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Author Topic: HP filter really needed?  (Read 5529 times)

Luca Rossi

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Re: HP filter really needed?
« Reply #20 on: June 30, 2016, 04:30:59 am »

Now reduce a LP filter or raise a HP filter while looking at the voltmeter.

You will see the voltage lower as the bandwidth gets narrower.

This is because there are not as many freq adding together-causing the voltage applied to the load to be higher.

I'm talking about signals like white noise wich has a flat response over the band. In this case when cutting out, the RMS voltage will decrease as i said, but not the Peak voltage... (at least for low-mid order filters). Of course if you use a brick wall slope and filter out with a very narrow bandwidth, let's say one octave only (50-100 hz), the Peak voltage will also decrease because there are not as many freq adding together. But this will never be the case.

Different story is when you have to filter out to prevent from mic's dropping, or when you have a mix with extreme bass content wich has 30 hz +15 db respect to 100 hz. In this case the 30 hz foundamental leads the peak voltage, so filtering out it, will also decrease the peak voltage going to amplifier.
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Ivan Beaver

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Re: HP filter really needed?
« Reply #21 on: June 30, 2016, 06:49:06 am »

I'm talking about signals like white noise wich has a flat response over the band. In this case when cutting out, the RMS voltage will decrease as i said, but not the Peak voltage... (at least for low-mid order filters).
I'm sorry-but I disagree.

Anytime you have 2 signals of different freq, the peaks HAVE to be higher than either of the originals. 

At some points.

Those points are where the signals are in phase (I am NOT talking about polarity here). They will "ride" on top of each other.

Of course at the points at which they are not in phase, the output voltage will be lower than either one.

So in order to reproduce both (or more) of the signals, the amp MUST have the voltage swing available to reproduce those peaks.

So the fewer freq that an amp has to reproduce-then the lower the possibility of the signals "stacking up" and increasing the maximum voltage (and resultant power).
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Ivan Beaver
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David Allred

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Re: HP filter really needed?
« Reply #22 on: June 30, 2016, 07:14:28 am »

ACTUALLY--------

By decreasing the bandwidth of a signal WILL reduce the power that it is putting out.

This is real easy to see for yourself.

Put in a noise source into an amp (no speaker hooked up) that is full bandwidth.

Hook a voltmeter to the output.

Now reduce a LP filter or raise a HP filter while looking at the voltmeter.

You will see the voltage lower as the bandwidth gets narrower.

This is because there are not as many freq adding together-causing the voltage applied to the load to be higher.

Does these analogie work?

A car has a certain amount of horse power available.  That HP is available going up an incline.  If the incline is "filtered out" does the HP increase?  No, but top speed and acceleration increase.

A pickup with a bed full of gravel will struggle to pull a hill, but if the load is "filtered out" the hill is no problem.

In both cases, the transmission lasts longer and the engine sounds better while in operation.

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John Roberts {JR}

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Re: HP filter really needed?
« Reply #23 on: June 30, 2016, 11:30:03 am »

Does these analogie work?

A car has a certain amount of horse power available.  That HP is available going up an incline.  If the incline is "filtered out" does the HP increase?  No, but top speed and acceleration increase.

A pickup with a bed full of gravel will struggle to pull a hill, but if the load is "filtered out" the hill is no problem.

In both cases, the transmission lasts longer and the engine sounds better while in operation.
The technical term for this is superposition of waves, and X plus anything will be larger than X.


For another analogy, imagine you are standing up in a row boat, in a river underneath a bridge and your head barely clears the bottom of the bridge. Just then a power boat races past creating a huge wake (the extra LF content).  As the boat bobs up and down in the wave created by the power boat, your head crashes into the bottom of the bridge...

That is the physical manifestation of inadequate headroom (pun intended).  ;D

JR
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Ivan Beaver

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Re: HP filter really needed?
« Reply #24 on: June 30, 2016, 12:14:06 pm »


For another analogy, imagine you are standing up in a row boat, in a river underneath a bridge and your head barely clears the bottom of the bridge. Just then a power boat races past creating a huge wake (the extra LF content).  As the boat bobs up and down in the wave created by the power boat, your head crashes into the bottom of the bridge...

That is the physical manifestation of inadequate headroom (pun intended).  ;D

JR
I like that analogy. :)
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David Allred

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Re: HP filter really needed?
« Reply #25 on: June 30, 2016, 12:22:32 pm »

The technical term for this is superposition of waves, and X plus anything will be larger than X.


JR

I will bow (pun intended) to you wave bob analogy, but if the "anything" added to X is a "negative anything" it will yield something smaller than X.   ;)
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John L Nobile

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Re: HP filter really needed?
« Reply #26 on: June 30, 2016, 12:23:16 pm »

The technical term for this is superposition of waves, and X plus anything will be larger than X.


For another analogy, imagine you are standing up in a row boat, in a river underneath a bridge and your head barely clears the bottom of the bridge. Just then a power boat races past creating a huge wake (the extra LF content).  As the boat bobs up and down in the wave created by the power boat, your head crashes into the bottom of the bridge...

That is the physical manifestation of inadequate headroom (pun intended).  ;D

JR

Thinking about that analogy makes my head hurt.
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John Roberts {JR}

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Re: HP filter really needed?
« Reply #27 on: June 30, 2016, 12:51:29 pm »

I will bow (pun intended) to you wave bob analogy, but if the "anything" added to X is a "negative anything" it will yield something smaller than X.   ;)
and i thought I was the pedantic one...  :o

For polarity or phase to have any affect on combined waveforms they would have to be the exact same frequency.

In the context of a HPF the summed waveforms by definition will be different frequencies  so combining will always result in higher peaks and more likely amplifier clipping.

QED

JR
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Ivan Beaver

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Re: HP filter really needed?
« Reply #28 on: June 30, 2016, 12:58:08 pm »

Thinking about that analogy makes my head hurt.
Here is another "real world" example.

I have 2 sine waves-both of equal amplitude.

But when I turn them both on, the result get larger and smaller-depending on how the phase relationship of the waves is.

This was done on an old CRT scope-so that is why some of the traces are not "complete".  You are just seeing what is being traced on the screen at the time the shutter on the phone is open.

But you should get the idea.

You can easily see how the higher freq wave is "riding" on the low freq wave and how the low freq wave is pushing the HF wave much higher in level (as in JRs example of a boat)

If the amplifier does not have enough voltage swing (power), it cannot reproduce the HF signal-resulting in distortion.

The top 2 photos are the individual freq.  Both are clean

The bottom left is when they are added together-with plenty of headroom on the amp.

The bottom right is when each signal is just a little under clipping.  So when they are added together, you end up with a greatly clipped signal.

If the lower freq signal was filtered out, then the higher freq one would still be nice and clean.

NOTE: I did readjust the gain on the scope channel to make it fit onto the screen.

In the first 3 photos, no gain was adjusted.
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Ivan Beaver
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Ivan Beaver

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Re: HP filter really needed?
« Reply #29 on: June 30, 2016, 01:12:45 pm »

And to JR

I do realize that my scope was not centered, and the "clean addition" runs off scale.

But since this is not a "paper" I did not worry about doing a great presentation.

Hopefully the idea comes across-especially since this is NOT a simulation, but actual signals and how they add.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

Ivan Beaver
Danley Sound Labs

PHYSICS- NOT FADS!
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