Hi-Fi speaker resistor/capacitor question

[Rob Strand]

- I checked the grey disk resistor : as you wrote, i got 1.6 ohm with my DMM, so I guess it's good. But it looks ugly, with stains on the coating. Maybe it's only cosmetic?
- I tried to put a metal wire across the resistor legs to see if it sounded different : it didn't.

Looks like corrosion.   They seem to test OK.   If you short your meter leads together that will measure the DMM  lead resistance.  You can then subtract that value from the 1.6 ohm you measure.   If your meter leads are 0.9 ohm then 1.6 - 0.9 = 0.7 ohm.   If you meter leads are 0.2 ohm and you get 1.6 - 0.2 = 1.4 ohm then it would be a sign something is wrong with the PTC (the grey disk).

I guess the biggest conclusion is you shorted it out, which removes any question of its effect, and the result wasn't enough.

Even when the PTC is 100% OK, the 0.7 ohm resistance of the PTC will reduce the level of the tweeter by about 1.4dB.   If you want more highs then it's probably wise to leave it shorted.    1.4dB is getting hard to detect by casual listening but at least it's a step in the right direction.

- Would you care to elaborate a bit about that 0.7 resistor? Its purpose is to act as a voltage protection, right? but, depending on the input impedance, it will also act as a smooth lowpass, that's it?

The reason for it being there is to protect the tweeter against over powering.   Not all all speaker include tweeter protection.  At room temperature PTC devices are at low impedance, in this case 0.7 ohm.  When more power is fed to the speaker more current flows through the PTC and it gets hotter.  When it gets hotter the resistance rises and that disconnects the tweeter from the signal source. 

It affect the overall level of the tweeter more than the frequency response.

- I noticed that the wiring on my system is "antiphase", fwiw.

The thing that's confusing for me is does it just look antiphase because of the wiring or is it really antiphase.  In other words if you measure the resistance from the speaker - input terminal to the tweeter terminals does the speaker - terminal connect to the tweeter + or the tweeter -.

The other way to look at it is simply do an experiment.  Reverse the tweeter wiring and see if it has more upper-mids/treble.
[Before you do that see my comment on the 20uF cap below.]

You might find both connections sound similar because neither connection produces a notch.   That means the tweeter and woofer are not in-phase or out of phase, they are in between.   That means the limitations of the crossover design are showing (The W18 link I posted earlier is about fixing those issues.)

- I have glued some cork sheets on the sides of the cabinet, and a rubber damper sheet on the cab base. I'm waiting for it to dry, and i'll proceed to some listening tests.

You will find dense materials on the outer walls have a very small effect.   Thick foam does work on the outer walls.

For a sealed enclosure you will find the stuffing has very little effect on the bass but generally has a very positive effect on the midrange.   For vented enclosures you can lose bass but there's no point having more bass if the mids are bad.

Thank you for the tip, but i don't want to go too deep with these speakers, and I want to keep things simple. I mean, I often like to stay faithful to the way things were designed, and I feel that if I don't like the way the speakers sound, maybe I should just use other speakers (just like there is no point in trying to make a Tubescreamer sound like a Fuzz Face. You can tweak a Tubescreamer, but trying to change it into something that much different is quite pointless).
After a deeper listening, I don't think the Dual are faulty at all. Maybe it's to be expected to have 1978 speakers (with a sweet mid bump) sounding fine on 70s music, but not as good on 2020s electronic music. Also, maybe that I am trying too much to compare my Dual against my bigger Magnat speakers, and that I should enjoy the Dual for what they are.

All this is very true.    I have no doubts it would be possible to improve those speakers.   The question is the effort required to do it.   I'm not against making small improvements.   The 70s was full of bad speakers it wasn't until around 1980 that modern design methods became the norm.   Modern drivers are generally better than 70s drivers.

One thing I noticed from you close-up shot is the 20uF cap is an electrolytic.     Using an electrolytic cap isn't a good thing on a speaker.   Secondly, electrolytics can fail over time.   While a failed 20uF cap won't reduce the level of the tweeter it can stuff-up the crossover on the bass driver and that would mean the crossover would not work as designed and perhaps cause the speaker to sound weird.   So it would be a good idea to check the capacitance and ESR of that cap.   Maybe even just replace it.    The 10uF is an MKT which should last forever.

[snk]

Thank you.

- I checked my DMM lead resistance, which gave me 0.03. So, 0.16 - 0.03 = 0.13, which is twice the expected value.
I think that if it is safe enough, I'll remove it, and put a wire instead.

- I might change the electro cap and swap it with two 10µF Wima wired in parallel, if that may improve the sound a bit.
I don't have any tool to measure the capacitance and ESR of that cap.

[snk]

Hi,
I don't know if it is the stuffing, better monitor placement, having spent much more time listening to music through these speakers or Placebo effect, but  i am starting to really enjoy them 
I have enabled loudness on my amp, for further treble and bass enhancement (after all, it's here, and serving its purpose), and it produces a nice, clear sound, with nice focus and instrument separation.

I think that the next time i'm ordering capacitors, i will grab some Wimas for the Dual, and use it to replace the electrolytics, but for now it's all good!
Thank you all for your kind help !

[Rob Strand]

I don't know if it is the stuffing, better monitor placement, having spent much more time listening to music through these speakers or Placebo effect, but  i am starting to really enjoy them 
I have enabled loudness on my amp, for further treble and bass enhancement (after all, it's here, and serving its purpose), and it produces a nice, clear sound, with nice focus and instrument separation.

You can adapt to a certain sound over time.   I've got some top-end Sennheiser headphones which helps stop you adapting off course.

It was also very common to use the loudness on 70's speakers but I suspect that was more to do with the bass and to some degree speaker designs not allowing for cabinet diffraction.

FYI, I did a rough simulation of the tweeter polarity and to me it looks like the tweeter polarity should match the woofer.  By that I mean the input -, woofer - and tweeter - should connect together.   


[I didn't simulate the woofer low frequency roll-off.   Also I'm assuming the tweeter and woofer have equal sensitivities.]

If you look at the frequency response document I posted earlier you can see a dip around 1.5kHz which matches up with the simulation.
http://www.hifi-archiv.info/Test/Dual%20CL-Serie%20Test%201977/1977-2-112.jpg

There's no suck-out on the manufacturers plot - there's no way they would intentionally do do that.     The thing that's confusing is the schematic at least implies the wrong connection.  The wiring has a flip but I don't know if there's already a polarity flip on the PCB or PCB polarity markings.

One good thing about those speakers is the woofers have a butyl rubber surround.  That's why the surround hasn't rotted out like so many of those foam surround speakers.

[Rob Strand]

OK I've managed to work out a few things.

1)  CL-240 crossover with 6.8uF tweeter cap.

This combination was seen  on a 1976 model.  The speaker was produced from 1976 to 1978.
The 6.8uF tweeter cap appears on the crossover for the smaller CL-230.
I can only guess two conclusions.  Early on they were going to use the same crossover on the
the CL-230 and the CL-240 and it didn't workout so well so they increased the 6.8uF to
10uF on the CL-240.  Alternatively it was a build error where someone used the wrong crossover,
the PCB's look exactly the same except for the cap.

2) 1970's vs Modern Listening Positions. 

The CL-240 has a 3/4" tweeter and an 8" woofer.  The box is 420mm high.
The CL-230 has a 3/4" tweeter and a 5" woofer.  The box is 260mm high.

It is very like the CL-230 would be place on a bookshelf so the listener would
be listening to the speaker with the ears roughly at the same height as the tweeter.

The CL-240 isn't large but it would be more likely to be placed on the floor than on a bookshelf.
The listeners ears would be somewhat higher than the tweeter.

Keep in mind in the 70's speaker stands were not common.  Today something like the
CL-240 might have a stand.


3) Factory tweeter polarity

Based on the schematic and wiring:
The tweeter polarity on the CL-240 is reversed from the woofer.
The tweeter polarity on the CL-230 is in-phase from the woofer.


4) Why is the tweeter flipped on the CL-240?

I had theory that this was done because of the different listening positions in the 1970s of the CL-240 vs the CL-230.

The large listening angle of the CL-240 causes the woofer to be moved back further from the tweeter.  That creates
significant phase shift.  Perhaps enough to cause the tweeter to be reversed compared to the CL-230


5) Simulation of the CL-240

I did a crude simulation of the CL-240 with the tweeter in-phase and out of phase with the woofer, and with the listener's ears on the tweeter axis (modern listening position) and with the speaker on the floor and the listener's ears at 1m.

FWIW,  to get the simulation to work I estimated some geometries and delays and matched-up the impedance plot from the article link I posted.  I also matched the (acoustic) crossover point which is supposed to be at 2kHz.



We can see why the out of phase tweeter works OK when listening in the 1970's position.  However when listening in the modern position with the ears on-axis to the tweeter the results are poor.   That kind of confirms why the CL-230 crossover wires the tweeter in phase with the woofer.

With the tweeter changed to be in-phase the modern listening position appears to be the best.   Also, when we listen in the 1970's position the crossover is still working well.   It seems as good as the out of phase connection but the two connections will sound different.

We might conclude that putting the tweeter in-phase with the woofer seems like the best connection on the CL-240.  Again you would need to listen to confirm.

[snk]

Wow, Rob, that's amazing!
Thank you so much for this rather deep analysis! 

When I will placing my next capacitors order, I will grab some caps for the Dual CL240, and try to see/hear how it sounds with the reverse phase.