Archive of the former Yahoo!Groups mailing list: MOTM

It seems my comment and Paul's response about getting the FFB out has
caused a flurry of FFB orders. [ Paul: Feel free to send me ten
percent! :-) ] Since I'm sure people will want to make informed
decisions, I've taken the liberty of reproducing my review of Paul's
FFB, which appeared (with the format all mixed up) in AH last year.
Paul's answer is also reproduced here, so be sure to read that as
well.

Mind you, I don't think FFB's make much sense nowadays. They were
created in Moog's day because a parametric FFB would have been too
costly and impractical. But people still seem to want them, so
hopefully this detailed review will help.

Johnm


MOTM-450 Review:


Yes, I know: the MOTM-450 Fixed Filter Bank from Synthesis Technology
isn't shipping yet. But it will be shipping ere long, and what good
is reading a review after you've bought the damn thing? Since I had
the good fortune to test the prototype and am in a mood to dish,
perhaps you will humor me and actually read this review. But before I
grace the world with the opinion it so deserves, let all be reminded
that my synth of choice is Synthesizers.com and my only MOTM module
thus far is a sub-octave mux (love it). Fortunately, the two systems
use identical power supplies and are pretty much compatible, so I was
able to test the FFB with no problems.

I have heretofore reviewed the dotcom FFB, so we won't rehash that
again. However, some comparisons will be made. How could I not, what
with two Texas FFB's sitting side-by-side begging for a shootout? So,
armed with my trusty Scopemeter and my trustier ears, I set about
putting the 450 through its paces.

SOME BACKGROUND

The MOTM-450 follows the venerated Moog 907A filter bank in that it
includes an LP filter, an HP filter and eight bandpass filters. But
whereas the Moog FFB used BP filters based on an LRC topology
(inductance, resistance, capacitance), the MOTM FFB uses FDNR filters
(frequency dependent negative reactance). This is the way to go if
you want to emulate an LRC filter without using inductors, which are
expensive and bulky and a general bitch to deal with nowadays.
How much the MOTM filter sounds like Moog, I don't know. I'm not even
sure that's important. What is important is that the FFB does what it
sets out to do and does it well--namely, providing a choice of fixed
frequency bands for spectrum enhancement and formant synthesis.

Controls on the MOTM-450 include a Wet/Dry mix pot and a
manual/remote bypass switch with LED.

THE GOOD STUFF

When all of the controls on an FFB are set to zero, you should hear
nothing, ideally. I guess that makes the 450 ideal in that regard.
With all controls set at minimum and the mix set to wet, there was no
detectable output, even with my amplifier cranked as loud as I dare.
Bugbears such as bleedthrough, hiss, and line noise must have been
out having coffee or something because hey sure weren't at the
output. The filter was also nicely resonant, lending an excellent
instrumental quality to otherwise electronic sonorities.

I didn't take a measurement, but my ear detected no attenuation of
the filtered signal, other than what would be expected from the lack
of overall spectra. What you dial is what you get, and at the output
level you expect.

The mix control was nice, but I hardly used it, given that my
interest was in purely filtered signals. The LED was also a nice
touch, as I like knowing at a glance whether something is active or
not.

The LP and HP sections performed as expected. The HP section gave
particularly crisp results.

THE BAD STUFF (not all that bad, really)

The module was a real power hog. For some bizarre reason I neglected
to measure the amperage, but suffice it to say that when I hooked the
FFB up "hot" it created a noticeable spark at the connector. I
actually had to disconnect six of the eight dotcom VCOs just to run
the thing.

The .com oscillators output a 10v p-p signal, which caused clipping
in the MOTM FFB unless attenuated. The LP section was particularly
prone to clipping and would not handle a signal greater than about 3V
p-p without distortion. The max input for the remaining sections was
about 3.5V p-p, beyond which was obvious clipping. This deficiency
was not serious, however, because a 3V p-p signal still represents a
lot of juice. If the filter were noisy, this would have been more of
a problem because SNR would be lessened. But given that the noise is
essentially zero, having to deal with lower than normal signal levels
(by .com standards) really wasn't an impediment.

My ear told me that some of the MOTM FFB center frequencies were off
the mark, and measurements with a frequency counter confirmed this.
At a target center frequency of 350 Hz, for example, the dotcom FFB
resonated at 353 Hz. The MOTM FFB weighed in at 422 Hz. At a target
of 700 Hz, the dotcom was resonant at 701 Hz (!). The MOTM FFB was
resonant at 802 Hz. Paul had some explanation as to why the MOTM FFB
was so far off the mark, but the brain cell with that memory is now
dead. He did say that the frequencies in the production version
would probably be changed to reflect the center frequencies called
out on the panel. I hope that change was made because not to do so
would impact the usefulness of the FFB. To get the primary formant
for an oboe or bassoon, for example, you need to be able to dial up
500 Hz. Any other value won't quite cut it.

One more thing: I think the switched bypass should be attenuated, so
as not to be so jarring when switched from filtered to non-filtered
output. But this is a minor nit that can be worked around easily
enough.

SHOOTOUT TIME!

So how did the MOTM FFB stack up against the dotcom FFB?

The MOTM FFB offers bands that are more resonant than what the .com
FFB offers, but the difference--while noticeable--was not dramatic. A
sawtooth processed at 1 kHz with a touch of reverb sounded virtually
identical on both systems (when the .com FFB Aid module was used).

The MOTM FFB offers 7 bands only, while the .com offers 12. From the
standpoint of synthesis, this isn't a major advantage for the .com
because the ear is less sensitive to the higher and lower frequencies
offered. All of the "business" frequencies are covered on the MOTM
FFB, and the HP and LP filters can be used to take care of the rest.

The dotcom FFB costs less than the MOTM FFB and the center
frequencies called out on the panel were far more accurate than on
the MOTM FFB (see above). However, my dotcom FFB has a serious
bleedthrough problem that spoils the filtering effect. Roger Arrick
claims to have a fix for this bug, but I haven't gotten around to
taking him up on it. Since I don't know how quiet the dotcom FFB
would be with the patch, I can't make a comparison on the
bleedthrough. But I doubt that ANY design could match the low noise
floor of the MOTM FFB.

Interestingly, the MOTM FFB reminded me a lot of the Doepfer FFB,
which was also nicely resonant. But while the Doepfer FFB offers far
more ranges and costs much less than the MOTM FFB, it is rather noisy-
making the MOTM FFB a better choice, even at the higher price.

CONCLUSION

The MOTM FFB is expensive and offers fewer bands than other designs,
but what it does it does quite well. Fans of "CD quality" analog
circuitry will be sent into an ecstasy at the astonishing quietude of
the 450. This is an excellent filter bank that MOTM users with some
cash to spend will certainly want.

Konkuro Rating is K:4 (on a scale of 1 to 5)

johnm

∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗∗

Just a few comments on John's through review:

a) the filter bank does use more power than any other analog MOTM
module, around 180ma. The average MOTM module draws about 25ma. I
could use lower power quad op amps, but they are 4X the cost of the
ones in there now.

b) the prototype tested is actually 'missing' 1 frequency band at
2100Hz. I ran out of pc board space :) But it will be included on the
production module.

c) about the filter band labels: they are the same as the Moog 907,
BUT....it turns out that Moog didn't use the ∗center frequency∗ for
his filter labels, he use the ∗lower 3dB point∗ on the bandpass. I
didn't realize this until too late. So, the production version will
tweak the capacitor/resistor values to shift all the filters down
slightly to match the center frequency with the label. This is how
the .com and Doepfer did it, and this is what users expect. I will do
likewise.

d) the prototype gain is indeed set wrong for full 10V pk-pk VCO
driving signals. I was using my bench test signal generator and Audio
Precision, both default set to 1V Pk-Pk. This is a 1 resistor tweak.

Paul S.