Add a master volume control to Roland JC-120

[aentresz]

Hi people,

Call me crazy (I know that I am), but I am wanting to put a master volume control on the JC-120 as I actually like the sound of the preamp overdriving, as it sustains really well, and has this weird lo-fi Albert King like tone (especially with the chorus on), it's much nicer than the inbuilt distortion (which is pretty putrid). Firstly, is it possible at all? I understand that the fact the amp is stereo probably complicates matters, and also the fact that it's solid state probably also would too, and secondly, would it be an expensive job to do ?

Cheers, Aaron

[J M Fahey]

No, not *that* crazy, not much more than any of us anyway 
As you know, we have a psychologist take sanity exams here, and if you pass it, you can not be a member  :lmao:
At least that's what the tube fanatics think about us, anyway.
Oh well.
You are not wrong with Albert King, he *does* use SS amps !!!!
Now, how do you know you are hearing "preamp" distortion?
If you play real loud, you are clipping the *power* amp for sure.
I fear that if you simply put a Master volume between preamp and power amp, you might lose a lot of distortion when turned down, unless you add some extra clipping.
Do your homework, google the amp schematic (there are a couple different versions, try to find the closest one to yours) and post it, so we all refer to the same.
I guess a couple red Leds soldered across the master volume or something similar will do, but let's see that schematic first.
They will provide a somewhat soft squarewave clipping, similar to your power amp being overdriven.
After all, most SS power amps are just "big" Op Amps and that's the waveform they provide, so essentially we are not changing things much here, just being able to lower volume somewhat.
At very low (bedroom) volumes, it will probably become somewhat buzzy, but most Master Volumes do, including Tube ones. .

[teemuk]

There are too many too different versions of JC-120 that I'd care to comment in detail. Somewhere out there in the Internet is a schematic pack for JC-120 and from it you can find at least five different JC-120 schematics, all of them completely different from one another. The estimation is that there are circa 8 - 12 different versions of that amp; some of the versions being quite close to others in arhitecture, some being totally different.

As for what comes to overdriving these things... The earlier ones actually had a FET differential pair -based soft clipping "pre-limiter" circuit built in and cranking the volume control could be used to overdrive that pre-limiter but the limiter circuit itself prevented the power amp from going to hard clipping.

That circuit, along with plenty of other more "tubey" voicing tricks, was however removed quickly from later versions of the JC-series amps, resulting into something that gave a reputation of that "sterile" JC-120 sound. The early versions were much warmer and much less "HiFi" than the ones that started to appear late 1970's and onward...

On that note, the distortion circuits in all these different versions can also differ quite drastically, some being plain diode clipping, some being overdriven FET stages, voicing of them being different, etc. so the distortion function in some JC-120 might totally suck while in another version it might sound entirely different and even passable.

So... These amps can be very, very different depending on the circuit revision thus I think it's really not wortwhile to make any comments about mods and all unless we know which circuit exactly we are talking about.

[aentresz]

Cheers guys for your replies ! It's the current JC-120, I bought it around a year ago. I've been running a graphic EQ, and just boosting that by 12db to achieve some moderate breakup, so I think it's the preamp that's doing the overdriving. I don't know a huge amount about modding amps, other than what I've learned just from tinkering around with bits and pieces, so I don't know how feasable this idea is; is there a way you can control the volume of the power amp, so basically you can turn up the preamp so some breakup occurs, while you are not deafening the entire southern hemisphere of the world ?

Cheers , Aaron

[aentresz]

I tracked down a schematic, it's from 1984, so it may not be exactly right, but it should give a general idea (I hope !) 


Cheers Aaron

[teemuk]

Sigh. When will folks learn...?

You need the proper schematic.

The JC-120 designs from different earas are widely different from one another. Schematics from different years than your amps pedigree won't really help you too much, rather than just confuse you entirely.

Also... The " '84 schematic"  circulating in Internet is for the head model JC-120H, and - as far as I know - for that model only. You can already tell that by looking at the amp's features and arhitecture and comparing them to those of your amp. This design is again very different from the JC-120 combos. ...or does your amp happen to be single channel, without speakers, or happens to have a hi-treble control and OpAmps in the preamp...? Doubtfully.

If your amp is the current JC-120 - that is JC-120U, JC-120UT or JC-120JT - then you need the proper schematic for that particular model. Contact Roland and give them the serial number and they will hopefully provide you the right documentation.

[joecool85]

You could always build an attenuator.

[J M Fahey]

Dear Teemu:

As for what comes to overdriving these things... The earlier ones actually had a FET differential pair -based soft clipping "pre-limiter" circuit built in and cranking the volume control could be used to overdrive that pre-limiter but the limiter circuit itself prevented the power amp from going to hard clipping.

That circuit, along with plenty of other more "tubey" voicing tricks,

do you have anything on that?
I find it very interesting.
Thanks in advance.
JM

[Polyinstrumentalism]

I just experimented with my own JC-120 (a more recient one with an effects loop) by switching my effects loop from parallel to series and sticking a compressor in the loop to control the level after the preamp. With the preamp cranked all the way, there isn't too much breakup in the signal. Based on this, you might be disappointed with the results that you would get from this mod.