Technics SB-M1 Introduction & Description
The SB-M1 cabinet is built like a tank with considerable internal bracing. It is immensely heavy and the baffle is made of 1-1/4 inch thick particle board. The main chamber is a bass reflex enclosure for the woofer and there are two sealed subchambers, one for the lower mid driver, and the other for the upper mid driver and the tweeter. All are lined with heavy sound damping material. Even though the 4 drivers are mounted in a vertical line the left and right side speaker cabinets are a mirror image of each other with the bass reflex port, control panel and Technics badge being on different sides. The left side cabinet is the one with the Technics badge on the right and the right side cabinet is the one with the badge on the left. Oh, so the setup in my room is all wrong! Well my back is telling me that is how its going to stay! There are four inset lifting handles in the back and the two rails on the lower front called "Diffuse Poles" can also be used for moving the SB-M1's around. I did find on my SB-M1's that the large bolts that hold the center of each diffuse pole in place were bottomed out and the washers were spinning around. When pulling on the pole you could see it move in and out, so I had to add three extra washers to each side. On the back of the cabinet although before the days of bi-wiring speakers there are two massive terminals for connecting the speaker wire that can easily accommodate 10 gauge cable and they match the terminals on the back of an SE-A3MK2. The Technics SB-M1(s) speakers do actually have front grills even though they are most often seen with the drivers exposed. The grills clip onto the diffuse poles to stay in place. They have a nice angled appearance.
All four of the drivers are made from the same flat honey combed design intended to make the diaphragms light but rigid. The two larger drivers have a foam roll surround while the upper midrange unit has an inverted surround. Difficult to say much about the tweeter as there is little to be seen except a heavy cast shield built over the diaphragm. Producing a large high quality and high power moving coil speaker system requires two main things of the drivers: Dissipation of the heat generated by the voice coils, and large but light diaphragms. The power and heat dissipation is managed in two ways, first by the division of labor in implementing a 4-way system and secondly by having a vented magnet on the three larger drivers. In the case of the tweeter, there is a resetable fuse that cuts the tweeter out in case of overload. These days such a tweeter would probably have a ferro-fluid design to conduct the heat away. I have to say, this is the heaviest tweeter I have ever seen, 3 pounds 9 ounces to be exact! The vent on the three larger drivers goes right through the center of the magnet and the back of the honeycomb disc can be seen through the hole. So in the case of the bass driver, quite some air passes in and out of the driver when it is working hard. The issue of providing a large and light diaphragm is of course achieved with the honeycomb disc. Both the disc on either side and the actual honeycomb appear to be made from aluminum. Then on the back there is an aluminum cone that has about half the diameter of the speaker where it meets the back of the disc. The magnets on each driver are very large and heavy. The basket or frame of each driver is made from very thick cast aluminum to maintain rigidity. Only the 15" woofer has the funky square orange suspension as shown in the pictures.
Crossovers and level adjusters
There are two crossover boards and both are build with very heavy components to handle all the power. The treble crossover for the tweeter and high mid range is mounted behind the speaker terminals as shown to the right. The bass crossover for the woofer and low midrange is mounted on the floor of the cabinet behind the woofer as shown to the lower right. The woofer is directly connected from the bass crossover but the other three drivers each have their own level controller mounted on the front baffle as shown to the left. This provides a range of -9dB to +1dB for the tweeter and the mid-low driver while the mid-high has a range of -8dB to +2dB. The heavy duty rheostat style potentiometers are mounded directly through the baffle and the face plate is just screwed over the top of the knobs. The controls have a nice heavy feel with a soft click for each mark on the scale. After years of sitting around, the controls do get a bit dirty and the sound crackles a bit when they are turned, so I cleaned them all with some switch lubricant. They can be accessed by removing the woofer and unbolting each one in turn from the front of the baffle. The contour graphs for each control are shown below.
I have to tell you that I had considerable trouble in finding a way to set these controls properly! Of course the proper way is to compare the sound with a reference and adjust the controls so that the sound of the SB-M1's matches the reference as closely as possible. In my opinion there is no better reference speaker for midrange and treble than the Quad ESL-63. Unfortunately my original Quad's were lost by Allied Van Lines on the way to Florida so I actually had to buy another pair on eBay in order to get the SB-M1's properly setup. As you can see from the controls on the left, a setting of -6, -3, +1 from the top down gives the required equalization to match Quad ESL-63's. This also now makes my reference CD's sound correct. This all leads me to wonder what reference the SB-M1's were designed to and why the correct setting is not 0, 0, 0. It does mention in the user manual that these settings can be used to compensate for the phono cartridge being used so maybe the cartridges Technics were shipping with SL-1200's at that time were a bit on the trebly side. Once I was done, I resold the Quad ESL-63's to a local buyer who happened to be a professional classical guitarist who was going to use the Quad's as a studio reference. He commented on how close the SB-M1's sounded compared to the Quads, so I think I have the correct setting at last! When set incorrectly, the SB-M1's can really sound quite annoying and it is probably the case that many potential listeners and buyers have been put off them because of listening to them with the wrong settings. Once set, different room settings and locations do not seem make much difference to the sound of the upper three drivers, it is mainly the bass that is effected. I can tell you though that when properly setup they sound simply stunning and -6, -3, +1 is my recommended setting for digital music sources! Obviously such large and heavy speakers have tremendous well controlled bass and the three upper drivers do a very good job of keeping up with the Quad ESL-63 electrostatic reference. The other unusual thing about the Technics SB-M1 is that even with all these crossovers burning power and all these drivers, the average impedance is 6 ohms rather than 8 and they are quite sensitive at 94 dB/W at 1 meter. This means that with just an ordinary SE-A5 they can really rock loud and not even an SE-A3MK2 can drive two pairs at the same time.
Anyone who has owned or seen a pair of Bose 901's from the 1980's will be all to familiar with the issue of decaying foam on the surrounds of the drivers. This issue actually completely decapitates 901's! Well there is good news and bad news in this respect for the now quarter century old Technics SB-M1's. The good news is that the foam surround of the 15" bass driver is not effected. The bad news is that the 9" Mid-Low EAS-22PL03S unit is effected. I have heard the same thing from other SB-M1 owners. It appears that when stored for many years the units come out looking like new, but after just a little usage the foam just crumbles and holes start to appear. In the case of the SB-M1 Low-Mid unit, this actually appears to make very little difference to the units operation unlike for the Bose 901. The suspension is quite stiff (not necessarily a good thing for a mid range driver) and it still operates perfectly well with the entire surround crumbled and gone! Even so, with the SB-M1's looking so good with the grills off I thought it worth repairing them just for the cosmetic benefit and the process is described below.
Because of the Technics SB-M1's rarity there are no specific replacements available for the foam surround like for the Bose 901 and others but I have found that the generic 8" kit from Parts Express looks and works just fine even though the driver is described as 9". The only minor issue is that the new replacement has a better grey color than the original foam on the 15" woofer which has a bit of a green tinge to it although the foam there is in like new condition. Even so, it sure makes a great improvement to the appearance of the whole speaker system.
The process described here is not quite the same as in the Parts Express instructions. The first step is to remove the driver from the cabinet. This requires a 5mm hex key (Allen key for the English). Undo one of the bottom hex bolts last as the driver may just fall out. Make a note of which color wire is connected to the + terminal on the speaker (Red on my units). Be careful not to damage the flat diaphragm surface when the driver comes out. The magnet is big and heavy and the unit is completely unbalanced with regard to its centre of gravity! Pull off the connectors and place the unit on its back on your work bench. Then remove the front gasket with a screwdriver. It is just stuck on with some glue and the whole thing has to come off. Be careful not to damage the front surface. Although located where the front gasket is on a regular speaker, it is not really a gasket at all but rather a decorative trim. Clean off any foam from the back.
The next part is real messy! There will be stick and dust all over if you are not careful! All the remaining foam has to be scraped from the speaker basket surround and from the diaphragm surface. I found that scraping with a craft knife the best way to do this. The old glue on the diaphragm edge is quite sticky and the crumbling foam mixes with it to produce a sticky black mess. After the scraping is done, clean off any remaining old glue with some Goof Off. Even with the surround missing, the diaphragm is quite stable and you do not need to use the shims provided with the kit. Even so, hold the honeycomb disc diaphragm with your finger and thumb when scraping around the edge. Do not let the voicecoil rub against the magnet. It is not necessary to get a perfect finish on the diaphragm front as the inner diameter of the new foam surround is slightly less than that of the original, so none of the mess will show when its done.
After the foam has been sufficiently cleaned, new the new foam surround can be fitted. This is done using the lightweight contact adhesive supplied with the kit. Contact adhesive works by being applied to both surfaces, drying for about five minutes, then bringing the two surfaces together into contact with each other when the adhesive is still tacky. First apply glue to the top outer side of the new foam surround and to the back side of the gasket where the foam will touch. The gasket makes a snug fit over the foam surround. Leave the glue to become almost transparent and tacky for about 5 minutes. Then fit the gasket over the surround and press the glued surfaces together. Make sure the foam surround is pushed all the way through the gasket all the way around. Don't worry about any glue that shows through, it will dry clear.
Next after the gasket and surround have dried for about an hour, the gasket and surround can be glued to the basket and diaphragm at the same time. Don't try to do this in two separate steps! Try putting the gasket and surround in place to see how it will look and make sure everything is OK. The gasket has 8 locators to lock it in place. Next remove the gasket and surround and apply glue to the inner back rim of the foam surround ONLY. Immediately with the glue still wet and white, lock the gasket into position on the speaker basket and then lift the diaphragm a little from underneath so it touches the wet glue on the back of the surround all the way around. Then immediately remove the gasket and surround. Now you will see a white ring of glue on the diaphragm edge in the exact right location. Swab it around and add any extra glue needed. Now apply glue to the outer back side of the gasket and to the speaker basket. Reapply any extra needed glue to the inner rim of the back of the foam surround and to the front of the diaphragm edge where the glue mark was previously made. Don't add too much glue so it goes more to the center of the front of the diaphragm as it will be seen when the job is complete. Leave the glue to become almost transparent and tacky for about 5 minutes. Then press the gasket and surround back onto the speaker basket and lock into place. With finger and thumb, gently go all around the diaphragm bottom and foam top and press them together. Don't worry if a small amount of glue is squeezed out as it will dry transparently. After about an hour, the speaker can be replaced in the cabinet and tested.
Well now that I'm looking at it I see that the four foam surrounds provided in the Parts Express kit are in fact two different pairs. The other pair are of a slightly lower profile and also a bit darker in color. Those would have probably been a better match with the surround on the woofer of the SB-M1's but I guess I'll just leave them as they are now. I can always change them again later if I need to. So at the very least, you should make sure you have a proper pair before you start. Knowing this, you can just about see the two different pairs in the photo above.
Well another way to avoid the problems of the EAS-22PL03S Mid-Low driver is to go for SB-M2's rather than the M1. These slightly smaller speakers are effectively the same as the SB-M1 although slightly shorter and are a 3-way design without the EAS-22PL03S driver. The power handling is reduced from 150 watts to 130 watts and the crossover frequencies are 750Hz and 4kHz. So for the tweeter everything is the same as for the SB-M1 but now the Mid-High (or just Mid in this case) EAS-12PM216S has to go down to 750Hz from 900Hz and the EAS-38PL32S woofer has to go all the way up to 750Hz from just 280Hz on the SB-M1. Although I don't know, I would venture to say that the SB-M2's probably sound better than the SB-M1 just because there are less crossovers and drivers. If you know anything about this, please let me know.
Pictures & Reference
Technics SB-M1 Speakers For Sale
I sometimes have SB-M1's for sale that I have refurbished or acquired by other means. If there are any, the following pages are what I prepare for eBay auctions and these items may or may not be currently listed for sale on eBay but you can find out by clicking here to see my current eBay auctions. If you would like to buy a pair that is listed here please let me know. I accept payments by credit card (PayPal), cashiers check or money order. If you have a pair of Technics SB-M1's that you would like to sell in the USA, please let me know and I'll come and pick them up!
Technics SB-M1(S) SOLD