Need advice on notch filters or BSC for FE207 simple BR

[jsb]

Dear listers

I have mad several searches and found some hints but not the definitive answer.

I have have built the fostex simple BR cabinets for the FE207E and use them on KR 300bs Tube amps. Even though they are wonderful, they visibly need some correction, a thing I expected. Based on your actual experience with these drivers and cabinets, which kind of filter and what values would you advise for this kind of tube amps?

I am quite a newbie as regards filters.

Best,
JSB

 

[Scottmoose]

Can't give you any values until you tell us how wide the baffle is. A basic crcuit with resistor and inductor in parallel in the + lead, and a zobel across the + & - leads will be fine.

 

[jsb]

Hello!

This is the fostex recommended enclosure

35 liters.

Vent: 80mm diam / 180 mm length

(sorry, it is in metric system)

AS you would expect, this sounds a bit middy, I wish to smooth a bit the upper mid and treble.

Best

 

[Scottmoose]

Assuming your box is about 300mm wide (give or take a few mm here of there won't make an audible difference), then try this:

Parallel a 3ohm resistor with a 1.2mH inductor in the + lead to the driver. After that, run a Zobel consisting of an 8ohm resistor in series with a 3uF cap between the + and - wires.

That should bring the response into line. You can adjust the amount of attenuation by altering the value of the resistor in the circuit (the one in parallel with the inductor) up and down: more resistance will increase the level of attenuation, smaller will reduce it a bit. As resistors aren't expensive, buy a few if you can and play around, though give yourself a good half-hour of listening before deciding to make any changes or not.

Hope this helps
Scott

 

[Bob Brines]

Parallel a 3ohm resistor with a 1.2mH inductor in the + lead to the driver. After that, run a Zobel consisting of an 8ohm resistor in series with a 3uF cap between the + and - wires.

It's interesting how different people come up with different values. Personal taste and room differences I suppose.

I use 6.8uF=8 ohm for the zobel and 2.5mH||4 ohms for the step filter. This is for an 11.5" wide baffle. With the FE207E, you might consider 1 uF || the step filter.

While you are zeroing in the step filter, you might wire the low side of an L-pad in place of the resistor. That allows 0-8 ohms for testing. Add a resistor if you need to go higher.

Bob

 

[Scottmoose]

Nothing like variety.

Go with Bob's -he's much better at this than I am.

 

[jsb]

Scott and Bob: Thanks a lot or your time and help!

My idea is to try the simplest tweaks first, so that I do not alter too much the output of my 300b-s. (Some people seem to add just a RLC on the + wire, is that a regular tweak?).

Bob: just to make sure I understand, as I'm really a newbie in that field
- Zobel : 6.8uF and 8 ohm in series.
- Step: 2.5mH, 4 ohms and maybe 1uF, parallel, on the + wire?

_ _________ // 2,5 mH // 4 ohms// (1 uF) ____ + _____ ____

\

6.8 uF
8 ohms

/

_______________________________ - ______ _______



I'll do some reading in order to understand your suggestions regarding the L-pad (... reeaally newbie )

jsb

 

[Scottmoose]

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

Use this circuit layout, with Bob's values for the 4 components.

 

[joval1]

Hi All

I am also in the process of experimenting with the BSC values in my 206 MLTL. I started with the middle values suggested in Martin King's table. Please correct me if am wrong but as I understand it, I need to increase the value of parallel component of the circuit if I want to have more bass and increase the zobel if I want to tame (reduce) the highs? Thanks in anticipation.

 

[Scottmoose]

Pretty much. The zobel is probably OK, I'd look to the parallels first. I don't like the 206 in this sort of enclosure much (207 is the better bet IMO) so I'd probably just go for max value on the parallels, starting with the resistor. See how that sounds, and then think about further tweaks if you're still not happy, initially with the inductor.

 

[jsb]

thanks a lot,
I've read the much interesting piece on bsc, thanks scott!
I'll check with my components retailer and will start with bob's values!
All the best
m

 

[Bob Brines]

Hi All

I am also in the process of experimenting with the BSC values in my 206 MLTL. I started with the middle values suggested in Martin King's table. Please correct me if am wrong but as I understand it, I need to increase the value of parallel component of the circuit if I want to have more bass and increase the zobel if I want to tame (reduce) the highs? Thanks in anticipation.

First, I really, really wish that folks would pick the right driver for the cabinet rather than try to force the driver-of-the-day into a cabinet it won't be happy in.

Anyway, this is the way I arrive at the values for the filter parts: First I measure the the far-field FR and the impedance of the driver IN THE PROPOSED CABINET. Then I use a cross-over design program (Sound Easy or Speaker Workshop in my case) and eye-ball the zobel so that the impedance curve is dead flat from ~2kHz up. Next, I determine where I want the knee of the BSC filter from the FR plot. I then eye-ball in the BSC part to give me a relatively flat FR curve. Finally, I build the filter and listen to the results. Usually, I will need to tweak the BSC resistor.

The FE206E presents an additional problem. The FR curve skyrockets above 1kHz. That is what give this driver the harsh, in-your-face sound. The simplest fix is to put a resistor in series with the driver BEFORE the BSC filter and the zobel. Increase this resistor (1-2 ohms usually works) until you get a sound you can live with. You will also probably have to go back and tweak the BSC resistor, as there is interaction. I strongly recommend that you do NOT screw with the zobel, unless you can do the simulation.

The final result is that you burn up the extra efficiency of the FE206E and make it sound exactly like the FE207E.

Bob

 

[joval1]

pre-bsc resistors

Thanks for the advise.

Bob, my initial thought was to go for the 207 but the vaunted 'details' of the 206 led me to it. However, the recommended BLH miserably failed in the WAF criteria so have to settle for the MLTL.

I would like to explore your idea of having pre-BSC resistors and will appreciate it if you can let me know the starting value. Am pretty much a novice in this area and as I understand it I will connect a series resistor in both + and - wires BEFORE the BSC? Please confirm.

Also I note in the earlier thread that your zobel values for the 207 are 8ohms|6.8 uF combination and I presume these are simulation results as Martin's simple spreadsheet yields 8.4 ohms|5.7uF, based on Re=6.73, Lvc=0.4 Baffle width = 11 inches.

 

[ronc]

Well Bob IMO the 206e is good in a proper BLH but with the series resistance required in either a BR or MLTL it still could be better than the 207.
I really shouldnt state this as i have never heard a 207. However the 206 has an inherent speed that is hard to beat.
The 208 sigma in my old Dallas (not the Dallas II) with a ribbon tweet is hard to beat for a larger horn.
ron

 

[joval1]

ronc

Can you point me to a properly tested BLH design that is no more than 280mm wide? Depth of up to 450mm (18in) and around 1 meter (3.28 ft) high will pass the WAF criteria. Cheers

 

[LeifB60]

I only use a resitor. I use one 10 ohm but you can test between 5-10 ohm depend how much bass you like.