Quad 2805/2905 dismantling: in pictures (part 2)

[wout31]

Nobody will touch Quads anymore as it's not worth the effort.

Personal opinions may vary. I think it is very very much worth the effort.

No way 0.2 mm on anything will make any kind of objective difference in the sound.

I wasn't talking about sound difference, neither were you if I recall correctly. You mentioned arcing due to loosened stator PCB's.

an occasional visible membrane puncture is nothing to worry about

It isn't if it was caused by arcing. Burned.
If you cold puncture it with a paper clip the total diaphragm will rip. Completely different subject.

 

[1audio]

"Nobody will touch Quads anymore as it's not worth the effort." These guys have built a nice business rebuilding panels and speakers: https://estatsolution.com/elements.html I have used them and can recommend their services. A burned hole in a diaphragm becomes a focus for voltage breakdown. The carbon conducts better and you get a corona around it that arcs easily. With an ultrasonic detector you can easily "hear" the corona from a burned hole.

I would agree that a new design using newer materials and manufacturing is overdue but discounting the assembly as using bookbinding glue is really dismissive. The adhesive needs to be able to stick to plastic (hard), mylar (very hard), paper phenolic, PCB solder resist and handle differential expansion without cracking and handle the stress of the diaphragm. This would be a specialty adhesive. There are new adhesives that might work better (after 40+ years) but you would need to test them to make sure it doesn't have other problems. Ross Walker tells a story about the damping screen. The vendor improved the product by making it conductive (its normally used a a filter medium) and it caused serious problems. Quad had to get the vendor to make the old version specifically for them. Manufacturing is never easy.

 

[n719z]

Ah, I see there is some confusion about the nature of the beast here, so let me clarify further. It is not the touching of the mylar membrane to the stator that one needs to fear. The mylar has GIGAohm-meter resistivity and very little stored energy when not pushing air. Any arcing from the mylar to the stator is self-limiting in a very benign way. Forty years ago I would play Kraftwerk too loud on my dad's '57s and the membrane would definitely slap the stators quite often. Those speakers still sound very nice today after a HV supply rebuild; same mylar. Granted, none of our hearing is ultrasonic anymore: that could explain the dearth of cats in the vicinity. Martin-Logans too acquire a greyish patina of carbon from all the dust that self-combusts over the years. Vacuuming regularly picks up most of that and keeps the membrane healthy for a very long time: you can hear the hiss when it becomes too conductive if neglected.

No, the dangerous failure mode is the two stators touching each other after coming unglued from the grids. If your monoblock is pushing a kilowatt of transient at 20 Hz, that could be 50 Joules stored in the magnetic field of the trafo that can all dump into that tiny spark in microseconds through a highly conductive pathway. That's enough energy to weld copper. Then the next weld will be in the trafo, then in your power amp, then if you're lucky the circuit breaker will pop before you get to replace your house wiring or your house. I suppose there may be some wisdom in never powering old Quads with anything other that 20 W of triodes!

Is it worth it? Well, dunno, ymmv. I just finished Elina Garanča's Carmen on the Quads and on the LS50's sitting right next to them, and I can tell you that the little KEFs have a surprisingly similar tonal character and pleasant performance, but the sound stage is a tiny joke compared to the Quads. Take me to the Opera dammit!

 

[john65b]

I have used hot glue in the corners / along edges of loose stator panels too, using a sharp hooked tipped awl to pull stator onto stator matrix (careful not to puncture mylar - I have not yet) . The other benefit is a tiny bit of hot melt glue gets thru the stator hole, runs down the inside of the stator, solidifies, and forms a nice "anchor" to the stator matrix - can't get that with epoxy...one must be careful with all the "stringy" leftover from hot glue, but thats why you thoroughly check before you button everything up...

I. for one, am looking for a better way to attach stator matrix to stator via "anchors" in all corners, instead of the short-sighted method of epoxy on all stator matrix surface area to stator, that will just eventually fail. These speakers are a technical marvel, but the way the stator matrix is attached to the stator is just...well...bad.

The thought of a set of quads I can really drive real hard like my Martin Logan CLS, without worry of breaking loose the stator from stator matrix would be the last speakers I would ever want to own.

 

[1audio]

"Ah, I see there is some confusion about the nature of the beast here, so let me clarify further. It is not the touching of the mylar membrane to the stator that one needs to fear. The mylar has GIGAohm-meter resistivity and very little stored energy when not pushing air. Any arcing from the mylar to the stator is self-limiting in a very benign way." doesn't match my experience. Once the air ionizes the temperature is hot enough to easily melt the plastic. Even though the current is limited to the available charge its still enough. I have seen holes in panels where the stators are still quite solidly attached. Big burn marks on the stators as well. The bias voltage (5.2 KV) is very close to the breakdown voltage of air for that gap (approx 7KV at 1 PA at 3 mm.) so it takes little extra voltage to get an arc.