The VCF or Voltage Controlled Filter

VCF - the Doepfer A-100

 The Voltage Controlled Filter,  High or Low?

The voltage controlled filter is another very important block in the design of the analogue synthesiser. (Digital control and processing of most blocks within the synth has become more popular because of lower pricing, mass production, comm etc).

The video below covers the history of the Minimoog with some of the artists whom shaped our musical destiny in electronic music.

Passive filtering using capacitors and resistors

Below, figures 2 and 3 show how a capacitor passes high frequencies more readily than the lower spectrum using the Xc or capacitive reactance formula – thus creating high pass and low pass filters in a potential divider network. VR1 would simulate the capacitor C1 in Fig. 2. Note that this is a passive bypass filter.

VCF - high and low pass filters
High Pass and Low Pass Filters – corner frequencies of fig 2 and 3 = 1 / 2.π .R.C

Active Filters

VCFs are controlled bandwidth frequency amplifiers allowing for high and low frequency cut-off, throughput of a specific bandwidth, notch-filtering and a Q-factor slope control for attentuation.

VCF - the Doepfer A-100
Doepfer A100  –  Nina Richards Creative Common Licence ZoeB

Notch Filtering

(Q-factor, or quality factor determines bandwidth of a filter).

Notch filtering is a great way to reduce a frequency which may be causing interference, a deliberate action to cause a specific sound and of course is sometimes used to reduce acoustic feedback.  (this is another subject though).

VCF voltage controlled oscillator
Voltage Controlled Filter – VCF

The circuit on the LHS, the High Pass Filter explanation is given below.

All filters will have some Resistance, Inductance or Capacitance component incorporated. A capacitance will pass more current as the frequency rises and in an inductance the opposite is true.

The NPN or BC549C acts as a buffer amplifier with very high input impedance and low output impedance. The PNP BC559C is where the action happens.  The PNP transistor acts as an amplifier where the collector and emitter is coupled to a capacitor which varies in resistance according to the control voltage i.e. the more negative Cv the less resistance.  As the resistance becomes less the circuit starts passing only the higher frequency spectrum.

VCF - Band pass and Band stop Filters
Band pass and band stop filters
Filters – Notch, Band-pass, High-pass, Low-Pass

The Q-factor or Quality factor of a circuit determines how tight the bandwidth is controlled. Whereas we add resistances into an LC circuit to reduce the quality factor (Q-factor) in radio circuits we need the LC (inductance and capacitance) Q-factor often to be as high as possible to improve sensitivity.


Equalisers used in audio allow for any part of the audio bandwidth to be cut or boosted, usually +/- 6dB or +/-12dB. Parametric equalisers control the center frequency and bandwidth range as well making for finer and more detailed sound.


There is a lot of confusion over dB and the measurement thereof. The Sengpiel Audio website explains the difference between dBu and dBV very clearly and has an online converter.

Filters, VCOs, PLL, Ring Mod, AM, FM all play an integral role in the audio synthesiser. Modern synthesisers have become extremely sophisticated with the use of microprocessing and the use of digital electronics. Indeed, the circuit board may look so much more sparse than the old Minimoog but such is the way of technology.

Next:  Envelope Generation and ADSR (attack, delay, sustain, release)

Further Reading:  Active Filters – Rod Elliott.

ADSR – Attack, Decay, Sustain, Release

The VCA or Voltage Controlled Amplifier

The VCO or Voltage Controlled Oscillator

Vintage Synthesisers using Ring Bridge Modulation

Total Harmonic Distortion meter

The VCA or Voltage Controlled Amplifier

The VCA  – a Voltage Controlled Amplifier which doesn’t amplify.

The VCA, in this case used in an audio instrument such as an audio synthesiser is usually the last block in the chain.  VCAs have a control voltage, on peak control voltage the entire signal is allowed through, on zero volts or lower the signal is blocked.  VCAs can also be termed voltage controlled attentuators.

The VCA is usually of 2 quadrant or 4 quadrant type, the output is blocked when the input control voltage is at zero. Although a VCA can be made to amplify an input signal, here the VCA is used and predominantly so as an attentuator. 2 quadrant designs have no output when the CV is at or just below 0V, 4 quadrant designs invert the output signal, and with gain set to an absolute value of CV below 0V the gain then rises. Four quadrant designs are used in amplitude and ring modulation effects. VCAs are therefore ideal for envelope shaping. The  MOTM-190 can be switched to either 2 or 4 quadrant mode.

  • VCAs are often used in circuits for compressing, companding (compressing/expanding to improve S/N) and limiting.
  • DCAs or digital controlled amplifiers are another variant, describing the controller type which varies the gain.

Although years back VCAs were usually of the discrete type; getting to grips now with most VCAs one would be looking at a high quality, multi-bit controlled unit in an IC package.

VCA using an LM101
VCA using an LM101

The VCA above is gain controlled by FET 2N5457 which acts as a variable voltage resistor. The circuit is not unique, many opamps may and will have better performance but this is a typical design where the linear voltage change across the FET ultimately results in well controlled gain of the LM101 (or other).

Another circuit showing similar principles below.

vca - linear voltage control
Linear Voltage Control VCA

Although many circuit designers are using operational amplifiers in their circuits there are some that feel with a discrete component approach the output quality can be greatly improved. See below, under further reading.

As discussed before, the VCA is used in limiting, companding and compressing resulting in the use of additional controls which vary attack, delay and sustain, release times. (In synth language ADSR) or AGC (automatic Gain Control).

VCA - 2 and 4 quadrant multipliers
2 and 4 Quadrant Multipliers (Courtesy Texas Instruments LM13700)

Above we have the 2 quadrant multiplier or amplitude modulator and the ring modulator both using dual operational transconductance amplifiers, the LM13700 with linearizing diodes and buffers.

For the AM or 2-quadrant multiplier refer to chapters 20 and 21 of the application notes which explains the VCA.

Note the additional path in the RM where a resistor (RM) is installed between out and invert input. RC for Vin2 is a potentiometer.

In our previous article on RM we cover the 4 diode bridge circuit used in SSB (RF).

In our next article we look at how VCF or voltage controlled filters work.

Further Reading: 

VCA Techniques by the honourable guru Rod Elliott. Good solid write-up, as always.

Analog Devices – AN213 / AD539

The Ring-Bridge Modulator


The VCO or Voltage Controlled Oscillator

VCO in synthesisers

The VCO – the beginnings of a new era in synthesis

The voltage controlled oscillator is a very common circuit often used in conjunction with a phase locked loop circuit.  VCOs often use a varactor diode to control the output frequency of the oscillator. They are cheap to build and are often used in modular synthesiser designs.

VCO in synthesisers
Rick Wakeman – the king of Moog (photo credit above, Aurelio Moraes, live, Sao Paulo, Brazil)

This page is not necessarily a continuation of the building blocks in a synthesiser as the VCO in it’s own right makes for a very versatile piece of test equipment.

The VCO can have a control voltage which is fixed to get a single tone or the control voltage can be that from a modulation input which causes shift in pitch synched with the input control.

The VCO in a synthesiser is normally tuned to one of either:

  • Volts per Octave, the common standard used in Moog where e.g. in sequence each volt equals one octave. This is currently the more popular configuration;
  • Hertz per Volt, commonly found in Yamaha where each octave equals double or half one Volt.

The VCO is very versatile and in modular form will have a multitude of outputs, usually being ramp (sawtooth), triangular, sine and square. VCOs can be connected together through the control voltage, the outputs can be wave-shaped and of course, modulated from an external (or internal source).

One of my favourite circuits is brought to you by Analogue Designs, using the AD654

the VCO - analog designs AD654
Analogue Designs – AD654 – Technical Information


Some circuits of the VCO 

Schematic of popular but ingenious VCO by Thomas Henry on Birth of a Synth – Scott Stites.

Nuts and Volts, as always, gives a very interesting view of OTAs (Operational Transconductance Amplifiers – Part II the LM13700). Article by Ray Marston.

Onsemi MC74HC4046A – VCO/PLL

Further Reading:

Muff Wiggler – Wave Shape Transition (beyond the scope of a beginner but very interesting, thanks to “Highcooley”).

Experimentalists Anonymous – Oscillators, Minimoog VCO etc

Vintage Synthesisers using Ring Bridge Modulation

Oscillators for the Newbie – Positive Feedback

The 741 Operational Amplifier

FM in Music Synthesizers


Vintage Synthesisers using Ring Bridge Modulation

Ring Bridge mixing of audio signals

In a previous article we cover ring-bridge modulation in SSB transmitters to get DSB-SC (double sideband suppressed carrier). Of course not to be  outdone it didn’t take scholars of audio synthesisers to add ring mod effects to their synthesisers. But where to first?

Vintage Synthesisers using Ring Bridge Modulation
Minimoog 1970 – wiki commons – credit Iko1992

Harmonics, pitch and loudness

The first synthesisers were of the analogue variety and yes, to get an audio signal out we will always need some sort of analogue contraption to amplify the signal.  Single note synthesisers are no fun of course. The timbre of an instrument is caused by harmonics, pitch  and loudness.  This can distinguish the difference between a violin and guitar.  The lowest note on the piano keyboard is A0 set at 27.5Hz. Middle C (C4) is 261.626Hz and the highest note is C8 at 4186.01Hz.  A4 is set at 440Hz and the tuning reference note.

Tuning Forks

Tuning forks were long used for the tuning of musical instruments because of the high energy tone at the fundamental. Concert pitch A440 tuning forks are therefore very common.

Although there is much to be said about the sophistication of the circuitry used in audio synthesisers, the stability of the oscillators, filters and amplifiers are infinitely more critical in the design and final result.

Polyphony and Homophony

Polyphony is a term used to describe the use of multiple voices or the ability to play multiple notes at a time. The single tone multiple frequency oscillator is an example of homophony. The Minimoog is an example of a monophonic synthesiser.


Of all the scallywags that make up the “voice” of a synthesiser the most important is that of “timbre”.  The Minimoog is a classic in that it can reshape, amplify, filter and process the signal to get the most uncanny and bizarre effects.  Often the intent to build a multiple oscillator synthesiser becomes a bad choice when one realises that all the effort is lost in the final result – lack of timbre. This is not say that the builder is incorrect, the intent may be to focus on polyphony with fixed frequency tones.


There are many ways to modulate a carrier but possibly the most important one in the scope of this article is the ring-bridge modulator.  Ring bridge modulators are famously used in SSB suppressed carrier transmissions and of course, the synthesiser.  By applying a 440Hz signal to the modulator input and with a 1kHz carrier input the resulting double sideband is that of 1000Hz + 440Hz (upper sideband) and 1000Hz – 440Hz (lower sideband) with the carrier at the output being suppressed.

But of course there is more to this. Your frequencies can be ramped (sawtooth), square, in fact anything you like.


The Ring-Bridge Modulator

Oscillators for the Newbie

FM Receivers – yesterday and now in South Africa

FM in Music Synthesizers

To be continued….  sidebands

FM in Music Synthesizers

Yamaha DX7

FM in music synthesizers – for the home tinkerer

Basics – the 555 timer IC

In this article we cover basic modulation and tone generation circuits using the 555 IC for the DIYer. Many synth based projects are based on the 555 IC and young hobbyists are encouraged to read, experiment and play with these fascinating chips.


The generation of electronic signals which are converted to sound is possibly the easiest description of the synthesiser (UK) or synthesizer (USA). Being pedantic this would also describe an electric guitar which is not a synthesiser, or is it?

Moog and DX7 – analogue to digital

Most people in the music industry would have heard of Moog and the Yamaha DX7.  When we think of Moog we think of two people, Robert Moog and Rick Wakeman. I would like to describe Rick Wakeman as possibly one of the most technically advanced musicians of our time, an absolute legend. Youngsters of today should switch off the rap n crap, pop and listen to The Six Wives of Henry VIII and Journey to the Centre of the Earth. Wakeman has played with the big names in industry, including Cat Stevens.  Oh, Yes!

Yamaha DX7
Yamaha DX7 – Wiki Commons – Steve Sims

As an aside but big news if you are a Trevor Rabin fan (ex-Rabbitt, Yes) we have the new formed group, ARW – Anderson, Rabin and Wakeman. AWR Tour.

Robert Moog was king of the music scene in the early 60s with his analogue synthesiser development which was used by most of the big bands at the time. Where Moog was king of analogue, Yamaha became king of digital with the DX7.  And no, Moog’s analogue models are still popular today and fetch a pretty penny on eBay. Look for one of his most famous models, the Minimoog of the 70s.

What’s in it?

In our previous articles on Ring Bridge modulation and FM Receivers it’s no wonder that it didn’t take engineers long to start using FM synthesis to generate unique sounds. Opportunities create wealth, right?

The common modules found in synthesisers are:

  • VCO – voltage controlled oscillator
  • LFO – low frequency oscillator, usually sub hearing <20Hz
  • Envelope shaping – ADSR, attack, delay, sustain release
  • VCA – voltage controlled amplifier, 2 quadrant multipler
  • VCF – voltage controlled filter
  • Gates – not a module but the electronic switching
  • Arpeggiator – sequences the notes of a chord (minimum 2 notes). Up, down, up and down, randomise- after defining a chord.
  • Sequencer – Plays notes in a series, mostly user defined.
  • Ring Bridge Mixer – 4 quadrant multiplier, suppressed carrier AM

Building blocks – modules

Music synthesisers form great building blocks for DIYers and programmers. The initial build could carry for example two VCOs and a ring bridge mixer with additional modules added later to generate more effects. (I include unique tones, even noise here as an effect).

Your hobbyist free standing synthesiser with out any form of midi controller input would not need to follow any specific design criteria if the objective is just to get a unique sound, possibly generated through voice (vocoder). Your synthesiser modulation or carrier wave does not have to use sine waves either – triangle, sawtooth and square with variable mark space ratios are indeed great for changing frequencies to get unique sounds.

The Nuts and Bolts – FM Synthesis

FM Synthesis - modulator
FM Modulator using 2 x 555 (LFO and Mod) – T-CAD [555 tutorial link]
Note that the circuit above was only simulated on LT-SPICE XVII and not tested. For best results see TI literature on the 555/556 chips.

Not intending to duplicate what one can find on the internet in droves, write-ups and warnings of sophistication on FM Synthesis striking fear into the hearts of many. It’s not to be, this is a DIY page, the hobbyists page. Buy a DX7 if you want but when fiddling there is nothing like having a Function generator on your workbench.  Since you are working with audio frequencies, building your own 10Hz to 100kHz FG is not expensive neither difficult. You will need the frequencies to be stable though.

See Yebo Electronics – Audio Function Generators Kit52S – I have not built this circuit.  Ask the shop assistant to check the diagram before shipping. Sometimes it’s not that legible. Pricing around R450.00 ex shipping.

The most practical circuits to build when experimenting usually incorporates the 555 or dual 555, the 556 from Texas Instruments.

NE556 – Texas Instruments

Further reading:  Possibly one of the best on the net, Indiana University – Introduction to Computer Music

Dintree – Synth DIY (interesting read – schematics)

Texas Instruments – 555  556  The best experimentation IC for oscillators and precision timers.

Music from Outer Space

FM Receivers – yesterday and now in South Africa

The Ring-Bridge Modulator