passive line level crossover for stereomour....

[orangealpaca]

hi,

my first post here, and i am no electronics genius.  : )

what i am wanting to do is create a pllxo for the stereomour, so i can bi-amp a kind of FAST system (sorry for all the jargon i picked up....!)  basically, run little full range drivers (alpair 7p) with the stereomour down to 120Hz crossover, where i feed some helper woofers (sdx7 in the same enclosure, separate sealed) run by an adcom amp for the low range.

firstly, does anyone have any thoughts, concerns, or encouragement?

secondly, i am considering a simple first order high pass for the stereomour - less components so more transparency - and stack the effect with sealed box resonance to the same 120Hz point.  so i have looked around, finding different figures - is the stereomour a 100k input impedance???

thirdly, is this basic idea right?  i have a selector switch in the stereomour, and from that to the volume pot - from there - after the volume knob (an alps now) - i take two lines - one to the stereomour preamp with the pllxo cap(s) to give high pass, and a separate line to the adcom with a low pass 2nd order.  does this make sense???  of course i would have a pot before the adcom to balance the sound also.... (or in the steremour if it ends up being the louder of the two amps!)

why???  sounds complicated, i know.  what i am trying to get - clarity and soundstage with the stereomour and full rangers, combined with good extension into 25-30Hz with helper woofers and adcom.  i am hoping this frees up a little stress for a touch more headroom and poetry with the stereomour, as well as giving me serious extension and quick punches down low.  i do not want to thrash this - i just want full range, clear and transparent, at good normal listening levels....  i listen to all sorts of music, some classical, post-rock, glitch and experimental that can venture down very low in the frequency range.  i want to hear it all.

happy to hear any thoughts / ideas....

cheers!
sean.



ps - i love my stereomour - i made a special copper chassis for it being a metalworker - it sounds great, and makes me smile when i listen to music.  : )

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[Paul Birkeland]

what i am wanting to do is create a pllxo for the stereomour, so i can bi-amp a kind of FAST system (sorry for all the jargon i picked up....!)  basically, run little full range drivers (alpair 7p) with the stereomour down to 120Hz crossover, where i feed some helper woofers (sdx7 in the same enclosure, separate sealed) run by an adcom amp for the low range.

You can undersize the parallel feed capacitor to remove bass content going to the 7P (probably a good idea).  You can run a feed from the output of the volume pot back to another set of jacks, then out to the Adcom.  There are in-line RCA passive filters that could be used at the input jacks of the Adcom to get you your LPF.

Still, a preamp makes things like this oh so much easier!

-PB

[orangealpaca]

thanks PB - this sounds like an easy implementation.  i am just not great at understanding if there is any difference in loading on the amplifier between decreasing the parallel feed cap, or actually adding another cap in front of the amp input....

i came across this old thread just now -

I must have mis-communicated - for crossovers, I prefer to put the filter as early in the chain as possible.

Putting it at the amp input means an additional capacitor, so that's a tradeoff.

so this has been looked at before - apologies.

but still, i am assuming the stereomour is 100k input impedance after the volume control (i have seen it listed at 25k and 50k also) - is that right?  if that is right, i can use the pllxo calculations on t-linespeakers.org to design what i need, placed after the volume control....

again, thanks - this is something i am not so sure about and need help with....

[Paul Birkeland]

Yes, the Stereomour has 100K input impedance. 

[orangealpaca]

thankyou.  i will start fiddling!

[2wo]

One note, the 100k input  impedance is ahead of the volume control...John

[orangealpaca]

ok - thanks greatly for that john.  so can i somehow apply a line level filter after the volume control in any way - splitting the signal to high and low (pllxo) after the volume pot?  it would be awful to have to change the two different low and high volume knobs every time i change overall volume...!

ps - i have changed my volume pot to a 50k alps - does this mean that my input impedance is now 50k and not 100k?

sorry if i have missed something - this is all fairly new to me....

maybe i should just get a preamp and use it as selector and volume, filtering between the pre and stereomour / adcom...?

cheers.

[Paul Joppa]

To maintain the low distortion of the Stereomour, I recommend putting the filter before the amplifier. Using the parafeed cap leaves all the bass going through the driver and power stages, and does not help significantly to relieve them of bass duties. To a lesser degree, using the interstage capacitor has the same problem, but affects only the driver stage.

If you take the signal from the output of the Stereomour volume control, the source impedance will vary with the volume setting. Keeping the filter impedance high (and using a lower-impedance volume control, as you are already doing) will reduce this problem.

It is not impractical for the low-pass function to cascade two RC filter stages the Q will be slightly less than 0.5 (Linkwitz-Riley) but not too bad. Higher orders are progressively more problematic.

I'm on vacation for another couple weeks but can help more after that if needed - this is IMHO an excellent approach to getting the best from flea-power SETs!

[orangealpaca]

thanks heaps paul - enjoy those wondrous holiday days and i will ask you for more help in a couple of weeks.... : )

i am looking forward to implementing this - i reckon it will be a great little setup!

cheers.

[RayP]

This site is useful and it has a spreadsheet you can download to help with the calculations.

http://www.t-linespeakers.org/tech/filters/passiveHLxo.html

I have used it in a FAST system and like the results but I still need to tune the bass a bit more.

ray

[orangealpaca]

cheers ray - nice and easy!
great to hear that the FAST is working for you - i have high hopes.  i hope it can combine liquid transparency and lower end punch.  we'll see....  : )