Doepfer

> week, including the A-123.  My initial reaction is that the A-123
> behaves in pretty much the same fashion as the A-121, and in fact
> could be slightly worse. I haven't really had time to check it out

I can happily report that the A-123 is in fact *considerably better* 
than the A-121 in this respect! (For my initial 'look-see'  I used an 
A-110, but since the sine wave is quite 'approximate', it gets badly 
mangled by the filter, which tends to obscure things a bit! The sine 
output of my A-111 is clearly much closer to a pure tone, and thus 
comes through the filter less mangled, so it's easier to see what is 
going on.) By setting the VCO to a specific frequency and monitoring 
the voltage levels out of the filters, I adjusted the frequency 
setting on the filter so that the given frequency was the -3dB lower 
cut-off frequency, then I increased the VCO frequency until I went 
through the passband and came to the upper -3dB point. (I had 
resonance set to minimum throughout.) In all cases I tried, regardless 
of the Freq knob setting, the A-123 upper frequency is at about 30kHz: 
for the A-121 it was as follows:

Lower cut-off freq = 25Hz (approx F=2.5 on 121, F=2 on 123), upper 
cut-off = 800Hz
lower = 50Hz (F=3.5 &F=3), upper = 1.6kHz
lower = 100Hz (F=4.5 & F=3.5), upper = 3.3kHz
lower = 200Hz (F=5 &F=4.5), upper = 6.6kHz
lower = 400Hz (F=5.5 & F=5), upper = 11.5kHz

Setting the A-121 to give the upper cut-off at 15kHz, gives the lower 
as 600Hz, at F=6. At low F settings, 0 - 2, on the A-121 there comes a 
point where you don't really get a well defined passband at all! This 
clearly supports M Dimm's findings, i.e. that the A-121 high pass is 
really more bandpass, especially at low F knob settings!

The intro paragraph to the high pass chapter of Don Lancaster's 
'Active Filter Cookbook' puts this problem rather bluntly: "In 
reality, there is no such thing as an active high-pass filter, for the 
upper-frequency rolloff of the operational amplifiers in use will 
combine to give a passband. If an active filter is to be good for 
anything, we have to save enough 'daylight' between the lower passband 
limit of the active circuit and the upper limit set by the op amp." 
The A-123 obviously achieves this, the A-121 less so. Haven't managed 
to fathom out why the A-121 configuration should be so much worse, and 
unfortunately there is unlikely to be enough info on the CEM datasheet 
to be able to work it out, so its likely to remain a mystery to me 
(unless Dieter is reading and can offer some explanation....)

Tim

[The views expressed above are entirely those of the writer and do not 
represent the views, policy or understanding of any other person or 
official body.]