T914 Extended Fixed Filter Bank

Introduction:

And yes - another Moog filter is cloned, the 914 Extended Fixed Filter Bank. Other synthesizer concepts call them Formant Filter, but anyway - these filters don't have any voltage control but attenuate or emphasize the signal content concerning a certain frequency band by manual control (potentiometer).

The T914 consists like the original 914 of 14 filter bands. There are a low pass filter, 12 band pass filters and a high pass filter. The center frequencies of the band pass filters are:

125 Hz
175 Hz
250 Hz
350 Hz
500 Hz
700 Hz
1000 Hz
1400 Hz
2000 Hz
2800 Hz
4000 Hz
5600 Hz

The behviour of the module is non-linear. If you process a square wave signal and enable all filter bands the output is no square but something complex with peaks and notches on the scope and it sounds much more "vivid" than the input does.

Circuit:

Again something for science fairs or boy scouts, as no capacity value of any filter stage exists in the circuit diagrams of Moog. The documents just say 'selected'.

So see below my list of capacity values I worked with:

Frequency Hz	C1	C2
Low	1u	1u
125	680n	570n (470n + 100n)
175	377n (330n + 47n)	470n
250	288n (220n + 68n)	330n
350	220n	253n (220n + 33n)
500	150n	150n
700	100n	115n (100n + 15n)
1000	72n7 (68n + 4n7)	68n
1400	47n	62n (47n + 15n)
2000	38n6 (33 + 5n6)	35n2 (33n + 2n2)
2800	22n	26n7 (22n + 4n7)
4000	16n8 (15n + 1n8)	19n7 (15n + 4n7)
5600	12n2 (10n + 2n2)	15n
High	1n5	1n5

Please refer to the original circuit for L-values or see the LTSpice circuit diagrams below for all component values.

Service: If requested I can provide sets of inductors / coils. Please send a mail request (mail address see below).

Schemas: Attention: Clicking a schema means acceptance of disclaimer on page bottom!

It is the simple architecture of input - processing - output. The input signal passes the input buffer around Q1 (amplification factor 1) and is processed by the following 14 parallel filters of the filter stage. The result is amplified by the 3 stage output buffer (Q2, Q3 and Q4, amplification factor 70) to gain the same amplitude as the input signal.
The low pass filter circuit consists of two cascaded 2nd order passive LRC low pass filters with a total slew rate of 24db / oct. The band pass filter looks like a dual coupled RLC band pass, but with a resistor as the coupling element. I didn't find any background information about it, but it works fine :-)
The high pass filter circuit is a concatenated architecture as the low pass filter, just inverse, also with 24db / oct slew rate I suppose. R2 can be ommitted.

Part replacements:

I tried to build my module as close to the original as possible, but I had to do some changes due to actual components I use instead of the originals:

All transistors have been replaced by the low noise type BC550C.
All capacity values have been changed to values of actual value ranges (e. g. C 80uF => C 100uF).

Circuit changes:

After my LTSpice analysis of the original circuit I changed some parts of the output buffer for my T914 clone. Well, to be honest, I analyzed the circuit with the new transistor types listed above because my aim was to build the module without vintage components (transistors) of course, so the circuit was not really 'original', but quite close.
After building the circuit in reality I changed some more things within the output buffer because the module worked slightly different compared to the simulation. So at the end I had changed the following component values:

22R resistor of Q4 amplification stage was replaced by 100R resistor
220k feedback resistor of Q2 amplification stage was replaced by 69k resistor

Frontend:

The human interface of the 914 shows one of the most characteristic module frontends I've ever seen. For me this frontend is one of the identifying elements of Moog modular systems - you see just this one, and you know: this is a Moog modular system, before you even had a look at the other modules.

A lot of knobs - arranged in a kind of face look - are reponsible for one frequency band each (see list above). They are not only attenuating a frequency band but also emphasizing it.
Two connectors - one for input, one for output are the interface to the system. No backside connectors this time.

Board of the T914:

Board dimensions: 145 x 195 mm
Upper side: Input buffer, low pass filter, band pass filters 125, 175, 250, 350, 500, 700 Hz.
Lower side: Output buffer, power supply, high pass filter, band pass filters 5600, 4000, 2800, 2000, 1400, 1000 Hz.
In contrast to the original concept of the Moog modulars I supplied the board with a +/-15V power supply. Voltage regulators on the board convert it to standard Moog +12V and -6V. Benefit of this is a higher stability of the power supply.

Click to enlarge

Complete module:

Click to enlarge

Sound example

The Swarm Colourful white noise. White noise is processed by the T914. It starts with Low Pass and High Pass enabled, then all other 12 bands are added randomly.

Please send questions or remarks to:
Carsten Toensmann

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