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.
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.
(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).
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.
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.
Further Reading: Active Filters – Rod Elliott.