Total impedance when running tweeter in parallel with a FE206E?

Hi all. I plan on building these speakers.:

http://www.quarter-wave.com/Project05/ML_TL_Enclosure.pdf

I also want to mount some Fostex FT17H tweeters directly underneath the FE206Es with like 1/8th inch between them. I plan on using a simple single cap crossover and a L pad. It seems that I read somewhere that this type of setup is still considered to be a 8 ohm load even though the tweeter and the driver will be connected in parallel. Something about the reactance of the cap and the inductance of the voice coils. Am I remembering this right?
 
Hello G, The general statement that this is still a nominal 8 ohm load is correct. Because of the natural rolloff of the fullrange, and the capacitor acting as a high pass filter for the tweeter, the amp sees the parallel combination as a nominal 8 ohm load. I say nominal because any speakers impedance is going to vary based upon the frequency, and based upon how hard it is driven. When the driver moves around it is moving the voice coil in the magnetic field, this will also cause some change to the impedance.
But in general, if you hook a tweeter and cap in series, then hook this in parallel with your full range (or dare I say it in this forum, mid range and woofer) the general impedance will stay the same.

Peace,

Dave
 
Thank you very much for your reply Dave. I just wanted to get that straightened out before ordering parts and building the amp that I plan on using with these speakers. I will have to do some studying to understand the theory behind the reality but I needed a quick answer to calm my worried mind. I'm sure you understand.:)
 
Just my 2 cents, I started with Martins MLTL with a 206E powered by Zen V9 and after following the half chang thread, decided to build them on Martins recommendation. The build is no harder than the MLTL. I finished my half chang's a couple of weekend ago and agree with Martins recommendation that they are a step up from the MLTL. There is clearly more base with a larger and more distinct soundstage. The half chang's are a little taller, but the foot print is pretty much the same.

Either build is great, just more base from the chang's
 

ronc

Member
2003-03-08 2:22 pm
It all comes down to the impedance (resistance) at the XO frequency, not the rated nominal impedance of the driver. This impedance changes with load and heat. As the transducer is required to produce more energy the unit (VC) heats and the resistance is greater. If there is more mechanical loading the VC also heats to a greater degree. Energy is never gained or lost, just transformed.

ron
 
ronc said:
It all comes down to the impedance (resistance) at the XO frequency, not the rated nominal impedance of the driver. This impedance changes with load and heat. As the transducer is required to produce more energy the unit (VC) heats and the resistance is greater. If there is more mechanical loading the VC also heats to a greater degree. Energy is never gained or lost, just transformed.

ron

I guess that makes the crossover cap critical huh? Can anyone explain to me how to calculate the crossover cap?
 
rabbitz said:
For a simple 1st order cap, C=0.159/(Rxf)

where
C is in Farads (F) not uF
R is resistance at the xo point (not driver nominal or Re)
f is frequency (Hz)

There are online calculators.

How do you figure out what the resistance at the crossover point is? I the PDF for the FT17H tweeter they show a diagram of the FT17H being used with a 1.5uF cap and a L Pad in parallel with a FE207E. Does that sound about right?
 
I guess that makes the crossover cap critical huh? Can anyone explain to me how to calculate the crossover cap?

I'd just use the calculator at the link below to get a rough point for the high pass. Then I'd tune by ear by paralleling cheap low value caps. Once the preferred result is achieved, replace with a good quality cap of close value.

1st order x-over calculator

Then again, I've not done it... tho intend to soon.

Cheers
 
rcdaniel said:


I'd just use the calculator at the link below to get a rough point for the high pass. Then I'd tune by ear by paralleling cheap low value caps. Once the preferred result is achieved, replace with a good quality cap of close value.

1st order x-over calculator

Then again, I've not done it... tho intend to soon.

Cheers

According to the calculator Fostex is recommending a crossover point of 13.25KhZ when using a FT17H with a FE207E. Does that sound right. I am using the FE206E. I was going to cross it over at around 16KhZ. Opinions? Thank you for the link BTW rcdaniel.
 

OzMikeH

Member
2007-03-18 9:22 am
I have plugged FE207E with FT17H. it rolls off drastically at about 10kHz. The 206 extends a LOT higher so all bets are off.
The first thing I did was add 8 ohms in series with the FT17H and treat it as a 16 ohm driver, this drops 3dB of efficiency to match the FE207E, then I started with my calculated value and didn't like it. Also the FE207E is 16 ohms at 10kHz which makes the combined impedance amp-friendly.

I was using a Mr Liang PP el34 amp at the time and I spent about a day in the loungeroom with a soldering iron, a tag strip and a fistful of caps.

Then I gave the tube amp back and went back to my Harman Kardon SS amp, and it sounded too bright again, now I'm onto another PP EL34 (diy hifi supply Ella) and it sounds balanced again.

FE207E is bloody awful with original dust caps. I expect the 206 will be the same.
Hold off on the high quality cap until you decide to keep your amp for a while.
Dont put the tweeter in the centre of the baffle either. centre of the tweeter in line with one edge of the whizzer would be about right.
 
G said:


How do you figure out what the resistance at the crossover point is? I the PDF for the FT17H tweeter they show a diagram of the FT17H being used with a 1.5uF cap and a L Pad in parallel with a FE207E. Does that sound about right?

On the manufacturer's graph, there should be an impedance plot. Have a look where that line is at the required frequency.... that's the impedance at the xo point and close enough without measuring. You'll find you will still need to tweak after installing the calculated value which is only a starting point.
 

kaan

Member
2007-08-03 7:46 pm
I tried this with the FE206 and XT-25 tweeters.
I put the XO at around 6KHz.
I took the tweeters off again after a week because its sounds a bit strange at times. I tried putting out some test tones and I concluded that a 1. order filter wasn't enough. Example: When listening to 10KHz, the sound would dissapear and reappear as I moved my head around. It needs a low pass filter for the FE206.
I'm going to try again with a 4. order filter sometime.