Experiment with a Crown XLS1500

[djtrumptight]

Sorry - that is what I have - I wrote it backwards.

Seeing that you are using an outboard DSP ahead of the power amp, have you then double-checked to make sure that any of the PureBand features in the XLS are completely defeated? If not, you could be inadvertently cascading filters.

That came to my mind too.I use a XLS 1500 and the lowest highpass filter it allows is 50HZ (in stereo anyway) so if that feature is being used the T-39 is not being allowed to go down to 45HZ.

[dswpro]

Just a guess, but I suspect you were not really driving the amp to full output with your kick drum signal when you were not bridged.

When you set the amp up for bridged mode, you effectively doubled the output voltage per millivolt of input.
So the same input signal got 2x the output voltage (or thereabouts). Hence the louder sound.

Try running your amp without bridging, and run a 60 Hz sine wave through it. Disconnect the T39 and measure the output swing of the amp to be sure you are hitting your desired limit voltage with the input signal applied. Turn your amp volume all the way down. Connect the T39, slowly increase the amp volume until your organs start shaking loose from your insides.

[yigba]

Filters on the XLS1500 are bypassed as are the Clip Limiters.

I tried it again last night with my band members as witnesses and it's definitely not me. There is a marked difference in volume with all input levels to the amp staying exactly the same and just switching between one stereo channel powering the T39 and the bridged channel powering the T39.

I'm getting another amp today and I'm gonna see if this phenomenon happens when I use that amp also.

Freaking weird.

[sine143]

You are limiting before the amp correct? in this case when you bridge all you are doing is doubling the output voltage. little surprise its getting much louder. like I said, download some kick drum samples to test with and play them at 0 db.

[whines]

Again - same exact voltage going into the amp - I tested it - amp limited to 50 volts whether bridged or not. Yet one (the bridged one) is stupidly louder than the other (and I assume this is what he output of the T39 is supposed to be). Why?

Silly question, but you did measure the output of the amp with a voltmeter both ways, right? I assume you did but you haven't said that you have, only that you measured the input.

I think someone suggested trying each channel seperately, that's a good thought, it's possible that one of them isn't working well.

I'm using an XLS1500 for two t39's (one per channel) and it only takes a little limiting for me to cut it down to 50v....I think it was 53 or so before limiting. No complaints here.

[Bruce Weldy]

There is a marked difference in volume with all input levels to the amp staying exactly the same and just switching between one stereo channel powering the T39 and the bridged channel powering the T39.

Of course there is - it's twice the power. Now, if you have set the voltage in the driverack to xx volts in stereo mode and the same amount of volts in bridge mode, the output should be the same when running at full output.

Sounds like you aren't measuring the voltage output correctly or aren't setting the limiter correctly.

[yigba]

You are limiting before the amp correct? in this case when you bridge all you are doing is doubling the output voltage. little surprise its getting much louder. like I said, download some kick drum samples to test with and play them at 0 db.

Now that would make perfect sense. BUT - I limit the bridged amp to 50 volts. That is my issue - both the bridged channel and stereo channel are limited to 50 volts via the DSP.

Is it just that the bridged amp, being more powerful, outputs the signal higher than an unbridged amp prior to hitting the limiter?

For example (I hope my math is correct) - if 1.4V is going into the amp (XLS1500 sensitivity is 1.4V and stereo channel amp gain is roughly 35) then we're looking at roughly 49 volt output.

In the same exact scenario for a bridged XLS1500 the amp gain is roughly 65.6. So 1.4v going into the amp would output 92 volts (if it wasn't limited). I limit it down to the 50 volt level with the DSP.

So at 1.2v input the unbridged yields 42 volts.
At 1.2v the bridged yields 79 volts - but it's limited to 50.

At 1v unbridged is 35v, bridged is 65 (but limited to 50)

Wow - I think I just answered my question. As the input decreases the unbridged channel's voltage of course decreases. The bridged stays at the same voltage level because it's over the limit threshhold anyway and is being limited to 50 volts.

And I have been a sissy with the input levels into the amp anyway when playing with the speakers hooked up and don't get near the limiters. So then the amp gain difference for the same input level is huge - Amp Gain of 35 unbridged versus Amp Gain of 66 bridged. That's why the bridged amp is soooo much louder.

Now this makes sense - am I correct?

[byacey]

I think you said it all in the first post: Amp clips when not bridged, even with limiter set to 50 volts, but doesn't clip when bridged. Either your amp has zero headroom at 50 volts output, or the limiter isn't clamping down fast enough.

I realize many people don't believe the output voltage changes between a loaded output and unloaded, but it can be a significant factor due to power line sag especially in conventional transformer type power supply amps. Your amp rails may be sagging enough that you are running into a zero headroom condition, causing the clipping and associated distortion.

As mentioned in one of the other replies, always measure the amp output voltage when switching between modes of operation, as the limiter threshold will certainly change between bridges and un bridged.

[Bruce Weldy]

Is it just that the bridged amp, being more powerful, outputs the signal higher than an unbridged amp prior to hitting the limiter?

Ok....this is backwards. The limiter comes before the ampilier, not the other way around.

In the same exact scenario for a bridged XLS1500 the amp gain is roughly 65.6. So 1.4v going into the amp would output 92 volts (if it wasn't limited). I limit it down to the 50 volt level with the DSP.

When you say DSP, what exactly are you talking about? Do you have a driverack or similar? Because the on-board limiter on the XLS series is not at all capable of limiting properly for your purposes.

Something sounds all wrong here.....you are trying use math to figure out what's happening. You need to be doing that with a voltmeter.

[yigba]

Ok....this is backwards. The limiter comes before the ampilier, not the other way around.

I was referring to the input level going into the amp.

When you say DSP, what exactly are you talking about? Do you have a driverack or similar? Because the on-board limiter on the XLS series is not at all capable of limiting properly for your purposes.

Something sounds all wrong here.....you are trying use math to figure out what's happening. You need to be doing that with a voltmeter.

The DSP I use is the BSS336 Minidrive. And I was using math to conceptually figure out what was happening so I could understand it.

It makes complete sense now, and that's all I wanted. How can it be wrong? The bridged amp is louder simply because of the amp gain factor.

I know Bill says not to bridge if you don't have to but in my case it looks like that is the way to go.

[byacey]

Bruce is absolutely correct. Ignore whatever the limiter increment values that display in your DSP, and simply adjust the limiter threshold while passing signal and monitoring the amplifier output with your voltmeter. Set it according to the actual meter reading, and nothing else. Make sure that if there is a choice, the limiter is set to hard limiting, and not a soft knee.

If you change the amp to bridge mode, you have to go through the limiter setup procedure again with the meter.

[yigba]

Ok....this is backwards. The limiter comes before the ampilier, not the other way around.

I was referring to the input level going into the amp.

When you say DSP, what exactly are you talking about? Do you have a driverack or similar? Because the on-board limiter on the XLS series is not at all capable of limiting properly for your purposes.

Something sounds all wrong here.....you are trying use math to figure out what's happening. You need to be doing that with a voltmeter.

The DSP I use is the BSS336 Minidrive. And I was using math to conceptually figure out what was happening so I could understand it.

It makes complete sense now, and that's all I wanted. How can it be wrong? The bridged amp is louder (when the input signal is lower than the amp's sensitivity rating) simply because of the amp gain factor.

I know Bill says not to bridge if you don't have to but in my case it looks like that is the way to go.

[yigba]

Bruce is absolutely correct. Ignore whatever the limiter increment values that display in your DSP, and simply adjust the limiter threshold while passing signal and monitoring the amplifier output with your voltmeter. Set it according to the actual meter reading, and nothing else. Make sure that if there is a choice, the limiter is set to hard limiting, and not a soft knee.

If you change the amp to bridge mode, you have to go through the limiter setup procedure again with the meter.

Did all of that, in each of my scenarios. Didn't bother looking at the limiter values, just the voltmeter.

[Bruce Weldy]

Bruce is absolutely correct. Ignore whatever the limiter increment values that display in your DSP, and simply adjust the limiter threshold while passing signal and monitoring the amplifier output with your voltmeter. Set it according to the actual meter reading, and nothing else. Make sure that if there is a choice, the limiter is set to hard limiting, and not a soft knee.

If you change the amp to bridge mode, you have to go through the limiter setup procedure again with the meter.

Did all of that, in each of my scenarios. Didn't bother looking at the limiter values, just the voltmeter.

Then, if the voltage was achieved by the amp in stereo mode was exactly the same as that in bridge mode the sub would indeed sound louder in bridge mode up until the limiter engaged - after that it will be exactly the same.

As an aside......you did check voltage across both red terminals when testing in bridge mode, right? Not the red and black like in stereo?

[byacey]

Then, if the voltage was achieved by the amp in stereo mode was exactly the same as that in bridge mode the sub would indeed sound louder in bridge mode up until the limiter engaged - after that it will be exactly the same.

As an aside......you did check voltage across both red terminals when testing in bridge mode, right? Not the red and black like in stereo?

I don't follow what you are trying to say; 25 V out in bridge mode will sound just as loud as 25V out in stereo mode. The input however, will be 6db more sensitive, if that's what you mean.

Good point on measuring between the + binding posts.