BillFitzmaurice.info

Does anyone have ideas on how to tell if my HPF is working correctly? I have it set to 45Hz with a LR 48 slope. I have limiter set using a 60Hz signal to 53V. Then when I run a 40Hz signal and play with the mixer settings, I can get to the limited voltage of 53V. I would have thought that this frequency would have been attenuated so that it wouldnt be able to get that high due the a steep HPF set at 45Hz. Not sure what I am missing here.

This may seem like a dumb question but, is the limiter turned "on"? I'm only asking because in the past I have set some settings without engaging them.

What device is doing the limiting and how exactly did you set it up?

While I don't know a great deal about the difference between the LR and BW slopes, and despite having different subs (T30's), the same theory should apply.

I just tried a little experiment with my system to see if I could mimic your results. My limiter is setup on my amp, set to 49V. On DCX I set hpf to 45hz with LR48 slope, ran a tone at 40hz and could not get a reading higher than 47V. If I slid the tone lower, the voltage dropped rapidly off, to almost nothing at 22.5hz (1 octave below 45hz), as expected. Even with the slope I use (BW24) which is less aggressive, I could not get higher than 48V at 40hz.

Maybe your limiter is not setup properly?

Without the speakers plugged in, try measuring the voltage coming from the amp when you play a 22.5hz tone (1 octave below 45hz). It should be almost nothing, even with your gains wide open on your mixer.

Another experiment you can try is with the speakers plugged in and the volume at below a comfortable listening level, so pretty quiet. With the hpf set to 45hz, play a 40hz tone. While it's playing, shift the hpf down to 40hz. You should notice the tone gets slightly louder, which means it's working as it should.

I did a test like this.
Set limiter with 60hz signal to 53V.
Then played a 22.5hz signal. voltage was very, very low, but I don't think the limit light was on. (Cant remember as I did this last night).
But, then I turned up the gain on the 22.5hz signal and maxed out my board and was able to get to around 45V (not good). I was thinking that the DCX would be able to clamp the output to follow the HPF set in the crossover section of the unit. If not, how does the unit really provide subwoofer over excursion protection?

laits wrote:I did a test like this.
Set limiter with 60hz signal to 53V.
Then played a 22.5hz signal. voltage was very, very low, but I don't think the limit light was on. (Cant remember as I did this last night).
But, then I turned up the gain on the 22.5hz signal and maxed out my board and was able to get to around 45V (not good). I was thinking that the DCX would be able to clamp the output to follow the HPF set in the crossover section of the unit. If not, how does the unit really provide subwoofer over excursion protection?

The difference between 22.5hz and 60hz is only a little over one octave. Crossover filters don't shut off frequencies below the crossover point, they merely attenuate them at varying slopes. At 24db per octave slope, you probably have enough gain in the board (maxed out as you said) to overcome the 24db to get your reading up to 45 volts.....but you aren't going to max out your board, are you?

Looks like it's doing just what it should......and don't worry, if you aren't playing 22.5hz content it won't be a problem. If you are, then you have the wrong subs.

laits wrote:I did a test like this.
Set limiter with 60hz signal to 53V.
Then played a 22.5hz signal. voltage was very, very low, but I don't think the limit light was on. (Cant remember as I did this last night).
But, then I turned up the gain on the 22.5hz signal and maxed out my board and was able to get to around 45V (not good). I was thinking that the DCX would be able to clamp the output to follow the HPF set in the crossover section of the unit. If not, how does the unit really provide subwoofer over excursion protection?

The limiter only limits the max voltage you set regardless of frequency. Having a limiter that can change that voltage depending on frequency is very $$$$$. You had to really max out the board to get the voltage below the HP filter to get that high. Under real world conditions you wouldn't max out your board because then the rest of the music (above the HP filter) would be so far into the limiters that the sound coming out would be absolute garbage (very noticeable).

Although you set up your outputs on the DCX, did you set up the input gains as well? When you show the first red on your board, adjust the input gains on the DCX to show red (0 dB) as well. You will then have to readjust the sub output gain to match, the limiter setting should stay the same.

Another thing is that when you maxed out your 22.5 hz signal on your board you were most likely distorting the signal extensively. These distortions would show up as harmonics @ 45 hz, 67.5 hz, 90 hz, etc. Those harmonics are well above your HP filter and would get through the filter without being attenuated.

Bruce Weldy wrote:

laits wrote:I did a test like this.
Set limiter with 60hz signal to 53V.
Then played a 22.5hz signal. voltage was very, very low, but I don't think the limit light was on. (Cant remember as I did this last night).
But, then I turned up the gain on the 22.5hz signal and maxed out my board and was able to get to around 45V (not good). I was thinking that the DCX would be able to clamp the output to follow the HPF set in the crossover section of the unit. If not, how does the unit really provide subwoofer over excursion protection?

Crossover filters don't shut off frequencies below the crossover point, they merely attenuate them at varying slopes. At 24db per octave slope, you probably have enough gain in the board (maxed out as you said) to overcome the 24db to get your reading up to 45 volts.....but you aren't going to max out your board, are you?

Let me ask you guys a question since I don't have a DCX. Can you set the HPF to come AFTER the limiter, or does it have to come before? The reason I ask is because if the HPF is set before the limiter, it would theoretically be possible to get the max voltage limit (in this case 53 volts) well below the HPF limit. However, if you set up the HPF after the limiter in the chain, then with a voltage limit of 53 volts and a 48 db/octave slope, it would be impossible to get over even 1 volt at 22 hz!

I'd want the HPF after the Limiter, then wouldn't have to worry about how high the mixer gain is set. You're right though Bruce and Corona, the audible distortion is going to tell you to turn it down if the mixer gain gets too high well before you would crank into the HPF.

I know we usually say the limiter should be the last thing before the amplifier, but the HPF is augmenting the limiter so it wouldn't be a problem to set it up like this, if it was possible to do so.

Ryan A wrote:

Bruce Weldy wrote:

laits wrote:I did a test like this.
Set limiter with 60hz signal to 53V.
Then played a 22.5hz signal. voltage was very, very low, but I don't think the limit light was on. (Cant remember as I did this last night).
But, then I turned up the gain on the 22.5hz signal and maxed out my board and was able to get to around 45V (not good). I was thinking that the DCX would be able to clamp the output to follow the HPF set in the crossover section of the unit. If not, how does the unit really provide subwoofer over excursion protection?

Crossover filters don't shut off frequencies below the crossover point, they merely attenuate them at varying slopes. At 24db per octave slope, you probably have enough gain in the board (maxed out as you said) to overcome the 24db to get your reading up to 45 volts.....but you aren't going to max out your board, are you?

Let me ask you guys a question since I don't have a DCX. Can you set the HPF to come AFTER the limiter, or does it have to come before? The reason I ask is because if the HPF is set before the limiter, it would theoretically be possible to get the max voltage limit (in this case 53 volts) well below the HPF limit. However, if you set up the HPF after the limiter in the chain, then with a voltage limit of 53 volts and a 48 db/octave slope, it would be impossible to get over even 1 volt at 22 hz!

I'd want the HPF after the Limiter, then wouldn't have to worry about how high the mixer gain is set. You're right though Bruce and Corona, the audible distortion is going to tell you to turn it down if the mixer gain gets too high well before you would crank into the HPF.

I know we usually say the limiter should be the last thing before the amplifier, but an HPF is sort of an augmented limiter in and of itself so it wouldn't be a problem to set it up like this, if it was possible to do so.

I don't have a DCX, but all of the processors that I've seen have the limiter last.....it makes the most sense there....you don't want to be able to manipulate the signal once you've set the limit.

ok, just looked it up.

The end of the chain goes:
Limiter
Polarity
Delay

The last two don't change the signal's voltage. So, the limiter is at the end of the line for any gain structure changes.

Bruce Weldy wrote: I don't have a DCX, but all of the processors that I've seen have the limiter last.....it makes the most sense there....you don't want to be able to manipulate the signal once you've set the limit.

It doesn't make the most sense there, as it relates specifically to high pass filtering and low frequency speaker protection. The only manipulation taking place after the limiter would be proper voltage to frequency rolloff, as set by the HPF.

Seems like its not possible with the DCX though, so nevermind. Ideally that is what you'd want though.

Ryan A wrote:

Bruce Weldy wrote: I don't have a DCX, but all of the processors that I've seen have the limiter last.....it makes the most sense there....you don't want to be able to manipulate the signal once you've set the limit.

It doesn't make the most sense there, as it relates specifically to high pass filtering and low frequency speaker protection. The only manipulation taking place after the limiter would be proper voltage to frequency rolloff, as set by the HPF.

Seems like its not possible with the DCX though, so nevermind. Ideally that is what you'd want though.

However, and my experience is with the dbx products (although I'm sure the DCX is the same), the function of setting the HPF includes a gain control as part of that module - you definitely wouldn't want that after the limiter....and since that's how they are designed and built, I'll stand by my assertion.

Now, if they totally changed the design to do what you suggest, that would be fine - but, it's not just a junction of moving the HPF (crossover module) after the limiter.

Ryan A wrote:
Seems like its not possible with the DCX though, so nevermind. Ideally that is what you'd want though.

It could be done by routing the sub output into input C and then using another output assigned to Input C and then using that highpass filter. You would have to be careful not to adjust the gains on that output though!

However if the DCX input gains were set right raising the mixers output to max wouldn't increase the voltage at 22 hz that much because then the input section of the DCX would clip preventing that.

Laits,
Can you please just stick to your thread "T39 and HPF" for the time being.

You absolutely need to get your DCX sorted out in terms of setting it up before you worry about anything else.