Function of this piece in a loudspeaker?

[NTL2009]

TL;DR Summary

I'm not familiar with this design, it looks like a 'tweeter dome' in a midrange speaker, but it is heavy and fixed in position.

My son asked me to look at his Logitech Z-2300 system (Amp, sub-woofer and 2 satellite speakers) that was acting up. He had already taken a lot of it apart to see if he could find a loose connection. In checking things out, I found this part in with all the screws and loose hardware. It's the center, shiny part in the middle of the speaker in the picture ("driver" is a better term in audio circles, "speaker" generally refers to the whole system of drivers(s), crossover(s), cabinet, etc).

I got that 1st pic from the internet, I'll try to get a screen grab of the loose part from the video I took to document as I went (sorry for poor quality). It is slightly smaller diameter than the voice coil, and then a smaller 'stud' at the bottom end. The bottom of the 'hole' in the driver is solid metal (looked like copper). Typically, a shiny dome like this is lightweight and part of the driver cone, meant to act as a stiff diaphragm for the high frequencies. But this piece does not move at all.

It would rattle a bit being loose, so I added a small drop of epoxy at the bottom to hold it in place. But I don't know the function, and I didn't find anything specific in my searches. Other then decorative, is it a heat sink? I forgot to check its magnetic properties, pretty sure it's all aluminum though, but if not, could it constrain the field for the voice coil? That pale red ring/washer looked to be fiber, maybe it was just glued or press-fit at that point? Is it meant as some kind of horn/wave-guide?

Any info would be appreciated, I'm just curious.

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[Keith_McClary]

central bullet or "phase plug."

 

[berkeman]

central bullet or "phase plug."

Ack, I tried following the link, but only got ads while I scrolled down a lot. Can you post some screenshots of the best parts of that link? Thanks.

@NTL2009 -- Can you point to which parts of a traditional speaker you think that is?

https://www.researchgate.net/publication/288727113/figure/fig1/AS:398589600780288@1472042575149/Schematic-of-a-general-speaker.png

 

[NTL2009]

See if this mod to that drawing helps. It's like there is no dust cap at all. This center "bullet" seems to be mounted to the base plate - it is fixed, it does not move with the cone. Small air gap between the "bullet" and the inner diameter of the voice coil.

I see references to a phase plug, but I'm not so sure. That seems to be applied with a compression driver feeding into a horn. I'll try to get some of those links, but first I'll post my mod to your drawing.

 

[NTL2009]

A few youtube links - In the 1st, a guy cuts out the dust cap and inserts a "phase plug" which is glued to the base, similar to what I'm seeing. He's just "some guy" though, I have no idea of the merit to what he's doing.



Some similar looking mods to woofers in this one, using a copper pipe, but he doesn't show the construction, but I assume the pipe is mounted to the base, it would be awful heavy for the cone.



And here's one that they call a "phase plug", but it moves with the cone. Confusing.

 

[sysprog]

The wikipedia phase plug article explains it reasonably well.

 

[NTL2009]

The wikipedia phase plug article explains it reasonably well.

I saw that, but it is almost all related to horn designs, with compression drivers. The section on woofers does talk about the phase plug being used in place of the dust cap, but again it is not clear (to me at least), if this is similar to my driver, they talk about it being us "in place of" or "in the center of" - but they don't specifically say it is mounted to the base with the voice coil going through/around it.

And my driver is the mid-range/tweeter, not the woofer, so I don't know.

 

[sysprog]

I saw that, but it is almost all related to horn designs, with compression drivers. The section on woofers does talk about the phase plug being used in place of the dust cap, but again it is not clear (to me at least), if this is similar to my driver, they talk about it being us "in place of" or "in the center of" - but they don't specifically say it is mounted to the base with the voice coil going through/around it.

And my driver is the mid-range/tweeter, not the woofer, so I don't know.

From the linked wikipedia article:

​

Phase plugs are commonly found in high-powered horn loudspeakers used in professional audio, in the mid- and high-frequency bandpasses, positioned between the compression driver diaphragm and the acoustic horn.​

However, a coned speaker can have a phase plug, too $-$ from reference '17.' in the article $-$ https://web.archive.org/web/20030414152835/http://www.preference-audio.com/phaseplug.htm:

In the Logitech Z-2300 system, phase plugs are present in the 2.5" drivers $-$ from the Setup guide at https://www.logitech.com/assets/35688/setup-installation-guide.pdf $-$

 

[NTL2009]

Ahh, very good, thank you! That second diagram explains why they would be used in a mid-range driver. And combined with the last link I'd say it's definitive - Logitech calls it a phase plug as well.

Mystery solved! And my drop of epoxy was probably the right thing to do.

Now to see if I "fixed" it. After I got it back together and it was working, I asked my son for more detail of the problem. I was thinking that loose phase plug was causing a weird rattle/distortion, but now he says it was just continuous white noise hissing, with or without any music playing. I didn't hear it, or see anything funny on my 'scope (noise levels can be tough to see, it's a cheap module that feeds a computer over USB). And, I'm 65, my high end hearing has dropped off, and I have mild tinnitus (like a fan running in the background all the time). But I still hear hiss from amplifiers, so I don't think this is too bad. My wife thought it would be too weird to ask a neighbor kid if I can borrow their ears :).

As long as I'm typing, when I was checking it with my scope (direct, not a microphone to pick up the speakers), it was amplifying signals way, way out to 100~150 KHz! I thought that even very good amplifiers with wide freq range purposely limited the response in the last stage to just a bit over 20 KHz, just to avoid oscillation and wasted power? Or to not burn out tweeters? And also something about not applying negative feedback for such high frequencies, that maybe one of the stages could not respond to, leading to some spurious distorion (Transient Inter-modulation Distortion? Or was that voodo? I think there was something to it.).

 

[sysprog]

One of the reviews (https://forums.tomsguide.com/threads/logitech-z-2300-the-last-of-the-titans.234775/) $-$ I'm taking some license in paraphrasis $-$ said that although Logitech had previously been known for making cheap non-audiophile-quality 'computer speakers' that didn't measure up to e.g. Altec-Lansing, on this rig the engineers had a free hand, and produced something that rivaled Klipsch, and, apparently mostly accidentally, just happened to turn out to be 'reasonably affordable'.

 

[NTL2009]

Thanks for that link - it brought back some memories. I saved it to my folder on this repair.

We purchased this unit as a gift for my son way back. I do recall that at the time, I did a fair amount of research, and it got great reviews for the price range. I'm a bit of an audiophile (even if my ears aren't anymore! :) ), and while this doesn't come close to what I want in a sound system, it is a pretty nice package overall for casual use. The sub woofer is pretty smooth, and reasonably balanced. I'm not that thrilled with these satellites, but they get the job done.

A few years later, we bought a slightly smaller Logitech unit for our daughter. She didn't take it with her on her latest move, so I have it on a 2nd computer now. It's not as good as the Z-2300, this newer one really pushes the bass out of balance. There is a bass level control, and at minimum it still booms. I looked into opening it up to mod the RC values to trim the bass down some, but it did not look easy to open, and I wasn't motivated. I could always add an external high pass filter or a little EQ box if I wanted.

OK, I looked, the smaller unit is a X-230, the sub cabinet is about 1/4 the volume of the Z-2300. But they pump it up so much (amp EQ or box/port design Q), the bass is way out of balance. I think they are just feeding the market that thinks more bass is better bass. Or gamers, where the bass rumble for sound effects is kinda cool.

OK, made me look. We paid $108 in 2004 for the Z-2300. The X-230 was $44 in 2006. But you can get a Z-2300 today for the low, low price of... $1,499.99 & FREE Shipping! Only 1 left in stock - order soon.

https://www.amazon.com/gp/product/B0002SQ2P2/?tag=pfamazon01-20

 

[sysprog]

We paid $108 in 2004 for the Z-2300. The X-230 was $44 in 2006. But you can get a Z-2300 today for the low, low price of... $1,499.99 & FREE Shipping! Only 1 left in stock - order soon.

https://www.amazon.com/gp/product/B0002SQ2P2/?tag=pfamazon01-20

$108 $-$ hey that's $$(1^1 \times 2^2 \times 3^3)$ $-$ you can get a brand-new Logitech Z-2300 system from a reliable seller on ebay for about half of the $1,499 current price that you quoted, and a gently used one for less than $200, but I'm happy with a formerly-over-$300 used Altec-Lansing M604 that you can now get used on ebay for less than $50.

 

[sysprog]

Ahh, very good, thank you! That second diagram explains why they would be used in a mid-range driver. And combined with the last link I'd say it's definitive - Logitech calls it a phase plug as well.

Mystery solved! And my drop of epoxy was probably the right thing to do.

Now to see if I "fixed" it. After I got it back together and it was working, I asked my son for more detail of the problem. I was thinking that loose phase plug was causing a weird rattle/distortion, but now he says it was just continuous white noise hissing, with or without any music playing. I didn't hear it, or see anything funny on my 'scope (noise levels can be tough to see, it's a cheap module that feeds a computer over USB). And, I'm 65, my high end hearing has dropped off, and I have mild tinnitus (like a fan running in the background all the time). But I still hear hiss from amplifiers, so I don't think this is too bad. My wife thought it would be too weird to ask a neighbor kid if I can borrow their ears :).

As long as I'm typing, when I was checking it with my scope (direct, not a microphone to pick up the speakers), it was amplifying signals way, way out to 100~150 KHz! I thought that even very good amplifiers with wide freq range purposely limited the response in the last stage to just a bit over 20 KHz, just to avoid oscillation and wasted power? Or to not burn out tweeters? And also something about not applying negative feedback for such high frequencies, that maybe one of the stages could not respond to, leading to some spurious distorion (Transient Inter-modulation Distortion? Or was that voodo? I think there was something to it.).

Are you still getting HF noise? Have you checked the caps? Ref: https://learnabout-electronics.org/Amplifiers/amplifiers33.php

 

[NTL2009]

Are you still getting HF noise? Have you checked the caps? Ref: https://learnabout-electronics.org/Amplifiers/amplifiers33.php

I don't hear anything but a very low level hiss, with my ear right up to the speaker and volume all the way up. But as I said, I have 65 year old ears with some high frequency loss and some mild tinnitus. And since it was apart when my son brought it over, I couldn't have him demonstrate the problem. I'm going to need some younger ears to check it, but I really don't think my hearing is that bad, so I really don't think it is exhibiting anything much above normal S/N ratios.

I expect the following outcomes:

A) Just taking it apart and putting it back together fixed a loose connection or something.

B) The problem is intermittent (though it wasn't according to my son).

C) The problem is in whatever he connected to it (though he said he tried different things).

D) It is still hissing and I'm just not hearing it.

I didn't do the usual 3rd degree since he just dropped it off when he was here for a family get together (of 10 or less!), and I figured I'd hear the hiss when I got it back together. Then I got sidetracked by that funny phase plug. He's pretty busy with job and family, so it might be a few weeks before he can get over and give it a listen. I'll update the thread when I learn more.