| Author |
"meModular" Modular DIY Synth |
| rolfdegen |
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| rolfdegen |
Hallo..
My first DIY Modul is an VCO (Voltage Controlled Oscillator). The basis is a CEM3340-G from Curtis. These chips were built in the 80s in many synthesizers. Because of digitization in music electronics, the chips were no longer in demand and disappeared from the market. In recent years, the demand for analog synthesizers has grown again and the chips have been re-produced.
In my first step i build a triangle and sine out for my VCO. The triangle output of the CEM3340-G is not symmetrical. For the symmetrical adjustment of the triangle output, I have provided a trimmer R28.
The trimmers R33 and R27 are used to set the sinusoidal shape and symmetry.
VCO
Sine sweep: https://drive.google.com/open?id=1z_BxgFnUachjezBpC2MiINbbsz35n0bq
spectrum analysis
Sine waveform
Development environment
In the next step I will expand my oscillator. FM, AM, an additional SubOszillator and much more ..
Greetings from germany. Rolf |
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| rolfdegen |
Hey friends ..
My second Curtis Chip CEM3340-G from Portugal has arrived. He probably had a longer journey behind and now gets a new nice home.
Greetings Rolf |
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| jorg |
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| rolfdegen |
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| Boofin Moonrocks |
| Excited to see you starting to develop some modules! I followed the degenerator development for a number of years (haven't gotten a chance to build it yet though). |
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| rolfdegen |
Thanks  |
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| tobb |
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| rolfdegen |
Did you have an AS3340 with the same temperature problem?
I'm looking for someone who can confirm that! |
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| guest |
| the warm up time on the AS3340 does not suprise me, what does suprise me is that you are not seeing any warm up time for the CEM3340. did you test them both the same way (both from a completely cold start - circuit and powersupply off for an hour or more)? |
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| rolfdegen |
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| guest |
| just to verify: the powersupply was off for an hour before both test? or was on for an hour before both tests? the chip is just one area of drift, the powersupply, and all other components on the board also drift. i ask, as my CEM3340 definitely took a bit to warm up. |
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| rolfdegen |
I measured at a room temperature of about 29 ° C. The case was open. The power supply is a switching power supply +12 -12 + 5V from Doepfer (Recom RAC10-12DA). ± 0.02% / ° C typ.
 |
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| rolfdegen |
Hello..
I have corrected some minor errors in the schematic and adjusted the frequency setting on the Coarse and Fine controls. The frequency on the Coarse control can now be adjusted over 12 octaves.
With a CV control voltage of 0.00V this corresponds to a frequency of 0.30Hz - 6500Hz. You can set + - 2 semitones on the Fine knob.
Small note: The housing of the potentiometers should be connected to ground (GND) in the circuit, as they cause small disturbances in the signal when touching.
The next step is now the construction of the 2nd oscillator and the modulation control.
Greeting Rolf |
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| rolfdegen |
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| rolfdegen |
Hello..
Today I was busy again and put the design of the VCO front panel into practice.
Above display with encoder for setting the oscillator functions eg Hard- u. Soft Sync, Modulation, LFO, Sub-Osc, Octave and many more.
In the middle the controls for Coarse u. Fine Tune and pulse width. Below inputs and outputs.
Greetings from germany. Rolf |
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| rolfdegen |
Hello friends..
The TFT display is working now. It is controlled by an Atmega328P in I2C (TWI) mode. A corresponding library for the display controller SSD1306 is available in the internet (see link). I think the looking at colors of poti knobs and tft display is really well.
Link: https://www.arduinolibraries.info/libraries/ssd1306
meModular VCO
A small problem occurred while operating the display. The display causes interference on the + 5V power supply which are also audible in the oscillator. To avoid the interference, a small filter consisting of a coil and capacitor must be connected in front of the + 5V power supply of the display.
Greetings from germany. Rolf |
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| rolfdegen |
Hello..
After a little summer break, we continue now. I have slightly expanded the oscillator circuit. Now the sync function can be controlled via the Display menu.
You have the choice between hard sync with positive and negative pulse edge or both. You can also switch to Soft Sync.
oscillator circuit
In the Curtis Manual for the CEM3340 / 3345 from the year 1980 there is still some information about the frequency synchronization:
"Frequency Synchronization
the oscillator frequency may be hard synchronized in serveral different ways. One way is to couple positive pulse, negativ pulses ore both, into the hard sync input pin (pin 6 on the 3340). A positive sync pulse will cause the triangle wave to reverse direction only during the rising portion of the triangle, while a negative sync pulse will cause direction reversal only during the falling portion. The resulting waveforms are shown in Figure 1, and provide a wider variety of synchronized sounds than possible through conventionally synchronized oscillators."
Figure 1
"Figure 4 shows circuitry for allowing only positive or negativ puls edges to synchronize the oscillator. The peak amplitude of the pulses actually appearing on the sync pin should be restricted to 1 volt minimum and 3 volt maximum for best operation."
Figure 4
"Another method of hard synchronizing the oscillator is shown in Figur 5. Negative pulses only are coupled into the base of the PNP transistor, with a peak amplitude of 8 to 10 volts for best results at VCC = +15 V. This method will produce the same waveforms generated by the conventionally synchronized sawtooth oscillators."
Figure 5
"Finally the oscillator may be soft synchronized by negative pulses applied to the threshold voltage pin (pin 9 on the 3340). These pulses cause the triangle upper peak to reverse direction prematurely, causing the oscillation period to be an integral multiple of the pulse period. The peak amplitude of these negative pulses shoud be limited to 5 volts maximum and positive pulses should be avoided entirely. If this input is not used for synchronization purposes, it is recommended that it be bypassed with a 0.1uF capacitor to ground to prevent synchronization or iittter to noise pulses on the Vcc supply line."
Greeting Rolf from germany |
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| rolfdegen |
Hello there
There are again changes in the oscillator circuit. The signal outputs now all have a symmetric +-5 Volt level (10Vpp).
Additionally a sub oscillator has been integrated. The square wave signal at pin 4 of the AS3340 is adjusted to +5V level with support of transistor T3 and fed to the Interuppt input PCINT8 of the ATmega328.
The square wave signal is divided by software and output to port C3. From there the square wave signal reaches the square wave output via the Mix potentiometer R58.
Greetings Rolf |
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| cackland |
This is a great project. Nice work. Keeping an eye on this  |
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| rolfdegen |
Thanks 
In the future there will be a 2nd AS3340 chip and internal sync and modulation functions and LFO via software.
Greetings Rolf |
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| rolfdegen |
Hello there.
There were still some problems with the overshoot in the rectangle signal. A resistor change from 330K to 150K on R56 prevents the overshoot (picture 2).
In the manual of the CEM3340 this problem is pointed out and a 1M resistor between pin 4 and pin 5 is recommended.
Picture 1
Picture 2
Picture 3
Greetings Rolf from germany |
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| alfa |
Dear Rolf
Maybe schematis work suitable, but it looks
that pin Vp is overloaded, parallel connection of:
R38 // (Vbe T3 + R54 ) !!! // R60 + R59 |
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| rolfdegen |
Hallo alfa
Thanks for your tip. The overshoot was due to a small R41 resistor. I've upgraded R41 from 1K to 10K and adjusted R59 and R60 resistors. Now there are no more overshoots (see Pic).
Greetings Rolf |
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| alfa |
Dear Rolf
Ok! I see that R54 is changed from 4.7k to 47k.
It was just a note about possible overloading.
Maybe You can remove R38 ? It looks You don't need it.
(Vbe T3 + R54 ) // R60 + R59 is the load for pin 4.
Additional note about T1 & T2 2N3904.
It looks, if You change them on AS194 / AS394 matched pair:
http://www.alfarzpp.lv/eng/sc/AS394CH.php
you will have more matched, more temperature matched
solution and maybe without trimmer R27.
In these block it looks (IC1b) using TL084A (offset 3-6mV) is not so good.
And very important is matching in resistors pair R31&R32 and
between R48,R25 &R34,R24. They must be as much precise/matched as
budget allows. |
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