Inside the cabinet there is no stuffng material used and all the wiring is soldered connections.
The Jensen P10PF driver was also used in Fender guitar amps and is known for its beautiful tone.
The closed back jensen tweeters are familiar to me and part of the reason why I bought the speakers. These tweeters are able to achieve something special compared to any other tweeter I've encountered. They're able to reproduce vocals and electric guitar with a realism and believability that lets you forget everything and just enjoy the music. Sure they have some limitations. They don't resolve complex music very well, they have limited power handling, and somewhat ragged frequency response. But they are wide bandwidth playing from 2 KHz up to 17 kHz. The natural Fs of the tweeter is 1.7 kHz, so a crossover point of 3 kHz is recommended if using a 2nd order slope.
These speakers sounded pretty unflattering when I first hooked them up. The bass was deep (down to 47 Hz) but there was a serious upper bass resonance coming from the cabinet.
The first order of the day is to stuff the box with some polyfill to help resolve this issue.
I went a little over board at first try. Stuffing the box right full resulted in losing most of the bass. Although the cabinet resonance was gone so was the bass!
I decided on a much smaller amount that seemed to quel the resonance just enough however not to steal the bass.
Ok so my conclusion here is that these could stand to have some much better enclosures.
Listening to the midrange it was obvious something was amiss. There was this nasty harshness that demanded some further investigation.
First step, measures impedance curve and frequency response.
Below is the frequency reponse of the speakers.
Ok so things don't look too bad, nothing seriously out of whack. I was surprised to see the drop off in the high treble but I would deal with this later.
In order to figure out where he harsh sound is coming from I needed to measure the woofer and tweeter separately. Being bi-wirable speakers this was easy to do.
Below is the frequency response of the low frequency and high frequency separately.
Below is the impedance curve of the Jensen tweeter.
Ok so yes there is a natural resonance at 1.7 kHz that likely is contributing to the harsh sound that I'm hearing.
This is indicated by the small bump in the impedance plot at our suspect frequency.
Ok, so at this stage what does one do? Replace the cap? The cap says 3.5 uF which is easy enough to replace but there is another easier thing to try first that could identify the solution.
Being bi-wire I can insert an additional capacitor outside of the cabinet to drive up the crossover point of the tweeters to quickly tell if there is a subjective and objective improvement.
Shown below is a 1.5 uF cap wired to the terminal lugs.
The resulting frequency response is shown below as the red line.
My subjective listening does confirm that the this bump at 1.7 kHz was the source of the harshness. The sound is much improved and the dip that is now present is slightly noticeable but not nearly as objectionable as what was before.
Also notice the green curve that rises from 7 kHz up to 17 kHz. This was a result of me bypassing the 16 ohm resistor found on the crossover! Why this resistor is ther is beyond me, maybe it was there to reduce turntable hiss and pops, I don't know. This alone was another giant step forward in improving the sound.
Sound continued... again
Doing these quick changes to both speakers and sitting down and listening to some good music revealed that these are some really great speakers. The bass is the weak point (I blame the cabinet) but from there on up everything is dynamic and detailed able to reproduce details such as brushes and natural decay in reverberation. Percussion in particular is very lively and enjoyable.
I bought these speakers because I wanted to evaluate mainly the Jensen P10PF woofer for use in another project where I was to build a large Transmission Line enclosure utilizing my DATS speaker test system to create a design. You see, older drivers although good, didn't have the Thiel & small parameters available so that you could design a proper box. A lot of the design was trial and error. However with technology available today it's possible to take advantage of vintage drivers and design them into projects. Hornresp is able to optimize a design that would otherwise take years.
Are the vintage drivers worth the trouble? So far my most successful projects sonically have been when using modern drivers such as the B&C DE120 compression driver. There is just no comparison to the sonic beauty that this driver can produce. Also, more recently the Celestion AN3510 driver proved out to be a clear winner. And it was designed using the latest in Finite Element Analysis software according to Celestion.
I would say that there is a charm to the vintage sound and I am going to continue to do some projects using vintage drivers simply because it is interesting to blend old with new and maximize the old in ways never possible before.
In the meantime I will replace the old capacitor as I suspect that it's value has drifted. (Maybe)
I'm not sure if the L-pad is required either. It was obvious that the tweeters didn't need attenuating at all. The tweeter pair struggle to match the efficiency of the woofer.
Apon writing this I just remembered why bypassing the resistor brought back the highs. Ceramic resistors contain a coil of wire to create the neccisary resistance. The problem is that this coil has inductance which causes the highs to fall off. I didn't realize this right away because I always use metal oxide resistors which have zero inductance. There ya go. Lesson learned. Never use ceramic resistors.