JH SYNTHI CLONE

I've built a Clone of the EMS Synthi A.

I've made the following changes:

• A slightly larger Matrix. You can get a stock 20 x 20 Matrix from Ghielmetti cheaper that a custom ordered 16 x 16 one.

• A second Envelope Generator. I chose the Korg MS-20's HADSR design for this, slightly modified to work from +12V / -9V supplies.

• A Sample & Hold and Glide circuit. It  has 3 modes of operation:

(1) Ordinary S&H with internal clock rate.
(2) Portamento / Glide (linear mode; actually a slew limiter)
(3) Glide & Hold. Clock source in this case is Pulse wave from Osc. 3.
The idea is to have only one potentiometer to control clock rate and slew rate.

• An Envelope Follower and a Signal Inverter. I designed a new Meter circuit for these functions. When the Meter is set to "Control Voltage", it will give an inverted output. (You could emulate this function one the original with external patching, btw.) But when the switch is set to "Signal Strength", the output will be a rectified and lowpass filtered signal for use as Envelope Follower. There is a switch to trigger the trapezoid and HADSR from the meter channel, too.

• A Precision Control path for the VCOs. Remember the original Synthi A goes out of tune when you change the number of modulation sources in one patch? Graham Hinton shows one way to fix this problem (on his EMS web page): Buffering the matrix and get precision summing instead of weighted summing of the modulation inputs. I'll try a different approach: Leave the matrix as it is, and introduce a second, independent control path for keyboard control voltage. The VCOs will have a 3-way switch for Keyboard Tracking / Free Running Audio / Free running LFO.

• The Interface to the outside world is changed a little, also. Gate and 1v/Oct inputs replace the multipin connector, the mains connector is an IEC type, and the headphone output doesl monitor stereo left and right (not output ch. 1 and 2).

Pictures

• Photo

• Screenshot of Front panel design

• Detail view of Matrix design

• Frontpanel Photo 1

• Frontpanel Photo 2

• Frontpanel Photo 3

• Looking Inside 1

• Looking Inside2

Schematics and Circuit Descriptions

• Sample & Hold / Glide
• An additonal Envelope Generator

The Trapezoid Generator contributes to the Synthi A's uniqueness, and I wouldn't replace it, but sometimes a "normal" envelope could be useful as well. I chose the Korg MS-10 / MS-20 style Hold-Attack-Decay-Sustain-Release circuit as an additional EG. Component values are changed to match the +12V / -9V supply. The 1 MegOhm potentiometers are a compromise. I've increased the integration capacitor to preserve the maximum time, but this also increases the minimum time. This is surely not optimal, and I may change it some time. (several MOSFETs in parallel ?!)

• Using standard components for the VC Reverb

I used what I found in a drawer - similar transistors should work as well - no need to get the exact parts. The reverb tank in my Synthi Clone is a standard Accutronics one (3 springs, long version), so it was no surprise that the tank's output level was different from the original one, and the resistor values for the wet / dry mix had to be changend, too. That's why there are more trimpots than in an original Synthi.
BTW, I love the design of this voltage controlled wet / dry mixer. If I would have designed it, I would probably have ended with two OTAs and several opamps. But look at this: Two transistors for the audio path, one acting as VC resistor, and a 4th for CV mixing. Not exactly precision stuff, but it does the job, and does it well enough.
 
• The "Precision Path" for 1V/Oct CV to the VCOs (might also be interesting for owners of an original Synthi)