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Author Topic: Ripple In DBX4800  (Read 6432 times)

David Sturzenbecher

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Ripple In DBX4800
« on: August 18, 2013, 10:18:53 am »

Hello,
I am in the process of pulling some JBL presets from a DBX4800 and creating ItechHD presets from the transfer functions.  When I took the TF of the DBX4800 I get alot of ripple in the HF.  My resolution is quite fine, and when smoothed even a little it goes away, but I am wondering if anyone can take a TF of the 4800 and see if you see anything similar. Setup was just a Octapre sending pink noise into the 4800, and measuring the output from the individual channels.
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Marcus Baeumler

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Re: Ripple In DBX4800
« Reply #1 on: August 19, 2013, 10:31:05 am »

Hello,
I am in the process of pulling some JBL presets from a DBX4800 and creating ItechHD presets from the transfer functions.  When I took the TF of the DBX4800 I get alot of ripple in the HF.  My resolution is quite fine, and when smoothed even a little it goes away, but I am wondering if anyone can take a TF of the 4800 and see if you see anything similar. Setup was just a Octapre sending pink noise into the 4800, and measuring the output from the individual channels.

Are the two signals time aligned? Since the 4800 probably adds some propagation delay the result in the TF will be comb filtering, which I suspect is what your measurement shows.
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David Sturzenbecher

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Re: Ripple In DBX4800
« Reply #2 on: August 19, 2013, 08:34:15 pm »

I don't know why the two signals would need to be time aligned. They don't ever get combined, so there would be no way for them to comb. I took the exact setup over to the ITechHD with a VI box and 8ohm resistor and there was no ripple.
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Jelmer de Jong

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Re: Ripple In DBX4800
« Reply #3 on: August 20, 2013, 05:53:20 am »

I don't know why the two signals would need to be time aligned. They don't ever get combined, so there would be no way for them to comb. I took the exact setup over to the ITechHD with a VI box and 8ohm resistor and there was no ripple.
If you don't sync your measuresignal to your referencesignal there is no way for the software to display a accurate line on screen. Since there is no ripple in LF or MF the delay to be set is probabbly pretty small (0.xxms)
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Adam Black

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Re: Ripple In DBX4800
« Reply #4 on: August 20, 2013, 11:15:47 am »

Can verify if it's a comb filter by looking at the phase. A comb filter should also be visible with an RTA, albeit perhaps not quite so obvious. A comb filter is created when there are two arrivals that differ in time. How deep the troughs are will vary based on how much the signals vary in level.

I'd first verify that the routing and processor settings are correct.
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David Sturzenbecher

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Re: Ripple In DBX4800
« Reply #5 on: August 20, 2013, 11:49:35 am »

Can verify if it's a comb filter by looking at the phase. A comb filter should also be visible with an RTA, albeit perhaps not quite so obvious. A comb filter is created when there are two arrivals that differ in time. How deep the troughs are will vary based on how much the signals vary in level.

I'd first verify that the routing and processor settings are correct.


Attached is a picture with phase.  I suppose it could be crosstalk or bleed in the Octacapture, but I am not sure why it wouldn't be visible when the octacapture was moved to the ItechHD.   
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Adam Black

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Re: Ripple In DBX4800
« Reply #6 on: August 20, 2013, 04:18:28 pm »


Attached is a picture with phase.  I suppose it could be crosstalk or bleed in the Octacapture, but I am not sure why it wouldn't be visible when the octacapture was moved to the ItechHD.   

Doesn't look like a comb filter to me. The Octacapture is quite configurable, I'd first make sure it's setup correctly. If you have another device, I'd test the amp using it as well. Anything to rule out pieces of the signal chain.
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Malcolm Macgregor

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Re: Ripple In DBX4800
« Reply #7 on: August 22, 2013, 08:58:09 pm »

The phase display shows a time-difference, so you might need to check the measurement delay.
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Mac Kerr

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Re: Ripple In DBX4800
« Reply #8 on: August 22, 2013, 10:00:43 pm »


Attached is a picture with phase.  I suppose it could be crosstalk or bleed in the Octacapture, but I am not sure why it wouldn't be visible when the octacapture was moved to the ItechHD.   

As Malcolm said, there is definitely a time offset between the reference signal and the DUT signal. When you are measuring a piece of electronics the phase plot should be mostly flat across the whole band. With no filters the phase plot should be completely flat, but with the band pass and parametric filters you have you will not have a flat phase curve, but it should not look like what you have.

Mac
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David Sturzenbecher

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Re: Ripple In DBX4800
« Reply #9 on: August 22, 2013, 11:55:11 pm »

As Malcolm said, there is definitely a time offset between the reference signal and the DUT signal. When you are measuring a piece of electronics the phase plot should be mostly flat across the whole band. With no filters the phase plot should be completely flat, but with the band pass and parametric filters you have you will not have a flat phase curve, but it should not look like what you have.

Mac

The idea was not to measure the 4800, but rather the preset inside the 4800, so i really wanted to capture the phase slope to make sure my signal delay would be consistent in the new device.  My measurements are not time windowed so I was under the impression that there would be no loss of data.

My concern is that if I add delay for each bandpass individually for the reference measurement to get a flatter phase it changes the time relationship between the band passes.  If i remember correctly from when I took the measurements I did set the delay to the Low frequency transfer function, then left it in place for the HF transfer functions.

Thanks for the input everyone, sometime in the near future I should have access to this 4800 as well as some others, plus I also use a USBPre2 that I can retake the measurements again.
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Re: Ripple In DBX4800
« Reply #9 on: August 22, 2013, 11:55:11 pm »


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