Korg Poly-800 External Audio In Mod.

[YashN]

I have a used Poly-800 and love the NJM2069 filter sound. I would like to mod it an add an external audio in.

It seems one can do that by switching near C24 to toggle between the noise source and the pure external audio.

How do I mod it to allow Mixing in with DCO1 and DCO2 as on the site?

Thanks

[Circuitbenders]

I don't use the noise input. If you do you have the advantage of being able to send the external audio signal though the DEG 3 envelope, but the noise DEG  is also shared with the VCF to sweep the filter, so you can't do a filter envelope on an external audio input without creating a volume envelope on it as well.

Try pins 20 and 21 of the NJM2609. They are the DCO1 and DCO2 input pins.

[YashN]

Thanks a lot, CrustyPaul, it makes a lot of sense to route it through the DCO1 or DCO2 Pins since I can then re-use DEG1 or DEG2 respectively.

Incidentally, this also means I could do two or three simultaneous External Audio Ins, which could be rather awesome, isn't it?

[Circuitbenders]

Annoyingly, DEG1 and DEG2 won't actually have any effect on an external signal injected into the DCO1 and DCO2 input pins. I'm not entirely sure why this is the case, but the only envelope effect you get is a gate on and off at the beginning and end of the envelope. The DEG's will still affect the DCO's.
I did have a 'eureka' moment a while back as to why this happens but off the top of my head i really can't remember. I do know i couldn't find a way around it though 

That does mean you could have two simultaneous inputs via the DCO input pins, and another via the noise input on pin 7.

[YashN]

Not being able to re-use the DEGs is a disappointment, but then, one could use a DIY module for the external audio too.

Doesn't Pin 19 also allow an input into the VCF?

So in all, we could have 4 simultaneous external inputs into the VCF?
1. DCO 1 input
2. DCO 2 input
3. Noise input (P 7)
4. VCF External In (P 19)

[Circuitbenders]

It seems that the envelopes for the DCO's are generated digitally inside the oscillator chip so you can never get an external signal to pass through DEG1 and 2, but i could be wrong about that.

On the Poly800 the VCF in on pin 19 is hardwired to the mixer / VCA out on pin 10. The best way to inject an audio signal would probably be to cut this link and wire the audio in to pin 19, but then you'll lose the other three inputs as i think they are mixed together on the chip and appear as a mix on the VCA out pin. You could probably come up with some kind of switching arrangement or use an opamp to mix the signals from the VCA out with the external input before they go into the VCF input.

It just seemed like too much hassle to me so i went for the DCO 1 and 2 channels.

[YashN]

Yes, that's correct from what I gather: internal envelopes and also P 10 wired directly to P19.

I do find it difficult to have to cut off the P 10 to P 19 wiring so I'll leave it as is currently and use either the DCO route or the Noise one for external Audio.

[YashN]

I did it!

Took two jacks plugged into the DCO1 Signal and DCO2 Signal pins into the NJM2069.

I am using them with outputs from my Kurzweil K2500 oscillators. I've recorded a test multi-track with the Kurz sounds passing through the Korg VCF both with some filtering and with the filter open (supposedly this should be transparent). The final mix sounds warm and punchy and I used some resonance on the high-frequencies of the drum track.

Perfect! Thanks for the help.

[noob666]

Hi, I just want to clear this up because I got a little confused about talk of the IC 2609, this chip is called the NJM 20690 in the service manual. On my board it's the KORG 2069AD.

Did anyone add volume control to their audio ins? In a youtube video I saw the person had a volume knob for the audio in. Although I haven't done this yet I find this mod really exciting. I can't wait to but other gear through that chorus (that works right?).

Also I have 2 more questions right now regarding the 800. Does anyone have a schematic or something for the moog layer mod w/ the on/off switch? I'm not great with electronics yet it would help me to see how it is done. My other question is does anyone know how a replacement for the Q1 transistor tr2sb731? I can only find it for sale on non english speaking sites.

[Circuitbenders]

We're all talking about the same IC here. Some versions are just labelled slightly differently, and i might have mistyped it up there.

For volume on the input i use a 4.7K pot with one side wired to ground, the input wired to the other outside pin and the centre pin wired to a SPDT centre off switch via a 1K resistor and 4.7uF cap. The other pins of the SPDT wire to the DCO 1 & 2 inputs on the 2069 chip so you can select the audio path.

Theres a moog slayer mod with an on/off switch?

Here a thread about the b731 http://www.circuitbenders.co.uk/forum/index.php/topic,2227.0.html .You can get them from a few places such as Nikko electronics http://www.dalbani.co.uk/catalogue/product_details.php?id=2291

[Synthetech]

Curios to see a schematic of this mod..

CB said..

For volume on the input i use a 4.7K pot with one side wired to ground, the input wired to the other outside pin and the centre pin wired to a SPDT centre off switch via a 1K resistor and 4.7uF cap. The other pins of the SPDT wire to the DCO 1 & 2 inputs on the 2069 chip so you can select the audio path.

attached below is a pic of the 2069's pins and signal paths to help with this discussion..

I was thinking it would be better to inject an external signal at where the noise source goes into the filter... at pin 7.
I suppose one could pull the IC out of its socket, bend pin 7 out (then reinsert the chip back into the socket.. Pin 7 sticks out the side) so you can attach a lead to it via a slipclip connector of sorts(or just solder direct to the pin).. connect that to a SPDT, then reroute the Pre-IC Noise signal to the switch(a wire from the PCB to switch).. the external audio brought in thru your volume pot/res./cap and then onto the SPDT.

This way DCO1&2 are isolated to either be on/off.  The noise VCA can be used to digitally control the ext. inputs volume(post the input pot).. and of course the VCF envelope(EG3) will alter the ext. audio's tone... just like it does for the two DCO's.

I suppose I could use both the DCO signal inputs, but they both will use the same filtering(just like the noise signal uses)... if I use the noise input, I can keep the DCO's audible and not have to shut down their individual Osc. tones.. I can control the DCO's volume independent of the Ext. audio signal.
I think this would provide a better control to balance the DCO's with the Ext. signal.

I dont know about other users of Korg's, but I seldom use the noise source in patches.  So I figured it would be better to select Noise/Ext. Signal rather than between DCO1/Ext. Signal or DCO2/Ext. Signal.



May seem a silly question, but where do you hook up the external signal's GND?

[Synthetech]

-- get's the defibrillator out---

CLEAR!!

--checks the forum for a pulse.....--



Anybody out there??

[Synthetech]

well I managed to do the mod temporarily to my Poly 800 today.

Not bad.. not bad at all..  check it out

http://youtu.be/V4QD1q94PTM


I did find out that I needed to input the audio at Pin 19.  The final input stage for the Noise source.
That way I am able to control DCO1 and DCO2 individually on or off, the noise on or off and have the external audio all at the same time if I wish.


I may check into what it will take to bring the external input in at the Noise sources first input stage.. where that internal VCA is at so I can control the External Audio's volume using the Korg's internal Noise Volume parameter..  that way automation of the control can take place via sysex msgs sent from a sequencer like Cubase.