freestompboxes.org

Hi folks
As promised my version of the KORG CX3 1980's portable organ Rotary Effect.
This is definately not a project for beginners ( not another fuzz face or thuper hard on ), but if you are up to it, you will be rewarded with beautiful Hammond Leslie speaker cabinet type tones.
I have kept it as close to the original KORG circuit as possible, with the following changes - an additional n channel jfet buffer at the input, a simpler fast/slow speed switching arrangement, and add on daughter boards to accomodate TDA1022 BBD chiips for those (including myself) who can no longer find the Panasonic MN3004 chips as originally used in the Korg.
The circuit also requires a 16V AC power pack ( it has it's own rectifier diodes and regulator chip on board)
I housed mine in a NSC 1790 case and bolted the 7812 regulator directly to it as a heatsink - works fine.
I used two TPDT footswitches - one for bypass - one for fast/slowspeed.
The slow speed is enabled by applying a small positive voltage to the marked pin on the board ( I used a 1K5 resistor in series with my three indicator leds and switched the earths accordingly - I took the the junction of the leds and the 1K5 for my small voltage source - +1.6v)
I was surprised when my build worked almost perfectly the first time I fired it up , however, one section - the amplitude modulation, or tremelo on the high frequency chain did not work at all. After considerable agonising, I discovered what appears to be an error on the original KORG service manual schematic - there is shown, a 4M7 resistor. I changed this to 4K7 and voila - tremelo on the highs
There are 5 preset pots on the board - the first two reading from the top down, set the bias voltage to +6v on the two BBD chips - pin 3 for the MN3004, or pin 5 for the TDA1022
The bottom two presets ( left and right bottom corners) set the frequency of the bass and treble oscillators maximum speeds - KORG reccommend using a frequency counter and setting the bass preset (left bottom) to 145msecs and the treble preset to 130msecs. I used my ears to determine the most realistic beating effect.
The last preset in the middle of the board towards the bottom, sets the amplitude modulation (tremelo) gain - again adjust by ear to get the most realistic effect.
Well - there you have it - good luck, if you would like to build it, but you sure aren't gonna make any quick dosh building and selling these - you could build 50 SHO pedals for what the parts will cost you
BUT - if you build it - you will love it, and that's for sure
Okay - lots of room for mods - split the BBD delay lines , alter the balance between treble and bass sections, variable speed oscillators, stereo outputs etc. etc. If you feel up to it, you are most welcome to experiment. I built it to evaluate it - i have many thoughts in my head as to where to go next - PT2399 digital chips, true sine wave ( it has triangle wave oscillators) oscillators etc., so do not be surprised if you see a completely different unit this time next year
Enough rambling - here is the PCB and layout pics - PM me if you require higher resolution pics
Cheers
baja mana mana

Hi again
I used 2N5089 npn devices for all the transistors - you could use BC549C devices as well - just make sure you get the pins around the right way on the layout
cheers
bajaman

do not fit the component marked with a question mark
bajaman

Holy Cripes Steve!!
Really REALLY Nice Job!!

I love it......thanks Baja.......!!!
Aharon

Holy mackerel!

Forgive my possibly stupid question, but isn't the CX-3 a keyboard? Is this just the Leslie sim lifted from it? Now, reasonable questions...

1) How convincingly does it do the Leslie thing?
2) What separates this from the Rotosphere, Roto-Machine, Destination Rotation, Spin II or any of the other Leslie sims?
3) Does the speed ramp up and down?
4) Are there separate controls for the rotor and drum?
5) What’s the topology (if any)? I looked at the layout quickly but did not see anything that looked like external knobs.

I went to the Korg site and listened to the CX-3 sound samples. They were pretty happening. Thanks for all your work.

Understand, the new Korg CX-3's are DSP. They have added a lot of sonic features (including improved leslie sim presumably). This leslie sim is from the original analog CX-3, which are not produced anymore.

Thanks Steve, nice job! This was undoubtedly a lot of work. This is the epitome of what DIY electronics should be AFAIC.

I wonder if making the separate bass/treble LFO frequency controls accessable on the panel would be useful? Also, I wonder how difficult it would be to create separate tweakable ramp times for bass and treble? This is a common complaint about leslie sims that the "horn" and "rotor" ramp up/down at the same speed. Also, I wonder if it would be possible to implement a "braking" function... Just spouting some ideas off the top of my head...

Would love to hear some sound clips if you get a chance.

Doug

Wow! Alot of work is right . Adjustable ramp times for the bass/treb(fast for the horn, slow for the rotor) i bet would sound great. crap, now i gotta build this.

@ Rotagilla

1. For keys?, Not bad, there def are better out there. Fast setting is not real great. This appears to be targeted at guitar use though, so how it sounds on keys might not matter.
3. The original does, so I would assume yes.

Thanks for the positive comments folks
It did take a few hours to shrink the original pcb size etc., but well worth the effort, and a completely different sounding unit to all the others that I have heard.

but isn't the CX-3 a keyboard? Is this just the Leslie sim lifted from it?

YES - to both questions. It is on a separate board in the old CX3 (KLM-246C), so it looks like they perhaps intended to produce it as a standalone unit or maybe they added it on as a late addition to the keyboard design.

How convincingly does it do the Leslie thing?

Subjectively speaking - very well

What separates this from the Rotosphere, Roto-Machine, Destination Rotation, Spin II or any of the other Leslie sims?

It is not digital for starters - basically it is, two filtered short BBD delay lines driven by independent adjustable rampable oscillators, with a small amount of amplitude modulation (tremelo) on the high frequency section. Most of the filtering is performed wby discrete low noise transistors, so it doesn't have that "op amp" sterility that the 5532 ICs give the rotosphere in my humble opinion. I have not heard the other simulators, so cannot comment further here.

Does the speed ramp up and down?

YES

Are there separate controls for the rotor and drum?

Yes - but only presets to fine tune the oscillators and AM gain sections

What’s the topology (if any)? I looked at the layout quickly but did not see anything that looked like external knobs

No knobs on the original Hammond Leslie - so, no knobs here too

I wonder if making the separate bass/treble LFO frequency controls accessable on the panel would be useful? Also, I wonder how difficult it would be to create separate tweakable ramp times for bass and treble? This is a common complaint about leslie sims that the "horn" and "rotor" ramp up/down at the same speed. Also, I wonder if it would be possible to implement a "braking" function... Just spouting some ideas off the top of my head...

I think once they are set, it is best to leave them alone.
As far as separate tweakable ramp times, I believe it already has these implemented in the voltage control speed section of the oscillators, I noticed different time constants, but I could be wrong - I will investigate further here.
I never fully understood the braking function - doesn't the bypass switch achieve the same effect - please clarify for me Doug

With regard to sound clips - you really don't want to hear my awful guitar playing I will post some sound clips when Stevie Ray Harrison comes around next week
cheers
bajaman

Hi snoof

Adjustable ramp times for the bass/treb(fast for the horn, slow for the rotor)

NO - it is the other way around - the horn spins slightly slower than the drum
bajaman

The Rotagilla wrote: 1) How convincingly does it do the Leslie thing?

It sounds real good with my cx3 in slow speed. Fast is not as good, although there's some thought that it might work better for guitar.

The Rotagilla wrote: 2) What separates this from the Rotosphere, Roto-Machine, Destination Rotation, Spin II or any of the other Leslie sims?

I have no earthly idea. Given that this is the first (that I know of) "guitar pedalization" of this cx3 circuit, I bet no one else does either- yet. edit: Except Steve.

The Rotagilla wrote: 3) Does the speed ramp up and down?

Yes

The Rotagilla wrote: 4) Are there separate controls for the rotor and drum?

What controls?

The Rotagilla wrote: 5) What’s the topology (if any)? I looked at the layout quickly but did not see anything that looked like external knobs.

There's another cx3 thread on here somewhere that has some of the service prints linked in. I believe he developed the layout pretty much directly from that. The schematics might give you more of a clue. My cx3 keyboard (IIRC) only has a fast and slow speed button. I believe the speed adjustments are internal.

According to Aron - we are evil
According to Jay Doyle and others - we don't produce anything new, we just reverse engineer boutique pedals and rob the makers of their livelihood
According to Zvex - we just fell into some shit
According to the people here - we are friendly and producing new stuff ALL the time.
cheers
bajaman

ps: don't tell the IP police at DIYstumpedboxes

bajaman wrote: I never fully understood the braking function - doesn't the bypass switch achieve the same effect - please clarify for me Doug
bajaman

I think we were both typing at the same time...

My understanding of the brake is that the speaker rotation slows until it stops. It's like just killing the power to the motor. Not exactly sure how you would do this electronically, but would be cool if you could.

I have a far clearer copy of the original KORG CX3 service manual - doesn't tell you much more about it's inner workings though.
PM me if you would like the PDF file
cheers
bajaman

ps: the original RE board was too big to fit in an NSC1790 case - took a few weeks working late at night to shrink it down to fit.

Thanks for the explanation Doug, regarding the brake function - I will give it some serious thought - may have to use three switches on my case
bajaman

Some more tweaking - To activate the slow setting I suggested running a feed from the junction of the 1k5 current limiting resistor and the led status lights. I forgot to mention that the feeder cable should also include a 10k resistor in series with it, to avoid dimming the slow indicator led. my apologies. I felt that the slow speed setting was too slow for a realistic chorale sound, so I increased the 1k resistors in series with the 1N4148 diodes on both oscillators to 2k7 - I think it is much more realistic sounding now. The amplitude modulation should be set to just under 50% of the trim pot rotation for most realistic effect sound.

The brake function should be simple to implement - a similar circuit to that used for the slow speed switch but with no series resistance should do the trick - I will test this out before posting a suitable mod or circuit board redraw.

One last thing - If you are using true bypass switching, please add a 4M7 resistor from the input pin on the board to ground (earth, dirt, whatever) to avoid switching pops.
cheers
bajaman

Funny... I created a new acct at diystomp and tried to post a link to this thread, since it is a cool diy project. But I guess the link to this site is outlawed or something as the url kept getting scrogged... Bizarre...

DougH wrote:Funny... I created a new acct at diystomp and tried to post a link to this thread, since it is a cool diy project. But I guess the link to this site is outlawed or something as the url kept getting scrogged... Bizarre...

You could post a tiny URL.

What's that?