what could be the reasons for High freq roll off after 14khz - Page 6 - diyAudio
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Old 7th February 2013, 05:27 PM   #51
llwhtt is offline llwhtt  United States
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I don't think that's the problem as 10 Ohms and 100nf is pretty much the standard Zobel network on most amplifiers, using 100pf may get the response up on the high end but it's only masking the real problem.

Craig
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Old 7th February 2013, 05:48 PM   #52
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yeah i do agree but I see that the response is good enough is there anything wrong why it could be behaving like that?
just that one capacitor is changing everything...
what could be the problem.... I couldnt get it since the gain setting is made properly so even the input rc constant is not a problem as 2.2uf and 2.2k is still decent

I could see only that one value... but the strange thing is that in the RTA it was showing rolloff after 14Khz and in the amp sim its showing before that... howz that possible?
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Old 7th February 2013, 06:45 PM   #53
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If your Zobel network and the output coil network both cause such heavy HF attenuation of the amplifier response, the error is in the amplifier circuit, preamp and/or simulation. We have been over this several time already and if you won't look at other designs to realize that virtually all class AB amps use these standard values, give or take, then you will get nowhere just trying crazy different parts like this in all the wrong places.

The amplifier shown, is designed and specified for a flat response within its bandwidth. If it doesn't do that with standard components as supplied, return it for a refund. If you actually want boosted treble response, the place to do that is in the preamplifier (assuming you are using an appropriate preamplifier and not feeding something poorly matched), never by surgery on the amplifier.

As advised earlier, you play with fire if you reduce the effectiveness of the zobel network by raising the pole into the RF bands. The result will be dangerous instability, as could well be the result of cutting the output coil back to a pointless value that will be less significant than even the speaker cable. It may be fine with trivial loads and short leads, but if you don't mind smoke and fireworks, try it again at full power in a real venue.

This is just being silly, whatever the apparent improvements. Why not tell us what sound card is being used or is specified for an on-board type. RTA is only as good as the card and wiring.
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Last edited by Ian Finch; 7th February 2013 at 06:50 PM.
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Old 7th February 2013, 08:37 PM   #54
JMFahey is offline JMFahey  Argentina
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That amp can feed many amperes into a speaker load, how come it can't drive a puny 100nF cap at 14 KHz?
At that frequency it still has over 100 ohms reactance.
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Old 7th February 2013, 08:46 PM   #55
llwhtt is offline llwhtt  United States
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The proper test equipment would probably tell the whole story, without it we're just guessing. This amp needs an o'scope.

Craig
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Old 8th February 2013, 02:42 AM   #56
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yeah do agree we need a proper test equip but im looking from the simulation point of view that if something is wrong at design phase we can correct it but lets get the test equp this time...

there is one more thing i wanted to tell that for the 1st order crossover for the tweeter ive used .47uf caps x 28 to get the total cap of 13.4uf for the tweeter..

i suspect that the freq rolloff is happening with the inductance caused by these huge number of capacitors at high frequencies but they are still in parallel would that be causing the rolloff? lets see..
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Old 8th February 2013, 03:16 AM   #57
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I havent read all posts and maybe things have moved on but I'm with those suggesting its the (laptop!) soundcard. My lenovo falls off a cliff @ 10khz.
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Old 8th February 2013, 04:01 AM   #58
JMFahey is offline JMFahey  Argentina
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Something that is "brickwall" bandwidth limited at, say, 22 or 24 KHz, can't be trusted above half that, simply because the system runs out of samples to depict *any* waveform.
So that's not so unexpected.
What *is* strange is that even in the pure, ideal world of Simulation, response falls off in such a way, without nothing warranting it.
And the resonant peak is even worse.
Searching for posible culprits: where/how did you get the equivalent model of your driving IC?
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Old 13th February 2013, 03:14 PM   #59
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check this ckt where there is not RL network at the output its Holton amplifier AV800 doesnt it create problem if RL is not used for such high power amplifier?

Hi JMFahey i got the Tina Ti model directly from the TI website just simulated the output from there...
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Old 13th February 2013, 05:06 PM   #60
JMFahey is offline JMFahey  Argentina
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OK, thanks.
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