[sdiy] OT - looking for roland JX10 chorus details

[Andrew Simper]

A quick analysis of the filtering around the chorus of the JX 3P: the
input to the chorus has a dc block at around 2 Hz followed by two 2
pole low pass sallen key filters that use pnp transistors for buffers
at around 10 kHz, the second with slightly more resonance than the
first to make the 4 pole total very flat in the frequency passband.
This is then followed by a one pole high pass (I don't know the
cutoff), and then a one pole low pass at around 7.5 kHz just before
the input to the BBD. The output has one pole passive low pass at
44kHz, then another transistor buffered sallen key 2 pole low pass at
around 10 kHz. The Juno 106 has two 2 pole low passes on the output,
but they just have the one on the JX 3P.

Andy
--
cytomic - sound music software

On 22 April 2012 03:19, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>
> Hi Dave,
>
> I don't have a JX10 to check, but I can give you the chorus details for my
> factory standard JX-3P:
>
> The voice mix is fed to two independently variable BBDs with 3.53ms
> average delay and +/- 1.91ms of modulation.
> The two BBDs outputs are panned hard left and hard right and mixed with
> the original voice mix.
> Each delay is swept between 1.62ms and 5.44ms by a triangle LFO wave.  The
> LFO triangle wave modulates BBD1 delay directly, and is inverted before
> modulating BBD2.
> When the BBD for the left channel is at maximum delay, the BBD for the
> right channel is at minimum delay, and vice versa.
> LFO period on my JX-3P measured at 2.2 seconds and is fixed.
>
> The wet chorused signals from the BBDs are lowpass filtered at about 5kHz
> with a 2nd order rolloff that takes a bit off the top end of the BBD delayed
> wet signals.
> If you can do it, you could also put a 1st order highpass at around 100Hz
> on the wet only.  There's also quite a bit of swooshing noise added too!
>
> As far as I know the Juno and JX series synths all use the same basic
> chorus setup, except for slight variations in the modulation.  The JX-3P
> chorus certainly sounds very close to the Juno-106 chorus, and very like the
> alpha-juno chorus too when it's set to it's more sane speeds!
>
> If you try these parameters and it sounds right then i'd say you've nailed
> it.  If it doesn't sound exactly right, then I doubt you would have to tweak
> the parameters too far to get the JX-10 sound.  If you still have the JX-10
> then it's not hard to record a low pitch sawtooth tone with the filter wide
> open and switch the chorus on and off.  You can then easily see the delayed
> edges from the chorus appear in a wave editor and work out the delay ranges
> for the left and right BBD channels.  (Thats what I did when analysing the
> JX-3P, 106 and alpha-juno.)
>
> I hope this helps,
>
> -Richie
>
>
>> I'm looking for clues as to mod depth, delay, LFO shape and rate - that
>> sort
>> of thing. If there is any EQ in there (a la Dimension D) it would be good
>> to
>> know too. I've got some way along the road, but any details would really
>> help.
>> FWIW, the JX10 appears to be two JX8P main boards shoved in a long box
>> with
>> a keyboard. It's rather cramped in there....
>
>
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