Hello,
I am designing a 32 band band-pass filter bank (eurorack standards) which consists of a tube Class A 2 stage mono preamp sent in parallel to the 32 bands and summed by two (stereo) active busses (VCA controlled). In each band there is a VCA controlling the volume that has an aux output connected to the input for feedback controlled through a pot and a switch for on/off, there is also a switch to bypass the filter connection to work as a summing stereo mixer. Another switch is found in the master controls to bypass the preamp sending the signal to the bands and instead be sent on a separate external output to let it be used as a separate pre.
I have been reading on ways of using different types of switches for less clickiness or pops when engaged, but a transistor or a relay seem to have too many drawbacks (distortion, noisy) so I though of using something like maybe a quick envelope to mute the VCAs when the switches are used but I have no idea if there might be a better way to also minimize part count.
The VCAs I'm using are the THAT2180 with a logarithmic amplifier to bend (slightly concave) the CV according to the power law (still trying to establish that one 😀).
I would appreciate any feedback as I'm fairly new to circuit design, thank you.
I am designing a 32 band band-pass filter bank (eurorack standards) which consists of a tube Class A 2 stage mono preamp sent in parallel to the 32 bands and summed by two (stereo) active busses (VCA controlled). In each band there is a VCA controlling the volume that has an aux output connected to the input for feedback controlled through a pot and a switch for on/off, there is also a switch to bypass the filter connection to work as a summing stereo mixer. Another switch is found in the master controls to bypass the preamp sending the signal to the bands and instead be sent on a separate external output to let it be used as a separate pre.
I have been reading on ways of using different types of switches for less clickiness or pops when engaged, but a transistor or a relay seem to have too many drawbacks (distortion, noisy) so I though of using something like maybe a quick envelope to mute the VCAs when the switches are used but I have no idea if there might be a better way to also minimize part count.
The VCAs I'm using are the THAT2180 with a logarithmic amplifier to bend (slightly concave) the CV according to the power law (still trying to establish that one 😀).
I would appreciate any feedback as I'm fairly new to circuit design, thank you.
The main source of pops when switching in the signal path is sudden voltage shifts. If you have a cap coming from some point in the circuit, and it just goes to a switch point. That cap is uncharged. When the switch is closed to some other point, that cap has to instantly charge to whatever voltage differential existed between the two points. Even if it is just a millivolt, it still results in a pop.
What we generally do is span the switch with a high value resistance. Say 1 meg or 470k maybe. The resistor keeps the cap charged, but doesn't allow signal leakage. Of course different circuits require difering interpretations of this basic approach.
What we generally do is span the switch with a high value resistance. Say 1 meg or 470k maybe. The resistor keeps the cap charged, but doesn't allow signal leakage. Of course different circuits require difering interpretations of this basic approach.
By span do you mean shunted after the cap? Ive found this circuit that I think uses what you are saying
So instead of using it as a bypass, the pole is coming from the preamp output buffer for instance and the dual throws one to the aux output and another to the bands, with the cap and resistor at the output of the preamp buffer? The pole would then be going to a jack for the aux or the input buffer on each band.
So instead of using it as a bypass, the pole is coming from the preamp output buffer for instance and the dual throws one to the aux output and another to the bands, with the cap and resistor at the output of the preamp buffer? The pole would then be going to a jack for the aux or the input buffer on each band.
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- Add a bleed resistor to the wire connecting the NC (bypass) contacts.
- Assure that DC is not introduced at the input and output by the attached equipment. E
- Assure that DC is not introduced at the input and output by the attached equipment. E
The only way to avoid a click when switching active signals is to cross-fade, simply removing any DC component isn't enough when the signal is present. Something like a JFET with the control signal transitioning over several milliseconds can provide a smooth transition between open and closed circuit.
Imagine a bass note - if hard-switched at the peak of the waveform you introduce a step at full amplitude, which has energy across the entire audio spectrum and will be audible as a distinct click.
Switching slowly provides a smooth waveform at the transition - far less energy sprayed across the audio spectrum.
If you switch always at a zero-crossing the click is vastly reduced, but this is complicated when switching between two signals as you have to time the switch point with the two signals being instantaneouly equal using a comparator/flip-flop circuit.
Probably the JFET way is the simplest, and there may be some chips that do the zero crossing trick specifically designed for pro audio equipment. Distortion _during_ the switching isn't a big problem, you just have to not click or clip.
In the digital domain its easy, a smooth band-limited crossfade is a few lines of code 🙂
Imagine a bass note - if hard-switched at the peak of the waveform you introduce a step at full amplitude, which has energy across the entire audio spectrum and will be audible as a distinct click.
Switching slowly provides a smooth waveform at the transition - far less energy sprayed across the audio spectrum.
If you switch always at a zero-crossing the click is vastly reduced, but this is complicated when switching between two signals as you have to time the switch point with the two signals being instantaneouly equal using a comparator/flip-flop circuit.
Probably the JFET way is the simplest, and there may be some chips that do the zero crossing trick specifically designed for pro audio equipment. Distortion _during_ the switching isn't a big problem, you just have to not click or clip.
In the digital domain its easy, a smooth band-limited crossfade is a few lines of code 🙂