Over at this website there is a whole load of Japanese brochures for all kinds of musical instruments including YAMAHA keyboards. The SS-30 debuts in 1977 and then appears for two more years. By 1980 it was gone though.
Now that I have built the filter module I showed in part 1 of Fun with Filters it's time to take a closer look at it. I decided to call it YAMAWAH and in a later part I will show you th finished article with a printed panel.
You will recall that I sawed off the end of a board from the Yamaha B75 organ and the offcut is the Brass Wah section. Obviously, Yamaha did not provide any details of this filter, other than to say that it's a wah effect for the brass voice of the. Also, I have not been able to work out what filter topolgy the iG02611 VCF chip is, so I could not easily guess what the filter type actually is from the schematic alone. Does it 'wah'?
Blessed Plots
When I was first experimenting with the filter I just noted that it was some sort of low-pass a that it rolled off around 10KHz, but I didn't get any further or measure it.The following is a brief look at the filter response curves comparing with the Roland 521 filter module. I used a white noise source from the Mutable Instruments Kinks module. All the traces are from WavePad and I recorded 5 seconds of audio in each case before plotting the spectrum.
Here's white noise as it comes from my MI Kinks module
White noise from MI Kinks
Here's the same noise through the YAMAWAH with the cut off frequency at maximum - 100%. As you can see there is a major resonant peak. This took me slighty by surprise, but then I realised what was going on. When I was first experimenting I use a spectrum analyser VST in Cubase to check the response. In that case there was no peak evident. Couldn't really hear the peak, although I did note a bit of 'bump' when sweeping through frequencies and it was obvious that something else was going on.
White noise through YAMAWAH VCF set to 100% open.
Contrast that with the same white noise passed through the Roland 521 VCF module in LPF mode with resonance set to 0 and the cut off frequency set to 7/10
White noise through the 521 VCF set to 70%
And now the same but with the resonance set to 5/10
White noise through Roland 512 VCF cutoff 7/10, resonance 5/10
Now setting the YAMAWAH cutoff control to 3 o'clock. There is still resonance and the peak has moved down to around 4KHz
White noise through YAMAWAH cutoff set to 83%
So far, it was much as expected. When the YAMAWAH cut off control is set to 12 o'clock something strange happens though. The resonant peak is around 450Hz, but over at other end, from 4KHz upwards there is an uplift!
White noise through YAMAWAH set to 50% cutoff
Moving back to the 521, a similar cut off frequency setting (4/10) look like this. Which is exactly what you'd expect to see in a low pass filter.
White noise through Roland 521 VCF set to 50% cut off and resonance
Back to the YAMAWAH, and here's the setting at around half past ten.
White noise through YAMAWAH set to 33% cut off
And at fully counter clockwise you can see there's been almost no change in the spectrum. In fact the cut-off frequency only seems to take effect from half past ten and upwards. This suggests that the control voltage is onlt effective over aroudn 2/3 of the Vee supplied to the iG02611, or about 8V of control voltage.
White noise through YAMAWAH - cut off set to 0%
Finally, let's look at a squqre-wave into the YAMAWAH. This is highly coloured by the 10KHz maxium cutoff frequency and the resonance.
Square wave through YAMAWAH VCF
Learnings
Resonance
The fact that the YAMAWAH is resonating proves that resonace is possible without connecting the C1s and KO/KN pins to C3, which is already fed back to the input. It makes me think that the connection between C1s and KO/KN is responsible for part of the resonance and if I break this connection the resonance will disapear. Adding a variable resistor between these pins could then lead to a resonance control.
I experimented with connecting C3 to KO/KN and seperating KO/KN from each other to no great effect, but now I'd like to return to this again.
First this is what happened when I disconnect the C1s from KO/KN. The resoanace peak disapears
And then at 50%
And at 0%
So, it really does seem that this is a notch filter. I suspect that changing the caps will move this around.
Anyway, back to resonance and adjusting the amount of current from the C1s to KN/K0 makes no difference. It's either on or off. Instead I again seperated KN from KO and that too kills the resonance. So, then I added a 100K variable resistor between KN and KO and now I have variable resonance!
Here's the plot with the 100K in line between KN and K0. There's still a bit of resonance compared to open circuit and the cutoff frequency has shifted a bit.
Here is the plot with 50% resonance - 50Kohm - and the peak is more noticable
And so I now need to add a resonance control!
Here's a video with a demonstration of the resonance control being tested
High Pass?
The second interesting thing about the plots is that almost band-stop behaviour as the filter cut-off moves down. I have been hoping that the iG02610/11 might be suitable for setting up as a high-pass filter as well as a low-pass and that behaviour makes me think that maybe it is possible.
It also reminded me that I had made a modification to the C2 pin. On the B75 the pin only has a capacitor to ground. On the CS-01 that pin is connected to Vee by a 10M resistor. I didn't detect and difference by adding this, but now I'm starting to wonder.
Well, I simply removed this resistor and nothing happened. The up-lift was still present, so whatever is going on it's not that resistor. I will have to start chanaging capacitors if I want to figure this one out!
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.
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.
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.
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?
As you saw in the Concert Mates post I'm very much thinking about filters at the moment. The SS-30 Brilliance control isn't only not a VCF it's also not even an active filter and is in fact just a single capacitor. The SS-30 is, lets not forget, a strings synth and not a proper synth.
The SS-30M though is a not only M for MIDI it's also M for Modular (where possible) and one of the enhancements I've recently added is a send-return (or insert) for each voice section. That means I can insert a filter and play that paraphonically just like in those video in the last post, but as part of the SS-30M signal path and straight into the Orchestra effect.
I have a bunch of different filters at my disposal too. Not to brag, but, as well as the Moog MG-1 filter I can call on the filters of the Roland MC-202, System 500 & System 1M and the Mutable Instruments Braids, and also the 303 clones from Freebass (MAM) and Cyclonic, which are all very much in the style of the Roland sound, or thereabouts. I've also got a modified Korg Monotron which can serve as a filter too. Nothing from Yamaha though.
I do have a CS1x and a CS Reface. Both are digital, of course and neither has a filter input.
I Have No (YAMAHA) Filter
Eurorack are modues exist for just about any style of filter you could want. Roland style, Korg and Moog, Oberheim, EDP WASP, Polivoks, EMS, Steiner-Parker, ARP, and so on.
Look for a Yamaha style filter and only three options exist. Studio Electronics SE88, Old Crow's RR480 and the Gotharman CS. The SE88 is a full-featured, several hundred dollar module that takes the CS-80 filter design and runs with it. It's based on the SE Omega CS-80 filter board, which is clearly a standard set of OTA and Op-amp ICs. The Gotharman used actual Yamaha chips, which you had to find yourself, and was made in very limited numbers. Old Crow's RR480 is also a recreation using standard components, but it's DIY only.
The Yamaha synths almost exclusively used a single VCF chip - the IG00156. This was the core of the Gotharman module. The CS range of synths all shared a common chip, but the overall sound changed over the years. By adjusting the external components it was possibe to change the character of the filter. It's not clear which one the Gotharman went with, but I guess the CS-50/60/80. The SE88 and RR480 recreates the IG00156 from first principles somehow, but no-one would really know exactly what's inside that chip now, so it's just a good guess on their part.
I had nothing to hand though.
Organ Transplant
So, I went to my garage to look at the organ donor.
Wah Brass Filter from B75 organ
I found that my B75 Electone organ also uses a couple of custom Yamaha VCF ICs. These are the IG02611. Some Electones did use the IG00156, but by 1981 only the
wonderous CS-70M was using that part. The IG02611 has a counterpart, the IG02610. The 2610 and 2611 are apparently the same, but there is very little info on them so who knows! The 2610 is used in the dinky little nephew of all the CS synths, the CS-01. The filter gets respect but no-one is doing much to recreate it in Euroland.
Anyway, this is a vintage Yamaha VCF and I have a couple on hand, so what shall I do?
Removing the boards
The organ has a curious 3-sided construction of boards.
Brass Wah section
On this board the Wah Brass filter section located at the end. So, I decided to saw it off!
Dirty off-cut Wah Brass filter section.
As with many old instruments, the boards are not just grimey with dust, it's probably had a cup of tea at some point!
Clean board
The board brushed up nicely with a bit of isopropanol. And now I had a board I could apply power and signal to and see what I got out of it.
Patio Electronics
It was a nice day, so I set up on the patio and started attaching wires to the off-cut Wah Brass board.
Wired up
Module-wah Synthesis
Back inside
Now I had the board wired up it was time to power up and see what I could get out of it!
This video shows what I managed to get working. With an inverting level shifter built around an 741 op amp I had to hand, I was able to get a reasonably good low-pass filter module on a bread-board.It starts with a cycling envelope from the System 500 fed with a square wave signal, followed by white noise and then a triangle wave and back to square wave.
I tried to get a resonant feedback going too, but without success, so I decided to keep it simple and make this into a eurorack module as it was.
The control voltage for cut-off frequency into the 2610 is negative going, so the inverting op amp is needed to mirror all positive goingvoltages to negative. Eurorack signals are either positive, like the envelope in the video above, or bi-directional with both positive and negative going voltages, used for LFOs or audio. The level shift is thus needed to move all negative input voltages into the positive. The inverting amplifier is actually a summing as well as inverting design, adding postive offset to the input shifts negative inputs to wholly positive before being inverted to wholly negative.
Simulation showing positive going input inverted to create the correct negative, mirrored, signal for the IG02611
I was a little concerned about postive voltage input to the IG02611 and what damage it might do, but it seems to cope okay with it. The shift is more important to avoid the signal being rectified and ensure the whole wave shape is fed through.
Here's the schematic (note green and red highlights match with the trace above).
Other features include an attenuator on the audio input and and LED to indicate negative input voltage. This has a slight impact on the current being fed to the IG02611 and adds a little proctection. A blocking diode would stop any postive going current to the IG02611, but I did not want to have any voltage drop across the input.
So! On with the construction.
Module construction
I made the wiring such that when disassembled the boards layout flat and can be worked on without disconnecting anything.
I used Front Panel Designer from Schaeffer to lay out a panel design and printed it to paper to check the fit.
I have a 3U aluminium panel handy so I chopped off a 9HP section from the end. Then I taped the paper print-out over the panel and headed to the pillar drill.
I was a bit worried about how well aligned the holes would be. It came out fine though! A bit of filing and the panel was perfect.
I decided to add the LED to warn when the input voltage was positive in a second revision.
This completes the first stage of construction. I need knobs and some print on that panel now though.
In the next part of fun with filters I will go deeper...
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