First try at active is a dud :(

[paulspencer]

I have built a 4LR 3 way active xo and was surprised to find that it hasn't worked very well

My speakers:
Vifa MTM TLs built from a kit with a 2.5 way passive xo (I think it could be improved with poly caps in place of electrolytic and possibly a lower xo point but they do sound good as they are)
Subs - 2 x Stryke AV12 currently placed to the side of the mains

Active xo:
4LR 3 way @ 80 Hz & 2.4k

Amps:
Tweeter: using a little mini system (unfortunately it goes through another preamp here) >>> to be replaced with my chip amp when I get rid of the hum
Mid: my trusty old Akai, a relaxed sounding amp
Sub: beefy Behringer EP2500

I'm also using Ultracurve to eq the whole thing flat. The problem is, it doesn't sound flat! The treble is in my face and the bass is weak sounding! It makes me wonder if the mic isn't happy with the 15V its getting from the phantom power from ultracurve. I tried to measure the voltage but it seemed to be very low.

Suggestions anyone as to where to go from here?

I don't have BSC on my active xo or anything more than dividing up the frequencies. I had hoped that Ultracurve would in fact do the trick. Should I start looking into making the active xo a bit more sophisticated? Are there other functions I should add to my active xo?

I'm thinking first I need to at least get it to actually sound flat!

[simon5]

I think you should crosscheck your measurements with a SPL meter, maybe your mic failed, I don't know.

Did you remove all the passive crossover network? Yes, I guess.

Maybe your little mini system is going into distortion so you get the in your face sound?

Good luck!

[Bill Fitzpatrick]

You did say you removed the passive crossover from your 2 way, didn't you?

Each of your amps has a volume control, right? Or at least the tweet and mid amp do, right?

Another possibility is that you wired the active crossover wrong.

[kelticwizard]

If your passive corossover was second order, the tweeter will have been wired out-of-phase with the mids.

Check the polarity of each of your drivers.

[paulspencer]

For interest, this is what Ultracurve is doing to supposedly get this thing flat:

[phase_accurate]

Another issue with filter orders:

If you have a 4th order LPF in front of the subs and a 4th order HPF in front of the mains then the signals won't add in phase, due to your main speakers' intrinsic highpass function.
You will need a HPF (of lower order than 4) in front of your mains, dimensioned such that it's response adds up (together with the speaker's intrinsic higpass response) to the desired 4th order highpass function @ 80 Hz.

If such things are sorted out than nothing will stop you from getting into "HiFi heaven".

Regards

Charles

[paulspencer]

Originally posted by simon5
I think you should crosscheck your measurements with a SPL meter, maybe your mic failed, I don't know.

Did you remove all the passive crossover network? Yes, I guess.

Oh no, not another thing to buy!!! :O

Yes I did remove the passive xo from both speakers. I put in some passive protection, thrown together quick which consists of a 2nd order HP to the tweeter at 800 Hz, which is 1.5 octaves below the xo point.

Originally posted by Bill Fitzpatrick
You did say you removed the passive crossover from your 2 way, didn't you?

Each of your amps has a volume control, right? Or at least the tweet and mid amp do, right?

Another possibility is that you wired the active crossover wrong.

The tweeter amp has a volume control as does the sub, the latter being on full, the tweeter turned back a little. The mid doesn't have a volume control. I fiddled with the trim pots on the active xo but there was a lot of turning and not much difference. For a quick test I was hoping ultracurve could compensate enough, using its AutoEQ function - it puts out noise, shows the response and sets about eq'ing til its perfectly flat at the listening position.

Originally posted by kelticwizard
If your passive corossover was second order, the tweeter will have been wired out-of-phase with the mids.

Check the polarity of each of your drivers.

Hmmmm I never wired the tweeter up like that anyway. If it was done wrong, what would I expect subjectively?

I was careful to get the polarity correct, nothing is reversed. I'm very careful about that stuff.

Apparently I'm supposed to set the gain of the active xo with the trim pots using an oscilloscope which I don't have, but I have downloaded software scopes. Will that work? Anyone know anything about how to do this?

[phase_accurate]

G'day Paul

You were posting while I was still typing.
The peak that the Behringer's EQ uses at 60 Hz in order to achieve a flat response could either be due to a room mode or the effect that I mentioned before.
Maybe you could make a provisional (i.e. quick and dirty) try and tap your highpass after the first 2nd order section of your 4th order LR highpass and look how things change. While this would not be an exact approach it might show some tendencies.

Regards

Charles

[Bill Fitzpatrick]

Originally posted by paulspencer

Yes I did remove the passive xo from both speakers. I put in some passive protection, thrown together quick which consists of a 2nd order HP to the tweeter at 800 Hz, which is 1.5 octaves below the xo point.

Remove the "passive protection" as you call it. It's part of your problem. If you need protection, stick in a n.p. electrolytic of about 100mfd until you get things squared away.

[paulspencer]

Originally posted by phase_accurate
Another issue with filter orders:

If you have a 4th order LPF in front of the subs and a 4th order HPF in front of the mains then the signals won't add in phase, due to your main speakers' intrinsic highpass function.
You will need a HPF (of lower order than 4) in front of your mains, dimensioned such that it's response adds up (together with the speaker's intrinsic higpass response) to the desired 4th order highpass function @ 80 Hz.

If such things are sorted out than nothing will stop you from getting into "HiFi heaven".

Charles, my head is spinning at the moment trying to suss all of that out. I have always paid little attention to that phase stuff!

My mains are tuned at about 40 Hz but I'm crossing at 80 Hz. Hence if I used a 2nd order filter, wouldn't it be worse? Unless I was crossing at 40 Hz, but I dont' want to cross that low. I don't see how I can achieve this. What is the nature of the phase effect here due to the rolloff at 40 Hz?

I think I'm in need of a little more elaboration here ...