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Robbie010

Type A replacement caps slightly off spec

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I am building DIY Type A crossovers for a clone la scala project. In place of the autoformers I will be using Lpads to attenuate the drivers. 
 

I have come across another issue in sourcing the capacitors specified (2uF & 13uF), whereby I can locally source polypropylene caps at 2.2uF and 12uF. 
 

Will this slight deviation from the specified caps be a major issue?

 

Thanks. 

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If the lpad you are using on the K-55 is designed to keep it's impedance as is (and just drop the voltage), then you will need a 26uf cap.

 

The 2.2uf tweeter cap should not be an issue.  It may measure 2.0 anyway, depending on the tolerance.

 

Mike

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6 minutes ago, mboxler said:

If the lpad you are using on the K-55 is designed to keep it's impedance as is (and just drop the voltage), then you will need a 26uf cap.

 

Mike


This is the Lpad that was revommended to me:

 

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F131570311873

 

Will this require a 13uf or 26uf cap?

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That's an 8 ohm Lpad.  It will work for tweeter, but not the squawker, which is 14 ohms (?).   

 

If you make your own 14 ohm Lpad, then a 26uf cap will be needed.  With a 16 ohm Lpad, it will be a little off, but now by much.

 

Mike

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11 minutes ago, mboxler said:

That's an 8 ohm Lpad.  It will work for tweeter, but not the squawker, which is 14 ohms (?).   

 

If you make your own 14 ohm Lpad, then a 26uf cap will be needed.  With a 16 ohm Lpad, it will be a little off, but now by much.

 

Mike


Thanks, Mike.
 

I thought the K-55 was a 16ohm driver? 
 

I was assuming I would need a pair of attenuators for each crossover, one for the tweeter and one for the squarker. 
 

Alternatively, I have found this:

 

https://willys-hifi.com/collections/l-pad-attenuators/products/l-pad-attenuator-50w-16-ohm-wirewound-volume-control


It seems that I could get an 8ohm Lpad for the tweeter and a 16ohm Lpad for the squarker?........ this is getting complicated!

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Here's a link.  The second post shows the k-55's impedance.  Varies all over the place.  Around 14 ohm @ 400 hz.

 

https://community.klipsch.com/index.php?/topic/65935-impedance-plots-for-k-55-v-and-k-77-m/

 

This load dictates the current running through the squawker circuit.  The lower the impedance, the more the current, the faster the capacitor will charge.

Using the -3db tap on an autoformer doubles the impedance of the squawker circuit.  A 13 uf cap is used.

Using a -3db lpad designed for the driver will not change the impedance.  Current will double.  The cap size needs to be doubled so it will charge/discharge at the same rate.  A 26uf cap is needed.

 

Hope that helps.  Yes, you will need a separate lpad for each driver. The 16 ohm lpad should be fine for the squawker.  

 

Mike

 

 

k55mimp.pdf

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2 minutes ago, Robbie010 said:

Thanks Mike, I’ve done a quick sketch, does this look right?
 

D2348A2D-2BD7-4491-87DB-E9E3205E5531.jpeg.eb3d51d1bb83a1307cfa85772de30e32.jpeg

Tweeter Lpad must be next to the K-77, just as the squawker Lpad is next to the K-55.

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17 minutes ago, mboxler said:

Tweeter Lpad must be next to the K-77, just as the squawker Lpad is next to the K-55.


So the 2uf cap should be before the L-pad, not after it?

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Yep!  Think of it this way...the Lpad is now part of the driver.  

 

Let's say you want to reduce the voltage to the K-77 by 3db.  That would require a 19.4 ohm resistor in parallel with the 8 ohm K-77.  Those two resistances in parallel equal 5.7 ohms.  Now put a 2.3 ohm resistance in series with that, and you are right back to 8 ohms.  The capacitor sees an 8 ohm load.

 

Mike

 

 

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8 minutes ago, mboxler said:

Yep!  Think of it this way...the Lpad is now part of the driver.  

 

Let's say you want to reduce the voltage to the K-77 by 3db.  That would require a 19.4 ohm resistor in parallel with the 8 ohm K-77.  Those two resistances in parallel equal 5.7 ohms.  Now put a 2.3 ohm resistance in series with that, and you are right back to 8 ohms.  The capacitor sees an 8 ohm load.

 

Mike

 

 


Mike, you are a star!
 

Thank you for taking the time to help me out with this, it is much appreciated!

 

I’m sure I’ll be back with other questions down the line. 👍🏻

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1 hour ago, mboxler said:

Yep!  Think of it this way...the Lpad is now part of the driver.  

 

Let's say you want to reduce the voltage to the K-77 by 3db.  That would require a 19.4 ohm resistor in parallel with the 8 ohm K-77.  Those two resistances in parallel equal 5.7 ohms.  Now put a 2.3 ohm resistance in series with that, and you are right back to 8 ohms.  The capacitor sees an 8 ohm load.

 

Mike

 

 


Sorry, Mike..... back with another query already.
 

A friend has just had a quick look over my diagram and commented that on the original schematic, the 13uf cap appears to be in the signal line to both the squarker and the tweeter, is this correct? Does the original spec show 13uf in-line to the squarker and 15uf to the tweeter or is he reading it wrong?
 

 

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Capacitors in series don't add.  They act like resistors in parallel.  A 13uf cap in series with a 2uf cap looks like a 1.73uf cap in series with the tweeter.

 

I've never been able to calculate, though, how the autoformer changes that calculation, if it does at all.

 

I'd stick with the 2uf cap. It will be fine.

 

Mike

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All that info may be correct but I think it's confusing stated that way.

 

Capacitors act as variable resistors based on frequency vs. capacitance value.  You want a value that has the same impedance as the driver (or driver plus resistor/l-pad, or transformer/driver) at the frequency you want the crossover to occur.  They will share the load equally there.  That means half the voltage each.

 

There'd be nothing wrong with having the l-pad before the cap in terms of crossover function except that the l-pad would be wasting energy over the entire audio spectrum instead of only that in the range of the driver.

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1 hour ago, glens said:

Capacitors act as variable resistors based on frequency vs. capacitance value.  You want a value that has the same impedance as the driver (or driver plus resistor/l-pad, or transformer/driver) at the frequency you want the crossover to occur.  They will share the load equally there.  That means half the voltage each.

 

 

I agree, except for the "half voltage each".  The impedance of the 2uf cap will be 8 ohms at around 10 khz, the crossover point.  Isn't it true that at that frequency, given a 2.83 volt signal,  the voltage drop across the capacitor and the driver will be 2 volts each?  Not half of 2.83, or 1.414  volts each.

 

1 hour ago, glens said:

There'd be nothing wrong with having the l-pad before the cap in terms of crossover function except that the l-pad would be wasting energy over the entire audio spectrum instead of only that in the range of the driver.

 

I always knew that the order of series components didn't matter, but you are right!  The parallel placement doesn't really matter either.  Looks like the impedance of the circuit gets wacky, but the ending voltage across the driver stays the same.

 

Thanks!  I learn something new every day.

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4 hours ago, mboxler said:

Isn't it true that at that frequency, given a 2.83 volt signal,  the voltage drop across the capacitor and the driver will be 2 volts each? 

 

Think about what you just stated...

 

2.83 minus 2 equals 0.83, minus another 2 ("2 volts each") equals -1.17!  Pretty hard to get into a hole ;^)

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Be careful to include the impedance impacts of the auto-former on the circuit.

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Yeah.  After all these years, including some comprehensively testing industrial transformers (would fit inside a two-car garage but for the height), I recently was made aware of what they do to impedance, as seen.  But as an excuse, I was never really a fan of Klipsch (Pro JBL!), and hadn't thus encountered autoformers in a loudspeaker...  And my first pair of Klipsch (Forte III) doesn't have them anyway.

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Thanks Gents, however, this thread is now kind of defunct, thanks to a very kind offer from @mboxler in donating a spare set of autoformers to the project! 🤩

 

This will make the crossovers much easier to build using the original schematic, I just need to obtain the correct value 13uf and 2uf caps, which may be easier said than done over here in the Uk.

 

👍

 

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