"Damping"resistors in X-overs. What do they do?

[Jonathan Bright]

In some passive X-overs there are resistors in parallel with some of the L&C components. I think I've seen them described as being there to prevent "ringing". What do they do? Is there a way for the DIYer to determine when they are required and how do you calculate appropriate values? Jonathan bright

[Svante]

Possbly, the designer has at some stage seen a bump or a dip or something in the frequency response, and figured out that it can be fixed by adding that extra L, R or C. Crossover design i built on basic theory (dBs per octave, butterworth/linkwitz, baffle step compensation etc) but is also a commonly used to fine-tune the frequency response. I suppose that this is what you have seen. It is hard to give general recommendations regrding those extra components, the best way of understanding them is to understand passive electric circuit design.
IMO any explanation that uses time-domain wordings, like "step response" and "ringing" should be viewed with the sceptic eye. I'm not saying they are wrong, but most often they don't help much but rather cause confusion and/or wrong conclusions. This does not apply only to crossovers.

[sreten]

They are often included for subtle adjusts to phase and
levels in the crossover region to help with subtle driver
integration issues.

They are not needed to prevent ringing or facilitate
a decent step response, this is mainly controlled by
the Q and order of the crossover alignment.

Is there a way for the DIYer to determine when they are required and how do you calculate appropriate values?

Not easily. You need good measurements and good modelling
software, and enough skill to only use extra resistors when
they are definetely needed.

sreten.

[Fred Dieckmann]

"Damping"resistors in X-overs. What do they do?"

I'll be damped if I know...........

[HeatMiser]

Actually, "ringing" is also a not-uncommon term to describe the oscillation that can occur in LC circuits, particularly in high-order crossovers comprising multiple inductors and capacitors. And a well-placed resistor could certainly be used to shift a troublesome resonant frequency out of band, or otherwise tame a problematic oscillation.

I believe this issue is also one reason why people sometimes substitute multiple small capacitors for a single large one.

Of course it's difficult to tell if that's what the designer intended without an actual example, but theory certainly supports this as a reasonable explanation.

[sreten]

Originally posted by HeatMiser
Actually, "ringing" is also a not-uncommon term to describe the oscillation that can occur in LC circuits, particularly in high-order crossovers comprising multiple inductors and capacitors. And a well-placed resistor could certainly be used to shift a troublesome resonant frequency out of band, or otherwise tame a problematic oscillation.

I believe this issue is also one reason why people sometimes substitute multiple small capacitors for a single large one.

Of course it's difficult to tell if that's what the designer intended without an actual example, but theory certainly supports this as a reasonable explanation.

Adding resistors to a badly designed filter is not the way to do it.

The Q (i.e. ringing and step response) of higher order LC circuits
can be completely controlled by correct component values.

Multiple capacitors will not reduce Q, due
to lower losses if anything Q will increase.

There is one case where damping resistors are required :
If you want a response between first and second order.

sreten.

[Svante]

Originally posted by HeatMiser
Actually, "ringing" is also a not-uncommon term to describe the oscillation that can occur in LC circuits, particularly in high-order crossovers comprising multiple inductors and capacitors. And a well-placed resistor could certainly be used to shift a troublesome resonant frequency out of band, or otherwise tame a problematic oscillation.

...and ruin the frequency response. This is about what I mean by saying that time domain approaches can lead to the wrong conclusions. Also, a resonance frequency cannot be changed by adding resistors. You'd have to change the L or C to do that.

Originally posted by sreten


There is one case where damping resistors are required :
If you want a response between first and second order.

sreten.

Would you have an example of an 1.5th order filter?

[sreten]

Originally posted by Svante
Would you have an example of an 1.5th order filter?

Not a good example really, as the astute will realise a low
Q 2nd order is the more sensible way to approach this.

http://www.users.nac.net/markowitzgd...tcrossover.htm

http://www.users.nac.net/markowitzgd/david/watt-x-o.gif

The CB17RC c/o is basically 2nd order but asymptotes to
first order at high frequencies, why ? who knows ?

But Thorsten is simply copying the original c/o.

sreten.

[HeatMiser]

Originally posted by sreten


Adding resistors to a badly designed filter is not the way to do it.

The Q (i.e. ringing and step response) of higher order LC circuits
can be completely controlled by correct component values.

Multiple capacitors will not reduce Q, due
to lower losses if anything Q will increase.

There is one case where damping resistors are required :
If you want a response between first and second order.

sreten.

I cannot argue with your statements, however I was merely pointing out what the term "ringing" strongly suggests (a time domain problem, an oscillation) and how a resistor might be used to combat this. I'll leave it to you to seek out the designers of these unseen circuits and tell them they are doing it all wrong.

[sreten]

Really should also note that for bass / midrange units any
resistor of in the area of 3R to 8R in parallel with a series
inductor is for Baffle Step correction.

And in crossovers resistors can be used for correcting the
overall response of the driver, and of course be used for
setting the level of the midrange and / or tweeter.

sreten.