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Old 26th June 2012, 05:47 PM   #6461
DF96 is online now DF96  England
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Quote:
Originally Posted by simon7000
How would it be worse? A class D amplifier is usually the worst case. For that it would really be -34 db.
Class D (or similar switching amplifiers) is probably the only architecture with a direct link between PSU ripple fraction and audio output ripple fraction. For everything else there is not such a link; the most we can say is that output ripple is probably proportional to PSU ripple but the ratio is unlikely to be 1.

Here is a 'proof by construction' (hinted at in my previous post).
1) Assume an amp with perfect PSRR in the output stage, but very poor PSRR in the input stage. Assume 1% ripple on a 50V supply, so the input stage sees 0.5V on its supply which it passes straight through to the next stage. Assume a gain of 50 from here to the output, so 25V of ripple at the output. Peak output is 50V so we have generated 50% ripple at the output from 1% on the PSU.
2) Now assume an amp with perfect PSRR in the input stage, but quite good natural PSRR of -20dB in the output (maybe it has a pentode or BJT collector output). So our 1% ripple (0.5V) appears as 0.05V at the output: 0.1% ripple wrt full output.

So 1% ripple from the PSU can produce 50% or 0.1% ripple at the output. One proviso, which gives you some wriggle room: full output means just below clipping, including any ripple-based clipping. Once you get hard clipping then the amp has become a switcher and then 1% PSU ripple turns into 1% output ripple.
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Old 26th June 2012, 05:50 PM   #6462
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If you have 1% ripple and a gain of 50 you still have 1% ripple!
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Old 26th June 2012, 06:07 PM   #6463
dvv is offline dvv  Serbia
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Quote:
Originally Posted by Pano View Post
Thanks DVV I see the voltages climb as the speaker sensitivity drops.

Are the voltages above what you measured directly from my test tone?
No, I'm afraid they are not, this was measured in 2002.

Perhaps you'll be interested to know that we also measured voltages at a sound pressure level we agreed was becoming uncomfortably high for any longer period listening. We tried varieties of music types, as I listen to just about everything but classical and jazz.

My own 1041 needed just 20 Vpeak to start ratlling my windows. The JBL required some 24 Vpeak, and the AR needed 27 Vpeak, but let me remind you that its lower impedance also required more than twice the current delivery on average, and a lot more at around 220 Hz, where its impedance cutve drops down to just under 3 Ohms, with a phase shift of some -46 degrees. They sound really good, but they tax the amps withou shame.
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Old 26th June 2012, 06:14 PM   #6464
Pano is offline Pano  United States
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Cool, thanks DVV.

Yeah, a.wayne doesn't need all that much voltage for his speakers, but gobs current for the 1 ohm load. That means a hefty PSU.
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Old 26th June 2012, 06:41 PM   #6465
bcarso is offline bcarso  United States
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Quote:
Originally Posted by nigel pearson View Post

Someone said about Hypex in half bridge . I think that's hype . If it wasn't it would be SE . I was glad hetrodyning was mentioned . I remember putting a class D amp on my scope for the first time . I couldn't believe what I saw . Sure my spectrum analyzer said it was OK ( ish ) in the audio band . Still couldn't believe it could work at all . If I made a conventional amp with problems like that I would doubt it could ever sound good . I would also be too ashamed to market it . Class D is the diesel of amplifiers . All very impressive . It is no jet engine .
That someone was me, thanks very much for the attribution .

"Half-bridge" = hype?? Say what? Around here a half-bridge is simply a push-pull arrangement of devices in a single-ended configuration. Typically half-bridge and full-bridge are from the vocabulary of switchmode power supplies and switchmode amplifiers. And since N channel DMOS always outperforms PMOS for the same silicon area, much effort has been made to use the same polarity device for the upper and lower switching duties, which has led to a lot of very clever circuitry for the upper device ("high-side") drivers. But I digress.

Your doubts about class D sounding good may be a surmise based on limited experience with specific systems and a lack of understanding as to how they work. I would agree that class D, particularly open-loop class D based on internal digital-domain modulators, are unlikely to really equal the sonics of a no-holds-barred nonswitching amplifier. And it may be mind-boggling to see the magnitude of the out-of-band spuriae after the typical two-pole LC output filter on an oscilloscope, but if done properly it really is out of the audio band. The opportunities for mischief based on signals that get downconverted to in-band signals need serious consideration, to be sure.

As well, if there is no loop closure around the output filter the inductor and capacitor will contribute distortion, and will as well entail an increase in the output impedance, which in turn will change the frequency response at the load. At an AES talk someone asserted that people listening to a particular switchmode amp remarked that it sounded somewhat tube-like, which led me to bark out that surely this was due to the highish output impedance interacting with the load impedance.

One of the benefits of the Bruno Putzeys UcD approach is that there is feedback around the output filter. BTW I receive no promotional considerations from Hypex

Brad Wood
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Old 26th June 2012, 06:51 PM   #6466
bcarso is offline bcarso  United States
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Quote:
Originally Posted by DF96 View Post
Class D (or similar switching amplifiers) is probably the only architecture with a direct link between PSU ripple fraction and audio output ripple fraction. For everything else there is not such a link; the most we can say is that output ripple is probably proportional to PSU ripple but the ratio is unlikely to be 1.

Here is a 'proof by construction' (hinted at in my previous post).
1) Assume an amp with perfect PSRR in the output stage, but very poor PSRR in the input stage. Assume 1% ripple on a 50V supply, so the input stage sees 0.5V on its supply which it passes straight through to the next stage. Assume a gain of 50 from here to the output, so 25V of ripple at the output. Peak output is 50V so we have generated 50% ripple at the output from 1% on the PSU.
2) Now assume an amp with perfect PSRR in the input stage, but quite good natural PSRR of -20dB in the output (maybe it has a pentode or BJT collector output). So our 1% ripple (0.5V) appears as 0.05V at the output: 0.1% ripple wrt full output.

So 1% ripple from the PSU can produce 50% or 0.1% ripple at the output. One proviso, which gives you some wriggle room: full output means just below clipping, including any ripple-based clipping. Once you get hard clipping then the amp has become a switcher and then 1% PSU ripple turns into 1% output ripple.
Another observation relevant to power supply ripple/noise and perceptible output spuriae, for both conventional linear and switchmode amps: we are usually far more sensitive to intermodulation distortion of PS and desired signal than simply a "breakthrough" of the bandlimited sawtooth ripple typical of rectified mains supplies, and this is even more obvious when the IM products both fall into frequencies of high aural acuity and are harmonically unrelated to the music. This would be yet again more obvious if we still paid attention to just-intonational tunings, as within equal temperament there are no pure intervals except octaves, but it's still pretty apparent.
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Old 26th June 2012, 08:15 PM   #6467
dvv is offline dvv  Serbia
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Quote:
Originally Posted by Pano View Post
Cool, thanks DVV.
No problem. If you need anything else, just ask. I like your test.

Quote:
Yeah, a.wayne doesn't need all that much voltage for his speakers, but gobs current for the 1 ohm load. That means a hefty PSU.
Yessir, he surely does. With Wayne, it's much more about the Amperes than the Volts.

While I would normally be sceptical about such arrangements, I remember the now long gone AR 3a Improved and the Yamaha NS 1000 Monitor. Both were very difficult loads to drive, so much so that Otala used them as examples of how tough loads can become, but both were speakers you never forget once you have heard them properly driven. Quite outstanding, both of them.

The trick was finding an amp capable of doing them justice. My reVox integrated A78 amp choked on the AR at anything over room volume, and the transients sounded, well, bland. It just couldn't cope, nor could your usual suspects (Kenwood, Pioneer, Sansui, Technics, etc). It was only when the reVox A740 power amp came that we (owner and myself) could really hear the AR come to true life.

I remember congratulating myself silently for buying AR 5, which were not as difficult and were not 4 but 8 Ohms - and a lot cheaper than the 3A Imoproved, way over my student budget, but admittedly better than my 5.
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Old 26th June 2012, 08:20 PM   #6468
tvrgeek is offline tvrgeek  United States
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Yup. Blew up my dad's Kenwood on a pair of AR-2's. Funny, the Knight tube amp never had a problem. The 3 almost had a tweeter if I remember.
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Old 26th June 2012, 09:25 PM   #6469
DF96 is online now DF96  England
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Quote:
Originally Posted by simon7000
If you have 1% ripple and a gain of 50 you still have 1% ripple!
True, but not very relevant. If you have 0.5V hum on a 1V signal, which was the first example I gave, then that is 50% - even if it arises from 1% ripple on the supply rail.

Quote:
Originally Posted by bcarso
we are usually far more sensitive to intermodulation distortion of PS and desired signal than simply a "breakthrough" of the bandlimited sawtooth ripple typical of rectified mains supplies
Yes. Hum is easy to hear, but I suspect our brains can fairly easily 'tune it out' and ignore it. Hum IM is more difficult to consciously hear, but I suspect that it may be responsible for some cases of 'muddy sound'.
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Old 26th June 2012, 09:50 PM   #6470
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Do really don't understand both issues?

If you have a 50 volt supply with .5 volts of ripple the ripple only is not passed to the next stage unless you are really trying something weird. If you just have a variable resistor powered through a fixed resistor you will have some loss of ripple along with the DC level. Now if you pass that to the next stage the ripple will decrease if it is an equal gain inverting stage. Only if by some not very common method you use a second non-inverting gain stage will the ripple increase.

HOWEVER ALL AMPLIFIERS THAT WOULD HAVE ANY RIPPLE PROBLEM USE ADDITIONAL POWER RAIL FILTERS ON THE INPUT STAGE! The question was about the main filter bank of capacitors and the effect on power output sags.

As to the second comment. the ear is not very sensitive to 50 or even 100 hz. signals. It is most sensitive to ones around 3,000 hz. What was said is that the intermodulation products up in the higher region sound really bad and are more easily detected than the hum. The hum would be masked by the music in actual use.

Last edited by simon7000; 26th June 2012 at 10:00 PM.
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