Filter adjustment


Estimated executing time: 2,0 hours.

Each filter print has 3 trimpots that must be correctly adjusted. This means that 3 separate adjustments should be performed for each filter, as futher described.

In this section there is no need to connect all wiring to the front panel, temporary interconnect the output jack connection with the front panel potentiometer connection as follows:

Do this for all band filters, including the low pass filter and the high pass filter. Without this temporary connection the vocoder will not produce any sound.


Prepare the backplane in the following configuration. Stricktly follow this order:

     O : Power supply
     N : Input/output module
     L : Filter to adjust (contrary to silkscreen indication on the backplane)

Start with the low pass filter and subsequently check the band filters one by one, finally end with the high pass filter.

Handy tip

Use a small screw bit that fits on the choosen trimpots to facilitate the adjustments. Put some heat shrink over the hex holder to prevent short circuits.

Step 1 - DC bias voltage for the inverting input of the OTA's

The purpose of this adjustment is to ensure that the varying DC bias voltage derived from the control voltage output of the analyser section when a speech input is present, cannot break through to the vocoded signal output.

P2 adjusts the DC bias level on the 8 band pass filters, P6 adjusts the DC bias level on the high pass filter and P10 adjusts the DC bias level on the low pass filter.

A steady noise signal should be applied to the speech input. One simple way to do this is to blow gently into the microphone. The bias trimpot on each filter (P2, P6 or P10) is subsequently adjusted for minimum output signal from the vocoder.

If professional measuring equipment is available, a more precise alignment procedure can be considered. Instead of blowing into a microphone, a test signal can be applied direct to the input of the module. A suitable test signal is a 500 Hz or less sinewave, superimposed on a fixed DC voltage.

The output signal from the vocoder can be observed on an oscilloscope, and the preset is adjusted for minimum LF output. Incidentally it may prove impossible to reduce the break-through to an acceptably low level. In this case, the OTA is almost certainly the culprit: in any batch there will always be a few that have too high a leakage from the control input to the output. The only solution is to replace them.

Step 2 - Preset voltage to current convertor for the OTA’s

This adjustment is intended to set the initial point of the control characteristic to the same level for all modules.

P4 adjusts the voltage to current convertor on the 8 band pass filters, P8 adjusts the the voltage to current convertor on the high pass filter and P12 adjusts the the voltage to current convertor on the low pass filter.

A suitable test signal is applied to the carrier input, white noise is perfect to do this job. A very low DC voltage of approximately 200 mV is applied to the input of the module that is to be adjusted. This calibration voltage can be derived from the +5V DC power supply by means of a 25:1 attenuator. 

The trimpot (P4, P8 or P12) can now be adjusted so that an output signal just appears at the main output. If the test voltage proves to be outside the adjustment range of one or more of the modules, the whole procedure can be repeated with a slightly higher or lower test voltage.

Step 3 - DC offset of the active low-pass filter

This is the easiest part of all adjustments and determines the DC offset of the active low-pass filter that is the last stage in the analyser section of each module.

P1 adjusts the DC offset on the 8 band pass filters, P5 adjusts the the DC offset on the high pass filter and P9 adjusts the the DC offset on the low pass filter. With no speech input signal, each preset is adjusted for minimum output voltage of the corresponding module.

In conclusion

With a spectrum analyser and a lot of patience, all filter characterstics are plotted and combined in the below picture. As seen at the left side, the characterstic of the low pass filter is followed by a neat procession of 8 band pass filter characterstics, finally followed by the high pass filter.

The minor differences in peak amplitude are caused by component tolerances. As can be seen, the filters provide a nicely regular division of the audio spectrum:

Well done

Congratulations, you have now finished the filter module adjustments, click here to navigate back to the assembly & adjustment instructions.


The contents of this document are provided as-is. The published information could contain technical inaccuracies, typographical errors and out-of-date information. This document may be updated or changed without notice at any time. Use of the information is therefore at your own risk without any liability for special, indirect, incidental or consequential damages resulting from or related to the use of this document.