1 - 10 of 10 Posts

#### 2500hd

·
##### Registered
Joined
·
119 Posts
Discussion Starter
While researching for my own passive system, I scored this (courtesy of Active Filters)

"The crossover design is not complete yet, because the tweeter's high-pass behavior and the midrange's roll-off cause phase shifts that are part of the crossover. In addition, the physical offset between driver voice coils causes delays and associated phase shift that must be corrected. This is best done experimentally and started with the driver offset.
The tweeter is approximately 2" (0.05 m) in front of the midranges and its input signal must be delayed by T = d/v = 0.05/343 = 146 us to be in phase with the midranges. The group delay of an all-pass circuit is used for the offset compensation.

The delay of the circuit changes with frequency, but the 1400 Hz crossover should fall into the flat region of the delay and therefore f0 > 1400 Hz. You can estimate the number of stages required from this inequality.
If f0 = 1/(2*pi*R*C) > 1400 and thus
R*C < 1/(2*pi*1400) = 114 us,
then a single stage would have to be operated in its sloping region to obtain 146 us of delay. Thus take two stages. The actual implementation in the tweeter channel provides 85 us and 104 us for a total of 189 us. The value is larger than estimated, because the low-pass behavior of the midranges moves their voice coils effectively further behind the tweeter than the physical distance measurement.
The final component values for the all-pass circuits have been determined experimentally from an optimization of the combined midrange and tweeter frequency response. The depth of the interference notch when the tweeter polarity is reversed, is an indication of how closely the two outputs are to being 180 degrees out-of-phase and of equal magnitude."

#### minbari

·
##### Registered
Joined
·
12,054 Posts
yup, in general a 6db/oct will give you 90°. 12dB/oct will give you 180°, 18dB/oct will give you 270° and 24dB/oct will give you 360°(or zero) this is certainly a consideration when designing passives, even more so if the slopes are not all the same in the system.

as for using passives for T/A, not really possible. you cant control the delays in any meaningfull way and the delays are in the uS range, T/A alignment is in the mS range.

#### 2500hd

·
##### Registered
Joined
·
119 Posts
Discussion Starter
So wait he's using the RC network to attenuate spl? Or change time?

I'm not good enough with analog schematics to figure out if I can help 18" on my tweet, 1.3 ms. Do you think it would be hard to design with this to hit 1300 us?

#### minbari

·
##### Registered
Joined
·
12,054 Posts
So wait he's using the RC network to attenuate spl? Or change time?

I'm not good enough with analog schematics to figure out if I can help 18" on my tweet, 1.3 ms. Do you think it would be hard to design with this to hit 1300 us?
those circuits are designed to attenuate SPL. (act as a x-over). in this case he is using them as a all-pass.(no x-over effect) not only do I think it would be hard, I think it would be impossible. for starters how do you know you need 1.3mS? grabbed a ruler, did some math? not gonna work. when you T/A a system you HAVE to do it by ear. so you have to have a way to alter the delay on a small step basis. (.1mS preferable although I have seen .5mS work, too)

last thing...........those circuits have a delay in the 114uS area. that is 0.11mS. you would ten of these circuits in series just to get 1mS

#### 2500hd

·
##### Registered
Joined
·
119 Posts
Discussion Starter
Thanks Minbari, you've been a huge help with my design so far.

I was assuming that this was a formula that I could use to add delay to the system. Plug in the frequency and delay and come up with a RC network to flip the phase right. Ugh, surely analog T/A couldn't be that simple...

#### minbari

·
##### Registered
Joined
·
12,054 Posts
ya, there is a reason T/A is strictly in the digital world. anything analog that could give the delays you wanted, would more than likely distort the original signal pretty badly too. be like putting a 1farad cap on your amplifier outputs. that cap do a good job of filtering out the signal entirely, lol.

as for phase, yes you can get phase correction with those. if you use a 18dB/oct passive on your speakers and want to flip the phase back, those can do it by adding an additional 90° phase to ge tit back to 360°. for the most part though, it is not a big deal.

#### coobah

·
##### Registered
Joined
·
36 Posts
...in general a 6db/oct will give you 90°. 12dB/oct will give you 180°, 18dB/oct will give you 270° and 24dB/oct will give you 360°(or zero)...
Do I have have to consider the same thing when using digital crossovers in my car ???

#### minbari

·
##### Registered
Joined
·
12,054 Posts
Do I have have to consider the same thing when using digital crossovers in my car ???
those numbers are for purely analog circuits. (passive x-overs or analog opamps)

#### coobah

·
##### Registered
Joined
·
36 Posts
And crossovers in HU are the digital ones or are those just digital controlled analog things???

#### minbari

·
##### Registered
Joined
·
12,054 Posts
more than likely they are purely digital.

1 - 10 of 10 Posts