RC crossover

[koushik]

We are all familiar with the concept of using inductor in the crossover circuit but we can also make filter circuit using only resistors and capacitors.

But I have never seen them in use anywhere.

The resistivity of capacitance to AC decreases with increase in frequency. So, a capacitance in series with tweeter to act as high-pass filter is commonly used but low-pass with capacitance in parallel with the woofer, with a resistor in series, is not often seen.

Any reason? Can we discuss the pros and cons? Removing inductor from circuit would lower the price of a crossover considerably.

Looking forward to valuable inputs from experts.

 

[pranavkumar]

Koushik,

The reactance (AC Resistance) of a cap decreases with increase in frequency, a series cap with a tweeter makes a FIRST order high pass filter.
A series inductor and a parallel capacitor across a woofer make a SECOND order low pass filter.

The main reason why we don't use resistors are because they dissipate a lot of power. out of resistors, caps and inductors... ideally speaking.. only resistors dissipate energy as heat. We dont want our amplified power(music) from the amp to go waste, hence we dont use resistors.

Technically speaking, all components have a series resistance associated with it, you might have heard the term : ESR (Effective Series resistance). While modelling the crossover networks, we have to take into account these ESRs of both inductors and capacitors, leading us to a Signal Response which is similar to a filter using (R C components) - Laplace Transform (if you know what i mean).

You'd commonly find RC filters in Active Crossovers, its because the inductors being bulky are not incorporated, and also the fact that the signal strength is not very large at the stage where the active crossover is situated, hence way lesser heat loss.

 

[koushik]

Thanks for your post.

To cross a 3KHz low-pass we need some 1 ohm in series with 63uF in parallel. I think 1 ohm is way too little to think about heat dissipation, or am I wrong?

On the other hand, that means, soundwise it all good, just the heat and loss of amplified energy? Is that so?

 

[pranavkumar]

well, any resistance is not worth putting in, unless it is specifically meant for attenuating the signal (in an L-Pad).
even if you put a 1ohm resistor, it will dissipate heat. the dissipation is not dependent on the resistor value as such as on the current through it and the voltage across it. (P = V.I).

yes, sound wise, it is all good... nothing better than a linear device (resistor).

 

[sashijoseph]

The effect on the amplifier's Damping Factor must also be taken into consideration.
A series resistor will spoil our pricey amp's lovely DF figures which we paid for...
OK even if the utility of DF is overhyped,still we should take care not to swamp it to below 2-digits.

Another consideration is the effect on Qts of the driver.The series resistance will raise the Qts of the driver and this will in turn affect the Qtc of the enclosure.So this interaction must also be accomodated for.

And finally for higher order xovers we have more series resistors,which means criminal energy wastage,apart from the above mentioned factors...

 

[koushik]

Thanks Sashijoseph.

Can't we optimise the value of series resistor keeping Qts and DF under control?

So can we do it this way? Put the same value capacitor (say 6.7uF for 3 kHz) in series with tweeter and parallel with woofer? Will this tax amp bit too much at woofer end?

 

[sashijoseph]

So can we do it this way? Put the same value capacitor (say 6.7uF for 3 kHz) in series with tweeter and parallel with woofer? Will this tax amp bit too much at woofer end?

See the filter is realized using a resistor and capacitor arranged to form a "L" section....ie. the resistor and capacitor are in series.
For the tweeter high-pass,the cap is in series with the tweeter,whose nominal impedance we conveniently use as the value of R,for calculating the cutoff.

But if you parallel the cap with the woofer,you don't have a filter unless you insert a series resistor and use the value of this resistor in your cutoff calculation.So 6.7uF will not work as the lowpass value.
The series element in this case is the sum of speaker cable resistance and the output impedance of the amp.This will have to be used to calculate the filter's -3dB!!!
But imagine having such dynamic values as part of your filter's calculation ...highly undesirable. :sad:

 

[kanwar]

Koushik,

Why resistors are not used in passive speaker Xovers especially in LF , see the points below:

1. They ruin the efficiency and power available for the driver from the amplifier.

2. They dissipate enough heat sometimes difficult to manage even when the resistor is 1 ohm only, connecting it between an amplifier say 100W and driver of 8 ohm impedance, it will dissipate around 14W worst case heat. Which is alot.

3. Every passive XO will ruin the damping factor whether you use resistor or inductor in series with driver. The back EMF generated in the voice coil needs a very low impedance path to get nullified. Having a resistance or inductor in series with it makes the DF poor and it is responsible for the coloration of sound associated with passive XO in the chain.

4. The resistance will cause a massive voltage drop also because the voice coil of driver has a finite RDC also, a typical 8ohm driver has 5-6 ohm RDC, which means you loose considerable amount of dynamic range also. This happens with inductors also but much less.



I hope this helps,

Kanwar

 

[koushik]

Thanks Sashijoseph and Kanwar. I understand your points and they making perfect sense to me.
So, what if I put a very small value resistor in series which will be equal to the resistance of the inductor? The heat dissipation should be same this time and we can't avoid that even using a inductor.

 

[kanwar]

DCR of LF inductor is in milli ohms range, which means adding a resistor with such a low ohmic value will serve no purpose.

 

[koushik]

Thanks Kanwar.