Finally; 6.5" MCM midrange build

Kornbread

Finally got around to working on the MCM Gradient axp-06 6.5" project.  Bought a few of these very reasonable when MCM went under.   

 

This is what DATS said of the 6.5" axp-06.  Ugh ... the midrange chamber was stuffed with denim insulation and triangular shaped, why all the squiggles?   



So decided to get a free air measurement.



   Is this going to be a problem? 

MartyH

Only if you let it be.

greywarden

did you try recalibrating the leads? or was there something vibrating on the surface you ran the sweep on?

jr@mac

Both look that way? I have a few in the shed, I can pull one and run a sweep as a sanity check.

jr@mac

Never mind, I see those are the high SPL version - the squiggles might be real. Not sure why, but a couple accordion drivers I measured had a lot of anomolies like that.

dcibel

It is just a result of the high sensitivity. Speakers are transducers remember, and the transducing goes both ways. High sensitivity of input voltage to output sound, is also a high sensitivity of input sound to electrical voltage. What you are seeing is the cone breakup, frame and motor reflections, etc.

Kornbread

Peerless da25bg08-06
Gradient axp-06 (MCM closeout)
sd215a-88

Thinking the squiggles might have been what dcibel said.  Measured it laying on the table, holding it in hand, and holding it tight between the legs, same results.  Did get in box measurements today so it's time to start working on the crossover.  I'll work up a theoretical crossover with the crossover points I think it needs, then use the minidsp to see if that's close, fine tune again and again, then order some crossover parts.  May have messed up in tweeter placement, there's almost a 4db dip centered @2.75k starting @2k, ending @3.4k.  Should have used the diffraction calculator before haphazardly placing the tweeter.  Fingers crossed it fills in at listening distance.

Question on the mid/woofer handover.  Figure the side mounted woofer will cross ~200hz and the sealed midrange rolls off plenty by then, is there any need to create a bandpass for the midrange?  Does the bottom of the midrange need rolled off any more or leave it elnatural?              

Got a little further on the project.  Made plenty of mistakes which pretty much seems to be on par for me.  You can see a little black accent around the border ... it's black paint from a paint pen covering up the area Ben123 seeped under the masking tap and onto the stain.  Paint is chalk paint w/wax, wood is plain ole lumber yard pine stained with Minwax Red Mahogany and a couple coats of Minwax lacquer.  Not that Minwax is the bomb, it's just what the local lumberyard carries ... along with the plain ole pine.  Can't tell it in the pics, but the paint has a leather sort of look to it.  Happened totally by accident, but in real life it looks pretty neat.          


 
 

  

jr@mac

Don't worry about tweeter dip. Benefits of not offsetting tweeter outweigh the single-axis smoothing you obtain by doing so. 

Kornbread

Krap ...

   mic@1 meter on tweeter axis. 

   Dark blue is outside on a stand about 15" off the patio floor w/o crossover. 

   Purple is in listening positoin inside crossed @150 18db w/minidsp. 

   Green is no crossover but inside and same position as purple in living room. 

   What hApPEned?  Is this due to floor bounce?  When listening to music you can tell the bass isn't quite right, not a lot of midbass to start with (mid) then it pretty much sounds like the pic looks.  The dip around 60hz is room related.  

   The da25bg sounds good though, the mcm axp-06 ... still trying to grasp what's going on there.              

 

   Just basic crossover w/minidsp, no correction other than, the mid pulled down 8db, tweeter down 6db.  Mid is ran full range on the bottom end but the issue isn't cancellation or the other measurements w/woofer only wouldn't look the same?  What's up?    

JasonP

Forgive me if I'm missing something obvious, but I'm having a hard time reading the first graph without context. Is this the Sd215A measurement? I can't tell from the pics, but knowing the characteristics of the driver I assume the box is vented? Did you do an in-box impedance sweep of the woofer? Seeing that might be telling.

Kornbread

My bad, yes the sd215 only, ported@35'ish hz.

Nearfield woofer and port response below.  Never mind the difference in output level, had the mic shoved into the port. 

 

The .zma;

   

So how do I go from this response to that big hole from ~35-80hz.?  No matter what order of crossover the minidsp is set on, the hole remains, even w/out a crossover.

I'm going to take these back outside and remeasure.           

PWRRYD

If you can shove the mic all the way into the box through the port, so it's hanging from the cord you will be able to capture one measurement that comprises the total system output (driver + port) and that is not influenced by the room at all.  You just don't want the mic capsule (measurement end) to be too close to the interior port opening.  Andrew Jones told us that tip, I tried it, it works great!  This measurement is only good up to about 200 Hz.

jhollander

When I look at this it looks like in room response vs. near field.  Your in room response is going to have multiple reflections and other anomalies esp. in the low frequencies.

Kornbread

Thanks for the tip PWRRYD!

Yep John, this room has some terrible nodes, then when they get to my seated position they all change again.  Need to treat this room badly.      

Getting somewhere.  I just 'assumed' I'd been measuring the same speaker inside as I'd measured outside.  Apparently not.  Left vs Right measurements taken inside in their listening positions.  Something is up, both speakers are not the same. 

Shaking head ... What have I done.  Like that frog in high school science lab, it's time to dissect.          

  

jr@mac

I see similar issues left to right, as well. 

Kornbread

Had the house to myself this morning so I've been at work on these.

 Measurements taken inside @1meter, gate was @3.14 The 2.75k dip remains but to my ears it doesn't seem to bother anything.  So far it's only taken a narrow notch @4170 and a shallow notch @2k and they're starting to sound rather decent.  If someone wanted to be picky, a few more notches could be added but my ears didn't hear enough difference to be worth the extra time/cost, as this will later be converted into a passive crossover.          

  

Inside but not gated.  Working the kinks out of the bass/midbass.  The sd215 is very picky on where and how it's rolled off.  If it's not spot on there's either a large hump from ~90-170hz or a deep null.  So far 24db@120hz seems to be the cat's meow. 

Odd thing, everything was the same in these two measurements except the purple trace is with the mid crossed 24db@80hz on the low end and the green trace is with the mid ran open on the low end.  Even though they measure the same on axis @1meter, they do not sound the same.  (purple trace)>After some time listening something was amiss, the speaker sounded more like a sub/sat system.  Not bad, just something was funny between the sub/midbass, it just sounded like two different speakers, not one.  (green trace)>Without changing anything else the 80hz crossover on the mid was removed and surprise, the speaker sounded as one.  Even though it didn't measure any different @1 meter, at the listening chair the area around (my best guess) ~150-300hz filled in nicely, maybe a little too much, but it connected everything.  It finally sounded like one speaker.  Initially, I'd cut about 9db off the mid and 6.5db off the tweeter to balance it with the bass, now I've been slowly adding them back in.  Expect the mid/tweet to come into line better with the bass on later measurements.                                             

sd2115 crossed 24db@120hz

axp-06 no highpass on the mid, lowpass 12db@1600.  So far this has worked best at pulling the rising response of the axp down and blending with the tweet. 

da25bg crossed 24db@2k.  I'm sure someone else could do it better but I'm impressed as how well the mid/tweet handoff sounds.

As they are now, I'm good with em' (other than some port chuffing at high spl).  Now it's time to convert all this into a passive crossover.               

Kornbread

Going to need some large capacitors, or stack of smaller ones,  for the sd215.  Can npe be used on the sd215 instead of poly?  Will the npe affect the sound? 

Going through some old threads to find those red audiophiler ebay caps.  May order an assortment of those unless someone else has a source for some good but cheaper poly caps.  

... doing some reading and it seems there are differing opinions on npe vs poly, especially when large values are going to be used in a woofer section.  It also sounds as if an npe could be used for the majority of the value and bypassed with a small poly to yield something better than just an npe.

      Comments ...                

jr@mac

Just received eight 100uF Audiophilers - all within 1.5% 

PWRRYD

I honestly cannot hear any difference between NPE and film caps used in woofer filters.

jhollander

I use NPE's on woofers and can hear some differences when the tail extends over 1,000 hz.  Obviously not the case here.  I'd also consider a high pass cap on the mid to keep the out of band freq from increasing the distortion.

Kornbread

There is a bit of port chuff just before the sd215 kraps.  Not too worried about this since the woofer is at its limit anyway.  But I've been running down a loud tapping problem during high bass peaks, in both speakers.  The right worse than the left. 

First thought was the wire feeding the other vc vibrating against the woofer.  Zip ties and some window seal foam tape fixed that. 

Noise still present. 

Puzzled.  Started to listen to different areas of the speaker, and the tapping sounded like it was coming from an area where the wires leave the mid/twt enclosure.  Got it, the wires were banging against the rear wall of the triangular upper enclosure.  Stuffed the area with polyfill making sure the wires were in the middle of the fill where they couldn't touch any wall.  Also put in some burnished and painted black, larger #10 screws in the woofers.  I've found heating the head of the screw till the shiny finish burns off lets the black paint adhere better.  Also did the glue in the screw hole trick. (sounds kinky doesn't it?)  Satisfied that fixed the problem, I sat back and turned up some bass heavy tracs.  Billie Eilish, You Should See Me In A Crown  has been revealing of the problem.

Well, that did eliminate some of the problem.

Keep it stupid simple.

Found this by moving the speakers while repeating that song.  The room has wood floors.  The speaker supports are outriggers with 3/8" socket head cap screws for feet.  You know where this is going ... the bolts sit on top of the wood floor, they don't penetrate at all.  If the speaker feet are not exactly level, one or more of the screws hammers the wood floor.  A few pieces of foam floor pad fixed the problem.

As they are now, driven with the cheap t-amps, I'm satisfied.     

Anyhow, have about 6 different value ebay 'audiophiler' caps with more on the way.  Should be able to order magnet wire next week if the company ever reimburses me for tuition.  Then I can get to rolling my own and start on the crossover.