Questions on the placement of BSC circuit

[marscoast]

Questions on the placement of BSC circuit

I'm trying to get a grip on where to place the elements of a BSC circuit in a simplified two-way design.

First, I'll set the stage - let's keep it simple and imagine we're living in a world that's pretty-much ideal.

I'm working from the simple equations provided by John Murphy's True Audio web site.

The speaker is an MTM design, crossed over at 2800, and let's say we're using only 2nd order electrical LR crossovers. All drivers are 8-ohm Znom.

The two woofers, in parallel, are 4-ohm Znom and have a published SPL of 91 dB 1W/1m. The tweeter has a published SPL of 90 db 1W/1m.

A Zobel circuit is in place, and let's just imagine a nice flat impedance line.

Baffle width is 8.5". According to Murphy's calcs, the F3 point for diffraction loss is 536 Hz.

8.5 inches is 0.2159 meters.
0.2159 * 4 / 1.021 = .8458
So L1 is 0.85mH.

So, I've got a BSC circuit to insert into my design; here's the diagram...


My first question is where do I put these components in relation to the Zobel circuit and the LP crossover?
My next question is, will this change the impedance presented to my LP crossover and thus require recalculation of those components?

This calculation is for the full 6bB compensation... what if I only want 3dB compensation?

So this is basically going to gradually pull down the woofer's response in those upper freqs that are not already pulled down by transition to 4pi space? Let's imagine that I used the 6 dB compensation, which would place the woofers (ideally, again) in the 85dB area... so the next step would be to use and L-pad to bring the tweeter down around 85dB as well, correct?

[dthomas]

Questions on the placement of BSC circuit

I'm trying to get a grip on where to place the elements of a BSC circuit in a simplified two-way design.

First, I'll set the stage - let's keep it simple and imagine we're living in a world that's pretty-much ideal.

I'm working from the simple equations provided by John Murphy's True Audio web site.

The speaker is an MTM design, crossed over at 2800, and let's say we're using only 2nd order electrical LR crossovers. All drivers are 8-ohm Znom.

The two woofers, in parallel, are 4-ohm Znom and have a published SPL of 91 dB 1W/1m. The tweeter has a published SPL of 90 db 1W/1m.

A Zobel circuit is in place, and let's just imagine a nice flat impedance line.

Baffle width is 8.5". According to Murphy's calcs, the F3 point for diffraction loss is 536 Hz.

8.5 inches is 0.2159 meters.
0.2159 * 4 / 1.021 = .8458
So L1 is 0.85mH.

So, I've got a BSC circuit to insert into my design; here's the diagram...


My first question is where do I put these components in relation to the Zobel circuit and the LP crossover?
My next question is, will this change the impedance presented to my LP crossover and thus require recalculation of those components?

This calculation is for the full 6bB compensation... what if I only want 3dB compensation?

So this is basically going to gradually pull down the woofer's response in those upper freqs that are not already pulled down by transition to 4pi space? Let's imagine that I used the 6 dB compensation, which would place the woofers (ideally, again) in the 85dB area... so the next step would be to use and L-pad to bring the tweeter down around 85dB as well, correct?

Place the circuit ahead of the low pass and zobel. If you want only 3db of compensation make R1 one half the value.

Yes you can use an Lpad to drop the level of the tweeter.

[marscoast]

Dave,

So for 3dB cut R1 in half but leave R2 the same?

Thanks,
Jason

[lhwidget]

Holler if I'm wrong, but the Zobel should be calculated & installed first.

The remaining design work should be carried out considering the Zobel as part of the driver it's attached to (in other words, draw a box around the driver and its Zobel, now consider that the "driver" for the rest of the XO design). Never install anything between the driver and its Zobel.

[dthomas]

Holler if I'm wrong, but the Zobel should be calculated & installed first.

The remaining design work should be carried out considering the Zobel as part of the driver it's attached to (in other words, draw a box around the driver and its Zobel, now consider that the "driver" for the rest of the XO design). Never install anything between the driver and its Zobel.

Not sure what you are saying by first I assume you mean ahead of the driver and its low pass circuit. If this is the case then that is right and what was suggested to the OP.

As far as the zobel it will not have much affect if any on the value of the inductor in the BSC circuit. Since a zobel is typically designed to bring down a midwoofers high frequency impedance rise it should have insignificant affect on the drivers impedance at 500Hz. I don't think anyone implied he should install the BSC circuit anywhere but ahead of the low pass filter and the woofer.

Reading between the lines on the original post it sounds like the OP is using a Zobel to get his woofers impedance near some nominal value in order to use a text book or premade crossover. Not ideal but the right thing to do if this is the design approach he is using. He then calculated a value for a BSC inductor. Not ideal design methodology but assuming the drivers are fairly benign it should work.

[daryl]

Questions on the placement of BSC circuit

I'm trying to get a grip on where to place the elements of a BSC circuit in a simplified two-way design.

First, I'll set the stage - let's keep it simple and imagine we're living in a world that's pretty-much ideal.

I'm working from the simple equations provided by John Murphy's True Audio web site.

The speaker is an MTM design, crossed over at 2800, and let's say we're using only 2nd order electrical LR crossovers. All drivers are 8-ohm Znom.

The two woofers, in parallel, are 4-ohm Znom and have a published SPL of 91 dB 1W/1m. The tweeter has a published SPL of 90 db 1W/1m.

A Zobel circuit is in place, and let's just imagine a nice flat impedance line.

Baffle width is 8.5". According to Murphy's calcs, the F3 point for diffraction loss is 536 Hz.

8.5 inches is 0.2159 meters.
0.2159 * 4 / 1.021 = .8458
So L1 is 0.85mH.

So, I've got a BSC circuit to insert into my design; here's the diagram...


My first question is where do I put these components in relation to the Zobel circuit and the LP crossover?
My next question is, will this change the impedance presented to my LP crossover and thus require recalculation of those components?

This calculation is for the full 6bB compensation... what if I only want 3dB compensation?

So this is basically going to gradually pull down the woofer's response in those upper freqs that are not already pulled down by transition to 4pi space? Let's imagine that I used the 6 dB compensation, which would place the woofers (ideally, again) in the 85dB area... so the next step would be to use and L-pad to bring the tweeter down around 85dB as well, correct?

That site is a waste of time and leading you in the wrong direction.

You might e-mail Mr. Murphy and ask what he thinks he's doing because his effort can cost you many hours of learning and in the end be very discouraging when you find you need to forget everything you thought you knew and start over.

Not only that but crossovers designed as described by such sites are very complicated and expensive as well as being garbage.

Formulae are never used for crossovers or zobels.

Most of the time you don't need a zobel circuit and if you did need one it likely would be more beneficial to align it differently than what you would for the flattest impedance (which means it wouldn't strictly even be a zobel).

A BSC circuit is not used in a crossover.

Instead you simply align the filter elements so that a suitable response is achieved.

What you want to do is download some free software from Jeff Bagby or maybe download Speaker Workshop or even buy Soundeasy if you like.

Learn some electronics and talk to the right people (a lot of them right here) and you'll be on your way.

[dthomas]

I totally agree...

[gmilitano]

Hi marscoast,

There is no hard answer on a BSC and it can vary based on the room, plus everyone has different preferences. I put together a simple Baffle Step Correction Circuit Calculator to do the math. I am usually happy with somewhere between 3 to 5 dB. Usually I will start with 4 dB and try different resistors till I am happy.

Also, try experimenting by pulling the speaker right up against a wall. That can occasionally eliminate the need for a BSC.

Cheers

[lhwidget]

Not sure what you are saying by first I assume you mean ahead of the driver and its low pass circuit. If this is the case then that is right and what was suggested to the OP.

I just ment that I'd model the Zobel first, before the XO network.

Also, you and Daryl are right, every time I started with a Zobel in a crossover design in SoundEasy, I finished without it. When you can measure the phase response of the drivers, it seems easier to achieve good phase alignment without the Zobel.