New Advent Tweeter - Measurements - Ferro Fluid Issues

[Pete B]

I measured several New Advent tweeters, as well as several

EPI, and Genesis tweeters that use the older thick ferro

fluid and there is a lot of variation in Fs probably due to

the ferro fluid. The thick fluid tends to raise Fs and also make

it temperature dependent, as was reported by Linkwitz many years ago.

I've got one New Advent tweeter with an Fs of 2979 Hz, another

with 2396, and two more very close to 2K. I believe that 1.9 to 2K

is correct and I wonder if there's anything that can be done

to salvage the other tweeters.

The EPI and Genesis also use the very thick fluid and one of the

EPIs has an Fs of 2786 Hz cold,

Fs = 2288 with 1W @ 2KHz for 4 minutes

Played for 1 hr on stereo, then allowed to cool, Fs = 2605 Hz

Played for 10 min on stereo, measured immediately, Fs = 2565 Hz

The matching tweeter also had temperature dependent behavior but

it also had a much lower Fs in general:

Fs = 2197 Zmax = 6.57

Fs dropped to 1656 after 5 min warmup with 1 W at 1800 Hz

There are reports of the Ferro fluid

being burnt by excessive input power, it might thicken with age,

or this could just be the behavior at a low room temp of about

68 degrees. I don't know if the high value of 2786 Hz for one

of the EPI tweeters was due to it sitting for many years unused,

or if it was overstressed.

The fluid is so thick in these older tweeters that there is hardly

an increase in the input impedance at Fs, it is just visible with

about a one ohm increase. In comparison, the two original Advent

tweeters without ferro fluid have Fs of 647.1 Hz and 635.6 Hz, with

a very high peak of about 26 to 27 ohms.

I've not tried any break-in with the very high Fs (2979 Hz) New

Advent tweeter, but I have a feeling that it is damaged or defective

at this point.

It would be interesting to disassemble a New Advent tweeter to see if

the thick ferro fluid can be removed and replaced with the newer

lower viscosity type, or left out completely to make it more like

the original tweeter.