varible LPF/ HPF/Baxandall EQ

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simonsez

simonsez

Well-known member
Joined
Nov 14, 2008
Messages
679
Location
Jakarta, ID
hello,

I like to build variable LPF with 2 pole, 6 position rotary switch.  At the sixth position will trigger the relay to by pass the circuit.
Instead of varying the resistor value, I vary the capacitor value to get different freq filter.
Is the schematic below will work?


regards,

Simon

 

Attachments

  • LPF.GIF
    LPF.GIF
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simonsez said:
hello,

I like to build variable LPF with 2 pole, 6 position rotary switch.  At the sixth position will trigger the relay to by pass the circuit.
Instead of varying the resistor value, I vary the capacitor value to get different freq filter.
Is the schematic below will work?
Click to expand...
Yes, it would. But why the hard by-pass? I would put very small capacitors (100pf) in the 6th position.
 
abbey road d enfer said:
simonsez said:
hello,

I like to build variable LPF with 2 pole, 6 position rotary switch.  At the sixth position will trigger the relay to by pass the circuit.
Instead of varying the resistor value, I vary the capacitor value to get different freq filter.
Is the schematic below will work?
Click to expand...
Yes, it would. But why the hard by-pass? I would put very small capacitors (100pf) in the 6th position.
Click to expand...

great! putting small cap would be nice, but this circuit will be follow by HPF filter, so hard by pass looks beter option to me (is it?)
Actually it's a part of my baxandall eq build .
 

Attachments

  • LFHPF.gif
    LFHPF.gif
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You probably want to add some discharging resistor between caps at the switch, 1M or bit higher maybe, so you don't get pops when switching between them.

Small cap is a good one, much lower complexity, switching noise, you get rid of the relay connected to your audio ground since there is no other option in the way you made it. And you probably will have a proper bypass for the whole unit and having some out of band LPF is a good thing to reduce noise of the system.

JS
 
joaquins said:
You probably want to add some discharging resistor between caps at the switch, 1M or bit higher maybe, so you don't get pops when switching between them.

Small cap is a good one, much lower complexity, switching noise, you get rid of the relay connected to your audio ground since there is no other option in the way you made it. And you probably will have a proper bypass for the whole unit and having some out of band LPF is a good thing to reduce noise of the system.

JS
Click to expand...

I'll use 4 pole 6 position shorting type rotary switches , do I still need discharging resistor?

relay will have flyback diode across  the coil, Is it necessary to put small resistor between audio ground and coil ground?


regards,

simon
 
simonsez said:
putting small cap would be nice, but this circuit will be follow by HPF filter, so hard by pass looks beter option to me (is it?).
Click to expand...
It is debatable, but I think most designers would do as i recommend. Nobody can criticize having a LPF with a 100kHz corner frequency and a near-Bessel response permanently on.
Actually it's a part of my baxandall eq build .
Click to expand...
I see you have come to great lengths with the servo-input, but for the output, you've given up and put a good old coupling cap. It doesn't make much sense to me; either you servo the output as well as the input or you don't.
Just the same, why the use of a DOA at the input?
The only justification for a DOA would be:
  • Optimization of the Source Optimum Impedance (SOI), which is not the case here because the source impedance is unknown.
  • Higher headroom (if powering with higher rails), which again is not case because the subsequent circui would not handle the extra headroom.
  • Higher current capability: that would be useless, the impedance presented by the filters being much higher than any opamp's limitation.
  • Adding "flavour": I wouldn't comment on that...
 
abbey road d enfer said:
simonsez said:
putting small cap would be nice, but this circuit will be follow by HPF filter, so hard by pass looks beter option to me (is it?).
Click to expand...
It is debatable, but I think most designers would do as i recommend. Nobody can criticize having a LPF with a 100kHz corner frequency and a near-Bessel response permanently on.
Actually it's a part of my baxandall eq build .
Click to expand...
I see you have come to great lengths with the servo-input, but for the output, you've given up and put a good old coupling cap. It doesn't make much sense to me; either you servo the output as well as the input or you don't.
Just the same, why the use of a DOA at the input?
The only justification for a DOA would be:
  • Optimization of the Source Optimum Impedance (SOI), which is not the case here because the source impedance is unknown.
  • Higher headroom (if powered with higher rails), which would not work because the subsequent circuit would not handle the extra headroom.
  • Higher current capability: that would be useless, the impedance presented by the filters being much higher than any opamp's limitation.
  • Adding "flavour": I wouldn't comment on that...
Click to expand...
 
your recommend is very simple solution indeed, but I think it's not good to apply in the HPF stage.
I feel not very good putting two stage opamp and HPF cap in signal path without doing anything .
LPF and HPF should have the same way to by pass (look at the face plate design).

The baxandall will have DOA with servo output. I put DOA at the input because I have a few of them in my hand.
The HPF output cap there just in case I need them, it easier to "jump" them instead of cutting the PCB trace and put cap,
I'm not sure whether it needed or not  with OPA2134 opamp.

I will upload the full schematic later if needed

 

Attachments

  • BAXX 2.jpg
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simonsez said:
joaquins said:
You probably want to add some discharging resistor between caps at the switch, 1M or bit higher maybe, so you don't get pops when switching between them.

Small cap is a good one, much lower complexity, switching noise, you get rid of the relay connected to your audio ground since there is no other option in the way you made it. And you probably will have a proper bypass for the whole unit and having some out of band LPF is a good thing to reduce noise of the system.

JS
Click to expand...

I'll use 4 pole 6 position shorting type rotary switches , do I still need discharging resistor?

relay will have flyback diode across  the coil, Is it necessary to put small resistor between audio ground and coil ground?


regards,

simon
Click to expand...

If you have extra pole you should use one for the relay to go to a different ground, that would be nice.

About the hard bypass vs the cap option, the thing is the relay adds quite extra complexity and cost compared to the cap, which is a really good solution and you get the potential advantage of filtering some noise out. You could well leave no caps and miss that or use caps so small that filter goes ridiculously high.

MBB switches will make it less of a problem, but switching from a small cap to a bigger one may gives the big cap to still be able to make some noise (more than switching the other way around) you may add the option in the PCB and not install them if not needed, would be a good place to use SMD resistors so the space is not a problem, this resistors won't add any noise or distortion if they are not the best metal film ones.

If you still go for the relay usually adding a series resistor with it is a good practice to limit the current but not necessary at all, if you want to reduce the dirt into the audio ground using an RC for decoupling the ground may be a good idea.

JS
 
You probably shouldn't listen to me but I would control the relay with a switch so you can bypass the band entirely and listen to what you're doing. Didn't read the other replies (sorry).
 
Zander said:
You probably shouldn't listen to me but I would control the relay with a switch so you can bypass the band entirely and listen to what you're doing. Didn't read the other replies (sorry).
Click to expand...

yes, that's the plan,  At sixth position will trigger the relay on to hard by pass the filter entirely
 
simonsez said:
yes, that's the plan,  At sixth position will trigger the relay on to hard by pass the filter entirely
Click to expand...

If you put it on the rotary, you'll have to alter the frequencies to get there except for the neighbouring position. No direct AB'ing. That's what I would do. Maybe not what you're looking for :)
 
Zander said:
simonsez said:
yes, that's the plan,  At sixth position will trigger the relay on to hard by pass the filter entirely
Click to expand...

If you put it on the rotary, you'll have to alter the frequencies to get there except for the neighbouring position. No direct AB'ing. That's what I would do. Maybe not what you're looking for :)
Click to expand...

Ah I see, but I'm okay with that, dangerous did that way too. I already order the face plate, so it's not possible to change the design.
 
Zander said:
You probably shouldn't listen to me but I would control the relay with a switch so you can bypass the band entirely and listen to what you're doing. Didn't read the other replies (sorry).
Click to expand...
In 50 years, I have never felt the need for individual by-pass in parametric EQ's*; for HP & LP even less  :).
Global by-pass, yes, because it can happen that two bands would tend to conflict, but on such a simple concept as a Bax+filters, it's useless. Having the end position correspond to a "virtual" or hard by-pass makes sense, though.
IMO, individual by-pass (by-passes?) are not needed because the processing done one one single band is clearly identified, there is no surprise when comparing in and out; this is not the case when several bands are engaged and interactions can produce unexpected effects.

*The parametric EQ's I designed (PFL52/104) had individual by-pass switches, because that's what the market "needed" but I think in terms of operation a single global by-pass switch is much more useful and sensible.
 
abbey road d enfer said:
Zander said:
You probably shouldn't listen to me but I would control the relay with a switch so you can bypass the band entirely and listen to what you're doing. Didn't read the other replies (sorry).
Click to expand...
In 50 years, I have never felt the need for individual by-pass in parametric EQ's*; for HP & LP even less  :).
Global by-pass, yes, because it can happen that two bands would tend to conflict, but on such a simple concept as a Bax+filters, it's useless. Having the end position correspond to a "virtual" or hard by-pass makes sense, though.
IMO, individual by-pass (by-passes?) are not needed because the processing done one one single band is clearly identified, there is no surprise when comparing in and out; this is not the case when several bands are engaged and interactions can produce unexpected effects.

*The parametric EQ's I designed (PFL52/104) had individual by-pass switches, because that's what the market "needed" but I think in terms of operation a single global by-pass switch is much more useful and sensible.
Click to expand...

Agree, but this gear will be use in the mastering room, when sometimes is more like audiophile talk
 
I'm not diligent enough to know if your eq is good to go as is,  but i like the concept. :)

I do like hard bypass in my eqs.
Individual band defeats are a nice to have, if you've got them but no show stopper either way.

Looking forward to hearing how you go with your build. It's certainly something I'd like to try.
The feature set is about right for a compact pcb.
 
abbey road d enfer said:
simonsez said:
Agree, but this gear will be use in the mastering room, when sometimes is more like audiophile talk
Click to expand...
What exactly do you imply?
Click to expand...

I don't know how to explain, but I think mastering guys (or audiophile guys ) prefer hard bypass than other way, even if they can't hear the differences.
 

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