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Electronic crossovers _ analog or digital ? Studio Monitors
Old 27th September 2011
  #31
Quote:
Originally Posted by O.F.F. View Post
I changed all op amps from TL072 to Burr Brown 2134
did you try other opamps?
I bought a bunch of OP275 for my K&H o98 but am open for suggestions!
Old 27th September 2011
  #32
Quote:
Originally Posted by O.F.F. View Post
Home | miniDSP

Personally I use modified BSS FDS360 analogue ones. I changed all op amps from TL072 to Burr Brown 2134 and removed the limiters as my amps contain better ones.
They are, as is usual for analogue, 24dB/oct L-R. This does me fine but I too have heard that 48dB should be better, on the other hand they replaced a 6dB hi pass so at least I don't have to worry about my tweeter frying.
I have rebuilt those used for Quested 4x10" systems. I used LT1358 opamps, that opened up the tops nicely. The EQ caps were changed to Wima FKP-2's. The coupling el caps were removed, that really helps stop the clouds.

Still, I'm a high quality passive crossover user. The amps are modified Adcoms. The crossover parts cost more than the speakers. A very high quality passive film cap combined with precision non-inductive wirewound resistors and copper air foil inductors easily beat any active design with their intrinsic THD, noise and errors of the active devices. A passive part is very hard to measure THD with, even with Audio Precision hardware. I can rip apart any active design and find errors. Digitize the signal through ADC's then DA's and more errors are introduced. A 48 db/octave slope won't buy you anything if the converters are not the best available.
Old 27th September 2011
  #33
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O.F.F.'s Avatar
 

Quote:
Originally Posted by Reptil View Post
did you try other opamps?
I bought a bunch of OP275 for my K&H o98 but am open for suggestions!
Jim will be the man to talk to about op amps.

I used those because everyone told me that they are drop in replacements and I got them dirt cheap in a tube of 50 since they were non-RoHS compliant old stock. They were like £0.70ea. instead of £2+! To get something substantially better I'd have to spend a lot more and there are a lot of opamps in 360s!
That said I use them mono only so I probably could leave some out but I had enough of them so I just replaced them all. Took all of 30 minutes (not counting the hours spent on the interwebs gathering info but I learned a lot too so it wasn't completely wasted).
A mate of mine who does repairs removed the limiters for me.

As for passive xovers I just never hear as much detail in the lower registers as with actives regardless of anything else. Some of the closest I heard were passive Tannoys but even then the difference between active v passive was much greater than the difference between passive with a crappy Arcam HiFi amp (the worst amp I ever had the misfortune to own) and passive with a MC2 MC450.
Old 27th September 2011
  #34
Gear Head
 

Quote:
Digitize the signal through ADC's then DA's and more errors are introduced
The idea is to enter the digital crossover directly with the digital source. This way we can have the precision of the digital crossover without any extra coversion.
Old 1st October 2011
  #35
nice, thank you for sharing!!
Old 2nd October 2011
  #36
Gear Addict
 

Hi again !

I read somewhere that a very telling test to assess the transparency of an electronic device is to see how it passes a square wave
I am asking this because often I read of judgement basis just on listening impressions/tests
For instance
I send a analog square wave through the cheap digital x-over Behringer via its analog inputs
I take the output signal from its analog outs (i.e. after AD-crossing-DA processing)
The square is pretty unchanged
This means that the unit is good quality ????
I like better measurements than listening tests
Regards,
gino
Old 11th November 2011
  #37
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Quote:
Originally Posted by ginetto61 View Post
I like better measurements than listening tests
But that's the thing Gino .. You do BOTH. You test and measure AND you listen and compare ..
Old 12th November 2011
  #38
Gear Maniac
 

You can also check out a VST version of an active 4-way crossover- the Frequency Allocator Light. It let's you import measurements and tweak filters and EQ's until the final acoustic response of your speaker is to your liking.

There is also a more complex version that has phase correction build in (Frequency Allocator), it will let your speaker pass a square wave correctly even with very steep filters. But it has inherit latency of 8192 samples, so it's not good for real time mixing, but would be OK for mastering.

If you already own a multichannel interface and a VST host software it's really inexpensive compared to hardware analog and digital crossovers.
Old 12th November 2011
  #39
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There's no best crossover or slope be it passive or active, analog or digital. There are pros and cons of active and passive solutions and you chose the best tools for the specific job which goes for the slope or transfer function as well.

An electronic or digital solution does not necessarily result in lower distortion.

When it comes to nonlinear distortion the only thing that makes sense is to look at the hole chain. An air coil coupled to a midrange results in lower distortion than what the midrange does on its own and also thermal compression is worse with a midrange (or woofer) coupled directly to a typical amp as compared to using a coil.

The actual response also depends on the intrinsic driver response. If you use a 24dB/oct you will not get a 24dB/oct transfer function.


/Peter
Old 14th November 2011
  #40
Gear Head
 

Quote:
There are pros and cons of active and passive solutions
What are the pros of passive crossovers?

Quote:
The actual response also depends on the intrinsic driver response. If you use a 24dB/oct you will not get a 24dB/oct transfer function.
+1 and I will also add that it depends on the driver response IN the speaker box. (including also phase, time alignement etc...)
Old 14th November 2011
  #41
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Quote:
Originally Posted by lulumusique View Post
What are the pros of passive crossovers?
Quote:
Originally Posted by Audiop
When it comes to nonlinear distortion the only thing that makes sense is to look at the hole chain. An air coil coupled to a midrange results in lower distortion than what the midrange does on its own and also thermal compression is worse with a midrange (or woofer) coupled directly to a typical amp as compared to using a coil.
Basically a moving coil speaker benefits from being driven from a high impedance in its upper working range and from a low impedance in the low range. The nonlinear distortion and thermal compression is worse in the mid/upper range when driven from a typical solid state amp with lowish output impedance as compared to when driven from a high impedance.

A high impedance can for example be an inductor (x-over coil) or a tube amp.


Quote:
Originally Posted by lulumusique
+1 and I will also add that it depends on the driver response IN the speaker box. (including also phase, time alignement etc...)
Yes of course.


/Peter
Old 15th November 2011
  #42
Gear Head
 

Quote:
Basically a moving coil speaker benefits from being driven from a high impedance in its upper working range and from a low impedance in the low range. The nonlinear distortion and thermal compression is worse in the mid/upper range when driven from a typical solid state amp with lowish output impedance as compared to when driven from a high impedance.

A high impedance can for example be an inductor (x-over coil) or a tube amp
Can you be a little more technical on the effect of the air coil on the Non Linear Distortions and the thermal compression?
Though, isn't it still easier to play with the output impedance (even by adding an inductor) when it's an active speaker? (the filter is before the amplifiers)
Old 15th November 2011
  #43
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Hi!

The voice coil current is modulated when the voice coil is moving in the gap. An ultra low impedance drive does nothing to deal with this which a higher inpedance drive does. In effect this means a decrease in HD and IMD.

Yes, you can manipulate the output impedance in the amplifier itself. Nothing for the novice though. And yes, using a hybrid approach of active electronic filters and passive filters (an inductor) can make sense.


/Peter
Old 15th November 2011
  #44
Gear Maniac
 

Quote:
Originally Posted by Audiop
Basically a moving coil speaker benefits from being driven from a high impedance in its upper working range and from a low impedance in the low range. The nonlinear distortion and thermal compression is worse in the mid/upper range when driven from a typical solid state amp with lowish output impedance as compared to when driven from a high impedance
Yes, and to address the lower range the higher output impedance provides less damping on driver inertial overshoots ( more low end ringing).

A coil (inductor) has the same effect on the low end due to series resisitance -- must use expensive air core, with many turns of low gauge wire (thick) to reduce series resistance.

Tradeoff's, tradeoff's......
Old 15th November 2011
  #45
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Quote:
Originally Posted by Geoff Waddington View Post
Yes, and to address the lower range the higher output impedance provides less damping on driver inertial overshoots ( more low end ringing).

A coil (inductor) has the same effect on the low end due to series resisitance -- must use expensive air core, with many turns of low gauge wire (thick) to reduce series resistance.

Tradeoff's, tradeoff's......
Hi!

Yes inductor resistance do increase Qe which means less electrical damping of the mass spring fundamental resonance. This is not necessarily a problem and the optimum depends on the Q before the inductor is added to the circuit. The R of the external inductor becomes a part of the total series resistance which inlcudes voice coil R of aprox 5-6ohm. A low Q driver may become more optimal with this extra R added.. all depending on target response.

So, some times a big (gauge) coil with low R is the right choice but it's also possible that you do best with a really small (gauge) coil.


/Peter
Old 15th November 2011
  #46
Quote:
Originally Posted by Geoff Waddington View Post
A coil (inductor) has the same effect on the low end due to series resisitance -- must use expensive air core, with many turns of low gauge wire (thick) to reduce series resistance.
Tradeoff's, tradeoff's......
Copper foil inductors overcome those problems. The series resistance is very low due to the large surface area used and the large amount of copper.
Hysteris THD is also lowered over any air coil design using round wire.
Old 15th November 2011
  #47
Gear Head
 

Quote:
The voice coil current is modulated when the voice coil is moving in the gap. An ultra low impedance drive does nothing to deal with this which a higher inpedance drive does. In effect this means a decrease in HD and IMD.
I'm not sure I understand, so let me know if I'm worng: The current generated by the driver is the opposite of the one generated by the source but with lower amplitude (lenz's law I guess? so it tends to prevent the driver from moving). And I also suspect that we can't consider this to be linear, right? I agree that by incresing the source output impedance we lower the current generated by the speaker motion but isn't it only because we also lower its motion? (because of the tension lost "on" the ouput impedance). The high output source transforms this current into tension, so doesn't it have the same effect on the driver?
Old 15th November 2011
  #48
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At driver resonance (where you typically see a big peak in the impedance graph) one can say that the generator function of the driver tries to output a current opposite to the amp current and that's why the impedance do increase there.

At higher frequencies you can see it as the modulation changes the voice coil impedance so that even though the voltage from the amp is stable the resulting current is modulated with this modulation in VC impedance. This results in distortion since motion/accelaration depends on the current.

I hope that helps!


/Peter
Old 16th November 2011
  #49
Gear Head
 

It does help!
Yeah, I don't know why I didn't think about the impedance changing with the speaker motion.
I think I got it. I was seeing this effect as a current source in the driver but it's more of a tension source (an electromotive force). So with a high amplifier output impedance, the current generated by the emf in the speaker impedance in serie with the amplifier output impedance is lower so it has less impact on the sound reproduction.
Since this emf is mostly dependant on the actual sound output (the driver's motion), the sensitivity to this phenomenon at a certain sound volume is lowered (with the overal efficiency, though).
Thanks for your help (sorry to the OP for the trhead polution...)
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