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The ultimate JC-120 thread

Started by teemuk, July 26, 2011, 01:33:27 PM

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...or hopefully something becoming one.

As some of you might know "the JC-120" amplifier is actually a collection of very different kinds of product revisions, all revolving around a design concept of basically featuring

- dual channels, clean + effect
- 2 x 60W stereo output
- stereo chorus or tremolo effect
- reverb and distortion
- uniform cosmetic styling

Everything else but these basic features have been evolving quite a lot since the amplifier was introduced in 1975. That also includes how these basic features were accomplished circuit wise. Since not much is written about this, but the topic seems to come around now and then, I hope we could combine a thread with some info of the different circuit revisions and their features, what S/N's cover those revisions, and hopefully, some user input of how those different revisions seem to have performed...


Here's a small and simplified list of differences I've found so far:

3rd Edition (1979) - applicable to
JC-120  S/N  380100 - 415099

- Gain stages are discrete, combinations of BJTs and FETs
- Distortion circuit is an overdriven FET+BJT stage permanently in circuit
- Features a FET-differential -based "pre-limiter" preventing power amp hard clipping
- Discrete BBD oscillator driver
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel.
- Line out tap at speaker output

JC-120   S/N   420600 – 471649
JC-160   S/N   440100 – 470549

- Pretty much similar to revision described above
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Also includes a trimmer for chorus adjust in rear panel.

JC-120   S/N   481650 – 502499
JC-160   S/N   480550 – 500849

- Gain stages are TA-7122AP, low noise integrated preamp ICs
- Distortion circuit is a switched on cascade of two BJT+FET based gain stages
- FET-differential pre-limiter preventing power amp hard clipping
- Current feedback power amplifier (meaning, the output frequency response is not linear but instead varies in interaction to speaker's impedance, similarly to many tube amps)
- Discrete BBD oscillator driver
- Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Trimmer for chorus adjust in rear panel.
- Line out tap at speaker output
- LDR-based muter for the chorus/tremolo

JC-120   S/N   512500 – 779499
JC-160   S/N   510850 – 282918

- Pretty much similar to one described above

4th Edition (1979) – applicable to
JC-120                 S/N   789500 – 248099 or 950100 – 283149 (USA versions)
JC-120A and R&P    S/N   810600 - 001099
JC-160                 S/N   782921 - 244509

- Gain stages are discrete, FET -based
- Pre-limiter circuit omitted, will be omitted from following versions
- Distortion circuit is an overdriven FET stage switched into the circuit
- "Main In" external inputs
- Current feedback used in the power amplifier
- BBD now with driver, pre driver made out of digital logic and discrete circuits
- Separate LF oscillators for vibrato and tremolo. Normal vibrato controls in front panel, chorus adjust trimmer in rear panel.
- Line out tap at speaker output

5th Edition (ca. 1982) – applicable to
JC-120   S/N   258100 and up or 293150 and up (USA versions)
JC-160   S/N   254510 and up

- Gain stages are discrete, FETs or combinations of FETs and BJTs
- Current feedback from PA is omitted and no longer featured in following versions
- Includes a bright switch
- Distortion circuit is a switched in FET gain stage with diode clippers in feedback loop
- "Main In" external inputs
- BBD + BBD driver setup
- LF oscillators for chorus and tremolo now combined to a single circuit. Chorus mode has fixed settings for vibrato speed & depth
- Line out tap at speaker output

JC-120H - the head unit (1984) – applicable to
100/117V      70126811
220/240V      70126814

- Gain stages are OpAmp –based
- "Single channel"
- Distortion circuit is a switched on combination of two parallel shunting diode clippers + switched in gain control in the preceding stage
- Revised reverb circuit with BJT drivers and current feedback
- "Hi-Treble" potentiometer control
- "Sub In", external mono input
- "Normal/Mixed" and "Effect" line outs preceding power amp
- Four speaker out jacks (this is a head)
- BBD and LFO setup similar to 5th edition of the circuit

JC-120 UT / JT (2nd Edition - these are the newest JC-120 amps)

- Gain stages are discrete, FETs or combinations of FETs and BJTs. FX loop circuitry also uses OpAmps
- Distortion circuit is a switched in FET gain stage with diode clippers in feedback loop, very similar to circuit of "5th ed". but some component values are different
- Modern FX loop with loop level selector and all other goodies
- BBD and LFO setup similar to 5th edition of the circuit
- Revised reverb circuit, similar to JC-120H model
- Output –tapped "Line out" feature omitted


Some pics that display cosmetic differences of JC-120's of different pedigrees:

From top to bottom

- Very old version of the amp: Notice the chrome corners and the overall green-ish colour scheme.

- Somewhat newer version (approximately 1982 and onward): Notice the addition of "bright switch" feature, plastic corners and a grey-ish colour scheme. Jacks are now insulated type with plastic cover (instead of old plain metal jacks) and the potentiometers knobs are different too (they have varied slightly throughout the series' history). The Roland logo in the grille is also bigger.

- Even newer one: Bright-switches are now push-button -type, the Vib/Off/Chorus -toggle has been converted to a rotary selector -type. Speakers are different and no longer have metal dustcaps. At some point of the revision history the metal "lever" -type rocker mains switch converts to modern plastic rocker switch.

Samples of rear panels differences.

From top to bottom:
1. Very early version. Note the lack of rear cabinet panel and that the labels were silkscreened to the painted chassis. Controls from left to right: Mains/polarity switch, convenience AC outlet, Chorus/Vib F/S jack, Reverb F/S jack, Distortion F/S jack, two EXT speaker out jacks, line out jack (mono).

2. Newer version. A rear panel with label stickers is added (this was probably a cheaper solution than silkscreening the chassis). The control/jack arrangement is the same except that a fuse holder replaces the mains switch and that the convenience AC outlet is removed. A briefly earlier version (ca. 1976 and not depicted) still had both fuse holder and the convenience outlet. A Chorus trimmer potentiometer is also added to the far right.

3. Late 1970's version. Controls from left to right: Mains fuse holder, chorus adjustment trimmer, two EXT speaker output jacks, line output jack, chorus, reverb and distortion footswitch jacks, two mains in jacks.

4. Very late 1970's / early 1980's version: Chorus trimmer is omitted, otherwise the panel arrangement is identical.

5. Bottom: UT/JT modern JC-120's: Two line out jacks, FX loop stereo return jacks, FX loop send jack and loop level switches, three footswitch jacks for chorus/vib, reverb and distortion.

The head model, JC-120H

This is actually the model of which's schematics are most commonly referenced when people request for a JC-120 schematic. An important point is that the schematic (often passed on with nickname "1984") doesn't match ANY of the JC-120 combo designs by a long shot. First and foremost, the gain stages are based in opamps only, unlike in JC-120 combos. Unique features incommon to combo versions also include:

- The amp is single-channel design
- "Hi-Treble" control, which is basically a bright switch replaced by a potentiometer
- Effect line out, which is basically a mono line output for the wet chorus effect only
- A more accurate labelling of the revised chorus effect: Manual/Off/Fixed
- Sub In jack, a line in jack that basically bypasses most of the preamp, excluding chorus
- Line out jack, a line out that mixes both channels (wet and dry) to a single mono output. The difference to certain combos that had line out jacks is the take off point. In combos the take off point is at the speaker output, in the head version the takeoff poinr is before power amp.

Rear panel arrangement from left to right: Fuse holder, two speaker out jacks (left), two speaker out jacks (right), Sub In jack, three footswitch jacks for chorus, reverb and distortion, effect line out jack, normal/mixed line out jack.


Another important difference are the speakers. I'm not too familiar with this topic, barely I know that speakers used in Roland JC-120's (and JC-series amps in general) have also changed throughout the years, a variable also having quite big impact on the overall tone of the amp.

The early amps feature Roland Corporation "Musical Speaker" drivers, made in Japan. These have a distinctive metal dustcap and either a black&blue or black&white label.

R&P speakers were developed in collaboration with Pioneer in the late 1970's, they basically appear in circa 1979 amp revisions. These drivers featured some fancy innovation but I'm not entirely sure what it was; basically they seemed to have a wider and HiFi'er response than generic Roland speakers of the time and the new technology was marketed keenly. The Roland amplifiers featuring these speakers were marketed as a unique top notch line, either indicated by "A" or "R&P" in the model number. Some amps even featured R&P insignia but I'm not sure if JC-series ever had that. These speakers again had metal dustcaps.

In the early 1980's Roland started to manufacture some of the speakers in California, USA. These Roland AD "Audio Development" Heavy-Duty Transducers seem to be featured in the newer JC-120's. These speakers also seem to lack the metal dust cap.

J M Fahey

Thanks a lot. :tu: :tu: :tu:


I agree, excellent information as always, Teemu!   :tu:
Life is what you make it.
Still rockin' the Dean Markley K-20X

Kaz Kylheku

Interesting stuff!

I started looking for some schematics because of this.

The vintage ones from the 1970's are harder to find. Here is a time saver for ya: google for the file name "Roland JC-120.zip".


ADA MP-1 Mailing ListMusic DIY Mailing List

J M Fahey

Hi kaz, thanks a lot for your posting/linking.
Very interesting.  :tu:

Hi Teemu: thanks for pointing to the early soft limiter.
It's very similar to somthing I had used as part of a tube emulation circuit, but this one has its own distinctive flavor.
It *is* a long tailed phase inverter.
Mine was the full monty , including the "magic" 82K and 100K "plate resistors" with both "plate signals" (which are balanced or at least symmetrical and out of phase) sampled and summed by an instrumentation type  differential input Op Amp circuit.
Somewhat complex for what I usually do, remember I'm a Minimalist at heart, but I always start "following the rules" and only after I get what I want, start chopping off the deadwood.
Of course, that "differential summing" is performed by the power tubes and OT in a classic tube amp.
It did sound good, adding some "tube flavor", distorting quite different to any classic SS circuit, showing that the base of a sound does not lie in "glass vs. sand" or "solid vs. vacuum" magic but on what the circuit *actually* does.
Later I found that I retained 90% of the sound by using the signal from just one "plate", much simplifying the circuit.
After that, I used a volume pot, which was equivalent to a PPIMV , which, as we all know, i9s not the perfect solution but noticeably better than an "end of the preamp" MV, which easily gets buzzy.
I see Roland took the same approach, but adding (very clever):
1) they do not use the Master Volume but with the preset "starve" the 2nd FET so It *just* provides the necessary signal to drive the Power amp.
I'd *LOVE* to read the service notes explaining its adjustment.
Specially if they show scoped waveforms.
2) the FET does not provide a "fixed" clipping level as a couple diodes or similar but somewhat tracks the +/- power rails, it's fed unregulated voltage derived from them.
I guess it must provide some "sag" at high power levels (when said rails voltage falls under load).
Not much, I guess, but anyway a step in the right direction.
They provide a simpler and much stronger similar effect in the Blues Cube.
The Sag switch kills the voltage rail feeding the last clipping FET.
Anyway, and as always, hats off to Roland (design geniuses) and to you for providing such useful info.  :tu:


QuoteI'd *LOVE* to read the service notes explaining its adjustment.
Specially if they show scoped waveforms.
There's not much to see, they tell you to adjust the trimmer to achieve results shown in few hand-drawn waveforms.
This [see attachment] is actually from the service manual of SB-100, a Jazz Chorus series look-alike but a bass amp. Pretty much all the Roland amps featured that FET circuit during that particular era.

J M Fahey

Thanks a lot.
Even though not explained, those waveforms tell me a lot.
I guess Vr5 still is the trimmer on the 2nd FET drain, of course.
Can you post SB100's schematic (or at least the limiter and power amp) so I am sure about it?
Thanks again.


VR5 is indeed the trimmer for the FET circuit.

Sorry, but the complete service manual is too large to attach. You can download the entire file from eletrotanya.com, though.


J M Fahey

Thanks a lot, it was very helpful. :tu:
I don't understand why they dropped it, surely it's not a cost decision, we are talking less than a dollar and sure it improved the final product.
Maybe they realized the average JC120 user *never ever* uses it at full power,  being typically a Jazz/session/Tango/etc. player, even a Rocker but searching a very clean sound, who has "something else" for his distortions.
I live in Buenos Aires and have seen Tango guitar players use them; in small shows they even plug a microphone in the spare channel and use it as a small PA system.
Doubt they use it over 20/30W if anything.


Personally, I was always fascinated by the "rivets".  I wasn't sure if it was supposed to look like fancy upholstery or kind of medieval theme, but distinctive, nonetheless.


I like that detail too, and at at least to me it sort of gives an impression of "function-defines-form" -type of design; an "armor-clad" amp that's ready to take some serious bumbs on the road like it's nothing. Personally, I have a thing for amps that look more or less like sturdy roadcases.


What makes the chorus and vibrato effects stereo? (other than the obvious, that they have two channels!) Are the left and right channels out of phase when using the chorus effect? If so, how many degrees out of phase? Do both channels change pitch in unison when using the vibrato effect or does the effect use the two channels in a different way somehow? I understand the way the effects are implemented might vary greatly between the different versions of the amp.