[sdiy] Low Pass filters and musically useful frequency range

David Ingebretsen dingebre at 3dphysics.net
Wed May 14 06:51:46 CEST 2014

Hi Andy,

Thanks for the thoughts.

The 914 Low Pass resonant peak is lower than the 907 and it is about 88 Hz. Right, I see and understand the adjustable HP filter cell in the main filter section. However, the output stage has a fixed high pass filter on the input with a low cutoff frequency. Jurgen Haible explains it better than I can. Quoting from Jurgen Haible's writeup on his 914 clone:

"... the original 914 has a different summing amplifier topology, too: Where the 907 uses an approximated virtual GND / inverting amplifier summing, the 914 uses passive summing with a noninverting high-gain amplifier to restore the level. This includes an extra quirk: a high pass filter between the passive summing node and the output amp."

That was the basis of my original question. Why put a HP filter there? 

When I tested the actual circuit I built and the simulation of it, it was clear lower frequencies were being blocked. Depending on the reference level, the output HP filter blocks frequencies starting at and below somewhere between 40 and 60 Hz.


>> -----Original Message-----
>> From: Andrew Simper [mailto:andy at cytomic.com]
>> Sent: Tuesday, May 13, 2014 8:22 PM
>> To: David Ingebretsen
>> Cc: synthdiy diy
>> Subject: Re: [sdiy] Low Pass filters and musically useful frequency range
>> On 7 May 2014 03:11, David Ingebretsen <dingebre at 3dphysics.net> wrote:
>> > 2. Why would Moog add such a high pass filter to the 914? I can see
>> > blocking DC, but why worry about 10 Hz getting through? Does it have
>> > to do with being musically useful? I don't think this high pass filter
>> > was in the 907, so why add it to the 914?
>> >
>> > Thanks
>> >
>> > David
>> I think there is some confusion here. The only "DC blocking" is done near
>> the buffers to remove the biasing.
>> The lowest frequency filter in the 907 / 914 is a 4 pole low pass filter with a
>> resonant peak at around 200 hz, there is a front panel knob to control its
>> input level, so it does pass DC when this level is up (ie no DC blocking from
>> it). Because a pair of LC low passes is used without buffering this makes for
>> a sloppy filter, so there is a bit of sag before the resonant peak. When the
>> level knob for this is adjusted you don't get a high pass filter, you just get a
>> different level of the low pass filter being heard, when the knob is at 0 the
>> "high pass" actually comes from the rest of the signal being filtered by
>> bandpass or highpass filters.
>> There is a high pass filter in the 907 / 914, it is at the highest frequency
>> which is around 4 khz, and it too has a volume knob so you can adjust how
>> much it heard when all the parallel filters are added together.
>> Andy

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