4345 filter modification for 8 ohms compression driver

[pos]

I need some help on the modification that would need to be done to a 4345 CC filter to accomodate a 8 ohms compression driver instead of a 16 ohms one.
63854

There is an 8 ohms lpad so I guess impedance modification would not impact the filter behavior, right?
Would removing the parallel 20 ohms resistor be a step in the right direction? (20 ohms // 16 ohms is 8.8 ohms)

There is always the possibility of adding an autoformer in front of the compression driver to multiply its impedance, but...

Any help would be greatly appreciated :)

[Chas]

I had the same question when I converted over to TAD Be drivers in my OEM 4345's. I thought I would first try it out as-is and then forgot all about it. It worked just fine.

[Ruediger]

I need some help on the modification that would need to be done to a 4345 CC filter to accomodate a 8 ohms compression driver instead of a 16 ohms one.

There is an 8 ohms lpad so I guess impedance modification would not impact the filter behavior, right?
Would remove the parallel 20 ohms resistor be a step in the right direction? (20 ohms // 16 ohms is 8.8 ohms)

<snip> ... </snip>


The LPAD expects an 8 Ohm driver and appears as an 8 Ohm load (with the proper driver impredance). Removing the 20 Ohms resistor is the right way.

Ruediger

[just4kinks]

I had the same question when I converted over to TAD Be drivers in my OEM 4345's. I thought I would first try it out as-is and then forgot all about it. It worked just fine.

Same here.

[ivica]

I need some help on the modification that would need to be done to a 4345 CC filter to accommodate a 8 ohms compression driver instead of a 16 ohms one.
63854

There is an 8 ohms lpad so I guess impedance modification would not impact the filter behavior, right?
Would removing the parallel 20 ohms resistor be a step in the right direction? (20 ohms // 16 ohms is 8.8 ohms)

There is always the possibility of adding an auto-transformer in front of the compression driver to multiply its impedance, but...

Any help would be greatly appreciated :)

Hi POS,

From your post I have understood that You want to use some different driver then expected here ( 2421 + 2307&2308), so its impedance would be different too, especially in the lower frequency range where much larger impedance peak(s) are expected (depending on the driver characteristics and applied horn). Instead of the pure resistive compensation ( 20 Ohm ) you can use a kind of "Zobel" network

http://users.ece.gatech.edu/mleach/ece4445/downloads/zobel.pdf
http://users.ece.gatech.edu/~mleach/papers/zobel/zobel.pdf

not only to compensate HF impedance rise, but more important to compensate the impedance peak(s) in the driver resonance frequency region to reach expectable impedance of about 8 Ohms (in order to prevent nasty drivers repose due to impedance mismatch with the network characteristic impedance in the driver resonant region). So a kind of used driver&horn combo impedance measurements would be the best solution.
As I have remembered, JBL considered that 242(4)x-J (16 Ohm) drivers impedance to be about 12 Ohms, while projecting the passive networks.
The Horn influence to the driver's impedance can be seen on the attached figure for JBL 2426H driver ( 8 Ohm) with 2370 horn and plane wave terminated tube.

The applied horn influence to the driver impedance response can be seen in , our forum member 4313B (http://www.audioheritage.org/vbulletin/member.php?15-4313B) valuable work:

http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=44874&stc=1&d=1269131737

http://www.audioheritage.org/vbulletin/showthread.php?28127-Truextent-Diaphragms-in-JBL-four-inch-compression-drivers&p=283241&viewfull=1#post283241

http://www.audioheritage.org/vbulletin/showthread.php?28127-Truextent-Diaphragms-in-JBL-four-inch-compression-drivers/page7

and here You can compare the same driver impedance response applying different horns: 2311&2308 (http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=44876&stc=1&d=1269132023) vs Yuichi-A290 (http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=44880&stc=1&d=1269132443)


regrads
ivica

[yggdrasil]

You could just add a resistor in series with the driver. 4-8 Ohm should get you close.

[ivica]

You could just add a resistor in series with the driver. 4-8 Ohm should get you close.

In such case, I would expect a kind of sensitivity reduction.

regards
ivica

[pos]

Thanks guys!

Ivica, wouldn't the lpad already act as a zobel?

[ivica]

Thanks guys!

Ivica, wouldn't the lpad already act as a zobel?

Hi POS,

Owing to quite low driver nominal impedance of about 8 Ohms, and if at some lower frequency it can reach, say 30 Ohms, and if LPAD is in the center region , then the equivalent impedance that is "presented" to the network would about 10 Ohms, around the resonant frequency

4 Ohm + (8 Ohm parallel to 30 Ohms) = 4 Ohm + 6.316 Ohm = 10.3 Ohm , instead of (ideal) 8 Ohm

Fortunately there are some resistors network after Hi-pass section of the VHF driver network so such would reduce impedance
'irregularities' more.

Just for the experiment, You can put say about 40 ohms instead of 20 Ohms, just to prevent unexpected large impedance value.

regards
ivica

[Ian Mackenzie]

The 20 0hm resister damps the impedance at the crossover point so as seen to be largely resistive.

The original equivalent with tapped choke is less sensitive to load impedance variations.

The network would otherwise need to be re optimised or the specific driver you have in mind?

I recall 4343b did this for a two inch JBL driver.

Another source of reference is the 4344mk11 network.

This is a more advanced network with impedance compensation compensation for the 2123 and hf driver.

Without measurements it's best to try removing the resister and give it a try.

If you wanted to look deeper I have Leap and I think Robert and 4343b does as well.

Hope this helps.
Imac



Hi POS,

Owing to quite low driver nominal impedance of about 8 Ohms, and if at some lower frequency it can reach, say 30 Ohms, and if LPAD is in the center region , then the equivalent impedance that is "presented" to the network would about 10 Ohms, around the resonant frequency

4 Ohm + (8 Ohm parallel to 30 Ohms) = 4 Ohm + 6.316 Ohm = 10.3 Ohm , instead of (ideal) 8 Ohm

Fortunately there are some resistors network after Hi-pass section of the VHF driver network so such would reduce impedance
'irregularities' more.

Just for the experiment, You can put say about 40 ohms instead of 20 Ohms, just to prevent unexpected large impedance value.

regards
ivica

[pos]

Thank you Ivica, using a high value for the // resistor is a good idea :)

Ian, thanks for the proposition for the leap simulation, I might drop you a mail in the near future :D
I wonder if xsim would do the job: http://www.diyaudio.com/forums/multi-way/259865-xsim-free-crossover-designer.html

By the way, where are you at with your horn project?

[bstleve]

I wonder if xsim would do the job: http://www.diyaudio.com/forums/multi-way/259865-xsim-free-crossover-designer.html


Hi Pos,
SpeakerWorkshop could be is a good candidate too. It allows you to measure the actual driver's impedance curve, and to draw the Xover schematic.
Then you are able to simulate the Xover's response curve.
The trick is to create a "new driver" using a flat response curve, associated to the measured impedance curve.
The result is quite accurate.
Regards.
Bertrand.

[Ian Mackenzie]

Thank you Ivica, using a high value for the // resistor is a good idea :)

Ian, thanks for the proposition for the leap simulation, I might drop you a mail in the near future :D
I wonder if xsim would do the job: http://www.diyaudio.com/forums/multi-way/259865-xsim-free-crossover-designer.html

By the way, where are you at with your horn project?

I would certainly try it out.

If the impact of the alternative driver is not thay great it may only need some minor tweak.

I plan to complete my mini Everest over the holidays.

Ian

[pos]

Hi Bertrand
Will look that up, thanks.

[pos]

I plan to complete my mini Everest over the holidays.
Now that's a teasing! :bouncy: