Reducing "Sibilance" from my Cosmic COVOX 4500 loudspeakers.

[Prasad Karpe]

Hi Folks
This is a cross over of my vintage Cosmic COVOX 4500 BS speakers (in the attachment). The speaker is a three way sealed enclosed speaker with all drivers of 8 Ohms. The woofer has got polypropelene diaphragm, the midrange is paper cone & the tweeter is metal dome. I was using this speakers in pair with Cosmic CO-100 Amplifier. The problem is that as my understanding of Hi-Fi increased I started to feel that the speakers are very bright with annoying sibilance. The sibilance becomes evident always with Indian Film Music.
This was the time when I came across this forum. The continued readings from the forum gave me the insight that my amplifier may be the weak link. With the help of my local vendor I upgraded my amp. with an old Yamaha receiver. The sibilance slightly came down. However after listening to some more Hi Fi & with the knowledge I had from the posts in the forum I decided to upgrade from a DVD player to a dedicated CD player. This is also a used gadget, Maranz -4000 CD Player. However I noticed that the low end response of the speakers improved I did not get any help in high end region. I realized a bitter fact of life Ignorance is bliss
After the DIY bug has bitten I started to experiment with speakers. I first covered the area around the tweeter front baffle with foam hoping that the excess high frequencies can get absorbed. Since this did not do the trick, I opened the speaker found that the mid range & tweeter are housed in a separate box. I lined the internal walls with foam sheet but found no respite. Then one fine day I thought that the midrange with paper cone may be the culprit. I then cut off the midrange & started listening to woofer & tweeter only. This considerably improved the low end. I was very happy but the happiness did not last long as I found that once I increased the volume level the ugly sibilance started appearing again.
I then ventured in to mysterious region of Cross Over. I removed the cross over from the box & started tracing the ckt. Since I am more of a mechanical guy it required huge efforts to understand the ckt. & reproduce in a respectable manner.
Now since these speakers carry a huge sentimental value in my life (bought from my first salary) I dont want to scrap & get rid of them. My request to the forum members that can somebody help me to reduce the sibilance from those speakers.
I am thinking on the lines of
1. Employing a BSC to the speakers. However I fear that in doing so it may rob the sparkle which I get with the music.
2. Reducing the power to the tweeters. Here also I think that if by any means if I could able to reduce the power the output as a whole will be reduced & the overlapping of frequencies which is the root cause may not go.
3. Shifting the cross over point so that their remains a definite gap without any overlap between the drivers.
Now here is the stage where I would require help from the Gurus in the forum. Since these are the drivers manufactured by Cosmic as per the label attached on the back of coil, I do not have any other information. The label only depicts COSMIC & 8 Ohms. I dont have any measuring instruments either, but after a close hearing I could make out that due to some cross over problem I can hear male voices both from woofer & the tweeter. Therefore I strongly suspect that the midfrequencies are getting played through the tweeter also.
I dont know whether my understanding is right or not. I therefore want help in deciphering the riddle.
Thanking you all in anticipation.
Regards
Prasad Karpe

 

[linuxguru]

Hi Folks

I could make out that due to some cross over problem I can hear male voices both from woofer & the tweeter. Therefore I strongly suspect that the midfrequencies are getting played through the tweeter also.

Probably not the problem, there will always be some attenuated mids that will be heard through both the tweeter and woofer.

Your problem may lie in the metal dome tweeter and/or a cabinet resonance in the upper-mids. Also check your line-level interconnects by swapping with some other known good interconnects.

Unrelated to this, there are a whole bunch of upgrades to the CD4000 that take it into fairly rarefied territory. Another bunch of upgrades can be done to most Yamaha receivers with fully-discrete power amplifiers. You can try those upgrades if all else fails with respect to tweeter sibilance.

I've used an upgraded Yamaha RX-300 receiver driven by an upgraded Marantz CD4000, with fairly cheap generic chinese interconnect, with no issues.

 

[keith_correa]

Can you stuff the mid enclosure with polyfill and report back on what you find/hear?

 

[murali_n]

Hi,

Put a 20K Pot in series with the tweeter and adjust it till the sibilance is gone. I have tried this in my earlier DIY speakers with much success.

N.Murali

 

[Prasad Karpe]

@ linuxguru
Thanks for the suggestion. I have a pair of silk dome "Bolton" 8 ohms tweeters. I will replace those with the metal domes over the weekend & observe. I use "Bandridge" make interconnects so there shall not be any issue regarding the interconnects. I will be highly obliged if you can suggest any upgrades for Yamaha RX-570 receiver & Maranz CD-4000.
@Keith
By polyfill do you mean the material used in the pillows. If so then i will try this trick.
@murali
I think Putting the pot in series will reduce the output of the tweeter. I also doubt that this may rob off a sparkle in the high frequency response. Anyway there is no harm in attempting a for a pot in series. I will try this also.
Thanks all for the suggestions.
Prasad

 

[keith_correa]

@Keith
By polyfill do you mean the material used in the pillows. If so then i will try this trick.

Yes, polyfill.

 

[Prasad Karpe]

Your problem may lie in the metal dome tweeter and/or a cabinet resonance in the upper-mids.

Yesterday i replaced the metal domes with "Bolton" Silk Dome Tweeters. Voila !there was an improvement of @ 20%.

However i could not follow how to eliminate cabinet resonance in the upper mids. Can you tell me more @ this.

Thanks in anticipation
regards
Prasad.

 

[Prasad Karpe]

These are the some modifications i have attempted

1) I first started with the volume control pot of 20K to reduce the power of the tweeter. Connected the pot in parallel with the tweeter so as to act as an L Pad. Though the mod. worked, it offered me very little range. The range of the resistor available for tweaking was only of 20% of the actual movement. I then tried with the 10K pot but the result was the same. A visit to a local electronics shop yielded me a 1 K preset resistor (lowest value variable resistor he had). Connecting the preset gave me a wider range to fiddle with. The tweak worked to some extent but I was getting the sibilance to some what lesser extent.

2) This improvement prompted me to reconnect the midrange back to its position. After auditioning the speaker I thought that maybe there was some kind of overlap between the midrange & tweeter. A pair of 4 microfarad electrolytic capacitor came to my rescue. Connected those in series with the tweeters & to my surprise there was no sibilance at all.

3) However there was another problem lurking from behind. The speakers were giving deep bass & good imaging with 8 o clock position of the volume control, but the moment I moved further the vocals were started becoming aggressive. This was the time to cut open the pillow & to use the polyfill. The damping gave me some punch but still the aggressiveness of the vocals was there.

4) Careful observation pointed that at higher volume the midrange was emitting more power than the other two drivers. This time I decided to put the 10K pot in series with the midrange. After some adjustment I was in total control & the problem was solved.
Now there is such an improvement that my speakers never sounded so good & I was wondering that how I lived with those for so many years. Thanks to the forum members who gave me the advice, it is because of you fellows that I went ahead & carried out the modifications.

@ murali_n

However I still have some queries,
Is it possible that the introduction of variable resistors in series with the drivers will generate heat. If so what will be the extent of it.

It is quite embarrassing for me that for all those years I did not notice that the position of midrange & tweeter is on the same side of both the speakers. i.e. the construction of the speaker is that midrange & tweeters are side by side above the woofer but the speakers are not mirror image of each other. I think the manufacturer supplied me left speakers from two different pairs.

Anyway once again many many thanks to linuxguru,keith_correa,murali_n for your valuable inputs.

regards
Prasad

 

[murali_n]

Hi,

There will not be any heat from the resistors.

N.Murali

 

[jazzlover]

Can anybody tell me what is the sensitivity of vintage Cosmic Convox 4500 spealers .:sos:

 

[Prasad Karpe]

I have some old pamphlet of Cosmic Radio from which I will be able to tell you that if it is mentioned over there.

 

[jls001]

Cheap and simple way to tame hot tweeter: put a layer of kitchen tissue paper in front of tweeter. Glue with masking tape. Try two layers for more attenuation. Works beautifully in my experience. Make sure the tissue paper does not touch moving part of tweeter. This is fully reversible. Downside is aesthetics.

 

[Hari Iyer]

With my past 6 months of further experimentation, simulation and design with speakers, I strongly now believe that "sibilance" is due to filter phase issues and phase-shifts in the cross-over. You can reduce them by padding with some passive components to give a false impression of reducing the "sibilance" with loss of other details, but you can completely remove them by tackling the "phase response" of the speaker system only. This may require measurements of your drivers in your enclosure and complete redesign of the cross-over which itself is a very tedious exercise.