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La Scala crossover


HiFifans

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6 hours ago, Don Richard said:

I don't believe the LaScala ever had the type A crossover, they used the AA crossover.

My pair of 1967 Lascalas have the type A crossovers.  I think the AA was used starting around 1972.

Bob Crites

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"Better" is a dangerous adjective.  Crossovers integrate the bandwidths of the specific drivers/horns.  The evolution of the A series accompanied the changes in the midrange driver.  The crossover was essentially trying "to keep up".

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Dear all,

Can anyone help me to confirm that the following diagram is a correct circuit diagram for Type A crossover which can be used in La Scala speaker ?

I am not living in USA. I cannot response to you on line. Sorry.

Thank you very much for your help.

La Scala Type A.jpg

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Hello WMcD and filmofreddy,

Thank you very much for your reply.

The inductor value is 2.5mH as shown in the above circuit. How about the current rating for such inductor ? How many mA or A ?

I am going to DIY a crossover for type A.

I got the specification for T2A autoformer as shown. Is it correct ?

If 50Hz or 60Hz Alternative Current (AC) is applied to pins 0-5 with pin 0 as ground, Vout is still measured at 10Vac ?

Thank you.

 

464280104_KlipschT2a.jpg.b9ddbbebab2969355c0bfb5b4683545f.jpg

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I would not apply 50 or 60 Hz to the autotransformer.  In the design it is handling 400 to 15000 Hz.  So try maybe 1000 Hz if you're experimenting.

 

As far as I know that is the info for the T2A.

 

The table re Vout is saying that if 10 volts a.c. are applied between terminals 0 to 5, the outputs are had between terminal 0 and 4, 3, 2, and 1 at the listed voltages.  In the A type crossover the terminals 0 and 4 are used as the output to the midrange.  This set up steps down the voltage by a factor of 6.8 to 10 (as shown in the table).

 

We usually use dB in audio matters.  The dB "gain" re volts is calculated by 20 log (6.8 / 10) which equals -3.35, or about 3 dB loss.  This is because the mid driver and horn is about 3 dB more sensitive than the tweeter or woofer.

 

You'll see the dB loss in the table is "3" and the use of quotes shows it is not exactly 3  (actually it is 3.35 by the above calculation).  

 

Maybe the above is a bit confusing in the minus sign.  + dB is voltage, power, current increase.  - dB is voltage, power, current decrease.  Here were stepping down voltage and we use minus gain or -dB.

 

I'll think about current rating for the inductor for a while or someone can chime in.

 

WMcD

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9 hours ago, HiFifans said:

Hello WMcD,

Thank you very much for your information. I learn a lot.

I hope someone can help me about the inductor current rating. 🙏

Not sure if you want to know what the current rating of the stock 2.5mh inductor is, or how much current can pass through the inductor.  I don't know the stock inductor rating, but...

 

I believe the K-33 can handle 150 watts.  If we assume the lowest impedance is 4 ohms, and use Ohm's Law, that equates to around 24 volts and 6.1 amps.

Since the inductor is in series with the K-33, current through both will be the same...6.1 amps.

 

Perhaps over simplified, but I hope that helps.

 

Mike

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To start, I have never seen a spec for the transformer-like inductor used in the A type woofer series.  

 

Also, current rating for inductors would probably be used when they are used as a "choke" in a power supply where they are used to choke off 60 Hz in a power supply circuit.  They have to pass amounts of d.c. current on a continuous basis.

 

In crossovers we are dealing with music with no d.c.  We have a complicated mix of a.c.   It is difficult to find a consensus on the equivalent power in a music signal.  It is certainly less than a continuous sine wave.  There is a standard someplace which I don't have.  But essentially, noise which a spectral distribution of frequencies is used to test drivers and, I suppose, determine at what level they fail.  That signal was to simulate music, very roughly.

 

My favorite example is an old rating of a T-35 tweeter which is about 2 or 3 watts of sine wave continuous.  Yet the tweeter in a Forte (probably similar enough for discussion) was tested up to about 1000 watts at it high freqs as I recall, if the sine wave of that power was pulse on and off so there was time for cooling.   So, it seems a tweeter is good for 2 watts of continuous sine waves and maybe 1000 watts if the input is band limited and not continuous.

 

Looking at the autoformer we don't have specs.  The Atlas driver is rated for 40 watts "power level."  I doubt that is 40 watts continuous sine wave.  I'll guess at 15 watts.  The autoformer might have a similar rating.  If you have to put a number on it.  It is probably the same for woofers.  A "150 watt" woofer rating is probably established with this noise simulation of music.  

 

Going to the woofer inductor.  Generally the main concern is the d.c. resistance.  Again, we don't have that for what Klipsch used on A series.  OTOH there are specs to be seen on more recent crossovers.  You can take a look in the Technical / Modification section for schematics.  I put together a compilation which appears at page 11.  There is, there, a requirement for less than 0.25 ohms for some.  

 

Looking at Parts Express we see that generally, iron/steel core inductors using 18 awg windings have higher but 16 awg have lower.  The more recent crossovers use inductions of this physical form.

 

https://www.parts-express.com/erse-super-q-25mh-16-awg-500w-inductor-crossover-coil--266-908#lblProductDetails

 

Another issue is at which current the core saturates, which is bad.  I've never seen a spec on that although you'll see ratings in the 100s of watts.

 

Some people are very concerned about d.c.r.  I'm not so sure.  We have 4 or 8 ohms in the circuit and then wring hands over 0.5 vs 0.2 additional. 

 

Overall  if you want to simulate an A, I think an 18 awg is okay.  If it bothers you, you could use a 16 awg.

 

WMcD

 

 

 

 

 

 

 

 

 


 

 

 

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Hi Gil/HiFi, 

 

I'm in Amarillo and can't look it up, but I have an interview of Mr. Paul where he said the woofer inductor saturated at 63 watts.  That should give you a means to calculate the practical current limit of the woofer inductor.  The horn loaded the woofer has an impedance of about 6 ohms.    He said the 63 watts would not be an issue for anyone but the gun shot enthusiasts.  I am assuming you want the value so you can replace the inductor.  Some here say that should be done carefully because it can change the Q of the woofer and change the response of the bass horn. 

 

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Good to hear from you John.  So if Power = I*I*R, we have 3.2 amps to make 63 watts into 6 ohms.  (Neglecting whatever resistance is in the inductor.) 

63 watts is 18 dB above 1 watt at which a K-Horn is rated for 105 dB.  So we would have an output of 123 dB.   The spec sheet says that max output is 120 dB.  Overall, It makes sense.  (I hope my math is correct.  Back when PWK made the statement the rated sensitivity was probably 104 dB. )

 

John, I look forward to seeing the interview if it is convenient for you to post it.  I recall reading one where PWK said he ran K-Horns with the inductor shorted out and "its worth a couple of dB."  (Wish I could find it now.)   But this means that the boss's favorite  inductor for an A in a K-Horn is no inductor at all.  I wonder if that goes for the LaScala too.  Smile.

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