Fun With Filters (Pt.2)

In the first part of Fun With Filters I chopped out the Wah Brass part of the YAMAHA B75 organ and made a low-pass voltage controlled filter module out of it.
That was quite satisfying, but there's more to tell and maybe more to do. There were two IG021611 chips in the organ after all and the module I built is not really the filter I think the SS-30M needs. But what does it need?? Ideally a YAMAHA filter. That would be the ideal companion, don't you think?

CS Filters- IG00156

As noted before though, each CS era had it's own sound. The IG00156 was versatile. As a multimode device it had LP, HP and BP outputs, but the different synths it went into had a range of colours. Even if I got a chip, which colour would I choose? Or could I design in options to use different colours?

The IG00156 is included in the Yamaha IC Guide, shared by Loscha many years ago. Searches for such a guide invariably lead to this revision. I have seen mention of a huge one from 1984, but it wasn't scanned in by the owner, and the trail is very old and cold now.

 What's perfect about this guide is that it shows a block diagram for the IG00156.

 This block diagram shows clearly that this is a standard arrangement for a state variable filter.
There's more on this design here: https://www.electronics-tutorials.ws/filter/state-variable-filter.html
The state-variable filter was first used on the early ARP synths and then the Obeheim SEM and is actually known as a Kerwin-Huelsman-Newcomb topology.

Gotharman CS Filter Module

Some years ago now, Gotharman used to make a CS Filter module which you could fit your own Yamaha IC to and get the filter sound of the CS range of synths.


https://www.modulargrid.net/e/gotharman-yamaha-cs-filter

http://www.gotharman.dk/Eurorack_CSfilter.htm

 This was specifically designed for the the IG00155/6 ICs. Here's what they say about each part:

• IG00155 will NOT work as a direct replacement for IG00156.
• IG00156 has a sharper resonance than IG00155.
• IG00155 has more bass boom than IG00156.

Appaemtly the '55 does not have the keyboard tracking input.
These chips sell for around £70 and are getting scarcer by the day, so Gotharman stopped making their module.

 I would quite like to get one of these modules second-hand and be done with it, but that would be expensive and they are rare. Or I could design my own version, but that would require spares of that already rather pricey chip. Also, using up much sought after parts feels a bit wrong when they could be keeping CS range synths alive.



If I'm not going to take an IG00156 then it's not really going to be a CS filter.


For now though, I'm going to focus on another YAMAHA VCF.

IG02610/11 VCFs 

Inside my disembowelled B75 were two IG02611 VCFs. They are pin-for-pin with the IG02610 and the service manual references the '10. I assumed the '11 was an improvement for reliability or some-such, but I then find that the C-605 uses both '10 and '11 parts for different modules and pre-dates the CS-01. In any case, they are apparently functionally identical.

The first thing to note about the IG-2610/11 is that there is no known datasheet on the internet at the time of writing.There's no IC guide with it in, no block diagram and only service manuals' schematics to go by.
 I shall have to reverse engineer the design and connections of the IG02610/11 from these schems and see what I can figure out.

CS-01

The first thing to note is that it's not clear as I type this what kind of filter it is. From the fact that it was used in the mark 1 CS-01 I know it is a 12dB design and can be used as a low-pass filter with resonance. The CS-01 only had two fixed resonance settings, but that can be replaced with a pot to get a variable setting.

Other things of note:

• The pin 1 'IC' is clearly the cutoff setting
• Input Control?
• GO is the output, but is it paired with GN?
• Similarly, there are pins KO and KN,  
• G & K? What are they?
• O? N? Output?? And what? 'iNput'?
• The resonance is an interaction between the input signal (clearly where the signal is fed back in to, C3 pin 14, KN pin 11 and then K0 pin 12, C1 & C1' pins 3 and 5. 
• The resonance amount is partly determined by how much of KN goes back to KO.

CS-01 MK II - IG05630

Before I go on, there is another YAMAHA VCF chip to note.
The CS01 mk II uses the IG05630 24dB filter IC. This is a 20-pin device and is properly labelled so the pins make sense. It's also clearly state variable, with outputs for LP, HP and BP filtering.

This chip is evidently the successor to the IG00156, but now they have caught up to the idea that 24dB is the way to go! Of course, this was just at the time that YAMAHA were about to kill off all analogue synths with FM. I can only find two hits on the whole internet for this chip ID, so I guess this post will be the third!

B-75 and other Electones - IG2610/11

The B-75 Electone Organ uses the IG002610/11 and I've also identified the A55N, C-55N and C-605 hosts. These organs are all 1981 models. The SK combo keyboards and Electones from 1980 all seem to use the IG00156, so it looks like Yamaha switched away from the '156 around this time and IG002610/11 took over. But, was it really a replacement?

The Mighty Wah!

The 2610 is mostly used in wah modules. Wah-wah is a normally a band-pass filter, so this gives me a glimmer of hope that this VCF can be configured as LP, BP and HP. Ideally, I would use one of the two chips as a LP and the other as a HP filter.

Yamaha advertising from the time is quite vague on what they mean by a wah  though (https://organforum.com/gallery/albums/userpics/78173/Yamamha_C-35_C-55.pdf)

 

You can hear the exact effect in this video of an A55N

I am not convinced that what you're hearing there is a BP filter, and the description of "swelling" is not typical of wah. The more I look into this, the more it seems that 'wah' was used to refer to low-pass and well as bandpass filtering.

In the Electone's service manuals the IG002610 pinout labelling is quite different to the later CS-01.

The changes are:

• Pin 2 is now ICD, not NC
• KO & KN and now F1 and F2
• GO is now simply OUT
• GN is mislabelled in this diagram and in the schematics is now NF

Now, let's look at the B75 schematics

B75 Rhythmic Wah

B-75 Brass Wah

The most interesting thing is that whilst on the CS-01 the IC pin 1 is the input for the cutoff frequency control voltage, and the ICD pin 2 is 'NC', on the B-75 IC pin 1 is tied off to the supply rail (via a 3.3M resistor) and the control voltage is being fed to ICD pin 2.
Guessing a little wildly, if the wah circuits are bandpass filters then does tying the IC pin to the supply make the 2610 BP mode? Well, in the (definitely) LP mode of the CS-01 ICD isn't connected to anything. I sense this is some sort of key to the behaviour of the 2610, but my guesses are not grounded in anything definite.

Inside the 2610?

We know that the 2610 is a configured for a two-pole, 12dB response on the CS-01, but there's no similarity with other VCF ICs.

The SS16124 is a modern VCA chip which can be used for VCF designs. Studying this application note provides many details on filter designs which use Operational Transconductance Amplifiers (OTAs) http://www.soundsemiconductor.com/downloads/AN701.pdf
Cascading combinations of these OTAs paired with Op Amps in Sallen-Key topologies creates higher order filter designs.The section on resonance and feedback is most illuminating, if you need to design such a thing, but there's precious little to compare with the IG2610/11.

Back to the 2610/11 and the main thing I have noted about the implemntations in all the schematics is that C1 and C1' are always tied together and with KO&KN / F1&F2.

Trying to make more sense of this I redrew the CS-01 schematic with a simplified block for the IG26010.

Simplified Schematic of CS-01VCF

It's easier to see now what the relationship between C1/C1' and the K' pins is and how the external components work together.
GN and C2 are both decoupled to ground, whereas C1/C1' appear to be part of the active filter. As the C1s do not run direct to ground there is some clue about what the filter topology in this chip is.

T?

Searching around for other VCF designs I found Ray Wilson's design on Music From Outer Space for a VCF using half a twin-T design.
http://musicfromouterspace.com/analogsynth_new/EXPERIMENTERBOARD/page3.html
Ray says  "...the "active" element in the active low pass filter..." "...is essentially the low pass half of a twin T active filter."

This is relevant because of two things. Firstly there are a pair of matched capacitors which are tied together and secondly further investigation leads me to a mention of 'k'.

The twin T design is actually a notch, or band stop/reject, filter. In some sense this is like the gap between a low pass and high pass filter. If the cutoff frequncies of the LP and HP are close, the space between them is a narrow band of frequencies which is stopped. Therefore a basic desgn would be simply an LP and HP where the input passes through both in parallel and both filters stop frequncies passsing in a band where they cross over. The twin-T has another trick which makes it perfect as a notch filter. The HP and LP outputs are 180 degrees out of phrase at the notch frequency so that  when their outputs are mixed the is a complete attenuation as their stop bands cross over and cancel each other out.

Looking further into these designs, the letter 'k' pops up. Here, k is the feedback fraction, set by a voltage divider. For more on that see here:https://www.electronics-tutorials.ws/filter/band-stop-filter.html

i

Could the IG002610/11 be a twin-T, or some other kind of T network filter? Well, it's not a twin-T if we're making a low-pass filter with it, and most importantly the low pass half of a twin-T does not have a pair of capacitors, that's the high-pass half. Umm, but why did the late, great Ray Wilson say his design was "the low pass half of a twin T active filter"? In fact there are other designs with twin capacitors that are tied together, e.g. 'Sub-Bessel' Sallen-Key, but they are all high pass. A scan through this article does not lead me to any other likely candidates. https://sound-au.com/articles/active-filters.htm


I refuse to accept that Ray is wrong, but I can't figure this out and I feel that although I'm closer to understanding thus VCF IC, I am missing something important.

Waaaaaaaah!

It pains me to have to report this, but I removed the other IG20611 from the B75, put in on a breadboard, wired it up in the CS-01 configurtion, got sound through it, and then killed it. Waaaaah! I'm pretty sure I connected the IC control input directly to the -12V rail and then, in a panic, felt it getting hot and took too long to find the problem. No more sound. No fun with filters there.

This closed off the option of experimenting with the chip and I'm not keen to risk my other IG002611 which is now safely in a module that works quite nicely. 

Conclusion

So far, the IG02610 and IG02611 remain somewhat enigmatic. The only things I know for certain is that the 2610 can operate as a LPF in the CS-01 and is a 2-pole filter. The use in Electones is harder to be sure, but I have a 2611 working as a LPF. On the other hand, there are some clues that there is more to these chips than that. Why would Yamaha stop making the multimode IG00156 and replace it with something with fewer features? The pair of capacitors (C1) tied together seems to indicate something contradictory about a low-pass design but I'm not able to discern what.
Now, I can either make it a mission to understand the 2610/11 ICs or just drop it and move on. Frying that spare chip means I'd have to buy a new one, either as part of another organ or standalone (at higher cost price!). I'm not keen. I think my journey with the chip ends here.

I still need a good filter for the SS-30M though. What to do?