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Question regarding shunt cap in x-over network

J

Johnny7

Member
I'm hoping someone can definitively answer this question.

Given a low pass filter, with a single shunt capacitor, what effect does lowering the cap's uF rating have on the frequencies passed.

Consider the attached schematic. 34uF PP film capacitors are virtually unavailable, yet that is what the speaker was designed with.

What is the effect of using a (commonly-found) 33uF cap?

In other words, are more or fewer high frequencies sent to the tweeter?

No need to comment on whether or not it's audible, or significant.

Thanks in advance to any and all who choose to chime in.

polkXover1.jpg
 
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Not sure on the exact change it would make.

But, I would run an 18uf and a 16uf in parallel to get 34uf of capacitance there.
 
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Typical film capacitor has a tolerance of +/- 10%. Wouldn't sweat the 1uF.

BTW, @z-adamson's suggestion is a good one. Parallel caps will have lower ESR. Within the same brand/series, the combined footprint will likely be smaller and cost less.
 
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The lower a capacitance value is, the higher the frequency allowed to pass and vice versa. That is, the impedance is indirectly related to the capacitance value at a specific frequency.

Xc = -1/ωC where Xc is reactance, ω is angular frequency, and C is capacitance See Electrical Reactance

For example, for a first order high pass filter such as used with a tweeter, lowering the the capacitance value raises the high pass crossover frequency for the tweeter.

In your crossover, the 34 uF cap is part of a second order low pass filter. As the frequency increases past a certain point, the signal is shunted around the woofer by the capacitor. Lowering the capacitance value in a such a low pass filter means that the capacitor will begin shunting at a higher frequency. That results in a higher crossover frequency for the woofer, which means the woofer will operate at higher frequencies than with a higher capacitance value.

Johnny7 said:
What is the effect of using a (commonly-found) 33uF cap?

In other words, are more or fewer high frequencies sent to the tweeter?

View attachment 2471739
Click to expand...

Changing the value of the capacitor and/or the inductor in the low pass filter will have NO affect on the tweeter. The high pass filter operates independently of the low pass filter for a typical crossover.
 
Johnny7 said:
I'm hoping someone can definitively answer this question.

Given a low pass filter, with a single shunt capacitor, what effect does lowering the cap's uF rating have on the frequencies passed.

Consider the attached schematic. 34uF PP film capacitors are virtually unavailable, yet that is what the speaker was designed with.

What is the effect of using a (commonly-found) 33uF cap?

In other words, are more or fewer high frequencies sent to the tweeter?

No need to comment on whether or not it's audible, or significant.

Thanks in advance to any and all who choose to chime in.

View attachment 2471739
Click to expand...
Here's your 34uF, https://www.soniccraft.com/product_...-4638?osCsid=1c210722e59f9e0d14bd6bedca20a37b
 
RTally said:
The lower a capacitance value is, the higher the frequency allowed to pass and vice versa. That is, the impedance is indirectly related to the capacitance value at a specific frequency.
Click to expand...

RTally -- Thank you for that succinct response -- perfect!

F1nut -- Thanks for the suggestion. I am fully aware of the Sonic Craft offerings, but would have to have my head examined to shell out nearly $175.00 for two capacitors. There are many far less costly 33uF caps which put those to shame.
Once you do the arithmetic and come up with the possible tolerance ranges for a 33 vs a 34, the choice is simple.
 
Johnny7 said:
RTally -- Thank you for that succinct response -- perfect!

F1nut -- Thanks for the suggestion. I am fully aware of the Sonic Craft offerings, but would have to have my head examined to shell out nearly $175.00 for two capacitors. There are many far less costly 33uF caps which put those to shame.
Once you do the arithmetic and come up with the possible tolerance ranges for a 33 vs a 34, the choice is simple.
Click to expand...
Please list the brands you consider better than Sonicap for less money.
 
F1nut said:
Please list the brands you consider better than Sonicap for less money.
Click to expand...

Here is a handful.

ClarityCap Capacitor 33uF 250Vdc CSA Series
Mundorf Capacitor 33uF 250Vdc MCap Classic (MKP)
Solen Fast Metallized Polypropylene Capacitor, 33uF – 630V
Jantzen Audio 33uF 400V Z-Standard Capacitor
Jantzen Audio 33uF 400V Crosscap Capacitor

There are, of course, others.
Bear in mind, this is a shunt capacitor -- IMHO, high-end caps are wasted here.
 
Johnny7 said:
Here is a handful.

ClarityCap Capacitor 33uF 250Vdc CSA Series
Mundorf Capacitor 33uF 250Vdc MCap Classic (MKP)
Solen Fast Metallized Polypropylene Capacitor, 33uF – 630V
Jantzen Audio 33uF 400V Z-Standard Capacitor
Jantzen Audio 33uF 400V Crosscap Capacitor

There are, of course, others.
Bear in mind, this is a shunt capacitor -- IMHO, high-end caps are wasted here.
Click to expand...
Having experience with those caps, none...not a single one puts the Sonicap to shame. In fact, just the opposite.

You don't think shunt caps matter, fine. My experience says they do.
 
While I agree that the shunt components in a crossover matter one has to consider if and when premium components are called for. In my a-hole opinion Sonicaps are overkill for this situation. Would these drivers be able to resolve the difference?
 
Maybe, maybe not. That schematic is for one of the vintage Polk Monitor series speakers. Most of the folks at the Polk forum would stick a 34uF Sonicap in there. In fact, Sonicap makes the 34uF specifically because so many of the vintage Polk speakers use it.
 
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So, essentially Polk originally used electrolytics to save money? Have we seen this movie before?
 
Guido57 said:
So, essentially Polk originally used electrolytics to save money? Have we seen this movie before?
Click to expand...
It wasn't about saving money as much as back then there wasn't much else other than mylar (which Polk used in the high frequency circuit) in a quantity large enough to meet the production requirements.

Even today most speakers use mylar and electrolytics.

Edit: without looking up the schematic the OP posted I believe it is for the Monitor 5jr, which was a budget model, hence the electrolytic in the high frequency circuit.
 
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Johnny7 said:
RTally -- Thank you for that succinct response -- perfect!

F1nut -- Thanks for the suggestion. I am fully aware of the Sonic Craft offerings, but would have to have my head examined to shell out nearly $175.00 for two capacitors. There are many far less costly 33uF caps which put those to shame.
Once you do the arithmetic and come up with the possible tolerance ranges for a 33 vs a 34, the choice is simple.
Click to expand...


Add in the fact, you are talking about a mid level basic 2 driver 35/40 year old Polk speaker....(I love many of the older Polks from this era,) but........I would find it hard to believe both speakers themselves are even worth that amount:)
 
F1nut said:
Edit: without looking up the schematic the OP posted I believe it is for the Monitor 5jr, which was a budget model, hence the electrolytic in the high frequency circuit.
Click to expand...

Close enough - it's actually from the 5B - I "removed" the polyswitch for the sake of simplicity, since the question was specifically about the effect of changing the value of the shunt capacitor in a low pass filter.
It was chosen at random as an example of a simple 2nd order crossover - there is no actual speaker behind the question.
 
If one is concerned that the cost of improving a pair of speakers will be greater than the value of the speakers you are missing the point.
 
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