JBL D-123

[Laurent Brondel]

Thanks guys for the links and plans! The JBL link doesn't load for me. Have you tried substituting the 3/4" ply for 3/4" pine or poplar for those designs? That would save quite a bit of weight, but be more "resonant", which could be a plus or minus, depending on goal.

[Teleguy61]

Thanks guys for the links and plans! The JBL link doesn't load for me. Have you tried substituting the 3/4" ply for 3/4" pine or poplar for those designs? That would save quite a bit of weight, but be more "resonant", which could be a plus or minus, depending on goal.

Sorry.
Try googling "JBL professional enclosure guide" and you should be able to
locate the pdf.

[Laurent Brondel]

So I received the D-123 pair and put them in a pair of brown Princetons ( a '62 and '63). I was expecting the extended range to brighten a bit those dark amps, but interestingly they're heavy on mids and the original 10" speakers (alnico and ceramic CTS respectively) do sound better in those amps, with better bass response and a more scooped overall EQ.
Otherwise the JBL's are quite efficient, and have a pleasing sonic signature for cleans, not so much for dirt (IMHO) as I found them a bit harsh. As some of you have pointed out, they probably sound better in a closed cab, as the bass response is a bit underwhelming in an open back (and small) cab.
I am thinking of building a 2 x 12 Thiele cab for them and see what happens.

BTW the phase on that particular pair of D-123's is "correct", with the push on the positive cycle, like all other speakers.

[RCGPNY1]

Hmmmm...thats kinda odd...JBL manufactured their speakers so that positive voltage on the red terminal would make the speaker go "in". They might have been reconed?

[Laurent Brondel]

No, original cones. I am not even sure the parts to recone them are available.

[Laurent Brondel]

So, I built a TL806 cab for the EVM 12L and I am loving it, for guitar and bass. That cab design makes the speaker sound terrific.

If I wanted to build a ported 2 x 12" cab for the D-123's, I assume I would need less internal volume than for EVM 12L's, right? How do I find out the right parameters?

[didit]

You'll find the Thiele-Small parameters in this PDF -- http://www.jblpro.com/pub/technote/thiele.pdf.

My 2x12" cabinet wasn't specifically designed for the D123. I bought it 2nd hand. It's really just TL-style without the fine tuned volume & porting. Inside dimensions are approximately 18" x 25" on face including port 1.75" high along the bottom, and 9" deep. Eyeballing it I'd guess it's about 3/4 volume per speaker vs a true TL-806. Last time I worked through a speaker design with detailed math was in the early '80s; and so paper/pencil and a calculator were my tools. You should be able to find applications or even useable javascript web pages out there that make it fairly easy.

Good luck and keep us posted on the results.

Best .. Ian

[Laurent Brondel]

Thanks Ian, do you think the T/S parameters of the D-123 would be comparable to the bigger E-120 and G-125? The cab/port measurements JBL gives for those are actually very, very close to the TL806.

[didit]

Thanks Ian, do you think the T/S parameters of the D-123 would be comparable to the bigger E-120 and G-125?

Hello - If you look through the PDF I'd posted a pointer to in my previous message comparing the T/S numbers for both D123 & E120 you'll see quite a difference. Roughly 50% across the set parameters. It's been far too long since I dug deep on the design impact of parameter values so I won't try, and doubtless mislead, by offering suggestions on how well specific designs would work. I will say that T/S was all about HiFi. Keeping close to that engineering produces best possible reproduction. In my case and I assume your's we're doin' rock n' roll, which is after a different thing all together. If I was building a new "from scratch" 2x12" I'd go back to playing a bit with T/S calculations but wouldn't be a slave to the results. Not helpful perhaps but that's how I'd suggest you proceed.

Best .. Ian

[Laurent Brondel]

So I built that 2 x12" Thiele design for the D-123's, and I much prefer them in the SFDR open back cabs!!! I just shrunk the depth to 12" and built it out of white pine. The cab is 47lbs, which is not bad.
I re-tried a pair of '60s Jensen C12N in the Deluxes open cabs, but I think my favourite so far are the D-123's, and a close second Altec 417 B's (in the SFDR open cabs). I A/B'd with the TL806/EVM 12L cab with a single Deluxe, the EVM of course has a lot more headroom and a tighter, more powerful bass, but the JBL D-123 compares favourably at low to medium volumes, and has more sparkle. I use the EVM with a late '60s Carlsbro 4xEL34's head.

[Laurent Brondel]

Rig:

[didit]

So I built that 2 x12" Thiele design for the D-123's,

That was pretty fast ...

and I much prefer them in the SFDR open back cabs!!!

Can you be a little more expansive describing what you didn't like about the JBLs in closed TS? I'm assuming you noted/heard some improvement in low end stability but also noticed some loss in the overall "voice" -- for want of a better word.

Best .. Ian

[Laurent Brondel]

Yes, exactly. The low end gained in overall tightness and especially in volume, although the JBL D-123 do not lack in this department even in a open cab (compared to, say, vintage Jensen C12N's or vintage Celestion G 12-65's).
But what I found slightly unpleasant was the dip in the mids and generally the exaggerated "hi-fi" character of the tone with also increased presence above 3 to 4k. Dirty and lead tones didn't sound good with the TL806Q inspired cab, and I found clean tones to get lost easily, again by the "loss of character" as you call it, and/or the lack of definition and presence of the mid registers, and the increased bass and extended high-end.

Essentially what I found amazing in the JBL D-123 (compared to an EVM 12L Classic): more character and presence in the mids, beautiful upper mids and a great but not harsh or brittle treble response, got totally lost in the 2 x12" Thiele inspired cab.

Interesting, as the single TL806 cab I built for the EVM 12L really made that speaker come to life and sound nothing short of great, when it didn't impress me that much in the Deluxe Reverb open cab.

Interesting experience, now I have 2 x white pine SF Deluxe Reverb head cabs I am not using at the moment… Although I am sure I will experiment again.