Dynaudio BM15 Passive Crossover

[kidb]

Went down a rabbit hole after my trusty ole Dynaudio BM6a finally gave up. I decided to turn them into passives, which is still in progress due to some parts being not yet in stock. But it got me curious about my Dynaudio BM15s. I took a peak inside and wanted to know if upgrading them was worthwhile. So I ordered the parts built a replacement board and I'm happy with the results. I modded one and kept one stock to A/B. I feel more low end coming through in a way that makes the stock crossover sound a little lacking and a smoother high mid thing thats nice and a tad less hard sounding. Debatably smallish but good improvements. 7~10% nicer???

I'm not going to say everyone should upgrade their BM15s, but what I thought would be interesting to share is the crossover for people who own the BM15a that want to go passive before or after the amps fail. The passive crossover has decent but not high quality components in it. I've had Dynaudios for 14 years now, feels good to refresh them and keep them going. I wanna help keep these things going for people who still want to use them, and save some BM15As from the trash bin. So I've attached the BM15 crossover, and will get to the BM6A after the holidays sometime.

Cheers

 

[kags]

Great work! I've been considering that with a few of my older, dead powered speakers. Did you analyze the DSP/electronic crossover function of the active/powered version before designing the passive x-Over, or did you simply work out the passive crossover from scratch?

 

[kidb]

I dont own the active version of the BM15, just upgraded the passive crossover! I have the BM6A however and starting from scratch there. It would be fun to do some measurements and tweak of course.. wouldn't mind diving into more crossover design, as I have an old pair of KRK E8's that need repair... or a crossover!

 

[kags]

Right on.

I’ve been debating for recreating am active x-over either as active or passive whether to go from scratch or start by measuring the response of a working unit. So many powered monitors die and aren’t worth the effort to troubleshoot and repair if I can simply use off-the shelf stuff to build a similar x-over. I would prob go class D and DSP for most systems today.

 

[k brown]

The only thing that could 'age' poorly in a passive crossover is the electrolytics. Certainly nothing that would qualify them for the trash bin.

If they sound OK now, you may have just gotten lucky; in most cases replacing electros in a passive xover with film types usually results in unpredictable changes in behavior. The designer 'tuned' the xover for electros. Changing them to film is not automatically an improvement in all cases.

 

[gyraf]

in the dynaudios, we've had much more problems-over-time with drying magnetic oils than with crossover wear..

 

[kidb]

Right on.

I’ve been debating for recreating am active x-over either as active or passive whether to go from scratch or start by measuring the response of a working unit. So many powered monitors die and aren’t worth the effort to troubleshoot and repair if I can simply use off-the shelf stuff to build a similar x-over. I would prob go class D and DSP for most systems today.

Totally agree

 

[Cqwet Dbdfte]

in the dynaudios, we've had much more problems-over-time with drying magnetic oils than with crossover wear..

Replaced magnetic oil in my 30 year old Vifa tweeters, got the old out with some mineral spirits and a business card, after removing the voice coil.
The new oil got sucked in from the plastic ampule like magic from a small hole.

 

[kidb]

The only thing that could 'age' poorly in a passive crossover is the electrolytics. Certainly nothing that would qualify them for the trash bin.

If they sound OK now, you may have just gotten lucky; in most cases replacing electros in a passive xover with film types usually results in unpredictable changes in behavior. The designer 'tuned' the xover for electros. Changing them to film is not automatically an improvement in all cases.

Curious what can be said about electros vs films performacne in crossovers, can you share more about that?
I assumed the crossover design probably had to go through price considerations, as companies do, and figured the electroltric was a cost choice over design choice.

 

[Cqwet Dbdfte]

Electrolytics have much higher ESR/ESL, higher DF than film caps, commonly larger tolerances, whereas film caps cost much more money and takes up more space. I doubt those ESR/ESL losses are a bonus used in setting a crossover frequency. In old budget speakers large value film caps would be a rare occurrence.

 

[JohnRoberts]

It would be very hard for a passive crossover to be an improvement over all but the very worst active crossover.

Passive crossovers have multiple tradeoffs.

JR

 

[k brown]

It would be very hard for a passive crossover to be an improvement over all but the very worst active crossover.

Passive crossovers have multiple tradeoffs.

JR

The BM15s he's speaking of have passive crossovers.

 

[JohnRoberts]

The BM15s he's speaking of have passive crossovers.

Thanx, I was not following closely, I thought he was trying to replace an active XO with passive.

Indeed capacitors in passive crossovers are the weakest link. Modern capacitors are likely better than older ones. It's difficult to find film capacitors large enough, but easier today than yesterday.

JR

 

[Cqwet Dbdfte]

. It's difficult to find film capacitors large enough, but easier today than yesterday.

JR

You can of course parallell film caps for the required values.

 

[JohnRoberts]

You can of course parallell film caps for the required values.

indeed but not to easily realize several tens of uF. I am trying to avoid opening the can of worms regarding paralleling small high quality capacitors across large lousy caps to make them well behaved. Back several decades ago my bench work determined that the small caps needed to be 10% or more of the final value to have any measurable impact.

I repeat modern electrolytic caps are much better and larger film caps are more available than they were back in the 70s when I did my bench work regarding this (I'm old).

JR

 

[Cqwet Dbdfte]

I have a large bag with very low ESR film caps, 7uF 300V I've been dipping in to.
But Digikey and others have plenty of choices.
If cost is an issue, yes new non polarized electrolytics should be an upgrade from the past.

 

[k brown]

The point of my first post was to caution that replacing electros with film doesn't always result in an improvement, since they have different characteristics, and the crossover was originally designed with electros.

It's a much safer bet to replace old electros with new of same value/rating, since electrolytics don't maintain their characteristics with age.

 

[Cqwet Dbdfte]

Replacing a damaged cap with a good one should be an improvement, replacing a good electrolytic with a film cap may not offer improvements, but offers no disadvantage, rather the opposite.
Electrolytics usually have higher tolerances and nobody would design a filter with losses as a positive artifact.

 

[k brown]

Replacing a damaged cap with a good one should be an improvement, replacing a good electrolytic with a film cap may not offer improvements, but offers no disadvantage, rather the opposite.

I beg to differ, in that cpas with different charcteristcs can alter the crossover behavior.

 

[mhelin]

BM15A and BM15 are totally different animals what comes to crossovers. The passive has 6db/oct filters whereas the active comes with
"1.7 kHz electronic crossover, 5th order phase aligned.
Instrumentation type balanced input section, with input
overload protection." as the user manual puts it.
You can dl service manual from
BM15A service manual

Instead of just swapping components of the passive crossover you might want to build the active crossover with or without the amps in BM15A. They give much steeper slope thus much narrower area in the output of the two drivers where the frequencies are overlapping and causing phase problems and hot spotting, meaning the FR changes when you move away from the sweet spot. So with steeper filters the sweet spot is wider. Also having overload protection is useful. Using DSP (MiniDSP and like) though makes things easier to control and adjust, even for some room eq.