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Old 27th September 2012, 01:08 PM   #7271
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Two questions . Ignoring the feedback argument which seems strong . Can we usefully apply input bandwidth limiting ? My by ear tests were inconclusive as I feel my amplifier has no real problems . If I deliberately made a low slew rate amplifier would some sensible input filtering transform it ? Theory suggests it should . As John said an MC pick up might generate 200 kHz output . I could imagine a 4 pole passive input filter is required ? These are things I know well so should not ask . Still very interesting to read the views of others as often the right thing's are done for the wrong reasons .

LTP degeneration . If we have the re of the Long Tail Pair set at 50 R is that less good than lets say 39 R external ( RE ) and 11R internal ( re ) . My instinct is to say the eternal is a real resistor and therefore better . I have always assumed 50 R ( per emitter ) in all my designs . It seems to be a good compromise . I found the inclusion of the resistor or cascode in the CCS very interesting ( tail ) . Almost a free lunch .
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Old 27th September 2012, 04:15 PM   #7272
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No, you cannot apply bandwidth limiting without adding noise and potential distortion. Just get over it AND design proper circuits and they can be fast enough.
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Old 28th September 2012, 07:09 AM   #7273
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No, you cannot apply bandwidth limiting without adding noise and potential distortion. Just get over it AND design proper circuits and they can be fast enough.
There, you see, Nige? Exactly what I've been telling you all along.

And you ask me why I think Beritish made amps sound slow and sometimes quite dead - band limiting is useful only way up there at or above 200 kHz. Most British amps use band limiting both down below at or below 20 Hz and up above from 50 kHz or so. The logic being "There's nothing there that we want." Not quite so.

I apologise for coming in so late to a subtopic I sort of initiated, but this forum has a wild way of sometimes NOT alerting me to new posts, and it has been dead silent for the last 6 days or so. I came in today to ask what happened, somebody die, or something, and I find you eager little beavers are at it full force.

John, thans for the hint, GK7 thanks for the link. I downloaded it so I can study it later, I don't want to just run through it.

From what I gathered from the discussion here, it seems that the criteria suggested in the Nationl Seminvonductor Audio cookbook, 0.5 V/uS per PEAK volt output is, as I took it even then, too conservative. Right from the start, I assumed double that value, i.e. 1 V/uS per peak output volt, or 40 V/uS for a nominally 100 W/8 Ohms. More can't hurt, unless it compromises something else.

I sort of knew H/K got it right when they made HK 680 integrated amps, nominally delivering 85/130 W 8/4 Ohms, but giving it a slew rate of 80 V/uS. Richard Miller, who is credited as the chief of its design team, obviously agrees with Otala, Jung & Curl. Hardly surprising, given that H/K employed Otala for some 6 or so years.

On a purely personal level, I am very pleased with this turn of events, I have always thought and spoken out loud that H/K is the one company to watch among all mass producing manufacturers, they were always at least one cut, and often more, in front of the rest of the pack. Citation XX is still thought by many to be one of the best power amps ever made by anyone, but unfortunately, few were made and it's really rare. I never even saw one live.
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Old 28th September 2012, 07:23 AM   #7274
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No, you cannot apply bandwidth limiting without adding noise and potential distortion. Just get over it AND design proper circuits and they can be fast enough.
Hallelujah and amin!

As ever since the beginning of time, it all boils down to use your head when designing it and get it right from the design stage, later added cure-alls will only ruin it.

Be balanced, favor nothing over the whole, and for the love of God, do not save on your output stage, be generous. Power supplies are assumed to be top notch.

Too bad Thorsten is not here, most of this is right up his alley. I've been talking to him quite a lot, and much of what I've seen here I also heard from him.

Thank God, common sense and good reasoning is still alive and well.
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Old 28th September 2012, 07:33 AM   #7275
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And of course the real answer, as John alluded to, for why really fast slew rate is necessary, is that it's required for the FB mechanism to function correctly. Funny concept, the amp has to work well enough to make sure the error correcting mechanism works well enough, hmmm ...

Frank
Honestly Frank, I don't find it strange at all. You can find the notion documented in Otala's papers published by IEEE in 1976 or 1977, not sure which.

As John quite rightly pointed out, Matti Otala and Walt Jung beat the subject to death.

I wish I could send you a copy of Otala's initial paper, but unfortunately, I no longer have a photostat copy of it, perhaps some good soul here has a link to it?

I guarantee it's WELL worth reading (or your money back ).
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Old 28th September 2012, 07:36 AM   #7276
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Let's discuss now who first started using Ohm's law in audio amplifiers.
No.... don't tell me somebody is still using that ancient waffle?
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Old 28th September 2012, 11:24 AM   #7277
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I suspect it was Lee de Forest who first used Ohm / Kirchhoff . It deserves an answer .

I came to the occlusion that input slew limiting required a filter far more sophisticated that is commonly used if to be of any use . For example a 4 pole Butterworth with 35 KHz cutoff frequency would be about -0.05 dB @ 20 kHz , - 3dB @ 35 kHz and -36 db @100 KHz . If this could be a offered in a preamp as an additional output then perhaps it has value ? NAD had the less filtered output called LAB .
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Old 28th September 2012, 01:18 PM   #7278
a.wayne is offline a.wayne  United States
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Originally Posted by dvv View Post
Hallelujah and amin!

As ever since the beginning of time, it all boils down to use your head when designing it and get it right from the design stage, later added cure-alls will only ruin it.

Be balanced, favor nothing over the whole, and for the love of God, do not save on your output stage, be generous. Power supplies are assumed to be top notch.

Too bad Thorsten is not here, most of this is right up his alley. I've been talking to him quite a lot, and much of what I've seen here I also heard from him.

Thank God, common sense and good reasoning is still alive and well.
Where ..? T I mean not common sense . Where are you communicating , you got pass the gate ...?

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Old 28th September 2012, 01:35 PM   #7279
jcx is offline jcx  United States
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Honestly Frank, I don't find it strange at all. You can find the notion documented in Otala's papers published by IEEE in 1976 or 1977, not sure which.

As John quite rightly pointed out, Matti Otala and Walt Jung beat the subject to death.

I wish I could send you a copy of Otala's initial paper, but unfortunately, I no longer have a photostat copy of it, perhaps some good soul here has a link to it?

I guarantee it's WELL worth reading (or your money back ).
actually Cordell did a lot to beat to death Otala's specific prescription for avoiding TIM/PIM with "flat loop gain" – with better explanation, Cordell built hardware both dedicated test hardware and low PIM but high feedback amp

CordellAudio.com - A MOSFET Power Amplifier with Error Correction

both Walt Jung and Marshall Leach were early Otala enthusiasts - later came to the conclusion that you don't have to use Otala's "flat loop gain" prescription - high feedback, sloping loop gains can still have vanishingly low TIM/PIM

perhaps Otala deserves credit for sparking the renewed slew analysis effort in audio amps but his actual recommendation, initial model have been shown lacking

Last edited by jcx; 28th September 2012 at 01:41 PM.
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Old 28th September 2012, 03:40 PM   #7280
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actually Cordell did a lot to beat to death Otala's specific prescription for avoiding TIM/PIM with "flat loop gain" – with better explanation, Cordell built hardware both dedicated test hardware and low PIM but high feedback amp

CordellAudio.com - A MOSFET Power Amplifier with Error Correction

both Walt Jung and Marshall Leach were early Otala enthusiasts - later came to the conclusion that you don't have to use Otala's "flat loop gain" prescription - high feedback, sloping loop gains can still have vanishingly low TIM/PIM

perhaps Otala deserves credit for sparking the renewed slew analysis effort in audio amps but his actual recommendation, initial model have been shown lacking
JCX, I am 59 years old. For the last 25 years, I have witnessed and in some cases participated in oh-so-many pro- and anti-Otala forums, I lost count. I am definitely NOT about to get myself into another Cordell vs Otala debate.

I do not claim or think that Otala got it all right, or even just got it all. Ditto for Cordell, or for that matter, anybody. I acknowledge that there are many roads which lead to Rome, that there is no one single way to do things, but at least several, or even many.

I believe in what Otala has said (the part I know of) because it has served me well and because I choose to do so. But this in no way means Otala is perfect, or that there are no others who may also have got it right, just used a different method. I don't know, I haven't researched them all, simply because of the old rule - if it ain't broke, don't fix it.

And I do have a beef with the extreme Otala camp people, who have somehow come to the conclusion that one should use no global NFB at all if possible. Otala never said "Thou shalt not use any global feedback". He simply said that global NFB should be used to sort of iron out the already clean drip-dry shirt, use it as icing on the cake. Using no global feedback usually means letting your output stage work with some error, although I have seen schemes to wrap the output stage inside it own local NFB loop. My feeling is that this is not worth the time and trouble, and my experience tells me that the best sounding (to me) amplifiers WILL have a global NFB somewhere in the region of 17 ... 26 dB, an open loop FULL power bandwidth of no less than 50 kHz, etc.

So, please, let's leave it at that.
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