All:
So I've been working on squeezing active sidechain filters into my 1176-alikes. I've attached schematics of the design I've been using. The three filters are HPF, LPF and a "Mid" boost placed in series. The HPF and LPF are both second-order Sallen-Key filters while the "mid" thing provides a boost centered at 1kHz intended to serve as a sort of "loudness" button, compressing only the midrange, leaving the bass and treble alone. So far it's been fun when two are stereo linked. It could also be retuned to provide a rudimentary de-essing function.
Op-amp is a TL074 operating on 30V with a 15V split. There are 10k pull-down resistors on the negative side of the blocking caps to prevent popping, with a 1M pull-down at the first position of the HPF for the same reason. Higher resistor so that other HPF settings, which will back-feed that first resistor/capacitor combo as another HPF, will see it further out-of-band. Changing settings on the HPF doesn't pop, just engaging it. The other ones are 10k because any filtering effects with the 22µF caps are still out-of-band and after experimentation, that value seems to minimize pops better than higher values.
The 15V "rail" is a voltage divider buffered by a TL072 elsewhere on the motherboard. The filters, btw, are on a separate card that interfaces with the motherboard via a molex connector, so I can remove it easily for tinkering and adjustment.
All rotary switches are shorting types, and the "mid" boost is engaged via a toggle switch.
Now here's the problem:
While the "mid" boost is whisper quiet, engaging the HPF and LPF introduces a fair amount of high frequency hiss (the stereotypical hiss, around 8kHz or so) into the audio signal path, and I can't figure out why. Any ideas based on this schematic? Could it have anything to do with the 15V "rail" being unregulated? How about additional buffering going into both the LPF and HPF? This is driving me absolutely nuts.
So I've been working on squeezing active sidechain filters into my 1176-alikes. I've attached schematics of the design I've been using. The three filters are HPF, LPF and a "Mid" boost placed in series. The HPF and LPF are both second-order Sallen-Key filters while the "mid" thing provides a boost centered at 1kHz intended to serve as a sort of "loudness" button, compressing only the midrange, leaving the bass and treble alone. So far it's been fun when two are stereo linked. It could also be retuned to provide a rudimentary de-essing function.
Op-amp is a TL074 operating on 30V with a 15V split. There are 10k pull-down resistors on the negative side of the blocking caps to prevent popping, with a 1M pull-down at the first position of the HPF for the same reason. Higher resistor so that other HPF settings, which will back-feed that first resistor/capacitor combo as another HPF, will see it further out-of-band. Changing settings on the HPF doesn't pop, just engaging it. The other ones are 10k because any filtering effects with the 22µF caps are still out-of-band and after experimentation, that value seems to minimize pops better than higher values.
The 15V "rail" is a voltage divider buffered by a TL072 elsewhere on the motherboard. The filters, btw, are on a separate card that interfaces with the motherboard via a molex connector, so I can remove it easily for tinkering and adjustment.
All rotary switches are shorting types, and the "mid" boost is engaged via a toggle switch.
Now here's the problem:
While the "mid" boost is whisper quiet, engaging the HPF and LPF introduces a fair amount of high frequency hiss (the stereotypical hiss, around 8kHz or so) into the audio signal path, and I can't figure out why. Any ideas based on this schematic? Could it have anything to do with the 15V "rail" being unregulated? How about additional buffering going into both the LPF and HPF? This is driving me absolutely nuts.