So What Let's My "Mermans" Sing?

So what are in the Merman’s and what is it in their crossovers that lets them sing?  There may be no others that function as these do and the parts selection has taken years of tweaking both by ear and RTA.

Components include a JBL 2241H (18"), a JBL 2251J (9.7"), and a vintage ESS Heil AMT from the 1D series. 

 

The 18’s are housed in separate ~6.5 cu ft boxes and I 3D printed the flared ports. 

The 2251s are in ~3/4 cu ft with a rear vent.  The Heil’s have been treated to several modifications including a "half round" placed in the rear "gap" and wave guides.  The effects are profound!

Caps were mixed for their "particular sound" and multiple values were mixed so as to produce the most common value (e.g., the higher of the two was always mixed with the lower of the two other caps when used in parallel).  The Sprague caps are vintage and now hard to come by.

 

 

On an Lpad, there are two floating wipers.  We are using the other and it goes from ~12 to ~36 ohms before it opens completely.  The circuit works best above ~18 ohms, hence R2, with ~20 ohms being near optimal.

 

If you saw the frequency response of the JBL 2251J, the circuit would make more sense.

 

 

I want to roll it off at about 3,500 Hz but the peaks at ~2.8 kHz and 7.5 kHz makes this near impossible to do cleanly.  I’m not concerned with beaming because I ALWAYS sit in the sweet spot and the more the signal "beams" the less prone it is to side wall reflections which hurts imaging.  But a ~3,500 Hz roll off lets the music stay in the 2251s all the way up through the vocal range and that of the fundamentals.  I believe this is partially the secret to the spectacular imaging and soundstage. The 2241 fills in the bottom (crossed at 245 Hz, or basically, the fundamentals of the bass guitar), and the Heil takes care of the overtones.  And because the signal to the 2251 passes through no caps, it does not pick up their coloration.

 

I had to come up with some very imaginative thinking and a bit of luck from experimentation to make this work!

 

The 2251 does NOT play nicely with the Heil.  You either wind up with a massive peak or suck-out at the crossover frequency.  So you need a steep slope on both the 2251 and Heil to get them to play together nicely.

 

With the Heil, I use "mechanical means" to modify the response and this results in a flat response down to ~3,500 Hz with a really steep slope below this point.  The primary modification consists of a "half round" placed behind the diaphragm that reflects the rear wave back through the diaphragm drastically modifying the response and slope.  The resultant frequency response is extremely dependent on this arrangement, almost regardless of the crossover setting.

 

What I do to increase the roll off of the 2251 is to "over capacitate" it.  If you use too much capacitance in a second order low pass filter, you will greatly increase the slope but also create a hump just below the roll off point.  The more capacitance, the bigger and broader the hump.

 

So, what we do is use this hump to fill in that dip between ~1,200 and 2,500, with a really steep roll off beyond that.  But this hump turns out to be too high when the roll off is really steep.  The shape of the rising hump is obvious on the RTA with the Lpad turned full up (open).

 

So I also let some of the signal pass through R1 or R2/VR1.  This portion of the signal is NOT IN PHASE with the hump and the more you let through, the lower the hump becomes until you have a fairly smooth response up to the ~3,500 Hz with a steep roll off beyond.  VR1 lets you selectively tune the hump.  Note that the hump lies in a critical portion of the frequency band, and adding resistance lets this portion of the band stand out, if so desired, while reducing upper band "noise/hash".

 

BTW, the "Tube Simulator" is a brand new feature and really helps take the "digital edge" off of recordings.