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Old 28th March 2013, 09:17 PM   #11
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if you choose LM3875, then go for the TF variant. (isolated from heatsink).

allso, Your problems are not going to be the chipamp and its phase shape.
But, getting a proper microphone, and a proper microphone pre-amp, and even something harder,
and enviroment suitable for mesurements are the real problem.
That is, if You are after freqvency response curves.
Actualy room modes, rezonating objects, room gain, reflections and many many other things will have larger impact.
and here is a hint, there are amny software that can actualy use the soundcard intput and output jacks to calibrate it self.
The idea is that is will generate a sine sweep , that goes into Your amp. The amp output is connected to the input of the soundacrd. The difference between the recorded and playd sounds are doe by the amp. So from then correction can be done.
Naturaly before this the input and output are connected with no amp in between to calibrate the stuff.

allso, if, that IF you know what You are doing, and do not have just a verry tiny DC offset You can omitt input caps on an amplifier. So it should go down near DC.
Upwards.. well most chip amps can actually go verry high. Even if they could not, software can correct the falling response curves to some degree.
I'm pretty sure LM3875 TF will be more than enough.
Usually a simple TDA 2050 is more than sufficient for the job.
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Old 28th March 2013, 10:35 PM   #12
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Quote:
Originally Posted by Arty View Post

allso, if, that IF you know what You are doing, and do not have just a verry tiny DC offset You can omitt input caps on an amplifier. So it should go down near DC.
Is it worth it, for the risk of blowing up something (next gen Revelator perhaps?). 10uF and 10k gives you less than 2Hz, which should be low enough.
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Old 29th March 2013, 12:30 PM   #13
AndrewT is offline AndrewT  Scotland
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10uF & 10k give 100ms RC time constant.
For reproducing music and LF sound effects to go along with some music then 100ms is sufficient for all very wideband speakers.

For measurement purposes I would guess one needs to increase that LF bandwidth by a decade to remove the filter effect from the measurements.
DC ability is probably not required but maybe an RC ~ 1s would give a good measurement system.
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Old 29th March 2013, 01:01 PM   #14
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if you know what you are doing then it is worth it.
if not, then don't even thinker with it.
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Old 29th March 2013, 10:14 PM   #15
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I agree microphone, and preamp contributes a lot to the improper measurements. So to minimize this, I have got the Preamp calibrated. For mic I have calibration file, which software uses for adjusting the errors. Despite of all the care taken, there is going to be some error creeping in. But I just tried to keep errors the lowest possible.

Regarding using LPF, I don't wish to use one but, would use it for safety against accidental offset DC ( wont know if all players connected while listening would have zero DC offset).

I feel 0.1Hz LPF should solve my purpose of near flat response and zero phase shift in the required frequency range .I have attached the screenshot pls view, and comment.
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Old 29th March 2013, 10:41 PM   #16
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I'm not sure how much protection 0.1Hz is going to afford you. What happens when you connect DC to the input? The capacitor charges for a few seconds.

I really don't think you need go down that far for anything to do with loudspeakers. At LF one generally infers the response from the parameters, which you can measure. And the speakers themselves are producing nothing to speak of at 5 or 10Hz - perhaps 30 or 40dB down.

You may want to measure around the LF resonance to see if it is doing what you expect and nothing in the accuracy of this will be affected by a pole at 1.6Hz. A different temperature in the room or of the voice coil will have a far bigger effect and, swamping them all, the amplitude at which you take the measurement will have the biggest effect of the lot, moving the resonance a semitone or two.
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Old 30th March 2013, 05:14 AM   #17
JMFahey is offline JMFahey  Argentina
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2Hz HPF is more than good enough for all the loudspeaker measurements you may ever make.

Any lower, only for earthquakes, Nuclear test blasts and whale communications.
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Old 1st April 2013, 06:22 AM   #18
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OK, so will be using 2Hz. HPF & will make a provision to bypass the filter. and compare the difference that can be seen in real life measurement.
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Old 1st April 2013, 06:30 AM   #19
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that seems like the ideal way, yes.
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