amplification

Sounds good. I just ordered some 0603 and 0805 smd kits won’t be fast shipping from overseas but I’ll give this mod a shot once I get all the materials.

Thank you very much for your inputs so far !

[lbovay]

So after discussing this with AI and doing some tests I concluded the hiss likely stems from the power amp. Also I didn’t wanna mess with the preamp as it handles the EQ and messing with it might lower the effectiveness of the EQ.

So I went ahead with replacing r60 and r134, originally 22k, with first 11k and finally 7.5k resistors.

With 11k the hiss was a bit quieter and the max volume was halved. Still too much to my taste so I dropped it to 7.5k, now the hiss is much lower, still noticeable but not as prominent anymore. The max volume is still plenty.

Let’s see how it plays over time but quite happy with it at the moment.

[jcg6523]

Ibovay, thank you for uploading your results. I am going to attempt this soon, I don't have any smd components on hand at the moment.

I just picked up this JC-40 yesterday, and it quickly developed an annoying hiss. It didn't start that way, it was dead silent in the room at first. It was a akin to the guitar just floating around the room. Then I plugged in a pedalboard of mine and I heard the hiss, I wrote it off as just a culmination of gain staging in the pedals somewhere. When switching back to just straight guitar in, I noticed the obnoxious hiss persisted. At low volume, high volume, no volume, nothing plugged in, unaffected by the eq or chorus/reverb section.

Also I plugged into the headphone out to hear there it and it sounds gorgeous, no hiss at all. I am guessing there was some component on the way out and plugging in this way went ahead and did it. I am honestly not certain. I know it is audible across the room at idle and is audible while playing as a texture behind the guitar.

I don't understand fully what would have caused the symptoms to evolve in this way. Pedalboard is on a high quality supply, I am just happy I can at least plug in to the headphone output and play clearly.

About to test the line outs as well, according to schematic they should be unaffected by the hiss but I have yet to verify.

[Tal_175]

JC-40 has an F/X loop. It's easy to isolate the power amp and see if the hiss is coming from there. Just plug your guitar directly into the F/X return (with the volume knob dialed down). If there is no hiss, then it's the preamp.