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'Nuther newb question:
Been poring over Dave's PLLXO Primer, and I have a question.
According to the primer, a first-order lowpass is formed by a series R followed by a cap shunt to ground. I can see the need for the R to set a minimum load for the preamp.
However, in a first-order highpass, why is a series cap followed by an R shunt to ground? The R seems extraneous to me. Is it there because the amp doesn't like to "look back" through more than a certain impedance?
Dart
Thanks, Peter.
I guess I'm still trying to understand why it's different from a speaker-level 1st-order XO where all you need is a series cap.
If I used a series inductor for a lowpass, would I still need the resistor to ground?
Also, what about line-level 2nd-order LC filters? Do they need resistors too?
I guess I'm still trying to understand why it's different from a speaker-level 1st-order XO where all you need is a series cap.
If I used a series inductor for a lowpass, would I still need the resistor to ground?
Also, what about line-level 2nd-order LC filters? Do they need resistors too?
In a passive, the series cap makes the x-over function WITH the series resistance in the VC of the tweeter.
IOW, tweeters that are identical (as far as possible) with the exception of one being a 4ohm and the other a 8ohm driver, will have different x-over functions with the same value of cap.
To end up with same x-ing the 4ohm driver needs to have a cap that is double the size of the 8ohm driver.
All x-overs needs a defined resistance to work in.
/Peter
IOW, tweeters that are identical (as far as possible) with the exception of one being a 4ohm and the other a 8ohm driver, will have different x-over functions with the same value of cap.
To end up with same x-ing the 4ohm driver needs to have a cap that is double the size of the 8ohm driver.
All x-overs needs a defined resistance to work in.
/Peter
Dartagnan said:
Basically the input impedance of the amp is so high that the impedance of the filter is dominant. This is why passive filters are a bit tricky -- you need to make sure that the load impedance is going to have negligible effect on the filter.
A good rule of thumb is to load the filter with no less than 10x the filter impedance. However, this can present problems if say an amp input impedance is fixed at 20k, as then you use 2k for the filter resistor, but then this loads your source too much.
Also, the source impedance is important as it will contribute to the filter effects. Again, the rule of thumb is that the source impedance should be 1/10 or less of the filter impedance.
/richie00boy -- likes active filters
Oookay, the light is beginning to dawn... Thanks for the informative posts!
I'm beginning to see that my tube preamp (Zout 1kohm) and a sixpack of SS amps (Zin 24.3kohm) leaves no room for PLLXOs. Is there no hope for this combo short of a buffer?
So if the amp Zin is low enough, can you lowpass with just a series choke? I have a pro amp with a 10kohm Zin...
I'm beginning to see that my tube preamp (Zout 1kohm) and a sixpack of SS amps (Zin 24.3kohm) leaves no room for PLLXOs. Is there no hope for this combo short of a buffer?
So if the amp Zin is low enough, can you lowpass with just a series choke? I have a pro amp with a 10kohm Zin...
Don´t be to afraid of buffers. They can be made very transparent I´d say. If you use batteries and only the best IC´s, or even discrete, you should be satisfied.
Tube output with your 1k driving 3 channels with PLLXO is not something I´d recommend, especially with that 22k load.
One solution is to use a passive solution for the tweeters HP section, and use opamps/buffers for the mid and lows. Typically it´s the highs that suffers most from an active solution if not good enough.
/Peter
Tube output with your 1k driving 3 channels with PLLXO is not something I´d recommend, especially with that 22k load.
One solution is to use a passive solution for the tweeters HP section, and use opamps/buffers for the mid and lows. Typically it´s the highs that suffers most from an active solution if not good enough.
/Peter
For 1st order filters, you can use an R approximately equal to the input impedance of the amp but you have to include the amp's impedance in the calcs. Art Ludwig gives the formulas.
http://www.silcom.com/~aludwig/Sysdes/Crossove_Design.htm#Bi_amping
http://www.silcom.com/~aludwig/Sysdes/Crossove_Design.htm#Bi_amping
Thanks for the link to the formulas.
These seem to be conflicting viewpoints (?)
If I go ahead and use a buffer, a battery power supply sounds intriguing.
So here's what I might try in place of a preamp:
A stereo ladder stepped attenuator followed by a buffer, followed by a PLLXO. Alright so far? I've got a 100kohm attenuator--will it work well before the buffer?
Again, I know little about electonics, so how should I go about constructing this buffer? What ICs would work well with a 6v or 12v SLA battery supply? Any preexisting threads with buffer how-to info?
Many thanks,
Dart
These seem to be conflicting viewpoints (?)
If I go ahead and use a buffer, a battery power supply sounds intriguing.
So here's what I might try in place of a preamp:
A stereo ladder stepped attenuator followed by a buffer, followed by a PLLXO. Alright so far? I've got a 100kohm attenuator--will it work well before the buffer?
Again, I know little about electonics, so how should I go about constructing this buffer? What ICs would work well with a 6v or 12v SLA battery supply? Any preexisting threads with buffer how-to info?
Many thanks,
Dart
Ah--I get it now. Thanks for the clarification, Ritchie.
Peter, I will certainly need to do some research--I'm not expecting a full schematic. Could you get me started down the right road by recommending an IC or discrete devices that would perform well in a simple circuit with 6-12 volts?
And until I can remedy my ignorance about buffers, I'll have to try to make a PLLXO work without one. So back to square one.
Peter, I will certainly need to do some research--I'm not expecting a full schematic. Could you get me started down the right road by recommending an IC or discrete devices that would perform well in a simple circuit with 6-12 volts?
And until I can remedy my ignorance about buffers, I'll have to try to make a PLLXO work without one. So back to square one.
One thing you must realize is that a passive linelevel xo will have a signal loss for the lowpass section. So you may need to work in something to set levels between amps. Also in the end you may need to have a buffer both after the attenuator and at the output from the filter. Depending on the impedance of the filters, they will not be very good at driving the interconnect and inputstage of the power amps.
I would go with a active solution, full out. Battery powered and with good ICs I think the result would be very good if done right. I realize that the passive may feel tempting, but I´d guess it would be hard to make it work good.
I´m not sure but possibly you could get away with one opamp for each channel. With opamps you can set gain to match the filtersections against eachother. The opamps would match the levels, do the filter and drive the output/cables. Not sure if one oamps would fix all those tasks or if you need several... I´m to tired to think now, maybe someone else can chime in.
/Peter
I would go with a active solution, full out. Battery powered and with good ICs I think the result would be very good if done right. I realize that the passive may feel tempting, but I´d guess it would be hard to make it work good.
I´m not sure but possibly you could get away with one opamp for each channel. With opamps you can set gain to match the filtersections against eachother. The opamps would match the levels, do the filter and drive the output/cables. Not sure if one oamps would fix all those tasks or if you need several... I´m to tired to think now, maybe someone else can chime in.
/Peter
Inductors have generally fallen out of favour many years ago. They are sensitive to magnetic field changes, amongst other things.
If your just talking buffers, they can be dead simple -- just an op-amp. But it's really no extra bother to design a full active filter using an op-amp, than a buffered passive filter. And the active one will provide you with much better performance and scope for response.
If your just talking buffers, they can be dead simple -- just an op-amp. But it's really no extra bother to design a full active filter using an op-amp, than a buffered passive filter. And the active one will provide you with much better performance and scope for response.
Passive networks with inductors are still used by some folks:
http://www.marchandelec.com/xm46.html
At line level the inductor values are very high....usually always above 1 Henry.
Dartagnan,
I'd agree with some of the other fellows......unless you know what you're doing I'd go with an active solution and forget the PLL. There are just too many interactions and potential problems that have to be addressed with PLLXO's.
Cheers,
Davey.
http://www.marchandelec.com/xm46.html
At line level the inductor values are very high....usually always above 1 Henry.
Dartagnan,
I'd agree with some of the other fellows......unless you know what you're doing I'd go with an active solution and forget the PLL. There are just too many interactions and potential problems that have to be addressed with PLLXO's.
Cheers,
Davey.
Hi Davey, et. al.
Thanks for the warning about the difficulties of pursuing a PLLXO solution.
Here's my perspective: I am not against going active, but I am getting the perception that, starting from electronics square one as I am, the learning curve/time investment required for a DIY 1st/2nd-order active solution is much greater than for a passive line-level one, even one arrived at largely through trial and error. (Please correct me if I'm mistaken).
Obviously, if I knew a bit about electronics, this situation would be reversed. Someday, I hope to reach that point.
This is why I persist in being interested in a PLLXO.
Cheers,
Dart
Thanks for the warning about the difficulties of pursuing a PLLXO solution.
Here's my perspective: I am not against going active, but I am getting the perception that, starting from electronics square one as I am, the learning curve/time investment required for a DIY 1st/2nd-order active solution is much greater than for a passive line-level one, even one arrived at largely through trial and error. (Please correct me if I'm mistaken).
Obviously, if I knew a bit about electronics, this situation would be reversed. Someday, I hope to reach that point.
This is why I persist in being interested in a PLLXO.
Cheers,
Dart
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