korg x911 guitar synth svc manual w/schems inside

[pinkjimiphoton]

this looks neat...is it doable? or worth it?

some crazy mofo up for a challenge?

http://www.korganalogue.net/korgother/X911/X911_service.pdf

ancient guitar synth things seem to be pretty cool in my humble experience....but too noob to glean much from this..

[Earthscum]

I think we should just do a forum synth project. I've got some bass synth stuff I've been working on here and there.

Maybe start off with a comparison of the building blocks, simplify them, etc? We have, now, 4 different NEW synth units popped up since the summer, 3 just in the last 2 or 3 months. I bet we could come up with some kind of DIY-friendly universal synth, given the tighter tolerance of parts available to us today.

BTW, this thing is gigantic! lol... not as much as the PhaseII, but still pretty big. I have seen one of these in the past at a shop, sitting with a couple other rack synth boxes. Wish I knew I'd be getting into this stuff, I would've picked up at least 20 pieces of rare gear for ultra cheap, lol. All the pieces I've passed up will never be the same as the '69 Hi-Flyer bass I stupidly gave to a girlfriend. I keep wishing I'd have a chance to get it back. Ugh. The things you do when you're a teenager.

[pinkjimiphoton]

Ugh. The things you do when you're a teenager.

like...trading off a ludwig phase II for a half a bag of columbian?

[Earthscum]

like...trading off a ludwig phase II for a half a bag of columbian?

Lol... yep. I haven't, myself, but I have known many others. Our band has a pair of SM57's through one of those transactions 

One thing I noticed the Korg uses more CMOS than the other synth boxes. I keep looking at these things and wondering why more CMOS isn't used (then I remember the dates these things were made). I mean, figure if you have to use any more than 2 or 3 schmitt triggers, save a ton of resistor/cap/tranny and use a 14-pin chip. Heck, anything that outputs a pulse or gate signal can probably be simplified with a 40106. as an example. I don't know if I would go as far as to sub 4049's for OPA's, but a large amount of synth  trigger and threshold and gate and ADSR, etc... could be simplified with CMOS. Heck, even used switched capacitor filters, now that I think about it.

[pinkjimiphoton]

ok...

so, in english that means the circuit could be simplified with modern components somewhat in a way that would be copascetic and pleasing to the ear in a way reminiscent of the original massive box before us?

that could be really neat! :thu:

[snufkin]

x911 is pretty cool and tracks ok but i would suspect a DIY version would end up costing quite allot and would be bigger than the original

the pitch to cv section of the x911 however is similar to the korg ms03 which would be a more doable project and let users pair there own synths as they saw fit (it has both hz/v and oct/v)

[UKToecutter]

It's a real shame the circuit diagram isn't clearer.
I'd really like to have a go at this (after the Ludwig Phase II of course)

Andy

[Mark Hammer]

The most critical part of the X911 is really the P/V.  Once you have that, there are plenty of other analog modules you can tack on for more sonic flexibility than what the array of buttons on the X911 provides.  I certainly wouldn't "kick it out of bed" if I came into possession of one, but for the work involved in a replication of one, there isn't really enough value-added to doing the whole shebang, AFAIC.

As an aside, There was a filter resonance mod in POLYPHONY which I added to a buddy's X911 back in 82 or 83 and it was a big step up in terms of flexibility of tones.

[digi2t]

Hmmm, three comments....

1) I'll stick to my GR-50....

2) Andy, you're a crazy mofo (but we love ya anyway)....

3) Refer to point 1). With some net vigilance, you can even find a GR-1 for less than what this sucker will cost to build. Besides being infinitately more flexible.

Not really rare, or unique enough for me to consider building something analog this complex. I've seen a video or two, and listened to some clips, but nothing really there to blow up my skirt. I'm having the same trepidation over the FK-1, and the Synth Traveller, although the FK-1 might be a bit more easily intregrated with other effects. It's a tough call.

[Jaicen_solo]

If somebody can clone the pitch to voltage section, and perhaps the onboard VCO, i'll build and test it. The X-911 is something i've been looking for for some time, but they're always way too expensive the few times i've seen them for sale.

[snufkin]

[digi2t]

This might help; http://manuals.fdiskc.com/flat/Korg%20MS-03%20Service%20Manual.pdf

I got it from here; http://manuals.fdiskc.com/flat/ . There's a crap load of synth manuals there 

[Mark Hammer]

Not that the Korg P/V circuit wants for anything, but I have always been curious about the Gentle Electric P/V unit, that was out around the same time as the Korg ones.  Have any of you ever seen the innards of one?

[HarryBBD]

Hey Mark, don't rub the lamp unless you want the genie to appear :^)   As I remember, the Korg X-911 (is that really 'nine-eleven' in the name, how fitting...;^)
did not work worth a sh!t in my opinion. (I'd like to hear from anyone who thinks it was acceptable...)   As for the "Gentle Electric" why as a matter of fact I
had one that was totally dead (someone reversed the power supply voltage) and I repaired it, including reverse engineering the "fundamental extractor module" so I have been 'under the hood'.  Its not suitable (imho) for guitar because of the lack of any pre-filtering of the signal, and it cannot accommodate the sharp harmonics that sometimes give multiple peaks of the same amplitude in the same polarity. You can expect octave hop galore with a guitar signal...   That said the basic design
of the P/V is quite good (more later...)
H^)

[Mark Hammer]

Korg had several units with "external processing" capability.  Examination of the MS-20 reveals a P-2-V circuit with adjustable passband (setting high and low corner frequencies) to improve rejection of the content outside the range of expected fundamentals.

FWIW, I etched up a board or two of the P-2-V circuit you had on Tom Gambles old EFM site, when he had all those analog modules, before he went VST.  The board is only partially populated, so I haven't tried it out yet.  As the obvious expert in it, would you recommend completing that population, or selecting another design?  I have two of the Penfold mono guitar-2-synth interface boards awaiting installation and final wiring.  

(P.S.: Still trying to reach Mike.  Can't get him on the phone or by e-mail, but I have his mailing address)

[HarryBBD]

more later... (sorry I had to do some WORK).   I just looked over the Korg X-911 schematics and I don't think they are complete. I don't see
the VCO anywhere. It should have a three position range switch and I didn't see that on any schematic page. If I'm all wet, let me know where
you think it is...

One thing to consider. Do you want to have a guitar synth that is strictly monophonic or polyphonic ? Most P/V converters either need a hex pickup, or you need to
have impeccable guitar technique to minimize tracking errors.  I did a DIY P/V converter for guitar some years back, its about as complex as the Gentle Electric
or the Korg (it shares some heritage with the 360 Systems "Slavedriver".  It uses a hex pickup and tracks the lowest or latest of the low three strings of a guitar in such a way that you can play chords etc and get a reasonable tracking bass line. The Electronic Peasant expanded the design to a five string synth-banjo (you can Google that). There is no reason the system cannot be used with a six string guitar, I just didn't find the three upper strings useful to track so i made it more simple.

All the reasonable P/V converters for guitar are about equally complex BTW, don't hope for too much simplification with 'modern' parts.  Modern parts are more for building FCP's (fvcking cell phones).

My best take on how to do guitar synthesis is to use (essentially) direct string to waveform conversion (like the Roland GR-50) and avoid P/V to VCO where possible. Normal guitar technique is that when you release or mute a string, the sound stops.  Trying to keep a stable pitch with no input is really a tough task.

Mark, I would not expect much of the PV-1 (EFM) circuit. It does not have the prefiltering you would need etc. I did front the circuit up with the preamp of the Shin-Ei octave box and it tracked pretty good, but it was slow (the PV-1 divides by 2 so you need on the order of 50ms to get a voltage). It could not hold the CV
after you end a note, that could be added.

Hey if folk want to do a guitar synth I'd be willing to assist, but you are in for a really big project !!!

H^)

[HarryBBD]

Oh yeah one more (or two more) things... The MS-20 is the queen of 'tachometer' P/V circuits, but not useful for tracking a guitar at uhhh... guitar speeds.
A good starting point for a full-range guitar synth (no hex pickup) would be the E-H "Deluxe Octave Multiplexer" with a P/V converter instead of the phase
locked loop approach.  In my opinion, the best way forward is to make a sawtooth wave directly from the guitar string and use that for synthesis. Its easy to
get fundamental, octave up, and octave down waves from that. OOPS, I should say "straighforward" not 'easy' (it is still very complex...)

H^) harry

[StephenGiles]

Or indeed the EH Guitar Synth itself for which circuits are on Mark's site.

[Mark Hammer]

Since it is hard to find, and since I did not wish to send people on a wild goose chase, I took the liberty of pasting Harry's
initial comments from the document for the old PV-1 he alludes to, since they give a little more detail about functioning of
P2V units in general, and I wanted to save him the time of having to re-explain what he's already explained nicely.

I have omitted the content related to the actual design of the PV-1, given the shortcomings he notes.

A pitch to voltage converter takes a frequency input and converts it to a voltage output. There are several forms of this
circuit, both digital and analog. Most of these can be grouped into two basic classes. The Tachometer circuit, and the Ramp Sample/Hold.
The Tachometer circuit is the simplest. The frequency input is converted to pulses of a fixed width, and these pulses are
integrated, or averaged over time. There is a trade off between how fast you can get to a new level, and how much feed
throughof the input frequency you can accept. The less ripple you wantin the output, the slower the circuit responds to a
change ininput frequency. Tachometer circuits are best for frequencies that :

1) You want to average over a long time anyway
2) Frequencies that are continuous (do not start and stop...)

Note that things like motors etc... are the most common use for Tachometer circuits.

The best example of the Tachometer circuit as applied to Electronic Music is the Korg MS-20 external signal processor. It
features a filter at the input and output that are linked... so the user can set the tradeoff between ripple and acquisition time to
best fit the signal they are processing. The circuit also has an extended range Tachometer circuit... at very low frequencies
the pulse width is fixed. As frequency increases, these pulses get closer together. At even higher frequencies the pulses
overlap, and occasional gaps appear in between multiple pulses. This allows the circuit to perform at frequencies above
the normal range of the Tachometer circuit. There is still a very real delay in how fast the circuit can track a rapidly moving input
frequency.

The Ramp - Sample/Hold is another class of circuits used to solve the problem of slow signal acquisition. The input
frequency is divided in half, and then a linear ramp is generated for one cycle, and then sampled and held during the
following cycle, and the ramp is reset. This happens over and over again... so the output is always one cycle behind the
input. No matter how large the step in input frequency (within reason) the output will follow within two cycles of the
fundamental.

The advantage of the Ramp circuit is speed of acquisition, and lack of ripple. The drawback is that it is more complex, and
sensitive to noise in the input frequency. This approach was used in the 360 Systems "Slavedriver" Guitar to CV interface
(mid 1970's) with some success. The noise problems prevented the circuit from achieving satisfactory performance at that
time. Speed of response is still an issue.

Consider the lowest note of a Guitar = 80Hz, which has a period of 12mS. The Ramp circuit needs at least twice that time to
process the Pitch Voltage, or 24mS. This is a physical limit for converting the pitch. If you have a lot of noise in the signal
(harmonics etc.) it will take even longer.

The Pitch to Voltage converter must have an accurate reference to derive the pitch from. Harmonics will cause false
operation, so inputs from real instruments like guitar, voice, etc. usually need signal processing before the pitch conversion.

[HarryBBD]

It would be possible to update the E-H guitar synthesizer with more available components. Some people might miss the availability of a CV (P/V) output.
I think from my simulations that the E-H has about half of the tracking covered with the adaptive filter, and the rest with the PLL VCOs. Just trying to extract
the fundamental from the adaptive filter stage does not work well enough (imho) for reliable pitch tracking.  There seems to be some room for improvement
here, but at the expense of MUCH more circuit complexity (I'm looking at a tracking filter loop modification that essentially makes the loop a track/hold when the input level falls below the compressor input range, the loop CV no longer remains stable and the filter starts to 'open', causing the dreaded 'octave hop' to start to appear. This might happen after most musicians have moved on to the next note... or maybe not.

The big question is, do people think that guitar synths have hex pickups, or not. For me, I insist that my guitars be playable polyphonically while still outputting a synth waveform (ie HEX pickup).  That makes things a lot more complex.

H^) harry