SubSub page: Filtertype MonoLadder (inspired by Moog concept)
designs started 2017
Introduction:
The famous 24dB/oct ladder filter can be made in a poor man's version using diodes as variable resistors.
More diodebased designs exist but according to me the one I made is the only one running on single supply 5V (compatible with microprocessor).
I also added a high impedance readout of the ladder, that improves bass response and prevents degrading Q (at low filter cuttoff).
Circuit description and explanation:
The diodes in this circuit should be seen as variable, current controlled, resistors with Rdiode=25k/I[uA].
The 4th order ladder filter is then formed by D1..D8 and C1...C4.
The audio input signal (V5, 100mVpp max, centered at 2.5vdc) is send into the ladder as a differential signal (opposite phase for upper/lower branch).
The frequency control signal (V14) however, is common (identical) for both branches.
By reading out the filter via a differential amplifier (X4-X5) the control signal is cancelled by subtraction and only the audio remains.
Voltage controlled frequency is then realised via V14 that sets the current through D1..D8.The diodes D9..D12 only form a current path to ground with higher impedance than a single diode.
At maximum diodecurrent (800uA) the total voltagedrop over the 6 series diodes is 3.5V which is ok at 5V supply when using rail-to-rail opamps.
Note that the highest cutoff frequency occurs for V14 at 0V, the lowest for V14 at 5V. In analog applications you could add an inverter, with microprocessor control it does not matter.
The voltage controlled Q is realised via feedback using D13 and D14, V15 sends a dc-current that sets their resistance.
If you check the feedback sign you will see that it is negative feedback but the trick is that exactly at the cuttoff frequency the total phase shift of the filter is 4x 45degrees
(4th order filter) resulting in 180 degrees and becoming positive feedback at that frequency only (giving increased peaking at F-cutoff).
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Before building, I made a circuit simulation model (SPICE, Microcap) to test the design.
Here you see the simulation results for sweeping the filterfrequency with V14:
Note that because a tuning voltage is applied to the diodestack, we get a nice exponential tuning curve (I=expV) for the diode resistance.
This gives close to 1V/oct frequency tuning here. It does have a temperature dependance (diode:-2mV/celcius) but
using a microcontroller it should be easy to add a temperature compensation because diode temperature drift is predictable.
Up to now I did not even implement temperature compensation because using it in practice does not give me problems.
Here you see the simulation results for sweeping the filterQ with V15:
In the MonoLadder1 synth it resulted in this circuit: (special Q--notch-mode removed for simplicity)
This is what it sounds like:
example q variation
Changing filterQ from low to high while LFO modulating filterfrequency with a 2Hz sawtooth
More soundexamples can be found on the page of the Monoladder1 synth
More text will be added here