Yamaha E-70/EX-2
Gosh, this is truly an absolute beast of a... synthesizer, really. And that's the second time I've typed this phrase on this website...
So lets take a deep look at what makes this organ so enticing!
Make sure to read this document too, it goes into deep detail about how P.A.S.S. works and the electronics involved.
THIS PAGE IS A WORK IN PROGRESS.
About the E-70 and EX-2
Yamaha has made a LOT of organs and synthesizers, the two often blending together in features and hardware.
This is why I've always kept an eye on their analog organs, feeling an hunch that they may well hide some glorious polyphonic analog synth hardware that usually is prohibitively expensive to acquire (looking at you CS synths).
To the left you can see the gorgeous fiberglass body version of the E-70 called the EX-2. It's exactly the same internally sans a few presets being different.
Marc Brasse kindly donated his E-70 to me, so I went from an hunch to a deep-dive and oh boy is there a lot of goodness in there.
He also helped build the case and frame for the Tower of Power.
Thanks Marc!
You may know Marc from his 'Son of GX' project, in which he expanded his E-70 with a bunch of new controls and a CSY-2 synth. Absolutely check out his website for more on that.
The glorious specs
Now what can this thing actually do that makes me rather hyped about it?
The specs:
- 12 organ voices with 9 octaves of sine waves,
- 14 orchestra voices with analog IG00155 VCF (very close brother to the IG00156 in the CS synths, bit more low end and slightly calmer resonance) and IG00151 VCA (same as used in the CS synths),
- Oscillators for both organ as orchestra are digital but do things very differently for waveform generation, evading typical issues with digitally generated audio. More on this down below.
- The ability to layer 7+7 voices, creating much more complex sounds and allowing for lush detuning.
- Technically multi-timbral as voices are dynamically assigned to either upper keys, lower keys or pedals.
- Digitally controlled but fully analog drums,
- And finally, of course: the ability to (easily) modify the daylights out of this thing :)
It's not a CS-80 by any means, but it has the potential to be a close sibling. So lets dive into the hardware and see what it is capable of.
The Digital Oscillators and Waveform Generators
P.A.S.S stands for Pulse Analog Synthesizing System, often abbreviated to 'System' by Yamaha. It's a digital system that turns a series of pulses into analog waveforms using some very interesting techniques.
The specific P.A.S.S. system used in the E70/EX-2 is the 'PASS-VCF System', which combines additive synthesis and subtractive synthesis for the various flute and orchestra tones.
To the left you can see a block diagram of this system, click the image to make it bigger.
The Flute tones use a combination of sine waves, but the Orchestra tones use square, pulse and sawtooth.
Using P.A.S.S. means digital oscillators but Yamaha did some tricks to avoid aliasing and other typical digital issues from those days, making it sound a lot better than many other digital synths of the time.
There's no set samplerate, instead a custom multi-stage resistor-ladder DAC is used which already has the waveform "baked in" if you will. This means the digital phase counter directly drives the DAC with the "sample rate" being 2048 times note frequency i.e. A440 would be just under 1MHz.
The actual digital processing takes place at 1MHz as well, since there's up to 14 channels that need to be processed in series so extra speed was required. Processing everything in series means only 1 set of IC's is needed to process everything, instead of 14 sets!
Each channel is processed in 1 microsecond, with the organ section taking 12 and the orchestra 14 microseconds in total.
Another trick the DAC has is that its minimum and maximum voltage are externally controlled by an analog CV (named IL and IH). So lowering the volume of the waveform does not lower its bit depth (11-bits i.e. 2048 steps), which avoids another common issue in digital audio back in the day.
To the left you can see the sine waveform generator for the organ tones (named 'Flute').
You can see two resistor ladders are used, to get the full 11 bits range. There's also external opamps for further buffering.
At the top and bottom you can see the envelope CV inputs. Interestingly these could be used to create DC offsets, but as far as I can see this isn't used at all.
Interestingly the waveform generators for the orchestra are slightly more complex as they allow for interpolation. Digitally generated audio had one more issue that needed tackling: aliasing in waveforms with high harmonic overtone content.
There's the same two resistor ladders, but there's additional control circuitry that will generate interpolation address data.
The way this is used is that the higher the note being played the stronger the interpolation and thus the weaker the harmonic overtones get. This prevents nasty artifacts such as aliasing on high notes.
This interpolation can go so far it can even turn the waveforms into near-sine, thus it's almost a digital VCF.
Below you can see a diagram of this in action.
For further details on how all the key-scanning, channel assign, etc works I highly recommend reading the P.A.S.S. guide linked in the intro of this page.
The Analog Glory (VCF, VCA)
This is the part where the commonly seen claim of "turn your E-70 into a CS-80!" becomes somewhat true!
The VCF:
The IG00155 used in the organs is the same as the IG00156 in the CS synths, but has a slightly "calmer" resonance and more low-end, which makes them very interesting filters. Personally at least I prefer these specs over loads of resonance but too much thinning out. Though the IG00155 of course still thins out at high resonance.
This isn't a problem however as there's an entire additive sine-wave synth in the organ that allows you to fill up the low end if needed.
Having heard Marc's Son Of GX in action I was surprised how full & rich the sounds are. The basses are absolutely thunderous.
I'm not sure how this compares to a CS-80, but a comparison between the two VCF IC's would definitely be interesting...
Either way: there's 2 of these bad boys per voice. One for high-pass and one for low-pass. This means you can have HP, LP and BP modes for your voices which makes things very flexible. The IG00155 does have its own dedicated outputs for these filter modes, but I think Yamaha wanted to be able to vary the width of the BP mode more easily, so they opted to use two filters in series with the other two outputs unused.
To the left you can see the frequency versus control voltage graph from the P.A.S.S. manual.
The filters are configured so that with the CV halfway the cutoff frequency sits at 800Hz. As you can see there's quite a large range on the filters, from 25Hz all the way to 15000Hz.
For the control voltages there are full ADSR envelopes with initial level + attack level, plus manual controls plus... keyboard tracking! Nice :)
The VCA:
This part is pretty standard, just a VCA to control amplitude controlled by an envelope. As they do. This is the exact same VCA as used in the CS synths.
The VCA's are configured to allow for mixing of all 14 orchestra + 1 pedal voice without clipping.
Control voltage is once again from an ADSR + initial level + attack level envelope
The Envelopes
These are a doozy, TODO
VERY moddable!
TODO
The insides of my E-70 and the Tower of Power (E-70 in rack format)
Here's a bunch of pictures I took and screenshots of the video from taking the E-70 apart to build the Tower of Power.
