Old Roland Jazz Chorus JC-120: Adding Stereo FX Loop

[heyniceguy]

Hello,

I own an older Roland Jazz Chorus JC-120, and i would like to retro-install an effects loop into the "MAIN IN" sections of the amp so i can place my Line 6 DL4 and MM4 in it and get full stereo capability out of the modelers. Id like to run the 2x12's in the amp as "LEFT" and another 2x12 cab as "RIGHT" on the other side of the stage. The DL4 has a cool ping pong and stereo delay and the MM4 has a rad stereo panner and other such stereo modulation effects. They sound great in headphones and i'd like to recreate that on stage. And i plan to make it a stereo send and return, so i can have the benefit of using the amp's built-in stereo chorus/vibrato.

At the effects send point in the amp, the nominal voltage of the signal is 800mV according to the schematic. I called Line6 and they said these pedals are designed to go "in front" of the amplifier, and not in the effects loop of the amp. They could not help me further because they say "it involves the use of the pedal in a fashion it was not designed for."

So basically, these pedals were designed to take a instrument level signal. So for a passive humbucker, it will expect a <100mV signal, probably closer to 80mV. That means i will have to make a 0.1 gain attenuator before my send jack (to drop the 800mV signal to 80mV), and a 10 gain amplifier after the return jack (to bring it back up to 800mV), if i want to use these pedals in the effects loop without distorting the signal in the Line6 pedals.

I plan to use a simple inverting op-amp attenuator before the send, and then an inverting op-amp amplifier to bring it back up after the return jack. Both the left and right send/returns will have this setup, totalling 4 opamp sections.

Does this sound like it will work? Is ~80mV a good nominal SEND voltage? Is there any problem with sending inverted (180* out of phase) signals out the SEND jacks?

here is the schematic showing the MAIN IN jacks where i will tap for the loop:

http://www.freeinfosociety.com/elect...C120-JC160.pdf

Thanks...
JB

[Mark Hammer]

No real need to go active, from the looks of it.

Looking at the schematic, Q5 in the top channel (and Q6 in the lower channel) is followed by a 1uf cap and 100k resistor to ground. You could simply replace that 100k resistor with a 68k and 33k resistor in series (68k first) and take the feed to what is labelled as "9" and "12" on the diagram from the junction of the two resistors. That should pad down the output enough to be friendly to the modeller pedals. If it is still too hot, then perhaps 82k and 18k would work better. Alternatively, if you have an inclination to install a hole for a pot, then sub a 100k log pot for the 100k resistor, and take your send feed from the pot's wiper.

The Line 6 pedals likely have electronic switching internally, such that the .082uf cap on the return path in the amp (C15) is never left hanging and susceptible to popping. So you can just run a cable to the pedal from the padded-down send point and leave the return point permanently patched into the pedal output without modification. In principle that ought to work.

The attenuation won't necessarily let you extract maximum output levels from the amp itself, but then do you actually do that now?

[heyniceguy]

good idea, mark. the only drawback, like you said, is that the overall volume output is permanently diminished due to the padding of the sends. this may present a problem for live performances.

i used to not use the JC120 preamp. my guitar was plugged into a pedal board, into a sansamp, into the stereo Line6 pedals, and into the two MAIN IN's of the JC-120, and then to 2 separate 2x12 cabs on opposite sides of the stage. I noticed a considerable drop in volume from when i used the jc-120 preamp, so i threw a stereo booster after the LINE6's. Worked well enough, but i'd like to use the JC-120 preamp while still getting stereo capability out of the amp.

in that case, i could use 2 dpdt button switches. one switch would have a pole to switch out the 100k vs 82k/18k network for the pre-send attenuator and the other pole to switch out a gain feedback resistor in a non-inverting amp stage immediately after the return jacks and before the cap. and then have the other dpdt switch for the other side. that way, there will never be an instance of one side of the amp being overloaded if i forgot to hit one or both of the switches. the worse to happen would be a clipped signal coming back from the modelers. which is subtle, but easy to hear if you listen for it.

another question: do i need a treble bypass cap on the 82k/18k voltage divider on the pre-send? i know for pickups, that cap is needed due to the output impedance of the pickups being so high. do i need any such cap in this case, or is the output impedance of the FETs low enough to handle it without losing highs?

[Mark Hammer]

You should not need any sort of tonal compensation such as you describe on the SEND signal. The pedals likely anticipate an output impedance higher than what the amp send will present.

You could insert a small FET booster after the pedals in order to restore level and crank the most volume out of the amp that you can. Just keep in mind that any gain recovery provided by an outboard booster will apply ONLY when you have a pedal whose output jack goes to that booster and back into the amp. Pull the phone plugs from the send/receive jacks and you lose that boost and retain the internal attenuation network.

[Enzo]

No reason the amp should be permanently diminished. Just because we replaced a 100k resistor with a voltage divider for output to the new send jack, doesn't mean we cannot take the signal from the original top of the 100k point and run it to the cutout contact on the tip of the new return jack.

That way when not using the jack, the contacts close and the original signal level is shunted through. Plug into the return, and the contacts open, leaving the amp to look for whatever is coming in the return jack.

[Mark Hammer]

Bingo.

[heyniceguy]

Update

So last nite I installed the SEND jacks at each MAIN IN jack. No resistor attenuator network just to see what level i was dealing with.

SETUP:

Korg MS2000 --> DL-10 --> AMP INPUT CHANNEL 1

Guitar --> TU-2 --> Homemade TS808 --> SANSAMP* --> GE-7 --> NS-2 --> AMP INPUT CHANNEL 2

AMP SEND (L) --> DL-4 (L) --> MM-4 (L) --> MAIN IN (L)
AMP SEND (R) --> DL-4 (R) --> MM-4 (R) --> MAIN IN (R)

*Sansamp is set for clean fender sound to give the otherwise sterile JC-120 some warmth. Drive and Level are fairly low on SANSAMP. This kept the signal lower than what it would have been had it gone straight into the amp. So, low voltage, but a strong enough current supply to keep the sound full.

Result = no breakup through the modelers. Now i have stereo effects for both the guitar and synth, while maintaining the built-in stereo chorus/vibrato of the amp.

I'm currently working on a way to switch the speakers from the stock "internal stereo" configuration to an "external stereo" config. As it stands, the 2 speakers are driven of the separate amps in the JC-120. i'd like to have a way to press a button on the back to activate some high current relays to switch it so the 2 speakers in the combo are now wired in series and powered by the LEFT amp, while the RIGHT amp powers an external 2x12 cabinet of similiar impedance through the RIGHT SPEAKER OUT jack. Stick those puppies on opposite sides of the stage and ping pong my way to rock-dom. I have a schem drawn up and i will let you know how it turns out.

[heyniceguy]

Update To The Update

hey all,

so i installed the speaker relays today. got some 125VAC contacts with 12VDC coil activation. Tapped off the 15V from the board. The relays switch great. In stock configuration, you cant tell that it's modded.


but when i activate the remote speaker option, the remote speaker sounds crappy. signal broken up. volume is there, but clarity is gone and a gross sounding buzz/hum over the signal. more to come...

ok bad cable. replaced. all is well.

[miamigroove]

how did it sound?

I know this is an old thread, but...
I have an older JC-120 w/o the FX loop as well. I was thinking about doing something similar to run my Del. Memory man + D. Electric Mistress through instead of going straight into the amp.

Remote speaker switching aside, how was the sound different going through the FX loop as opposed to going straight into the amp? Im trying to decide if it merits enough to have to do the work involved in adding the stereo FX loop. I simply love the amp as-is, and unless it were to provide a better sound, noise reduction, etc Id rather not tinker with it. thanks

[heyniceguy]

basically, all i did was insert a SEND jack before each MAIN IN jack. I did no attenuation of the SEND signal and was therefore giving the Line 6 pedals a hot signal (~800mV). I was afraid i would hear clipping after the signal returned from the Line 6's. But none. It sounds just as good as it did in front of the amp, but in stereo. when bypassed, i could tell no change in the 120's unmodded signature sound.

[Mongo]

JC-120H Stero Inputs?

Heyniceguy,

The way you wired in the loop seems like a sweet setup. I have a JC-120H that I would like to use with a stereo preamp. There is only the mono inputs or a sub in.

Do you think there is any way to place an insert option to utilize the stereo capabilities of the head (and chorus) from an external source?

Mongo

[rpape2601]

basically, all i did was insert a SEND jack before each MAIN IN jack. I did no attenuation of the SEND signal and was therefore giving the Line 6 pedals a hot signal (~800mV). I was afraid i would hear clipping after the signal returned from the Line 6's. But none. It sounds just as good as it did in front of the amp, but in stereo. when bypassed, i could tell no change in the 120's unmodded signature sound.

Ok, so I don't have any main in inputs or a sub in. Is it still possible to add a stereo effects loop to my JC120?