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View Full Version : Changing JBL SR4702X over component values to suit 16 ohm HF diaphram



robmuso
02-06-2017, 11:34 AM
Hi everyone,

If any JBL tech's read this, I would be very grateful for some help and advice.

www.jblproservice.com/pdf/SR-X%20Series/SR4702X.pdf (http://www.jblproservice.com/pdf/SR-X%20Series/SR4702X.pdf)

I am building this JBL xover network circuit for the following components in a monitor speaker.

LF Driver = 2206H
HF Driver = 2426J (16 ohm)
Horn Flare = 2305 Perforated Horn Lens (Known as the Potato Masher).

It's not the JBL OASR flare but it should be ok.

I already have the LF circuit sorted ok.

The only problem I have, is I will need to change the Xover component values to suit the 16 ohm HF driver instead of the recommended 8 ohm driver.

In the spec sheet above, the 8 ohm HF circuit shows a 2.2 uf cap to the .4 mH inductor
From that point, there are 2 caps in parrellel totaling 29 uf to the HF driver.
Also across the driver is a 12 uf cap through a 7.5 ohm resistor.

I believe the approach when using a 16 ohm driver, would be to halve the value of the caps and double the inductance of the inductor.

I'm thinking that the following values would apply.

The 16 ohm HF circuit would show a 1.1 uf cap to a .8 mH inductor
From that point, there would be a 14.5 uf cap to the HF driver.
Also across the driver would be a 6 uf cap through a 15 ohm resistor.

The spec sheet shows that the Xover point is 1200 Hz.

Could anyone please tell me if the acoustic output would be the same with these changes to the circuit?

Could anyone also please let me know if these component values would be the correct changes to make?

Please help ???

Thank you

Kind regards,

robmuso

Earl K
02-06-2017, 03:26 PM
I'm thinking that the following values would apply.

The 16 ohm HF circuit would show a 1.1 uf cap to a .8 mH inductor
From that point, there would be a 14.5 uf cap to the HF driver.
Also across the driver would be a 6 uf cap through a 15 ohm resistor.

The spec sheet shows that the Xover point is 1200 Hz.

Could anyone please tell me if the acoustic output would be the same with these changes to the circuit?

Could anyone also please let me know if these component values would be the correct changes to make?

Please help ???

Hi Rob,

You've correctly deduced the ( new ) LC values from the old .

But no, the acoustic output won't be the same due to your using a diaphragm with a higher impedance ( than the original design ).

Your derived LC values will duplicate the OEM slope & shape / but at a reduced level .

And ( to further complicate matters ) there's no padding ( in the form of resistors ) to re-adjust that HF output back upwards ( there's no Lpad because all the db loss is accomplished by the LC values ) .

One piece of good news is that most of this db loss ( in the HF leg of this network ) is determined by the very first capacitor ( ie; 2.2uF in the SR schematic ).
- You can double it's size ( even triple it ) to effectively turn up the complete horn/driver combo.
- The (following) stock coil & much larger cap, ultimately determine the shape of the "knee" around the crossover point .
- You may need to play with the coil size a bit ( varying it between .56 & .8mH to get the flattest "knee" .

One way or another, be prepared to play with the LC values until it sounds correct ( assuming you can't measure things ).

Remember, this SR circuit that you're using wasn't meant for your 2305 ( Potato-Masher ) or for the higher impedance driver that you've opted to use ( so, some component fudging is required ).

:)

PS; I'd first try; 3.3 uF, with .62mH, then 18uF, followed by the 15R-6uF combo.

See my pic ( Right-Click to open to Full-Size ).
- Remember ( when playing with caps ), you can directly add the cap values ( in parallel ) to make up any value.

http://www.audioheritage.org/photopost/data//500/JBL2427J_on_152iClone_3-Pole-Simple.PNG

robmuso
02-07-2017, 02:54 PM
Thank you so much Earl.

Before I started looking at the SR circuit, I had tried a 2.2 uf cap to a .8 mH inductor with the 16 ohm HF driver (on the 2305 flare) connected across the inductor, and it sounded a pretty good balance between HF & LF drivers.

This was basically similar to the JBL G series G-791 HF pack, so if I don't succeed further, I can at least settle for that.

I will now try the circuit values that you have suggested, and build the SR circuit and see if that works better and keep you informed of the outcome.

Your advice and help is very much appreciated and I wish to thank you for your time.

I am a JBL freak. Love the vintage components in particular.

Been a working muso for 45 years and strive for the best possible sound.

Thanks again mate.

Kind regards,

Rob






Hi Rob,

You've correctly deduced the ( new ) LC values from the old .

But no, the acoustic output won't be the same due to your using a diaphragm with a higher impedance ( than the original design ).

Your derived LC values will duplicate the OEM slope & shape / but at a reduced level .

And ( to further complicate matters ) there's no padding ( in the form of resistors ) to re-adjust that HF output back upwards ( there's no Lpad because all the db loss is accomplished by the LC values ) .

One piece of good news is that most of this db loss ( in the HF leg of this network ) is determined by the very first capacitor ( ie; 2.2uF in the SR schematic ).
- You can double it's size ( even triple it ) to effectively turn up the complete horn/driver combo.
- The (following) stock coil & much larger cap, ultimately determine the shape of the "knee" around the crossover point .
- You may need to play with the coil size a bit ( varying it between .56 & .8mH to get the flattest "knee" .

One way or another, be prepared to play with the LC values until it sounds correct ( assuming you can't measure things ).

Remember, this SR circuit that you're using wasn't meant for your 2305 ( Potato-Masher ) or for the higher impedance driver that you've opted to use ( so, some component fudging is required ).

:)

PS; I'd first try; 3.3 uF, with .62mH, then 18uF, followed by the 15R-6uF combo.

See my pic ( Right-Click to open to Full-Size ).
- Remember ( when playing with caps ), you can directly add the cap values ( in parallel ) to make up any value.

http://www.audioheritage.org/photopost/data//500/JBL2427J_on_152iClone_3-Pole-Simple.PNG