Transmission Line Project (Planning Stage)

[Mike C]

Well it's most likely to be a TL, and I expect to plan for a few months first but here
are my thoughts.

The system is Teres/Moerch/Zyx TT; Isabelle DIY valve phono stage (better than
SFP-1); DIY valve pre and 300B power; and IPL S3TL transmission lines.
I really love this system! I like classical, jazz, folk, female vocals, and some rock
(not a headbanger) so I'm sure people will appreciate how well the system matches
my needs.

However, some knowledgeable friends reckon that the system is capable of more.
Specifically, the speakers are said to have something of an 'old fashioned' kind
of sound ..... whatever that means! They are 15 years old. These IPL S3TLs use
a SEAS 165mm doped paper cone woofer and 1 inch fabric dome tweeter whose
number I forget but it might be H398? Sensitivity is said to be about 88 to 89dB
which is a bit marginal for a 300B amp - but hell, it sounds good. By the way, the
amp is a good one, with several things done for good drive and dynamics, so it
is subjectively the most powerful I've ever had (Naim 140;ST-40 EL34 PP). Weird!

So, I'm not looking to correct any fault, or make a significant change; rather, I'm
looking to evolve on and update what I have, for a slightly more upbeat, open,
lively nature, and better imaging. Note the word 'slightly'. Also, a bit more
efficiency would be useful (and may well give the kind of change I'm looking
for).

Now, constraints!
I really don't want anything larger than the present speakers; 8 inches wide,
40 inches tall. This pretty well limits me to 6.5 inch drivers.
Though the above mentioned friends point out issues in the bass, I much prefer
it to their Naim type bass which is indeed great for rock ... but I prefer TL bass
for my kind of music even if it's not perfect.

So what I come down to is a smallish TL using a 6.5 inch driver, aiming at say
91 to 92dB efficiency, and using very high quality drivers for top sound quality
in the mids and highs. I'll accept a limit on the low frequency, and I don't need
rock type bass dynamics; but I do want TL tunefulness, ease of bass and
efficiency.

One option is just to get the current drive units and Xover supplied by IPL for
the current S3TL, and fit to my box ... tempting as it's *easy*! But I'm trying to
aim even higher, and I'd like some more efficiency.

Another option is to use the Fostex FE167e; good results and 94dB. But I'm told
the high end of this unit isn't that good (remember, I'm aiming very high) so I
tend to rule it out. So far ....

My favoured option may not even be feasible (if the driver doesn't match the
design). And it looks a long hard path. It's the Focal 6W4311 mid/bass driver;
chosen for very high quality and decent sensitivity. No choice of tweeter yet
but I guess I'm looking at Scanspeak, Raven ribbon, Hiquphon etc (just so you
get an idea of price and level).

I've already done a *lot* of looking incl of course t-linespeakers.org, and will
make use of the rules and resources there when it comes to a design. I'm also
aware of the Thor, Ariels and most things out there.
But first I need to know if the plan makes sense!
If so, I can work the TL design; but I'll need a lot of support and help with the
crossover .....

Of course one option is to keep and enjoy the speakers I have and change the
friends instead but that's a bit drastic ...

Any comments are welcomed!

 

[Dave Jones]

If your cabinet making skills are up to it, consider Lynn Olsen's Ariels. They are somewhat taller than your 40", but the sound is everything you want. I own a pair.

If you want to go with a fullrange driver, hop over to the fullrange forum and take a look at the Jordan Jx92s MLTL's. There are two versions, both by GM. One is 31", the other 48". I imagine you could design one that was 40". Download MJK's Mathcad worksheets. For starters, take one of the existing designs, lengthen or shorten the pipe to taste, while keeping the internal volume about the same. Then experiment around with driver placement and port length. After you've done that, give up and ask GM for help. Works every time.

 

[ScottG]

After you've done that, give up and ask GM for help. Works every time.

that IS funny! (and true..)

One thing to consider - because it sounds like you are thinking along the lines of midbass driver T-Line (could be wrong though), you should make sure that stuffing is far enough away from the driver's rear so that the driver's "stroke" isn't impeaded by airflow resistance. In particular I like using an oversised pipe (typically an inch larger than the driver's surface diameter) cut to length (depending on the depth of the loudspeaker) and stuffed with drinking straws. Provided the pipe length is enough, this should remove the problem as well as providing other benefits (like reduced reflections on the rear of the driver's diaphram).

 

[Dave Jones]

I played around with the MJK worksheet for about five minutes and came up with the following:

Pipe length 40in
Pipe cross section 26in^2
Driver position 8in
Port radius .75in
Port length 3.75in
Stuffing density .4lb/ft^3

 

[Greg B]

Focal drivers tend to work extremely well in TL's. I don't have the specs for the 6W4311 handy, but it may work OK with a low DF amp such as a 300B SET. If it were me I'd probably use dual drivers and go with a paper cone model.

There's really not much point to the old ROT and cut and try methods anymore, with MJK's mathcad sheets available. Don't neglect the amp's output impedance or the baffle step...

GB

 

[Mike C]

Thanks (and more thoughts)

Thanks Dave and Greg for your work and opinions; it's encouraging.

I take the point about the Rule of Thumb methods and I'll do a better analysis
using Martin's application (if you read this Martin,big THANKS for that).
The ROT based on Fs and cone area isn't a design, it just established general
feasibility within the desired 'package'. In fact, pipe length based on Fs and a
pipe section from 1.2 to 0.8 x cone area fits the space like a glove but as I say,
that's just one starting point for a design.

There's lots to consider; I'm just picking up ideas at present and gawd only knows
what I'll build if anything.
Example: I saw about the Meadowlark Osprey which looks very high quality and
reasonably SET-friendly. A feature which I guess may help here is the use of
first order crossovers; I assume that, in this design, 'less is more'? But I guess
that a first order Xover would have problems with a 6.5inch bass and tweeter ....
ah, no doubt that's why the midrange driver is in there. Looks a well worked out
design, well engineered, to me (but I'm a novice here!).
This kind of design has appeal to me, but I know I don't have the skills to design
it and tune it to work properly (two crossovers for a start) so I guess I'll pass on
this one.

Whatever I do, I need to learn a lot about crossover design; any suggestions
where I can learn about the subject?

Thanks and regards