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Restoring / Modifying Goodmans Magnum K2s Advice Needed
Restoring / Modifying Goodmans Magnum K2s Advice Needed
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Old 28th August 2021, 10:44 AM   #131
david83 is offline david83  United Kingdom
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Join Date: Oct 2020
True. But why limit them to average listening levels? And what if someone did turn it up a bit? If a speaker is capable of, for example, 16 to 32 watts then surely the best thing to do would be to design the crossover to be capable of that? If we ever find ourselves in a situation where we want them cranked up (garden party? Hall hire?) it would seem sensible to make sure they are able to run like that. Totally agree that for average use theyíre not going to be that loud, but I couldnít trust that theyíre going to not accidentally be turned up over 1-2 Watts. Whilst I might be able to remember that, Iíd put money on the wife and kids not remembering. If the Mrs cranks it up and the house burns down, I think Iíd be getting a divorce pretty quickly.

Last edited by david83; 28th August 2021 at 10:56 AM.
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Old 28th August 2021, 11:12 AM   #132
Galu is online now Galu  Scotland
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It's not a problem with domestic speakers cos the manufacturers don't wrap their bass coils in insulating tape.

So, basically, the problem is solved if you don't use tape!

The inductors they use are designed with the maximum power rating of the speaker in mind.

Low resistance coils won't get as hot as high resistance ones. What is the resistance of your bass coil I wonder?
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Old 28th August 2021, 11:19 AM   #133
david83 is offline david83  United Kingdom
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Oh sorry, I see what you mean.

The resistance is lower than the spec Steve provided. I went for thicker wire than I probably needed. I probably could have used 0.8mm, but I used 1.0mm.

The 3.3mH on the Bass circuit is 0.4R, it can be up to 0.8R.

For the air coils I am using 1.25mm to get a lower resistance there too.

Last edited by david83; 28th August 2021 at 11:23 AM.
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Old 28th August 2021, 11:22 AM   #134
Galu is online now Galu  Scotland
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That will keep the I squared R heating low.
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Old 28th August 2021, 12:47 PM   #135
Galu is online now Galu  Scotland
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Attached, is a Wharfedale domestic loudspeaker crossover from the 70s.

Shock, horror, Steve!

Look at the tape around the coils!
Attached Images
File Type: jpg XO Interior.jpg (683.5 KB, 43 views)
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Old 28th August 2021, 03:16 PM   #136
david83 is offline david83  United Kingdom
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The lacquer is here.

While we wait for some input about the suitability of the lacquer for the coils, I was thinking about the sticker on the back:

13715112-5770-4075-9DC9-5E50ABEAB2DB.jpeg

Iíve kept it there because itís part of the charm, but I should probably label it correctly now itís been modified.

What would be a sensible thing to label it with now? 8ohms? How many watts? Not really sure how the relationship between the drivers works and how to determine the new maximum wattage and ohm value?
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Old 28th August 2021, 05:30 PM   #137
Galu is online now Galu  Scotland
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You measured the DC resistance of the bass driver as 6.5 ohm. Adding on one third of that figure, as is the rule of thumb, suggests a nominal impedance of around 8 ohm.

The power handling of the bass driver will not have changed.

Even your mid's DCR of 5.4 ohm indicates it has a highish nominal impedance of around 7 ohm.

And then, at certain frequencies. the impedance can drop below the nominal value.

Consequently, I would leave the label exactly as it is 4 - 8 ohm/40W.
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Old 28th August 2021, 07:51 PM   #138
david83 is offline david83  United Kingdom
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Got it.

Iím ready to rebuild these coils but I think I might put that on hold and work on the grilles instead. Varnishing coils with any sort of varnish I imagine is a fairly 1 way road so donít want to start if itís a no go.
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Old 28th August 2021, 09:00 PM   #139
Galu is online now Galu  Scotland
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When you rebuild your ferrite rod coils, why not cut a couple of squares of plywood, or what have you, drill holes in the middle of them to match the diameter of the ferrite rod, then glue them onto the rod to provide supporting end 'cheeks' which will retain your coil windings?

A sort of homemade bobbin I mean, and no need for varnish. Simply drill a small hole in each wooden cheek through which to pass and help secure each end of the insulated copper wire.
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Old 28th August 2021, 10:18 PM   #140
david83 is offline david83  United Kingdom
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Good idea. I think Iíll do that.

I noticed that even on the air coils, which do have a bobbin, it was still quite difficult to get the wire to sit butted up against the previous turn, continue wrapping and maintain an even tension. This meant that it was entirely possible to not Ďproperly fillí the length of the bobbin and on subsequent rows the pressure of the turns could Ďforceí its way down in between the turns of the row underneath. Iíve only done one air coil so far, but it was much easier to keep it neat. I think the next job will be to knock up some ends as you suggest.

I took the masking tape (at least thatís what it looks like) off one of the Goodmans air coils that came with these, and itís quite chaotic. Itís loose and all over the place. Not uniform and neat at all. Makes you wonder how the thing maintained a static value and didnít cause noise. Maybe the values werenít high enough.

I didnít get very far with the grilles this evening. The fabric (having a distinct weave) was really tricky to get the rows/columns aligned. Without a lot of pulling, holding and checking before stapling, it was extremely easy to get a wonky result which looked crap and was immediately noticeable! Iíve just about got one covered now, but the edges arenít trimmed and the badge isnít fitted so thatíll be a job for another day (probably next week now).

What with work / summer hols and various other commitments, I donít have anywhere near as much time as Iíd like to spend on these. Iím sure theyíll be worth it in the end!

Left to do:

Finish grilles
Add Velcro to cabinets (old stuff was shot)
Build air coils
Redo ferrites
Build crossovers
Several layers of clear varnish on cabinets
Add spikes
Assemble
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