Thermin- part 3 build - completion

This is part 3 of the Thermin build.

The earlier parts are  here:

Part 1

Thermin - Silicon Chip 

Part 2:

 Thermin - Silicon Chip - build pics 2

 

This is the build so far.

Resistors, caps, IC headers, trimmers are in.

There are two types of Diodes:

1n4004 x 4

1n4148 x 2

+ Diodes - Basic Info 

.

The 1n4004 form a bridge rectifier.

 

 The power supply is a 12V AC  job.

These rectify the AC to DC.

 

 It can also be powered with a DC battery.

But don't use a 12DC switchmode power supply

 

 

 

 

 

 

 

 

 

 

 

 

 

These are replacements for the inductors used in the original schematics.

IF oscillator Coil.

Its commonly used in radio receivers.

These coils can be bought to different specs.

Usually colour coded

In this project a black and 3 white are used.

..

The IF coil  is also known as an inter-frequency transformer.

The basic characteristic of every coil is inductance.

Their inductance can be adjusted by stretching or squeezing the coil turns.

 

...

The metal casing contains two coils. 

The first coil on the left is connected to a capacitor forming an oscillator circuit. 

The second coil transfers the signal to the next stage.

Install the voltage regulator.

Its a 7809 delivering 9V to the circuit.

final components... volume pot, power jack, switch, etc

 

 

 .... first tests

 

 

The antenna.Volume plate.

Its made from 1mm gauge aluminium.

...

 

                                                                                          Pitch antenna

connect speaker.

..

Tuning

The T1 reference coil doesn't need much adjustment.

 

Thermin - Silicon Chip - build pics 2

Some build pics of the Silicon Chip Thermin.

 

This kit has been sitting in the bottom of a draw for about a decade and I only dug it out

after a friend asked for advice re building a thermin from the old Moog designs.

Though this kit isn't the same I thought I'd finally put it together to learn about thermins in general.

I think this old Jaycar kit is discontinued.

 

This looks like a very easy project that

is great for beginners.

All through hole components.

This is part 2 of my Thermin notes.

The first part of the build notes are here:

• Thermin - Silicon Chip

More links:

https://www.siliconchip.com.au/Issue/2018/January/Make+your+own+Theremin+%E2%80%93+then+make+music%21

 

I'll discuss the circuit as I build the synth.

All Thermins start with this same basic circuit.

 

The basic circuit comprises 3 oscillators (reference, pitch & volume).

In this case they all operate at 455kHz.

Each oscillator is essentially identical.

They comprise a JEFT (junction field effect transistor) and a pre-wound IF (Intermediate Frequency) transformer. 

The JFETs are all 2N5485.

In this case the Reference oscillator = Q1 & T1 

The Pitch Osc = Q2 & T2

Vol Osc = Q3 & T3

 

The original Moog circuit used 2N 168 & 2N1086 transistors.

These are old GE  intermediate- and high-frequency transistors intended for use in radio receiver circuits.

I reckon these would be difficult to find today. 

 

+ Transistor - Basic info  

 

T1,T2 & T3 are the pre-wound IF (Intermediate Frequency) transformers.

These are a modern substitute for the inductors used in the vintage circuits.

Looks like you can buy these on Aliexpress, etc

 

The Tone Oscillators (ref & pitch)

The Reference & Pitch Oscillators are mixed in IC1 (called the product detector) to produce a audible tone.

IC1 = MC1496

It's a balanced modulator/mixer.

You won't find this in the original Moog circuit where the two oscillators are mixed through two capacitors and rectified through a diode.

 

Its a pretty common chip .

Pretty old ... dates I think from the 1970s. Easy to find on Ebay.

The MC1496 is used here as a mixer.

The signal inputs are on pins 1 & 4 .

Carrier inputs are pins 8 & 10.

Outputs are pins 6 & 12.

 

The deeper I compare this circuit with the original Moog circuit, the more I realise

how components have changed over the last 70 years.

I guess this is to be expected.

I wonder if its practical finding all those old vintage parts if you wished to stay true

to the original schematics.

Anyway, back to the build.

The VR2 trimmer looks like an new addition to the original circuit..

I think this is a basic wave shaper ???

It varies the coupling between the pitch & reference oscillators that produces the final voicing

& adds some waveform distortion. 

The addition of this shaper circuit changes the final timbre.

It can now be varied from a pure sinewave (which is not characteristic of a Thermin) to  a much deeper sounding square/ramp tone, which I think has a lot more harmonic content.

 

The output from IC1 is fed into IC2.

IC2 is a LM358 op amp.

It acts like a low pass filter.

Again, you won't find this in the vintage Moog Thermin. 

That circuit has no ICs. Its all transistor based.

 

The output from the filter is applied to the power amp (IC3, LM386)

 and this drives the speaker.

 

IC 3 = LM 386

IC 2 = Lm 358

IC 1= MC 1496

The Volume Oscillator.

This is made up of  T3 & Q3

The secondary winding of T3  is connected to the base of Q4 (BC548)

The collector of Q4 is  attached to T4 (and its capacitor).

T4 is another IF coil.

To be continued ......

Thermin- part 3 build - completion

 PS: do let me know if there are any mistakes or omissions. Cheers Jono

Thermin - Silicon Chip

 Some pics of a build... the Silicon Chip Thermin.

https://www.siliconchip.com.au/Issue/2018/January/Make+your+own+Theremin+%E2%80%93+then+make+music%21

This kit has been stuck in a draw for over 10 years ??

This design is from 2009.

A friend recently asked for some help in building a thermin  using Bob Moog's design.

In the 1960s Moog had 4 designs :

The Vangard, (Vacuum tubes)

The Professional (Transistors)

The Melodia (Kit & built vesions)

The Troubador

http://moogarchives.com/moogth2.htm

The Melodia could be purchased as a kit.

I though before I embark on that project I'll build this and see what I can learn.

This is the inside of a Melodia.

This old instrument seems to be made up of transistors, oscillators, coils and wires which create electromagnetic fields around the instrument's two antennae. 

Thermin's rely on the human body's capacitance. When you move your body in and out of that electromagnetic field, you change the frequency of the oscillating circuit.

You are actually making a variable capacitor with your hand being one plate, and the antenna, the other.

What a thermin contains are basically two primary circuits:
1) a pitch circuit.
2) volume circuit.

These two circuits are connected to individual antennas.

Vol antenna is on the left. pitch on the right.

The antennas need to be separated by at least 20 inches & be about 3.5 feet from the ground.

According to the build guide these antenna coils are machine wound by a special method called

"progressive universal" and cannot be duplicated by hand wiring. 

 

It looks like though the original circuit is quite simple, its full of hard to find parts.

The most difficult to obtain being the inductors.

More about inductors are here:

+ Inductors, ferric beads, noise reduction methods, etc

 

Inductors are also called coils or chokes.
They are passive two terminal components.
Like capacitors, they store energy.

However, in this case, energy is stored as a magnetic field.
They are usually made up of a insulated (copper) wire, wound round a core (magnetic ... iron or ferrite).

 

 

The schematics indicate (L4) 10mH volume and (L1) 75mH pitch antenna inductors.
These are wound coils that I think were only available only from Moog.
I can't find any manufacturer’s part number.  
They might be custom-made for Moog ???? 

 

According to the vintage DIY manual:

"satisfactory substitues for the antenna coils can be made by connecting

three 25mhy ferrite chokes in series

and

for the volume: connect two 10mhy chokes in series. 

It seems like the rest of the inductors (the 4 within the metal case) can be hand wound.

The pitch circuit used two oscillators:
A variable oscillator and a fixed oscillator.
The variable oscillator produces a range of frequencies and is connected to the vertical antenna.
The fixed oscillator generates waves at a static frequency.
 
Signals from the fixed and variable oscillators were mixed together
and then amplified and output as sound.

The second circuit (the volume circuit) controls the volume of the pitch circuit.
To do this it uses an oscillator connected to an antenna.
Disrupting the electromagnetic field around this antenna changes the volume of the tones from
the pitch circuit.

---------------------------------------------------------------------

Some Links:

https://technitoys.com/building-a-replica-moog-melodia-theremin/

how to make your own inductors

https://youtu.be/d-E12DlzGGc

 

https://youtu.be/awyLYgu6ODM

 

 This webpage is really useful:

https://rimstar.org/science_electronics_projects/coil_design_inductance.htm

 

 https://www.cs.nmsu.edu/~rth/EMTheremin.pdf

 

--------------------------------------------------------------------------

I'll start the silicon chip thermin build in another post

 

MetroModular Power 202 PSU

Some build pics of the MetroModular Power 202 PSU.
It's a eurorack power supply. Model number: MM1537 v0.1

The various voltage regulator specs are

+12V 1A

-12V 1A

+ 5V 1.5A

Justin's Power 101 board combined the PSU with the distribution board. 

Eurorack Power system 101

It's a very easy build. There are a few surface mount components, but they are 0805 and 1206.

Justin may have an new version of this by the time of posting, so check his website for updates.

Mouser 894-shhd001a3b41z

Its a DC/DC converter 18-75V in. 12V out 1.3A

The next 2 regulators are 580-OKI78SR12/1W36C

 Non-Isolated DC/DC Converters 12Vout 1.0A VSIP

Input voltage 15V to 36V

 

tHE 5v converter

OKI-78SR-5/1.5-W36-c

Input voltage 7V to 36V

 

 

 

These diodes are important to help reduce voltage spikes (flyback).

Gotta love  the choke.

580-51105C

Learn more about inductors here:

 Inductors, ferric beads, noise reduction methods, etc
 

There are 2 capacitors used in this build.

A 10uf and a 22uF ... both SMD.

The caps have a very large footprint... tricky  to solder on.

Don't forget to solder the remote

I'm using this Meanwell PSU

709-gst60a24-p1J

 

It's output is 24V DC @ 2.5 A

Input Voltage VAC:

90V AC to 264V AC

Some Links
+ Metromodular Facebook
+ Metromodular Youtube
+ Metromodular website  

Eurorack Format

 1. Superbooth 2018

 2. Eurorack Power system 101

 3. Eurorack Busboard MM1521A 

Noise, LC Filters, Inductors, etc

noise, Noise, and MORE NOISE.

Filtering noise is really important when building your synth.
Lots of ways to do this and the method may vary depending on the scale ... does the noise effect the whole synth (the PSU) or a small part of a circuit.
Decoupling capacitors, Ferrite beads, Inductors, LC Filters ??

I guess before you do this, you need to ask the questions;

1. Are you better off filtering the noise, or preventing noise in the first place?
2. Is there actually much noise to begin with?
3. Is any LC filter you might install making more noise than it is removing?

On the macro level, a common method is to use something like a LC filter.  This often is found in the psu (the main power supply). An LC filter consists of inductors ( represented by the letter L) connected in series with the power flow and capacitors (represented by the letter C) from the filtered voltage to ground.

Ferrite Beads
A ferrite bead and capacitor is another form of LC Filter. It is often used to filter power for specific power pins on an IC.  They are frequently used on sensitive parts of a circuit like PLL’s & analog sections.

 LC filters are used to keep noise in one section of the circuit from getting to another section.

Decouplig Caps.
A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect it has on the rest of the circuit. (Wikipedia)

Also known as bypass capacitors.
They act as energy reservoirs ... you will often see them close to ICs ... they help to smooth out any voltage fluctuations.

You can also use decoupling caps on your main power distrobution board.
Here is an example for eurorack:
NLC power distro board


Below is a LM7805 voltage regulator.
Notice the two decoupling capacitors:
The capacitors are placed between the power line & ground.

The 0.33uF helps to smooth out any low-frequency changes in an input voltage. 
The 0.1uF helps to smooth out any of the high-frequency noise at the output.
Combining these two caps helps to deliver a smooth uninterrupted voltage to your circuits.


As mentioned earlier, decoupling are often used with ICs. Logic circuits tend to do lots of sudden switching ... between on & off with not much in between..So decoupling caps help to smooth  and stabilize the input voltage..... absorbing excess voltage if the voltage spikes suddenly, and providing more power to the IC should the voltage suddenly drop.   

The decoupling capacitors are connected between your power source, whether that’s 5V or 3.3V, and ground. (Generally it's recommend to use a 100nF ceramic capacitor and a larger 0.1-10uF electrolytic capacitor for each integrated circuit).

Inductors
These are also called coils or chokes.
They are passive two terminal components.
Like capacitors, they store energy.

 However, in this case, energy is stored as a magnetic field.
They are usually made up of a insulated wire, wound round a core (magnetic ... iron or ferrite).

We measure inductance in units of Henry (H).
1Henry = 1 volt of EMF across the inductor with 1 Amp of current.
The larger the number, the higher the inductance.
The higher the inductance, the more energy we can store and provide.
It will also take longer for the magnetic field to build.

 

This is a inductor from a Metromodular Eurorack PSU

Eurorack Power system 202  

The inductor will remove any voltage ripple .

Notice also the diode> this eliminates voltage spikes if the power is suddenly switched off.

 

Here is another inductor - Its SMD

Ferrite Beads.
Also called Ferrite chokes, cores, rings, blocks, EMI filters.
These are passive components that suppress high frequency noise.
They are often wrapped around cables to  prevent the cable from acting like an antenna and receiving interference from other devices.

These are snap on ferrite beads. The upper pic shows one used at the end of a USB cable.

Ferrite beads convert Radio Frequency (RF) energy to heat. They are like a filter.
(Contrast this with inductors, which by design do not convert RF energy to heat, but rather offer a high impedance to RF.) 

 On the left is an inductor. To the right is a PCB ferrite bead.
You will probably have seen many of the PCB ferrite beads in DIY synth modules.

------------------------

Splitting ground planes.

I recently read a great article from Texas Instruments re splitting ground planes as a way to reduce digital noise.
http://www.ti.com/lit/an/slyt512/slyt512.pdf
A ground plane is an electrically conductive surface, usually connected to electrical ground.
On a PCB this is a large area of conductive material which is connected to the power supply ground terminal and serves as a return path for current from different components on the board.

 Hoikka1 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

Placing the ground planes below the relevant components and signal traces is also helpful. ...
maybe on a seperate layer of a multi-layer PCB .
This should help reduce electromagnetic interference, cross talk & ground loops.

I often see single ground planes on pcbs (with just analog components).
But if there are both analog and digital circuits on a single PCB, best practice seems to be to:
"separate the ground plane on the back layer into a digital section and an analog section, but leave the two planes connected near the return to the power supply. This ensures that digital signals do not follow a return path beneath sensitive analog components....

It is a bad idea to completely split a ground plane and try to bring the sections to the same potential using something like a ferrite bead, as this creates more EMI and noise problems ." (Altium resources)


I've always wondered why Don Buchla used two grounds on his power distro busses.

This may be the reason.
Two separate grounds for analog and digital.???
He never used this on the earlier 200 series.
This type of distro was first used in the 200e which of course uses digital components.

Other ways to reduce noise:
+ Use two separate supplies .. one for the analog & the other for digital
+ keep the analog & digital components separate & don't let their grounds touch.

But I think splitting your ground planes is a really neat idea.

Links
+ Altium
+ Texas Instruments