Realizing Active Crossover Networks (XO) without additional Transistors or OP-Amps

[Andersonix]

So a very elegant way to do this is a TL based bass unit driven full range by amp 1. And a full range top driven amp 2 fitted with a limited bandwidth (high pass cap coupled SE Class A) for the top. Match the frequency by trimming the cap and adjusting the levels.

I'm waiting for you to do this with Nelson's new ACA Mini class A amp for driving a tweeter above 2.5 - 3kHz.

 

[tiefbassuebertr]

Many years ago I tried this idea with a LM3886.

Results was an amplifier that was oscillating very nicely.
I would expect that any other kind of amplifier would have the same issues.

I kind of missing the point what is against an actual proper active filter?
Since it has more advantages than disadvantages.

Oscillating is a general issue at all integrated and discrete power op-amps. Reasons are manifold, but you are right, because if there are additional networks in the NFB (i. e. additional equalizer/filter parts except two resistors), the risk increases significantly (I have observed this on integrated amps while reducing the treble on a Baxandall tone control in NFB network to minimum, because the NFB resistor from the output gets smaller. Such observations occurs in general, if both line and power amplifier stage isn't suited for unity gain mode.

As long as you make sure the filter stage performs one or two orders of magnitude better than the power amplifier, it can be seen as non-existing from a signal point of view.
Which isn't so difficult to achieve anymore these days.

This is true in most cases, especially regarding the bass and midgange frequency area. But if you have a single ended power stage for a good dome or horn tweeter and you add an additional gain stage or Sallen-Key filter stage, this is definitely not true. Sonic quality suffers always. This was the reason for my mentioned approach under
ZEN include active crossover without additional OP AMP for ultimate sounding PHL1230
The only chance to use additional filter stages without audible disadvantages in sonic results could be the conversion of the topology from Sallen-Key (mostly used multiple PFB) to MFB (multiple NFB).
The use of already present input capacitor is also not the best solution due only 6db slope (I want to have 18db).

 

[b_force]

Oscillating is a general issue at all integrated and discrete power op-amps. Reasons are manifold, but you are right, because if there are additional networks in the NFB (i. e. additional equalizer/filter parts except two resistors), the risk increases significantly (I have observed this on integrated amps while reducing the treble on a Baxandall tone control in NFB network to minimum, because the NFB resistor from the output gets smaller. Such observations occurs in general, if both line and power amplifier stage isn't suited for unity gain mode.


This is true in most cases, especially regarding the bass and midgange frequency area. But if you have a single ended power stage for a good dome or horn tweeter and you add an additional gain stage or Sallen-Key filter stage, this is definitely not true. Sonic quality suffers always. This was the reason for my mentioned approach under
ZEN include active crossover without additional OP AMP for ultimate sounding PHL1230
The only chance to use additional filter stages without audible disadvantages in sonic results could be the conversion of the topology from Sallen-Key (mostly used multiple PFB) to MFB (multiple NFB).
The use of already present input capacitor is also not the best solution due only 6db slope (I want to have 18db).

The audibility of "sonic quality" is debatable at the very best, while the audibility of a poor frequency response and directivity is MANY orders of magnitude more significant (see Floyd Toole et all).
This can be even (easily) picked up by the lesser trained listener.

Since a crossover is always needed to get a proper frequency response as well as a good directivity, there is no way around it.

I find it very strange to choose something to avoid that debatable sonic difference over a very significant audible factor.

One needs at least one highpass filter to begin with anyway.
Although that's the very bare minimum and I highly doubt you will get a good freq resp and directivity out of it in general.
(there will always be very specific exceptions, but most likely also many more other trade-offs).

In the very vast majority of speaker designs, it's something not to worry about, since there are many many many many other things that are more significant and harmful.

 

[abstract]

the audibility of a poor frequency response and directivity is MANY orders of magnitude more significant (see Floyd Toole et all).
This can be even (easily) picked up by the lesser trained listener.

Since a crossover is always needed to get a proper frequency response as well as a good directivity, there is no way around it.

I think there are some cases where audibility of EQ variations is also overrated. There's a reason why amplitude vs frq plots are nearly always log-log.

Adding an L-pad to a tweeter would be a common example where EQ gets sole credit for the improvement, when in actual fact the amplifier's output impedance is also increased and most likely produces a noticeable improvement in harmonic distortion as well.

Even if, for the sake of argument, we say the tweeter is already perfect in terms of self-produced THD, its reactance still interferes with (most) amplifiers' negative feedback, causing the amplifier to under-perform compared to what it could achieve with an ideal resistive load.

It seems like a major oversight when people go to all the trouble of simulating their amplifiers in spice, but never bother to replace that ridiculous 4-or-8 ohm load resistor with even a basic LR-|LCR| network.

 

[Boden]

So a LPad increases amplifier output impedance?

 

[abstract]

Effectively yes. There is more than one way to increase output impedance, but series resistance is certainly one of them.

 

[tiefbassuebertr]

The audibility of "sonic quality" is debatable at the very best, while the audibility of a poor frequency response and directivity is MANY orders of magnitude more significant (see Floyd Toole et all).
This can be even (easily) picked up by the lesser trained listener.

Since a crossover is always needed to get a proper frequency response as well as a good directivity, there is no way around it.

I find it very strange to choose something to avoid that debatable sonic difference over a very significant audible factor.

One needs at least one highpass filter to begin with anyway.
Although that's the very bare minimum and I highly doubt you will get a good freq resp and directivity out of it in general.
(there will always be very specific exceptions, but most likely also many more other trade-offs).

In the very vast majority of speaker designs, it's something not to worry about, since there are many many many many other things that are more significant and harmful.

That's I know all. The reason I started this thread wasn't to optimize audio components and speakers from economy- resp. average -class with not perfect frequency response.
Rather, the reason was to optimize already present active loudspeakers in the tweeter- resp. high frequency rail by replace the exist push-pull class AB amp together with the associated high pass filter in Sallen-Key topology (replace by a filter according
Multiple Feedback Low-pass Filter Design Tool
integrated in a single ended power amp so as additional passive filter parts for 18-24 db slope).
Speakers like those:
SCM100ASL Pro | ATC Loudspeakers
or
A21-M - Perfectly accurate mid field monitor - PSI Audio
Minimizing semiconductors in signal pad provide still an audible improvement in sound.

 

[tiefbassuebertr]

The usual procedure for realizing active loudspeakers is to add an active crossover network device between the pre-amplifier output and the inputs of each power amplifiers.

However, this is always associated with the disadvantage that additional semiconductors (transistors or in most cases operational amplifiers in the signal pad) add their own sound signature. This disadvantage is audible only in the upper frequency area, so that this approach would be of no advantage for subwoofers but of great advantage for the high frequency range especially with very high resolution tweeters.


What about the approach to implement the associated filter network in the already exist power amplifier (and not in additional introduced gain stages) as describe under the below URLs ?
TDA2030 active speaker audio systems circuit with 60 watt output power
and page 12/23 under
https://www.st.com/resource/en/datasheet/cd00000129.pdf

Are there any experiences ?

I have implemented this approach only once on a "ZEN" - go to
ZEN include active crossover without additional OP AMP for ultimate sounding PHL1230
but have recorded a considerable increase in sound quality compared to the normal procedure of using before (i. e. a separate device for the active crossover network equipped with OPA627 and Sallen-Key topology).

Any news ?

In the moment I have for a VTL compact 100 MONO - go to post #2 under
https://www.diyaudio.com/community/threads/vtl-compact100-monoblock-blows-fuse.170592/
the need to replace the (op amp based with classical and modern TI op-amps) 18db 700hz high-pass sallen key active crossover against a solution with better sound and less active gain stages. Voltage gain factor according the NFB network is a bit less than 100 (< +40db)

This possibilities there are:
1) the use of a passive version as direct replacement (PLLXO) similar to
https://www.marchandelec.com/xm46.html
Disadvantage: inductors sensitiv to induced hum.
Advantage: no additional active gain stage in the signal pad and no inductor core effects if air coil comes in use.
2) implementing the sallen key network in the VTL compact 100 mono - basically as described under
http://www.electroniccircuitsdesign.com/audio-circuits/tda2030-active-speaker-audio-systems.html
Advantage: lowest effort regarding parts - possible
Downside: maybe oscillating and additional changing THD character
3) Modify this mono tube power amp to one for an inverted mode and implementing a MFB topology
Advantage: due only NFB best THD behaviour
Disadvantage: heavy efforts to find best topology therefore.

Maybe one of the members have implement such an active crossover network in a tube power amplifier.

 

[Andersonix]

https://ampslab.com/blog/2022/04/08/biamppro-bench-test/

Did you see the 'bi-amp pro' which uses the chip amps as part of the active xo and no extra opamps?

 

[phase_accurate]

If I understand the OP correctly he is mostly concerned with the high-pass branch of the crossover.
There is something like an "intermediate solution" between an ordinary active filter and a PLLXO: He could build a 3rd order high pass using two coupling capacitors in series and a "simulated inductance" to ground inbetween. The simulated inductor is less prone to picking up hum and is also cheaper to build. Its effect on the signal is decreasing with rising frequency. Maybe this is worth trying out.

Regards

Charles

 

[tiefbassuebertr]

If I understand the OP correctly he is mostly concerned with the high-pass branch of the crossover.
There is something like an "intermediate solution" between an ordinary active filter and a PLLXO:

He could build a 3rd order high pass using two coupling capacitors in series and a "simulated inductance" to ground inbetween. The simulated inductor is less prone to picking up hum and is also cheaper to build. Its effect on the signal is decreasing with rising frequency. Maybe this is worth trying out.

Regards

Charles

under
https://www.allaboutcircuits.com/te...-introduction-to-second-order-active-filters/
there is such a circuit (fig. before "Sallen-Key").

sometimes also to find as low pass filter in DA converter - go to page 10 under
https://www.sowter.co.uk/dacinfo/PCM63.pdf
and
http://www.chuacircuits.com/PDFs/Antoniou Inductance-Simulation Circuit.pdf
for calculating.
Maybe also of interest - thank you for this advice.

 

[phase_accurate]

There is an even simpler version with just a unity-gain buffer, a capacitor and two resistors. It should even be possible to built it with an emitter follower if you want to avoid an OPAMP.

Regards

Charles

 

[nautibuoy]

Sometimes folks design an amp to have limited bandwidth to do this. For example, I think Nautibuoy, is making a choke loaded SE Class A SIT ampwith a high pass around 150Hz or so by using a small undersized output coupling cap. This will then be used on the full range driver for the top and I assume some other more powerful amp with a low pass filter will be used for the woofer. Very similar in concept to a passive line level filter but taken to the next level where the amp itself is frequency modified.

Indeed X, this post (and the one following it) illustrate what I'm working on;

https://www.diyaudio.com/community/...oaded-2sk180-lamp.366312/page-17#post-6673284

The bandwidth limiting is achieved by setting both the input and output caps to achieve the roll-off and an additional benefit is I have used 'smaller' chokes.

I'm getting close to be able to actually hear the results, probably in the next week or so when a coule of parts arrive and I can allocate some time to installing them.

Incidentally, I'm using a pair of your single rail SLB Cap Mx supplies in the build.

PS. Just to complete the picture I plan to use the amplifier (actually it has no voltage gain) to drive a 'full-range' speakers in a Pass SLOB project. I will use, at least initially, a 100W chip amp for the bass drivers and have a Dayton Audio bass DSP device that I'll use to explore getting the sound optimised, after which I'll replicate the configuration with the LF sections of the LX Mini active crossover from the DIY Audio store.

 

[tiefbassuebertr]

I'm waiting for you to do this with Nelson's new ACA Mini class A amp for driving a tweeter above 2.5 - 3kHz.

I think, you mean the circuit of this project:
https://www.firstwatt.com/pdf/art_aca_mini.pdf
unfortunately this amp works not in an inverted mode like circuit from fourth ZEN version
https://www.passdiy.com/project/amplifiers/zen-variations-4
If you use this amp topology as tweeter driver, you can easy implement a MFB network - go to
https://www.diyaudio.com/community/...-op-amp-for-ultimate-sounding-phl1230.154703/

 

[Joel Wesseling]

These will be delivered soon. There's a very positive review on youtube
Mine will be 300hz in a 3 way, passive xover for the tweeter hi pass and mid low pass..
I hoping Xkitz are a solution for 3way and 4way active systems where tweeter is always passive.
It will be interesting if BSC works effectively.
Transparency in the upper frequencies???

https://www.xkitz.com/collections/a...-way-active-crossover-fully-assembled-xover-2