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Old 22nd August 2012, 05:08 PM   #761
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Ap2,
I don't think that if we had loudspeakers that actually produced usable output to even 20hz flat with the power output of the rest of the bandwidth of our speakers we would have any happy neighbors. I know that even at higher frequencies it is hard for our homes to contain those low frequencies. I am not talking about using sub-woofers, those are typically such narrow band devices that I don't consider that a normal part of a regular loudspeaker.
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Old 22nd August 2012, 05:19 PM   #762
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Quote:
Originally Posted by AndrewT View Post
Have you noticed that many "HiFi" amplifiers claim to be flat to 4Hz? This is generally the F(-1dB) value indicating they have chosen F(-3dB) of ~2Hz.
I believe that to get good audio performance from a speaker trying to reproduce down to 20Hz (even though it's frequency response is down by 20dB relative to the 1kHz output) the amplifier must be able to perform well down to ~ 2Hz to 3Hz. This just happens to be very close to the "mainstream" 4Hz value, quoted in specifications.
Andrew I agree but I see the lower limit ~2-4Hz as set to minimize any phase shift introduced at the lower speaker range of 20-40Hz.
So at least for the amplifiers PSU I wouldnt expect one to have to size ripple at this lower 2-4Hz.

Thanks
-Antonio
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Old 22nd August 2012, 05:20 PM   #763
AP2 is offline AP2  Italy
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Quote:
Originally Posted by Kindhornman View Post
Ap2,
I don't think that if we had loudspeakers that actually produced usable output to even 20hz flat with the power output of the rest of the bandwidth of our speakers we would have any happy neighbors. I know that even at higher frequencies it is hard for our homes to contain those low frequencies. I am not talking about using sub-woofers, those are typically such narrow band devices that I don't consider that a normal part of a regular loudspeaker.
yes, I agree with your post. I was referring to the amplifier and consequently to the psu. 4Hz (if true) requires a lot of work on the amplifier, according to the psu dimensioned and calculated for this frequency. I mean, that too easily declare 4Hz flat on amp's.
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Old 22nd August 2012, 05:22 PM   #764
gootee is offline gootee  United States
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Originally Posted by DF96 View Post
In most real amplifiers the ripple will be small in % terms, and the approximate formulas should be good enough. After all, big caps usually have wide tolerances anyway. For the worst case, full amplitude LF square wave, the load is a constant R (at least for a while, until it switches to the other side of the output). I think the problem is that people design with Idc, when they should design with Ipk=2 Idc.

Testing/simulation with square waves might be more tricky, as it would be harder to spot changes in harmonic structure when there are plenty of harmonics already present. Using sine waves, as you are, creates the problem of phase alignment as you have found. Full amplitide square waves could be used if you look for mains-related IM suddenly appearing, although you would have to choose your frequencies carefully. Maybe 25Hz, and look for 95Hz and 145Hz sidebands? You might even see some 35Hz and 85Hz, if the PSU becomes temporarily unbalanced. Testing with square wave RMS values equal to the sine RMS will show nothing, as you have found. You need equal peak values.

Transformer parasitics could be an issue, but these (apart from DC resistance) are usually unknown and could vary from one manufacturer to another.

I realise that the full equations get messy, especially with a varying load. However, a varying load is not the worst case so we don't have to design for it. This assumes, of course, basically good PSRR in the amp circuitry so we only have to worry about output stage limiting. For PSUs, unlike the rest of the circuit, we are not interested in calculating exact values because the exact values don't matter and we can't buy exact components anyway. Therefore we don't need the full equations.

Yes, I think we are mostly on the same page. I commend your effort. You probably know better than this, but I suspect some people are still hoping that some 'magic numbers' will drop out of your simulation. I don't expect this. At best you will confirm what the algebra tells us. I will shut up for a while, unless I can come up with some useful equations.
I DID do some square wave testing at the same peak amplitude as the sines, but only for the middle three cases in the most-recent table.

I was able to find the minimum capacitance for those cases, too, within 1000 uF I think. I am at work and the numbers are at home but I believe that they were 10000uF, 11000uF, and 6000uF. I would need to verify those, if anyone wanted to rely on them. And I think I did those with a positive-only square wave, for some reason, and then started messing with the .meas commands and never got back to continue with the square waves.

But whatever the minimum C numbers are for squarewaves at the same peak amplitude as the sines, using those values (or more) for reservoir capacitance should give very high confidence that the C value is at least not too low for the power supply to handle the peak output signal, regardless of waveform type.

Regards,

Tom

Last edited by gootee; 22nd August 2012 at 05:40 PM.
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Old 22nd August 2012, 06:30 PM   #765
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Quote:
Originally Posted by AP2 View Post
yes, I agree with your post. I was referring to the amplifier and consequently to the psu. 4Hz (if true) requires a lot of work on the amplifier, according to the psu dimensioned and calculated for this frequency. I mean, that too easily declare 4Hz flat on amp's.
The signal itself would/shouldn't have to go as low as 4Hz. That marker is just to reduce phase shift of the roll-on filter at the "usable" start of the audio band, around 15Hz and up signal wise. I have the roll-on filter on MF-80 set at 1.5Hz. Try it, it will be a whole new bass experience when you're 'used' to higher frequency roll-on filters.
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Old 22nd August 2012, 08:30 PM   #766
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MagicBox,
I think that we are all in agreement that a wide bandwidth in the amplifier is important and that we don't want the band pass filters to get into the audio range. How much difference there is between 1.5 hz and 4 hz I don't know without doing the math and seeing how far that shifts the high pass frequency where it is still flat in the pass band. I don't think that anyone is going to argue this point, at least I hope not.
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Old 22nd August 2012, 08:54 PM   #767
DF96 is offline DF96  England
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LF rolloff is not really relevant to this thread, but there may be good reasons to limit the bandwidth. There is little point in forcing a transducer to attempt to reproduce a frequency which it cannot handle. There will already be significant LF phase shift from the loudspeaker, and this would be a second or third-order rolloff. A first-order rolloff from the amplifier could be harmless in comparison.
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Old 22nd August 2012, 09:18 PM   #768
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DF96,
Isn't that what MagicBox is referring to in his statement about the roll-on frequency he is setting at 1.5hz? Perhaps I am missing something here.
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Old 22nd August 2012, 09:23 PM   #769
DF96 is offline DF96  England
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A roll-off of 15Hz might be more sensible than 1.5Hz, unless you have a very big room, very big speakers and definitely never use a turntable. I was disagreeing with your suggestion that we all agree about wide bandwidth. I prefer appropriate bandwidth.
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Old 22nd August 2012, 09:36 PM   #770
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DF96,
I agree and often wonder about some who insist on needing a system that actually produces bass down to 20hz and over 25khz. I have good hearing and I know that 25Khz is out of my range, though I still get up to about 20khz. And what besides a pipe organ or a steam train or synthesizer has anything musical at 20hz? Not a stand up bass or anything like a normal drum head or any orchestral instrument that I can think of. And yes I do still have a turntable and albums in my collection. Nothing like the rumble of a low note getting into feedback with the turntable and arm. I have been to many a live show behind the mixing console and I always see the low frequency cut set higher than 20hz. Bass feedback through the mics can be worse than a high frequency feedback if it just barely is perceptable. I can't even stand the sun roof open in some cars, the low frequency rumble will drive me nuts.
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