Converting active monitors to passive

[Albatross]

I have an old pair of Alesis M1 active MK2 monitors. The amps are shot and they have been in the cupboard for some time.

I was thinking about pressing them back into service as HiFi speakers and had a look at some replacement power boards but the price is prohibitive.

I thought it might be a good idea to get some old s/h crossover networks and make some back plates with binding posts and just drive them off an amp.

Any thoughts?

[Hugh Robjohns]

If memory serves, the originals crossed over around 2kHz, but with fourth-order (24dB/octave) slopes. While such steep slopes are possible with passive crossovers, they end up being physically large and pretty expensive.

Most passive crossovers are designed to crossover higher than 2kHz -- most are around 3kHz -- and with much gentler slopes. The gentler slopes mean that the drive units have to work over a wider bandwidth, potentially getting into regions where their responses become non-linear and with much higher levels of distortion.

So... while the active M1 MkII was a pretty good monitor for the money, the chances of achieving the same level of performance in a DIY passive mode, even using the same drivers and cabinet, are slim to nil.

On the other hand, if you want something to tinker with, with a view to learning about speaker integration and crossover design, then it could be quite fun...

The other option, of course, is to use an external digital crossover and external amps, so rebuilding the active design with external hardware... but going down that route is not going to be more expensive than buying a replacement board for the original electronics! Again, good fun from a learning point of view, but not a cost-effective solution.

H

[Albatross]

Ok thanks, I'll have a ponder.

[hashwood]

Hello, but of one uses then original crossover from pasive MK2, can it work?

https://instrumentalparts.com/crossover ... 1-and-mk2/

[resistorman]

I put a passive crossover into a RCF ART 910 A when the amp blew and it's been working surprisingly well for years. I chose a higher frequency crossover point because of the 12 db/ oct thing, but don't remember the details. It doesn't have the original specs or power handling, but it's perfectly fine sounding for my purposes.

hashwood's link looks interesting.

[Wonks]

Hello, but of one uses then original crossover from pasive MK2, can it work?

https://instrumentalparts.com/crossover ... 1-and-mk2/

Yes, it can work, but they won't be as good as the active versions. The passive crossover has 12dB/octave filters, whilst the Mk I actives had 48 dB/octave filters and the Mk II active has 24 dB/octave filters, both active with electronic time aligning.

But the modified speakers should then work as well as the Passive versions.

Note that the Passives had a 4-ohm rating. So, assuming the speaker units are the same, then you'll need an amp that is happy driving into 4 ohms, which most hi-fi amps aren't. So you may well need to buy a power amp in order to drive them. To get the most out of them and have headroom for transients you'll want around 100-120W per channel. If you do need to buy an amp, then by the time you've bought two crossovers and and some terminal backplates and the amp you, may well be spending the best part of the cost of some better sounding new (or nearly new) active monitors. Monitors have come quite a long way since 1999 when the Monitor Ones first appeared.

[ef37a]

Albatross, you say "the amps are shot". What exactly do you mean by that?
Are the power supplies intact and the small signal stages and the filters?

If so I would look around for some amp modules. The speakers seem to use a 25W and a 75W amplifier per speaker. Those power levels are pretty small beer these days so some modules should not be too expensive.

There is a snag however, when I looked for some amp modules a week or two ago I found it hard to find anything but class D switching amps.
Now, peeps here tell me that top class D amps are very very good but the best ones are expensive. Budget class D would be a bit 'gritty' I would imagine!

I still have a chassis I built around two ILP 60W modules and that sounded fine. If ILP are still going...way to go?

Dave.

[Albatross]

Ashamed to say that eight years on they are still in the cupboard. Christ this is an old thread, where does all the time go?

Dave, I'm buggered if I can remember and would have to get them out to have a look.

[resistorman]

didn't realize this was a necrothread! I usually check...

[hashwood]

Well, I repair digital cameras and lenses, and I have colleagues that are repairing other advanced electronics.
I have a spare amp, 7.1 Sony that looks like it fell of a cliff but it's like new inside and it works well.

My plan is to use those speakers in a room where I listed rarely, so I am not looking for that much quality. I repaired my 15 years old Active Mk2 with the usual resistor/capacitor problem. But this pair were a gift from a friend and they have the power supply board with lots of problems. My colleague who is advanced electronics repairman tried to repair them for the other friend, but he gave up. So passive conversion is the way.

can you please recommend a crossover schematic that I can DIY with similar characteristics with Passive original one ?

[ef37a]

Well, I repair digital cameras and lenses, and I have colleagues that are repairing other advanced electronics.
I have a spare amp, 7.1 Sony that looks like it fell of a cliff but it's like new inside and it works well.

My plan is to use those speakers in a room where I listed rarely, so I am not looking for that much quality. I repaired my 15 years old Active Mk2 with the usual resistor/capacitor problem. But this pair were a gift from a friend and they have the power supply board with lots of problems. My colleague who is advanced electronics repairman tried to repair them for the other friend, but he gave up. So passive conversion is the way.

can you please recommend a crossover schematic that I can DIY with similar characteristics with Passive original one ?

https://sound-au.com/articles/crossover-tables.htm

That does not give a 2 way 24dB/oct schematic but the 18dB jobby might serve if you are "not that fussy" and raise the frequency to the tweeter a bit?

Even then you have to source (or wind) the inductors. All in all it would be easier to build the filters in the Sony but it is all going to be a bit of a kludge!

Dave.

[James Perrett]

There's a schematic of the crossover for the passive version of your speakers at

https://www.diyaudio.com/community/thre ... 850/page-2

Scroll down to the bottom of the thread.

It looks like the high and low frequency drivers possibly have different sensitivities so whatever you do you will have to compensate for that so you can't just use a generic crossover.

[hashwood]

thanks a lot