AR v. Yves Usson:
Yves Usson is one of my synthesizer heros. He has amazing designs and is a truly nice guy. He has always been helpful to me.
Yves designed an all IC version of the Moog 914 based on the basic topology of the 914, but he was also able to compare to a real 914 and he shared this on his web site. I highly encourage you to visit Yves’ site. Twelve of his PCBs are available from The Bride Chamber, another site I can highly recommend. I actually built three of his 914s, and still have one because it is a really neat module.
Having a module as close to a 914 as I was going to get, I thought it a good idea to compare the audio output of my version of the 941 to Yves’. Not in a “whose is better”, but here’s what two of them sound like and does mine measure up to Yves’? Here is audio comparing my 914 to Yves Usson’s design. I compared my inductor version. There are a couple of things to remember:
- Yves uses two cascaded filter cells, each one having the same center frequency frequency but different “Q”. Mine has two cascaded filter cells as well but also follows the Moog where the first cell’s center frequency is a little lower than the second cell’s cutoff. The “Q” of the second cell is essentially fixed with a resistor and the trimmer on the input to the first cell is adjusted to change the overall “Q” to get a “Q” of 3.7 (per Yves’ research) which seems consistent with the component values on the Moog schematics (although there are a couple of errors on those schematics, no surprise). One glaring mistake is a missing resistor on the second cell which sets the “Q”. Somehow, Jurgen missed this. I caught it when I was unable to get a close visual match to the Moog frequency response curves so I did a detailed study of the only photographs I could find of the Moog 914 and worked through the math.
- The Moog has a fixed high pass filter with an effective cutoff of about 60Hz (if memory serves me) between the summing node of the filter cells and the input to the output amplifier. The 907 did not have this and I don’t think Yves included this on his 914, but I could be wrong. Jurgen included it on his 914. This affects the low pass cell response and helps eliminate unwanted low frequency rumble.
- Yves changed the 700Hz cell to a center frequency of 750Hz. I think the actual cutoff frequencies for the HP and LP cells are a bit different in my 914 as opposed to Yves.
In the audio, my 914 is in the left channel and Yves’ is in the right. There is some kind of funky echo or ray gun sound at the beginning of the audio. I have no clue what artifact it is. I recorded the two using a Tascam DP-24. The output of each filter was fed directly into the DP-24 with no additional EQ, or other modification. I’m driving the filters with the same sawtooth from a Steiner Type “A” VCO whose frequency is driven by a sine wave from a Steiner Type “B” VCO. The resulting audio files were mixed down to a stereo track using the DP-24 then uploaded here.
I start with all cells and let you hear the YU 914 alone, then mine. I then take out each cell one at a time HF down. I then compare each cell one at a time. I end by adding all the cells back in and then letting each filter play on its own again, YU first then mine. Saw tooth with frequency swept by a sine wave.
The most exciting 4 minutes of synthesizer sounds you will ever hear! :)
Here is a link to the uncompressed audio. It’s 40 MBytes.