Highest Qts for TL design?

[goody75]

What is the highest Qts driver that you could successfully put in a transmission line cabinet? I've read some people actually think around 0.7 is best because it produces too much low end and when damping the peak above the cabinet tuning, the end result is more flat. This is not the best explanation but my point is a 0.7 Qts is possibly considered an optimal value by some people.

If I want to accentuate the bass on a vintage full range driver with a Qts closer to 1, and want to avoid a sub or helper woofer, is a TL cabinet an appropriate choice?

I've also read that you can use resistors in parallel with a driver to get a higher effective Qts, does that mean that I could use a resistor in parallel with this driver and lower the Qts? If this is possible are there any tradeoffs I should be aware of?

[Don Hills]

There is a very serious tradeoff. Remember that the amplifier output is in parallel with the driver, and the amplifier's "resistance" is (in most cases) very nearly a short circuit. There is no practical value of additional parallel resistor which would make any significant difference to the Qts. It is possible to design the amplifier so that it appears as a "negative resistance" which will lower the overall Qts, but this is not a straightforward approach. A more practical approach might be to calculate a Linkwitz Transform filter to suppress the underdamped resonance peak.

[DrBoar]

There is a trick of using a serial capacitor, this will lower and flatten the bass somewhat. The cap is interacting wiht the inductive part of the drivers peak ( the left side of the driver peak, lowering a bit and thus getting more energy in the driver slightly below Fr).

There are two TLS schools. The first, the infinite line way is to have a lot of damping material in the pipe and "lose" the fundamental resonance in the pipe. Here a high Q driver works well. But a similar result can be obtained by resistive vents as Dynaudios Variovents.
The second school treat the TLS as any other resonating quarter wave pipe and try to keep as much as possible of the fundamental resonances radiation resistance (minimal cone motion) and selectively reduce the unwanted harmonic resonances. This is done by placement of damping material and folds of the pipe.

If you extend the bass of such a high Q driver below Fr without using things like negative resistance you will get quite poor transient response regardless if you use a pipe or bass reflex.

I like TLS but it is not a magical thing that will turn a boomy driver to deliver controlled deep bass.

[woody]

Placing a resistor in parallel with the speaker should lower QTS just a little not raise it. But placing a resistor in seriese with a speaker will raise the QTS. How much resistance is needed depends a lot on the driver. I have one driver 8ohm with a QTS of .186 that would need a a 20 ohm resistor to raise it to .522 . On the other hand
just an ohm or 2 may be very effective in raising the QTS of some drivers. But you waste a lot of power this way.