A-1?? Switched Capacitor Low Pass Filter (SCF)
Vorankündigung / preliminary

The preliminary version (prototype) will be completely redesigned. The module will be probably divided in two separate 30dB switched capacity filters with separate audio inputs, outputs and resonance circuits but with one common frequency control section. This allows the simultaneous usage of the two 30dB filters e.g. as anti-aliasing and clock suppression filters for digitally based. If desired the two filters can be daisy-chained to obtain a 60dB low pass.

Prototype Frontpanel 

The picture shows the preliminary front panel of a new filter that works completely different from all other A-100 filters that are available so far. It is based on the so-called "switched capacitor" technique. Instead of variable resistors, transistors, diodes, vactrols or OTAs (operational transconductane amplifiers) capacitors are switched at high clock rates (several kHz up to 1MHz) to obtain the filter function. The main advantage of this technique are the steep slopes. The SCF module can be probably switched between 30 and 60dB slope.

The SCF module is equipped with the usual filter controls and inputs:

  • manual frequency control

  • frequency CV input without attenuator

  • frequency CV input with attenuator

  • audio input with attenuator

  • audio output

  • resonance control

In addition an external resonance input is available that allows to process the feedback loop with external modules (e.g. VCA for voltage controlled resonance). With the switch the slope can be selected between 30 and 60dB.

The module also has a CV output that outputs a voltage that corresponds to the frequency of the filter (i.e. the sum of the manual control and external control voltages). And this is the reason for this output: the SC principle is a time discrete technique. This means that samples of the audio input signal are taken (similar to BBD, sampling or digital delay) and processed by the switched capacitor network. The relation between the sample clock rate and the filter frequency are about 100 (i.e. 1kHz filter frequency corresponds to 100kHz sampling rate). For low filter frequencies the sampling rate becomes audible (e.g. 100Hz -> 10kHz) similar to the clock rate that can be heard for low sampling frequencies of the sampler module A-112 or the BBD module A-188-1. To suppress the clock frequency an external conventional low pass can be used. As the difference between filter frequency and clock frequency is very high (typ. 1:100) a simple 6 or 12dB low pass is sufficient. It's frequency is simply controlled by the CV Out of the SCF. In this way it follows the frequency of the SCF. But for certain sound effects it may be desirable not to suppress the clock frequency (user's of the A-112 or A-188-1 will know what is meant ...).

Internally the SCF is made of a high-speed VCO that controls a switched capacitor integrated circuit.

Remarks: Our personal impression from the first results is that the sound of the SCF is a bit boring (sound examples will follow soon, probably after the NAMM in January 2008). At first sight there is nothing that cannot be obtained with the already existing A-100 filters. The difference between 30 and 60dB mode is not very exciting. We will probably manufacture this module only in the 30dB version and only if we obtain sufficient pre-orders (as soon as some sound examples are available).

The following mp3 files are the very first audio examples of the prototype .....  coming soon

Breite/Width: probably 8 TE/HP (still without obligation), Strombedarf/Current: ~ ? mA (still without obligation)

Preis / Price: approximately in the 120.00 Euro range
Liefertermin/date of delivery: no release date so far 
All features, specifications, prices, date of delivery are still without obligation
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