[Rob Babcock]

Is there such a thing as a tubed crossover?  I have never seen such a thing but it would be hard to imagine they don't exist.

[TheChairGuy]

Oh yes, and you're in luck Rob - it's on sale this month from Marchand Electronics.

http://www.marchandelec.com/specials.html

[dwk]

Yup - I have one made by 'Lux' corp. All-triode 3-way unit with filters adjustable via plug-in cards. Pretty cool stuff. I haven't really seen much info about the unit, although 'Gizmo' did praise it when talking about the Marchand unit.
 It's been sitting around for a couple years and I've been meaning to clean it up and maybe look for a buyer

[Scott F.]

Hiya Rob,

Yep, they exist. I use the Audio Research EC-3. Its a three way active. They also made a tubed two way. In addition to the AR and the Marchand John mentioned, Heathkit made one also. It was a two way.

These things are out of site price-wise. I bought mine about 5 years ago for about $600 and I thought I was over paying but now on the rare occasion they hit eBay, one in good shape sells for double to triple that amount.

Want some schematics? I've got them for both plus some stuff from Tube Cad Journal on tubed crossovers.


Oh, the really bad news....my EC-3 uses twelve 12AX7 tubes. 

[JoshK]

Indeed, I have read some really good 'thought projects' on tube based crossovers.  Basically they are complicated & expensive because of the number of stages, but there is nothing to say that they can't be top notch performance wise.   In fact some think that they are the holy grail of crossovers.

Myself, I'd like to approach such a project down the road, but I have far too much on my plate at the moment.  Here is what I'd do though.  Take a look at Linkwitz's site and really map out the filters needed to do everything needed in the xo, including delays, baffle-step correction and any needed notches or eq.  Then map out the sallen-key filters needed to do such.  Then determine a good tube stage for wrapping around filter sections.  I've seen a number, and the tube cad journal (tubecad.org) presents a good approach, but there are others too (google).  Then I'd make up such a circuit using a solid but inexpensive and easy to come by tube, maybe such as the Russian 6N9P and buy a lot of them for back up too.  Cause you are going to need a lot of them. 

You need something with a bit of transconductance I believe, lot plate resistance, good linearity and good harmonic spectrum (want to minimize colorations).  Don't use more gain then you have to for each section.   Pay ultra close attention to the PSU, because you have a lot of stages where noise can wreak havoc.  This is where it gets expensive.  CCS's, Aikido noise cancellation or other techniques for PS rejection will be wanted. 

It doesn't have to be hard if you break it down into pieces and take care, but any commercial unit will be expensive.  I do want to try this at some point in the future.

Basically all the same principals apply to tube xo as to a tube based phono stage.  Instead of RIAA filter you have your sallen-key filters.  Low noise is paramount here too. 

[Scott F.]

......Take a look at Linkwitz's site and really map out the filters needed to do everything needed in the xo, including delays, baffle-step correction and any needed notches or eq.  Then map out the sallen-key filters needed to do such.

......Pay ultra close attention to the PSU, because you have a lot of stages where noise can wreak havoc.  This is where it gets expensive.  CCS's, Aikido noise cancellation or other techniques for PS rejection will be wanted. 

You know, as much as I respect SL, I'm not convinced that you need BSC's, delays or notches in an active XO. If you carefully design your enclosure you will minimize the need for a BSC. Same goes for time delays, some quick experimentation with your drivers and pink or white noise will let you know what the acoustic centers are and when they are aligned. Then design an exclosure around those offsets.

When it comes to S-K filters, I really like my Pioneer SF850. It has 6, 12 and 18db filter slopes all adjustable via a switch. Based on the drivers you use, tuning frequency of the enclosure and personal tastes, this gives you tons of flexibility when it comes to XO points. Heres a 'for instance', my Lowthers seem to sound best 6db low pass at 150Hz when mated to my Goodmans are set at 12db at 100Hz.

To me, this type of adjustment in an XO is a must. Its fairly simple to impliment on a decent switch. To me and my 'pureist' veiwpoint, I'd rather not have delays and other filters in my XO. It makes it too complicated and since you are designing you own speakers for use with an active XO (in many cases), make those compensations in the enclosure.

You are absolutely right on the power supply. Thank goodness for guys like Ron Welborne and his kit power supplies. I'm getting ready to replace the PS in my EC3 (which hums like a mofo) with three PS8's.

[JoshK]

Scott,

I'd agree with you that if you do it in the enclosure that would/could be better.  Less is more inside the filtering, so enclosure optimization will yield better results, but not everyone has that flexibility and I was talking in general.  I may disagree with you on the BSC though but it is hard to generalize because so much is design dependent. 

I personally think too many audiophile speakers don't use enough BSC so as to not encourage a room mode (which would lead to bad reviews and no sales) or to increase perceived detail.  So in the end, most audiophile speakers sound aenemic or constipated IMHO.  There is also those that use too much to make a small monitor sound like it makes big bass.

[Rob Babcock]

Thanks, guys.  I knew someone here could 'link me'!