The Secret of the Yamaha NS-6490 speakers

TomTom4049

Member
I've done experimentation on two models of Yamaha speakers I own. In both cases I wasn't happy with their performance and ranked them as the worst I owned. They didn't sound horrible, just not even close to my better sounding speakers. I have a pair of Pioneer CS-511A's I bought back in 1978 who's cone tweeter gave up the ghost so were replaced with soft domes. They still rock to this day thanks to a treated cloth surround on the woofers and a special formulation for the woofer cone. I have a pair of JBL 2080's that I very much like, a pair of Advent Futura's that I also like a lot. None of my speakers are "beasts" but sound truly nice. Back to the Yamaha's. There is a secret to the NS-6490 I want to share with owners of these modest speakers. Their sound can be greatly improved through modifications.

First, if you dislike the forward personalities of the midranges and tweeters as I did, you can add cement (sandblock) resistors in series with them to pad them down somewhat. On the mids I have three sandblocks in series, two 4 ohms and one 6 ohm (each 5 watt types) for a total resistance addition of 14 ohms. For the tweeters I added two 4 ohm 5 watt sandblock resistors for a total resistance addition of 8 ohms. The next section addresses the bass performance.

If like me you have found yourself yearning for fuller, richer, deeper bass from them then read on.
The first thing to do is gather up your courage, put your fears aside, and be determined to succeed. I promise that once you have finished and reconnected them to your system for a listen you are in for a very nice surprise. The difference has made me say. "Holy Cow", "What a difference", and just plain "Wow!"

DISCLAIMER:
I bare no responsibility for the work you are about to undertake and you do so at your own discretion and free will. If you do something wrong and do damage to your speaker then you are responsible for any and all damages. If you don't feel comfortable drilling then ask someone who knows how to do it for you.

* You will need a clean spot to work
* You will need a drill
* You will need a wide drill bit
* You will need set of Torx drivers to remove the woofer
* You will need a bass reflex port tube (Parts-Express.com) & you can even add a second identical bass reflex port if you want to open the bass up even more. The second one can be mounted centered straight up and inline with the first one but above and between the midrange and tweeter drivers. Again making sure it's edge clears both drivers as described below.

These two locations are ideal so the ports have a clear and unobstructed discharge path through the grills. The upper port is only covered a tiny bit by the grill frame but not anywhere near enough to hamper it's performance.

I used these port tubes from Parts-Express.com:
Speaker Cabinet Port Tube 1" ID x 4" L Flared Part # 260-470 $2 each

* You SHOULD also add some damping material to the cabinets but don't absolutely have to though it cuts down on boomyness and kills a hollow sound made by speaker cabinets that lack it: (also from Parts-Express.com)
Part # 260-516 Acoustic Sound Damping Foam 1-1/2" x 24" x 18" UL 94 $7.90 each
My foam came from work and was going to be thrown out so I re-purposed it.

I recommend you remove or cover the tweeter and midrange with something. A old margarine tub cover may do fine.
I bought my drill bit at my local Harbor Freight Tools as a set of four. One of them was the perfect size. It is item 69017, just under $8 (your price may vary depending on location). The size to use is the one marked 1 & 3/8.

Remove the speaker grills of course and locate the area between the midrange, tweeter, and woofer. This is roughly a irregular but triangular shaped spot. Center the tip of the drill bit as best you can and slowly turn the bit by hand while you look to be sure the distance between the outside edges of the cutting blade and the three driver frames is pretty much the same. If it is too close to one or the other, pick the drill bit up and shift it to make it more center and check again. Don't worry that this will make a small mark or two because you'll be removing 1 & 3/8 inches of material anyway so those will be gone.

Once centered begin to drill steady and evenly until the blade of the bit goes through to the inside. Vacuum or blow the fine debris away from the area.
Using a Torx driver, remove the four woofer screws. I put my fingers of one hand right against the head of the screw so if the driver slips off it won't damage the woofer surround or the cone. Carefully disconnect the two wires at the woofer and set it aside. The connections are sized differently so you can't mix them up when you reconnect them. Set the woofer safely aside. Reach inside and remove the biggest round "plug" of wood and discard it. Vacuum out any debris from inside that has fallen in there when you drilled the port.

At this point I recommend you use a strip of foam almost as wide as the cabinet itself (the inside is a little smaller than the outside). I like to use the dark gray foam that has mountains and valleys over the surface on one side and is flat on the back. I cut it so it overlaps onto the inside top and bottom of the cabinet. None will be on the sides unless you have enough and want to try adding a little more. Folding the foam in half to find the center, I cut a slit horizontally from one side with scissors until I reach the middle and cut out a little circle for the speaker terminal cup on the back to protrude through. The slit allows you to slip it over the wires without disconnecting everything. No need to glue it down. Just push it in place top back edge and bottom back edge with the flat side of the foam against the back, top and bottom.

Next slide the bass reflex port in from the front through the new port you just drilled. You will have to push down on it will a little force until the lip seats against the front of the cabinet. If you didn't center the hole exactly perfect you will see this when you attempt to seat it and you may need to slide it back out and use a file on it on one area of the lip so it fits against the driver it is too close to. Take heart, I had to do this beside the midrange with one and next to the woofer on the other speaker. It isn't noticeable and doesn't affect the sound.

Now reconnect and replace the woofer along with it's four mounting screws.
Repeat this on the other speaker.

Now you're ready to connect them to your system and give them a listen. At this point I want to mention speaker connections. Whenever there is more than a single speaker in use the issue of polarity (or phase) comes into play. You must take care to connect the stereo amp/receiver positive terminal to the positive terminal on it's connected speaker and likewise, connect the amp's negative terminal to the speaker's negative terminal, and this must match with the other speaker as well. Some folks don't use a red & black speaker wire pair and have wire with a white stripe on one wire (or white dots or dashes) or like me they have what looks like clear lamp cord with a copper positive conductor and a silver negative conductor. If one of the speaker's connections are reversed there will be a noticeable bass-cancellation. This will be most noticeable when you are centered between the two speakers but this effect fades as you move closer to one side or the other. If you absolutely can't tell or have wire that is unmarked, just place the two speakers on the floor facing together but separated by three inches or so. Turn the stereo on and listen to any source material with the treble turned all the way down and turn the bass up a bit. If one of the speaker connections are backwards there will be little bass. When you swap one speaker's wires the bass should improve quite a bit. The reason for this is a simple one. When incorrectly phased, one woofer moves outward while the other woofer moves inward. One literally cancels out what the other produces because they aren't working together. On dual sets of speakers, system A can out of phase with system B. After comparing the second set using the same method described above, next pair one of the set A speakers with one if the set B speakers.

Congratulations on converting your acoustic suspension speakers into a bass-reflex and discovering the richer bass these speakers are capable of producing! You'll wonder why Yamaha didn't do this in the first place.

Enjoy! :thmbsp:

MORE MODS

Today I removed all three drivers in preparation for disconnecting the two factory crossover capacitors that are attached to the inside of the speaker wire cups, and connecting Infinity Reference 4 crossover networks. I had to remove the Infinity crossover from the black housing they are attached to and mounted the inductor that looks like a transformer to the circuit board with nuts and bolts. After soldering the red and black wires into the Yamaha speaker wire cups I secured the board to the back with a couple of wood screws. I ended up putting the padding resistors back in after I got home and tried them because the mid and tweeter were again too bright. After this they sound amazing like much larger speakers. Lots of powerful bass now. A buddy who owns a stock pair brought his over or an A/B test and he said mine totally stomp his in the dirt. He is begging for the same enhancements to his.
 
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This is cool. I have a pair and my midrange is nowhere to be found! I wonder if the cabinet would be okay for some beautiful Hivi drivers and a ribbon tweeter.
 
I read this thread a while back but was unwilling to experiment with any my current speakers. I just picked up a single 6390 to experiment on with this in mind. I plan on testing the port modification first and actually remember a different post regarding upgrading the crossovers that I want to find and reread before going this route. I have to say though, I didn't find an issue with the speakers sound that I couldn't balance with my AVR's equalizer.
 
I had a pair of these years ago. Mid bass wasn't what it should be, but other than that these are decent for their price. I left them sealed but I stuffed a pillow's worth of stuffings in each and that improved the bass, and took most of the cabinet resonance out of them too. My son has them now.
 
I actually lined all of mine with a single layer of those blue furniture pads you can buy at uhaul stores, it seemed to do nicely, just got the spare one I acquired to the house earlier today, waiting on the port tube to arrive now and I'll give this a go.
 
So I got my port tubes in today and ported the extra Yamaha NS-6390 I picked up and I honestly cannot tell the difference between ported and not. Now, the speaker I ported has an empty cab while the non ported one is lined with a single layer of blue furniture pad from the uhaul store. I intended to line it and then compare thinking maybe the pad and port tube both had a similar effect on the bass level. I'm most likely going to do that tonight still but due to some thunderstorms rolling through this area I have everything shut down and unplugged for the night so the second round of testing will have to wait until morning. I don't really expect it to change anything, I just want to give it a truly fair test. Hope I am wrong and am back on here tomorrow saying how drastic of an improvement it made but not holding my breath.
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I placed the port tube upside down on the speaker and put a 1" paddle bit inside the tube to mark the center before drilling the hole with a 1 3/8 paddle bit. I put a piece of Kent Seal N Caulk SP1 around the flared lip of the port tube just to make sure it sealed well. I picked it up from parts express. It's part number P10310. I think it comes in multiple sizes, the box I got is like 12" long strips 1/4" wide and 1/8" thick. Was very inexpensive and I think it will work fantastic for many different things including this.
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So after lining the inside of my sacrificial speaker I finally got around to testing it again this morning and after playing a bass track at medium volume I thought maybe this time I heard a bit of a difference in the bass. So to make sure it wasn't wishful thinking I placed both speakers in the exact same spot while testing each, ran them off the same channel of my AVR even, and measured them at a distance of 3ft with an app on my note 8. Turns out through 5 separate tests the ported speaker peaked an average of 2db higher than the non ported. Is that worth the effort to port them all? I haven't decided, but I do have 6 more ports, and it is raining today...
 
Nicely done! I don't think that port placement is the ideal location, but where else is it going to go, except maybe on the backs down low? Did you ever finish the experiment? I know it has been awhile.
 
I actually never did get around to porting any of the others. Honestly it seemed like too much effort for no real affect seeing as I have 4 10" subwoofers in my system as well. I eventually used the woofer from the extra speaker I ported to replace one with a damaged surround in a pair my friend picked up somewhere.
 
have been reading the post and find some good suggestions here. I bought the Yamaha NS-6490 bookshelf speakers before reading anything about them. what i discovered when i first listened to them was the dominance of the mid and high range. the bass seemed to be a mid-bass. I recall reading advertisements that said these speakers would be best in a 5.1 or 7.1 etc... setup which leads me to believe a subwoofer would be part of the system. I bought them as a stand-alone set of speakers for my garage and i did not want to complicate the set up with anything. So, to my point. I added a 2.5-inch I.D. port tube to the back of the box. I measured from bottom center and up four inches then drilled the hole with a three-inch hole saw. with a little sanding after that the port tube fit nice and snug. The results were very good. it opened up the bass range to a much lower level. I also put acoustic foam into the box covering the backside. Thank you to everyone who experimented and took the risk and posted your findings. have a good one !
 
Although this is an old thread, I would add a cautionary note for anyone who wishes to pad down a driver: use an L-pad. If you simply add a resistor in series with a driver, in addition to attenuating the response of the driver, the Q of its crossover filter will be increased. In other words, there will be a peak in the response in the vicinity of the crossover frequency. Also, if the driver's nominal impedance is 8 ohms, use an 8 ohm L-pad, and so on.
 
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