what could be the reasons for High freq roll off after 14khz - diyAudio
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Old 3rd February 2013, 11:24 PM   #1
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Default what could be the reasons for High freq roll off after 14khz

I have a lm4702 2sc5200 1943 pair amp. I have measured in room using a RTA with berdynamic mic.. we were playing through sony laptop which sound ashtonishingly good but we have seen a rolloff from 14khz to 20khz

im using vifaxt25sc ring radiator using a first order crossover but used a first order crossover with 28 .47uf in parallel to achieve the capacitor.

is the rolloff because of the possibilities..

1. the amp has a 7uH inductor approx ( 10 turns of 5mm dia 1mm thick coil ) at the output
2. the capacitors in parallel are making effective inductance because of leads ( infact they have to get reduced coz they are in parallel)
3. the mp3 320kbps format might not have enough HF information
4. Do room acoustics play role ( since we have kept the mic on axis at 1m)

i would like to know that is the inductor at the otuput makes any difference?
what if its removed? any problems? could the rolloff be happening because of that?
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Old 4th February 2013, 12:45 AM   #2
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I found couple of things..

1. the capacitor datasheet specifies that there will be rolloff after 100khz
2. I have found the inductor value as .18uH and checked what kind of imp it would give at different frequencies I found that..
the imp at the following frequencies..
at 1khz its 1.13
at 10khz its 11.13
at 15khz its 16.96
at 18khz its 20.36
at 20khz its 22.62

is it the reason why its attenuating the upper frequencies...
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Old 4th February 2013, 12:47 AM   #3
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oops sorry the measurement is done using pink noise with presounus live 16.4.2 so its not related to mp3s..
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Old 4th February 2013, 02:48 AM   #4
Bare is offline Bare  Canada
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Erm Measured OR heard ?
IMO genuinely unlikely you actually heard the problem.. IF.. over 45 yrs old and a Human Male
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Old 4th February 2013, 02:55 AM   #5
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measured...
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Old 4th February 2013, 12:26 PM   #6
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i found where the prob is coming frm... its coming from the RL network on the output when i connect the output before the inductor everything is working perfect all the hf information and transparency opened up... what values of RL recommended?

what if the RC network is removed..
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Old 4th February 2013, 12:33 PM   #7
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Location: Fosser, Aurskog-Holand, Akershus, Norway.
What values did You have there?
Linkwitz-Riley-network usually consist of a 10 Ohm resistor in series with a 0uF1 film capacitor. Different values and configurations are also seen, but I guess about 98% of amplifiers use this one.

I would not recommend to remove the filter.
Would give "infinite" bandwiith out with all problems that would occur in Your speakers and amp.
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Old 4th February 2013, 02:06 PM   #8
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Ive tried removing both the filters just straight but I dont see any problems its playing fine and clear. Do you mean that if I increase the volume then the amp would oscillate?

anyway there is a NFB so why will be the bandwidth infinite?
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Old 4th February 2013, 02:15 PM   #9
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You can most times remove the RL filter. The one with coil.
But you are playing with fire if you remove the RC filter.
Sooner or later there will be smoke.
Keep 10R in series with 100nF
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Old 4th February 2013, 02:21 PM   #10
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Quote:
Originally Posted by TANDBERGEREN View Post
Linkwitz-Riley-network usually consist of a 10 Ohm resistor in series with a 0uF1 film capacitor. Different values and configurations are also seen, but I guess about 98% of amplifiers use this one.
It is a Zobel network, not a Linkwitz-Riley one
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