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Old 1st August 2002, 06:49 PM   #1
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Hi Jan

I intend on using a subtractive crossover consisting of a 2nd order lowpass and a derived 1st order higpass with a Manger broadband driver and an Audio Technology woofer.

It could be woth a try since it is not very complicated and it is transient perfect.

Look at http://www.passlabs.com/pdf/phasecrx.pdf

Regards

Charles
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Old 1st August 2002, 07:03 PM   #2
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Active Subtractive XOs

Moderater's note: 23-aug-02 This thread is the result of splitting off the subtractive XO posts from a previous thread on a Jordan speaker. It was getting a bit OT from the original thread but was good stuff.

dave (with my moderator hat on)

The whole concept of active subtractive XOs i find very interesting so hopefully this will give the subject a little more visibility.

dave (with just my regular diyAudio cap on)

Quote:
Originally posted by phase_accurate
I intend on using a subtractive crossover consisting of a 2nd order lowpass and a derived 1st order higpass with a Manger broadband driver and an Audio Technology woofer.
I used one of these (based on the old colony boards) in the latter half of the '80s with a set of Quads & a single sub.

I am trying to figure out how to do the same thing with tubes... have some ideas -- need to try them out.

dave
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Old 1st August 2002, 07:49 PM   #3
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Yes, any proposals would be welcome, I'm just cutting my teeth here. Usefull, that Pass piece on phaselinear xovers.
The examples you guys are talking about are passive ones, right? That's what I need.

Thanks, Jan Didden
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Old 2nd August 2002, 08:24 AM   #4
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Quote:
Janneman wrote:
The examples you guys are talking about are passive ones, right?
No, I was trying to convince you to stay active.

Even though you were using a passive crossover in front of your amps the final outcome is almost the same (i.e. it has some slight advantages and disadvantages compared to an active filter) as if you had used an active filter.

IMO the main reason (beneath others) why many people prefer single-driver solutions is the high transient accuracy when there is no crossover introducing time-smear.

There are only a few options to implement crossovers that are amplitude and phase accurate .
One of them is the subtractive active filter as described on the Passlabs page.

If you want to go the passive way and stay transient perfect then there are two solutions: either first order (parallel or series) or second order with overlap and equalizing. Discussions on all of these you will find on John Kreskovky's Homepage: http://www.geocities.com/kreskovs/John1.html



planet10

What was your experience with the subtractive crossover ? or ?
What kind of subtractive crossover do you want to implement with tubes (i.e. which path do you want to make higher order and which one derived 1st order), maybe I can do some brainstorming as well ?

Regards

Charles
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Old 2nd August 2002, 04:46 PM   #5
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Quote:
Originally posted by phase_accurate
planet10

What was your experience with the subtractive crossover ? or ?
What kind of subtractive crossover do you want to implement with tubes (i.e. which path do you want to make higher order and which one derived 1st order), maybe I can do some brainstorming as well ?
My recollections are positive. It wasn't what you would call a hi-resolution system used in a business environment (Nak Casette, NAD 7020, subtractive XO, ILP module w Tangent PS into a single sealed 12", XO back into 7020 to drive Quad 57s hanging upside down from the ceiling)

At this point both my applications need the 2nd order on the Low Pass.

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Old 5th August 2002, 12:24 PM   #6
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planet10

I've done a little thinking and came to the circuit you will see attached below.

Since I was way too lazy to look for an appropriate model of a triode, I used JFETs for the simulation, since this was the closest thing to a triode that was on hand. It wouldn't be that difficult to adapt it for tubes. Depending upon it's use some input and/or output buffer stages will be necessary.

Whoever tries to implement it in real life has to pay attention to the DC-path between ground and input (i.e. gate/grid). I omitted this in my simulation circuit simply because it is intrinsic to the signal source I used for the simulation. Also the values of resistors # 1, 2, 12 and 13 are a little lower than optimal for maximum headroom.

The lowpass hasn't got any positive feedback, giving a Q of 0.5 approx. This can of course be changed easily.

The lowpass output can be taken from C11 (R19 represents the input impedance of the next stage).


At C10 (which has to be reasonably large !!) the inverted version of the lowpass filtered input signal is taken off and added to the input signal via the two summing resistors R17 and R18. At their summing junction the derived highpass signal is available, although damped by 6dB approx.
If everything were ideal then R17 and R18 would be of same value. But the signal level at J1's drain was a little lower than at it's source so I lowered R17's value a bit. Maybe R17 should be made at least partially adjustable anyway.

At C12 the highpass output can be taken off. The stage around J2 has a gain of 6 dB approx and is delivering an INVERTED output signal (which is still a little low by the fraction of a dB).

The DC Blocking capacitor C10 can be moved to another leg of the summing network, depending on how the DC path at the input is made (from the optimal-signal-summing point of view it's best place would be after the summing point but then the DC current flow through R17 and R18 has to be taken into account ! And don't forget another resistor from gate of J2 to ground when doing this!).

Although this little circuit may not be perfect it is at least a usable starting point.

Regards

Charles
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Old 5th August 2002, 12:35 PM   #7
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And here is it's simulated frequency response in linear and logarithmic representation:
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Old 5th August 2002, 12:53 PM   #8
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Hi phase_accurate,

I have been looking at the response graphs, but it seems that at xover there will be a hump in the combined response???

Jan Didden
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Old 5th August 2002, 01:07 PM   #9
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Hi Janneman

Although there is a hump in the derived branch of a subtractive crossover the combined response doesn't have a hump. Additionally this hump does NOT come from any form of resonance as would be expected from the first glimpse.
The combined response is both flat in terms of amplitude AND phase !!!
It will of course not be that perfect anymore as soon as drivers are connected to it (with amplifiers in between of course, but their error is marginal compared to the drivers).
The drivers should also be positioned reasonably close.

Most of this is mentioned in the article on the passlabs website.

Regards

Charles
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Old 5th August 2002, 06:40 PM   #10
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Charles,

This is good to chew on -- you have gotten the thing simplier than my sketches to date, but it is quite similar. I like that there is only one follower (getting a cathode follower to sound good takes some care).

As the order of the filtered stage goes up, the bump in the bottom of the derived 1st order filter gets larger -- hence the recommend to restrict them to 2nd & 3rd order.

The addition of an all pass filter in front of the derived part can be used to get higher orders -- and the derived filter becomes the same order as the reference filter.

dave
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