Audiophile active crossover

My amplifiers are near complete. I am using Dynaudio D28/2 and Dynaudio MW160. Crossover frequency around 2k. Currently I am using a Phoenix gold MX3i automotive 3 way crossover. I have been told that it is a good quality unit and is a candidate for upgrades. To me it sounds very good but I can not compare it with anything. I am contemplating some 2 way crossover PCBs to mount in my enclosures. I seen them on ebay and am unsure of the quality. I will also have to find another place to separate my subwoofer. I could buy a rackmount type of crossover but am unsure of which to choose. Many of them are music store units. In tems of sound quality, I have always been left wanting from any playback item that has come from a music store. I have also considered scrapping any resale value of my phoenix gold and mounting it in a rack with a nice clean 12Volts, audio grade caps and good quality ganged pots instead of independent left and right level controls. I am appreciative of any suggestions.
Thank you.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
Hmmm ... and yet Doug Self (who is certainly an accomplished and respected designer) thinks they are more than adequate to design a preamp, with phone stage, using them. Perhaps if there is a problem with that unit, it isn't the fault of the op-amps?

As for listening, many have been in disagreement with respected designers.

As for opamp fault, in active crossover, the opamp is not only one or two, but many, and it matters.

As for the unit, the circuit used in the said machine is not better than ESP active crossover. At Linkwitz site if I'm not mistaken, it was mentioned that without a delay network an active crossover is pretty unusable.

If there is a reference opamp-based active crossover, I think it is the one used by Linkwitz' speakers. Note that the crossover has been part of certain speaker design.

I haven't heard the Orion or the newer speaker, but I think the strength of the speaker is not the opamp. The speaker should still has the opamp sound quality weaknesses but the strength is a top design that is almost impossible to be achieved with single amplifier or passive crossover.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
So then the first thing would be to swap in some OPA134?

Theoretically NE5532 is sufficient for the job. I don't think OPA134 will give quite an improvement, if any. I have many opamps and I have the crossover but I didn't do anything except using better power supply which I have on hand.

Well, sure you can improve anything. I just think that "audiophile" label doesn't fit well with this kind of circuit.
 
Hi,

The requirements for phono are very different to those in an active crossover.
Phono is one of the few areas in audio where high gain is required at the same with filtering functionality. Another would be DC-servoes. Both's amplitude response is quite similar to the openloop amplitude response of OPamps.
A active crossover On the other hand may be constructed the most from buffers utilizing a gain of just 1. You may search the web for 'unity-gain Sallen-Key' filters.
In most textbooks you'll find the typical triangular OPamp icon for the active part in the filter, sometimes a '1' added to it. You can easily replace this OPamp by a discrete buffer made as simple as from 2 transistors, either bipolar or JFET. I prefer JFETs, because of circuit simplicity and sound. The transistors would be configured as emitter- or source follower, loaded by a constant current source. Nothing complicated, nothing expensive but much more musical and true. Such a simple buffer may not be totally transparent THD wise, but the added THD is low and THD distribution is optimal. Cascoding JFETs, thereby increasing the parts number count to 4 JFETs per buffer would allow for a THD transparent circuit structure.
A second important point is to reduce the number count of active stages.
If you compare active to passive crossovers almost all active filters are just that ... filters, sheepishly following textbook ideals. Passive filters on the other hand include equalizing functionality, which is what any speaker requires.
Sometimes one can find additional EQs in active filters, but simply in form of additional circuitry. Nothing hinders You to deviate from textbook formulas and to design the filter response as required by the speaker. If for example your Midrange driver requires a slight boost at the lower bandwidth limit for a linear amplitude response, a butterworth filter with a Q of 0.7, or even worse a Linkwitz-Riley filter with a Q of 0.5, would certainly be the wrong choice. A non-standard filter with the amplitude response and Q taylored to the requirements would be alot more convenient, elegant and lower on parts number count. And the best for last .... it'd sound better too.

jauu
Calvin
 
At Linkwitz site if I'm not mistaken, it was mentioned that without a delay network an active crossover is pretty unusable.
One can not say this is a true statement for all cases. Sometimes you can do completely without delay compensation. As an example, how many passive crossovers include delay stages???

What you want is for the phase response of each of the two drivers that you are pairing with a crossover to be "tracking closely" around the crossover point. You can state the design goal as something like "the relative phase angle between the two drivers when connected to the crossover network should stay within 45 degrees whenever one driver is within 40dB of the other." There is more than one way to attain this goal.

Since filters have a (non-constant) phase response, one can use one or two extra orders of HP filter on the tweeter to better align the phase response with the mid/woofer. This approach works for both active and passive filters. The asymmetry of slopes is of no concern. As long as the phase tracks relatively well and you can get the combined amplitude around the crossover point to be relatively flat (e.g. by adjusting the Q and corner frequency of filter stages) the result should be more than satisfactory.


-Charlie
 
I like the topic of an audiophile active crossover a lot, because that is my present puzzle.

An issue that will certainly be more important than the difference between a NE5532 or an OPA2134 is a correct choice of filter slopes. It is too easily assumed that with an active crossover, driver-in-enclosure parameters drop out of the equation. Nothing less is true.


Every driver in a speaker will have its typical roll of points on the low and high ends. These natural roll offs will have the typical phase shifts of a minimal phase system. In other words, on the low end drivers normally have 12dB/octave high pass slope, and that causes 180 degrees phase rotation around the -3dB point. Now, there are only two ways to go. Either you stay well away (2 octaves) of this -3dB point, or you incorporate it into your filter design. This is where it starts. Of course you need to do measurements for this, but the capability to do so has become democratized to a high degree.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
how many passive crossovers include delay stages???

As many as good active crossover available :D Just kidding.

What you want is for the phase response of each of the two drivers that you are pairing with a crossover to be "tracking closely" around the crossover point. You can state the design goal as something like "the relative phase angle between the two drivers when connected to the crossover network should stay within 45 degrees whenever one driver is within 40dB of the other." There is more than one way to attain this goal.

Since filters have a (non-constant) phase response, one can use one or two extra orders of HP filter on the tweeter to better align the phase response with the mid/woofer. This approach works for both active and passive filters. The asymmetry of slopes is of no concern. As long as the phase tracks relatively well and you can get the combined amplitude around the crossover point to be relatively flat (e.g. by adjusting the Q and corner frequency of filter stages) the result should be more than satisfactory.

Active is not as flexible as passive crossover. Most often (like the one discussed in this thread) you only have pots that determine the roll-off frequencies. The slope order is fixed. You may have a boost at around 45Hz (often also in fixed dB). What kind of drivers blending that can be achieved with such tools?

With passive, solutions are usually integrated (there are many ways to achieve the goal). For example, you don't need to create a separated BSC circuit to compensate for baffle loss. So is with phase. Different case with active crossover. With active you have to ADD a dedicated bandpass filter to alter the phase. But you cannot cross at any frequency you want and get a perfect phase tracking.

Phase is imo the major issue with crossover design, and with active design this is a really really terrible issue.


the result should be more than satisfactory.

May be not so for audiophiles.
 
Hi

Jay, I totally disagree. Passive filters are way less flexible than active. You have to account for the electrical impedance of the driver. This limits the available range of solutions. Than follows the sourcing of parts. There's hardly an affordable high quality high power pot on the market. Passive technology also is ruled out if You require a high-Q filter. Cascaded stages influence more on each other, while the buffers in active stages isolate the stages. And last, passive filters cost on power efficiciency.
The probs You are talking of are probs related to implementions of the technology, not drawbacks of an technology in itself.
I assume you are talking about the same reasons I critizised. With passive technology probabely no one would design the filter after textbook formulas and add several EQ stages.
No, of course not, because there's no good reason to do so. If just a larger value of a series inductance allows for the baffle step correction, it' d be rightous foolish not to use it.
I don't know where it came from, that the common sense thinking with active crossovers is often put away. But there's no reason to design an active filter not just after the same design principles as an passive filter. A 'integrated solution' as You seem to have named it. I like that term, as it describes exactly the difference between typical active designs where filter functionality and EQ functionality is separated and good active as well as passive designs where filtering and EQing is combined. The separated design leads to more complex circuitry with more parts (OPamp graveyards as we call them over here), cost and effort and it seriously costs on sonic quality. I assume this design principle as the most prominent reason that active systems are rather a niche technology, since it kills most of the possible sonic advantages of active technology. If active doesn't sound better than passive then the designer hasn't done his homework right.

jauu
Calvin
 
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Calvin, fully agree, you can only design an active filter well if you know exactly the drivers and the enclosure you are designing it for. In that sense, off the shelve solutions don't work, just like textbook passive filters usually don't work. It may be easier for some to tweek a passive filter, but it is not that hard using active technology. And, you can just do more actively, at lower cost.
 
There is far more here than I understand. I am a musician that is unsatisfied with the cosumer/pro equipment that is available to me and I am a new electronics student. As much as what I have currently is far better than what I have EVER purchased. Responses to this forum have made it clear that I can get much more out of my speakers. I am using a dynaudio D28/2 with Dynaudio MW160 in a Foccus enclosure. Is anybody willing to deign a crossover for me? It is beyond my knowledge and ability at this time.
 

Jay

Banned
2003-02-11 9:02 am
Jakarta
Jay, I understand the point, but there has always already been a series of opamps in the signal path before it gets to the consumer. Fortunately, they are very good.

That is a very common argumentation. But I think there is a flaw in the logic.

When a music is recorded with many opamps (during mixing), it doesn't mean that there will be no problem when the music is reproduced with opamps for the second time.

What can be perceived during listening (e.g. fatigue), not all can be captured in a recorded material.

MP3, FM radio signal, all are far from perfect. But they will sound different through an opamp. Because the problem is not only in the recorded material or the source, but in the reproduction of the material.
 
Hi,

It is just that not everyone can accept a series of opamp in the signal path.
Right ;) And that´s why we can easily replace OPamps by simple discrete source followers (Emitter-, cathode-followers) in unity gain SallenKey filters which allow for music not just replay :D

Put all things together active is not necessarily cheaper, but rather costier, since it means more effort regarding power supply number count, numbers of amplifiers etc. It is just that in tendency there are less boutique parts around which often cost incredibly much more, but still can´t keep up with the small sized lowvoltage devices required for active technology.

jauu
Calvin
 
somebody forgot about the increased dynamics of active vs passive..
and not having the parasitic losses of all the passive components between eh amps and drivers..

now since my DIDDEN modified dcx2469 sux so bad.. I'm going back to designing a 4 way analog x/o to mimic the digital.. op amp or discrete.. dunno

probably gonna be a mix and depends on the final filter circuits ..
i'll take active any day over passive. except for cost of all those amplifiers..lol