Awesome filter!

[sidmonster]

Hey guys,

I recently did a bunch of mods to my Korg Poly 800 and I LOVE the filter. This filter is VERY dramatic sounding and the resonance kicks so much butt it will blow your speakers quite easily! We could totally control it from an AOUT, maybe even J5... it has 12/24 DB (2/4 pole) and it sounds really really good. I have a SID with good OSCs and a blown filter, so I'd love to use it in conjunction with one of these.

I have a Korg DW 6000 with a few broken keys and I don't use it much so I'd like to reuse the filters. It has a total of 6 of these NJM 2069 filters in it. I will put the pinout for this chip below, if someone can help me realize the circuit needed to power this baby, I will send you one of these chips free of charge.

here's a link to how awesome this thing sounds, and after the break, you will find the pinout for the chip:

the mods I did: http://www.synthmod.net/korg/moog_slayer/

some sound examples: http://www.circuitbenders.co.uk/synthmod/POLYEX.html

and the pinout:

1 Audio In GND (GND)

2 Mixer Signal 1 Level CV In

3 Mixer Signal 2 Level CV In

4 VCF Resonance CV In

5 VCF 12dB/oct Audio Out

6 VCF 24dB/oct Audio Out

7 VCA Signal In

8 VCA Level CV Log In

9 VCA Level CV Lin In

10 VCA Signal Out

11 Vss (GND)

12 -Vee (-5v)

13 VCF Cutoff CV Lin In

14 VCF Cutoff CV Log In

15 VCF cap C4

16 VCF cap C3

17 VCF cap C2

18 VCF cap C1

19 VCF Ext Audio In

20 Mixer Signal 2 In

21 Mixer Signal 1 In

22 +Vcc (+5v)

here is a link to a page that has more info about the filter: http://www.sounddoctorin.com/synthtec/korg/njm2069.htm

and attached to this post is a .pdf of the entire schematic for the poly 800 with some pinouts and other details so you can see the rest of the circuit that's used with this filter, I compiled this layout so it's very nice with actual photos and stuff, check it out!

Thanks in advance for your interest, and again if anyone can help me design a circuit to use this filter, I'll hook you up with one or two of these chips!

poly schematic for upload.pdf

[Flying Panther]

Based on your information I'd say that the most simple version of the filter (mono filter that makes no use of the Mixer and VCA) should connect the SID output to the VCF Ext In (probably using a capacitor according to this Yahoo group topic).

I'm not sure if CV Lin and Log should both be used or that you can choose between them. In the schematic they both are connected. If either Log or Lin is required, you can connect Cutoff CV Lin and Resonance CV to the AOUT_NG. Furthermore get the right capacitors for C1 to C4 and connect them between pin 15-18 and ground. Then connect -5V/GND/+5V to the right places and you're all set to go ;)

Of course there's more to it, but I think if you can get that working, you can easily add functions to use the mixer and VCA as well (as you have more AOUT channels anyway).

Also, you might find the schematics in this Poly 800 service manual more easy to read (higher resolution) and you might also find some usefull information in the DW-8000 service manual (as the DW-8000 uses the VCA after the VCF like you would expect, not as an additional VCF input attenuator).

[sidmonster]

Based on your information I'd say that the most simple version of the filter (mono filter that makes no use of the Mixer and VCA) should connect the SID output to the VCF Ext In (probably using a capacitor according to this Yahoo group topic).

I'm not sure if CV Lin and Log should both be used or that you can choose between them. In the schematic they both are connected. If either Log or Lin is required, you can connect Cutoff CV Lin and Resonance CV to the AOUT_NG. Furthermore get the right capacitors for C1 to C4 and connect them between pin 15-18 and ground. Then connect -5V/GND/+5V to the right places and you're all set to go ;)

Of course there's more to it, but I think if you can get that working, you can easily add functions to use the mixer and VCA as well (as you have more AOUT channels anyway).

Also, you might find the schematics in this Poly 800 service manual more easy to read (higher resolution) and you might also find some usefull information in the DW-8000 service manual (as the DW-8000 uses the VCA after the VCF like you would expect, not as an additional VCF input attenuator).

It's really cool of you to help with this man! I built the circuit on perf last night and I'm gonna dig out that DW-6000 today and try it out. Your version of the simple mono-filter setup was exactly what I suspected, but what was extremely helpful was the page you linked to that helped me figure out which capacitor to use before the input. PM me if you would like one of these filters.

I've got a new question about building an LFO, I'm going to start a new thread for it in "design concepts"

[Flying Panther]

Be sure to check that the audio and/or CV signals are not too high. I don't know the values that the chip can handle, but it would be a shame to fry the chip because of too high input signals.

[sidmonster]

OK,

This may sound like a silly question, but how can I invert the voltage to get -5 for this chip? My power supply has only +5

I'm thinking it requires an op amp, and I have a few, I'm just wondering what's the best way?

[jojjelito]

Google voltage inverter and you'll find some (noisy) circuits based around a 555 timer. Or you use a dedicated IC for lesser noise like a LM2687 for instance. The best way to do this without noise or various switchers is to use a bipolar supply.

Please be aware that you can't use an inverting opamp to get -12V from a +12V supply. It will never go below GND at any event. You can use a trick like connecting +6V to a local virtual ground, use 0V as -6V, 12V then becomes 6V. However, as soon as you connect this to the outside world where 0V is ground you'll be out of luck unless you do some fancy AC coupling trickery.

Also, in real life you won't get 5V out from most opamps when they are fed +5V DC. The max output will be 5V - 0.7V (one diode drop) = 4.3V unless they are of rail-to-rail type which can go full swing.

In short: Get a bipolar supply to be safe.

[nebula]

If you need to mail-order a centre-tap transformer, you can always start off using two 9V batteries, or two wall-wart adapters wired in series!

[jojjelito]

Great idea Nebula! Always go for quick and dirty (doesn't everyone have a bench supply, what's up with that?).

Here we go for a quick and reliable solution. Add a 7905 regulator for some -5V fun to the -9V side (and reverse polarity protection diodes if you're ultra-picky!):

TFTFY :flowers: