PLLXO - Am I doing this right?

mortron

Member
2011-08-20 6:02 am
Town
So this morning I woke up with the intention of plugging in some numbers and heading to the electronics shop to get resistors and caps to build a PLLXO. After playing with the sheet in the past, I gave it little thought.

This morning I am more concerned :p

So after using the values with my amps and setting the XO points, I got my results. I am planning to use both second order filters. The impedance of my HP amp is only 25Kohm... and the bass amp is 10Kohm. I see the LP amp is okay, but the 25K is too low? So my can of worms goes like this:

-The stuff in the far right column confuses me and makes little sense.

-When it says use Ramp for R1 or R2 in HP, does that mean, no resistor at all, or that I need a resistor with the same value as my amps input?

-Can I make this with 1 input, 3 outputs: one with a full range output, one with HP and one with LP?

The sheet did not make clear what the drawbacks would be to the impedance of the HP amp, and am not sure how to compensate.
 

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-When it says use Ramp for R1 or R2 in HP, does that mean, no resistor at all, or that I need a resistor with the same value as my amps input?

In the HP circuit use Ramp as R1 (1st order) or R2 (2nd order) as the resistor needed in the circuit.

Unless your pre has a really low output impedance, and lots of drive, less than 50k amp input for a 2nd order filter is an issue.

dave
 

mortron

Member
2011-08-20 6:02 am
Town
Thanks Dave...

My gain on the pre is 7db and output impedance about 2000ohm. Here are specs from a review:

Review: Bottlehead Quickie tube preamp | Sound and Vision Magazine

So if I am understanding, the ramp as R1 would mean if my amps input is 25kohm, I'd use a resistor with the same vale as R1?

And in my situation, what would my options be with a PLLXO? Could first order work? I am following xo points from a thread (mjk) using slightly different drivers than I have. Should I try for a first order? There is a rising response about 250-300hz on my driver in its baffle. (Visaton B200 and dual Goldwood 1858 Hframe per side.)

Im building the hframes Sunday, so have all day today and tomorrow to mull over a temporary option. Thanks.
 

mortron

Member
2011-08-20 6:02 am
Town
Voltage 36v :(

I got bits at the shop... They were cheap. Hopefully the caps I got will suffice until I can buy nicer ones.


If I am implementing the values in my original post above, R1 at 10k resistor, I have that, but I messed up and R2 for the 1st order HP I don't have the 16.6667k resistor required, but have two 25k resistors, which is rAmp. Can I use these in the R2 position of the hp? There is a note at bottom but not sure if that applies here.


As well, do I just run the input to a common point and each filter hp/lp branches off from there? Thanks
 
If I am implementing the values in my original post above, R1 at 10k resistor, I have that, but I messed up and R2 for the 1st order HP I don't have the 16.6667k resistor required, but have two 25k resistors, which is rAmp. Can I use these in the R2 position of the hp? There is a note at bottom but not sure if that applies here.


For 1st order HP set R2 (leave it out) to infininty and use the value of Ramp as the R for the calculation. ie HP consists of a "single" added cap.

As well, do I just run the input to a common point and each filter hp/lp branches off from there?

Yes.

dave
 

mortron

Member
2011-08-20 6:02 am
Town
Gotten this far...

I haven't run any wiring, they're just soldered to PCB, with the strips all common to each other... Just in case it doesn't sound good, I figured this enclosure will suffice.


Inputs I'm just wiring right from rca to the board, as I am trying to just use parts about the house, only had to buy resistors n caps.


Do I need to isolate the rcas from the case or is it okay as is?

Sorry it's upside down... I give up on tablets and phones.
 

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mortron

Member
2011-08-20 6:02 am
Town
It's up and running... No hum.

BUT!


I am testing one channel of the high pass. I have my preamp connected to the filter then to the power amp. I run one channel into the preamp, and both channels to the filter, but only one RCA to the amp. With one channel connected to a speaker.


One channel works as it should and displays all proper characteristics. When I disconnect input, and put it into the other channel on the preamp, and turn it way up, to maximum (loud) I get some signal still coming through. I assume this is crosstalk.


Is this the filter causing this? Like due to a common ground? Or will it go away when both channels are connected?


I'm scared to try more, as I'm still unsure if its doing anything bad. Otherwise, I'm quite impressed.
 

mortron

Member
2011-08-20 6:02 am
Town
I have it up and running now... very impressive and problem free.

Now I am going to try more XO frequencies out. This is a great way to get into active amplification, and once again, hats off to you for planting the seed Dave.

I see the issue with loss on the LP. I have my amp turned to the maximum volume setting in order to blend with the HP, which is set to 1/5 volume, if that. :eek:

I am just using what's at the electronics shop for components, but once I get a setup I like, is there any benefit in buying more audio specific caps and resistors?

Is the 90 degree phase discrepancy with a 1st/2nd order combo a major issue? I have tried to read up on this, but the acoustic/electrical slope distinction has me lost, and couldn't find anything that would answer my questions easily (probably because it's not that easy :D )
 
but once I get a setup I like, is there any benefit in buying more audio specific caps and resistors?


Yes, but don't go crazy,

Is the 90 degree phase discrepancy with a 1st/2nd order combo a major issue?

Text book issues fall aside when you start talking about an actual implementaion. Could be insignificant, or worse, or anywhere in between.

dave
 

mortron

Member
2011-08-20 6:02 am
Town
Thanks Dave,

Really appreciate people championing ideas like this. Tomorrow im grabbing caps to try different cutoff frequencies. The affordability is in line with the DIY spirit as it allows experimentation. And many ideas need to be tried to be realized.

Next up is using Duct Seal on my Visatons.
 
A single pole filter introduces a 90degree phase to the signal.
BUT !!!!!
at the crossover frequency where the signal is down by 3dB, the phase is only 45degrees.
If the treble has a high pass then the phase has changed 45degrees and the bass with it's low pass is also changed by 45degrees, but in the opposite direction. The two drivers are being presented with signals at the crossover frequency that are 90 degrees out of phase.

Let's advance to a 2pole filter for each half of the crossover.
The high pass introduces 90degrees of phase at the -3dB frequency and the low pass introduces an opposite 90degrees of phase at the same -3dB frequency.
The two drivers are being presented with a signal that is in exactly 180degrees of phase opposition. There will be a big "null" in the output where the listener/microphone is exactly the same distance from each driver's apparent source point.
Reverse the phase of ONE driver and this will result in the phase matching at the crossover frequency.

Back to the single pole, where we had that 90degree phase error.
This creates a lobe pointed down, or up, from the directly in front position.
If you stand up you can hear the null or reinforced signal due to the phase induced lobe.
Here a sloping baffle may give a partial cure to the single pole phase anomaly.
 

mortron

Member
2011-08-20 6:02 am
Town
So in listening to this... It is apparent there is much above the xo point on the low pass. Did I do something wrong or is it just inevitable I will hear higher noises? Now low end of vocals but like a lot of music. It's supposed to be 60hz second order electric, but not acting as such.