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Old 21st September 2012, 07:03 AM   #7201
dvv is offline dvv  Serbia
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Originally Posted by nigel pearson View Post
Strange thing is Proton who made them tried to market an improved version when their contract was up . It was a pale imitation . 3020 would drive 2 ohms and attempt 92 watts briefly ( Hi Fi Choice test ) . I was told 3020 is full of junk that was selected carefully . Other amps are filled with treasure that often doesn't work . 2 x 3020's is ideal . One as pre and the other as power amp .

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Oddly enough, a few days ago I had the opportunity to audiotion two Proton integrated amps, a 45 and an 80 WPC models. The 45 WPC model sounded all right, no vices I could hear, and had a good sense of contained power, meaning it sounded like it had a lot more than it did according to the specs under the belt.

The 80 WPC model appeared to me to be a very serious player, not to be taken lightly at all. Not perfect, but surprisingly good. All the more so since it was made in the mid 80ies and hasn't been touched since, I can only imagine what it would sound like if properly refreshed and adjusted. Electronic button switching and all that. It also gave a feeling of appearently limitless power available on demand.

I must say I was pleasantly surprised by both models. The 80 WPC model had a more "modern" sound, while its smaller sibling was quite vintage sounding, smooth overall and very pleasant, but at the cost of some finer detail.

Admittedly, everything was auditioned via AR9 speakers only, and one swallow doth not a spring make, but on the other hand, AR9s are not an easy load to drive (like most AR speakers are awkward loads).

I don't know anything about the company, so if someone could supply some details, I'd be grateful.
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Old 21st September 2012, 08:00 AM   #7202
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If I am right they also built a very interesting valve hybrid called AMC . The output 2 x EL34 with computer fans cooling the valves . Very nicely made in NAD out of Audio Research type of look ( horse breading term ) . The EL34's were Siemens with the valves soldered in on exchangeable PCB modules . The drive circuits were transistor / fet . This is how the guitar amp people see things . They will drive EL84 with op amps to get the sound they want . Hi fi people have valves driving transistor outputs which seems very logical if wanting a hybrid . AMC felt the hardest to do option the better one .

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Old 21st September 2012, 04:29 PM   #7203
bcarso is offline bcarso  United States
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The 3020 parts quality is low-end, but it's a series of compromises that are somehow in nearly perfect balance. For example, if the power transformer were larger the rails wouldn't sag as much, but then other things like the output devices might overheat.

Keith was intrigued enough to look at some of BEE's later designs, and he said they weren't quite as good. And I have bought other NAD products that failed early or were defective (one standalone preamp had a workmanship error that led to high hum, which I thought was something associated with the overall system until I got inside it a year later and fixed the problem). The CD player that Corey Greenberg went crazy about had a fundamental defect, and my sample of it also had the display fail early on.
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Old 21st September 2012, 04:43 PM   #7204
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The 3020 parts quality is low-end, but it's a series of compromises that are somehow in nearly perfect balance. For example, if the power transformer were larger the rails wouldn't sag as much, but then other things like the output devices might overheat.

Keith was intrigued enough to look at some of BEE's later designs, and he said they weren't quite as good. And I have bought other NAD products that failed early or were defective (one standalone preamp had a workmanship error that led to high hum, which I thought was something associated with the overall system until I got inside it a year later and fixed the problem). The CD player that Corey Greenberg went crazy about had a fundamental defect, and my sample of it also had the display fail early on.
That rhymes with a general sentiment. The 3020 was definitely a one-off, they never managed to get even close later, even if some of the later models were reasonable.

But no matter what they did later on, they never managed to pull off the Giant Killer trick again. Not even with the BEE series, or the Monitor series before that.

I don't know, I guess I was plain lucky in guessing that the 3020 was a one off, and to me, NAD is just another wanna-be company. Some friends have their products, including the 150 WPC integrated amp, which will deliver 1.2 kW in peaks into 1 Ohm, but tremendous load tolerance notwithstanding, it sounds murky and bland. I'd never buy it. Same goes for each and every CD player I ever heard from NAD.

I'd much prefer one of the two aforementioned Proton amps any day.
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Old 22nd September 2012, 07:48 AM   #7205
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I have a question for everybody, the more, the merrier. It's something that's been nagging me for years on end.

Say you just designed a power amplifier. More likely than not, you will some compensation points in it. In my case, I will almost certainly have my VAS working into a resitor ballast (swamping the predriver), thus limiting its amplification factor to what I want it to be; probably other things as well, but VAS load at the very least no matter what else I may do.

Now, those resistors (in case of a fully complementy design) will do their job just fine, but at some point, I will see a rise in delieverd voltage and current at something like 200 kHz and above. Thus, it would be opportune to bypass those resistors with small value caps, in the pF range (rarely less than 33 pF, rarely more than 100 pF), to further reduce amplification at very high frequencies. I do this routinely, even though there is not supposed to be anything way up there, in fact, there is not supposed to be anything of any appreciable value much above 20 kHz.

Now, let's also assume that beside the was load, there is another compensation point somewhere, as there usually is.

My question is this: when declaring specifications, I have noticed that some manufacturers make statements like "with the output network short circuited" (e.g. H/K), "without the input filter", etc. Is this the right way to go? Because after all, the customer WILL have all that in circuit, so what's the point of excluding it, except for the sake of getting better published specs?

Specifically, should open loop performance be measured with or without the compensations one has used for closed loop?

Without compensation, the figures will look better in many cases, but will they be realistic and truly representative of the unit?

For example, in his 1205 power amp, John has a small cap (7.8 pF if memory serves) from collector to base compensation in his VAS - when declaring your amp for open loop, do you include that cap or don't you, John?

Personally, I look at two companies whose logic I like. Sony gives us two figures for say slew rate, the internal one (i.e. no input filter) and the absolute one (with the input filter included), and so did reVox. I have adopted that logic anyway, I get the power amp to have a full power bandwidth out to say 300 kHz, and then install an input filter which has a -3 dB cutoff point at 200 kHz. That way, my amp is always faster than the fastest realistically possible incoming signal, if there even should be anything at 200 kHz.

Views? Thoughts? Comments?
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Old 22nd September 2012, 10:42 AM   #7206
DF96 is offline DF96  England
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Your question seems to combine different things as though they are the same thing. You start by talking about loop compensation, and perhaps LF loop gain restriction (by resistive loading of the VAS output). Then you switch to asking about 'black box' performance figures (presumably closed loop), but with part of the input or output circuit omitted - nothing to do with compensation. Then you ask about open loop performance figures, which few people state anyway.

If your circuit normally runs closed loop, and to do this requires compensation components then these should be in place when you make any claims about open loop performance. In addition you should, strictly speaking, ensure that not only is the feedback path broken for open loop but that extra components are temporarily installed so the places where the feedback components attach to the circuit still see exactly the same load. For example, if you have a 20k feedback resistor with 47pF in parallel with it feeding into a 470ohm bottom resistor then 'open loop' actually means separate networks like this attached to both the output and feedback points. In practice people often ignore this, and often it makes little difference, but it could sometimes turn and bite you if you don't know that it ought to be done.

Regarding slew rate and input filters, the important thing is that it is clear what is being claimed. If you say X V/us then is this at the box input terminals or the base/gate of the first transistor? Similarly for output. People who make extravagant claims for, say, damping factor with the internal output inductor shorted are either stupid or dishonest. They also assume that their customers are impressed by big but almost meaningless numbers.

Here is my question: with a given peak signal voltage followed by a first-order low pass filter, what signal before the filter will give the highest slew rate after the filter? (i.e. input waveform shape, frequency?)
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Old 22nd September 2012, 10:49 AM   #7207
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Originally Posted by DF96 View Post
In practice people often ignore this, and often it makes little difference, but it could sometimes turn and bite you if you don't know that it ought to be done.
Common example: feedback RIAA stages.
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Old 22nd September 2012, 11:35 AM   #7208
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Originally Posted by DF96 View Post
Your question seems to combine different things as though they are the same thing. You start by talking about loop compensation, and perhaps LF loop gain restriction (by resistive loading of the VAS output). Then you switch to asking about 'black box' performance figures (presumably closed loop), but with part of the input or output circuit omitted - nothing to do with compensation. Then you ask about open loop performance figures, which few people state anyway.
Not the same thing, but the same measuring principle. Personally, I like to measure open loop full power bandwidth WITH everything in place it needs to be.

Perhaps I should have mentioned that I never bypass the feedback resistor with anything, it always goes in alone and is followed only by its companion resistor divider. It's a habit I picked up from H/K, I have never seen anything of theirs since 1980 taht was bypassed (RIAA network excluded). On the other hand, I rarely use more than 26 dB of global NFB in anything, except the RIAA circuits. Most often around 20 dB. And my open loop full power bandwidth is typically at 70 kHz or above WITH everything in place. My ommission.

Quote:
If your circuit normally runs closed loop, and to do this requires compensation components then these should be in place when you make any claims about open loop performance. In addition you should, strictly speaking, ensure that not only is the feedback path broken for open loop but that extra components are temporarily installed so the places where the feedback components attach to the circuit still see exactly the same load. For example, if you have a 20k feedback resistor with 47pF in parallel with it feeding into a 470ohm bottom resistor then 'open loop' actually means separate networks like this attached to both the output and feedback points. In practice people often ignore this, and often it makes little difference, but it could sometimes turn and bite you if you don't know that it ought to be done.
Well, good news here, I do not forget to keep the differential circuits in full balance as if the NFB loop was attached. Seems logical to me to measure things under actual operating conditions, or as near as possible. Otherwise I am likely to get odd results.

Quote:
Regarding slew rate and input filters, the important thing is that it is clear what is being claimed. If you say X V/us then is this at the box input terminals or the base/gate of the first transistor? Similarly for output. People who make extravagant claims for, say, damping factor with the internal output inductor shorted are either stupid or dishonest. They also assume that their customers are impressed by big but almost meaningless numbers.
That's the trouble with slew rate, it has, to the best of my knowledge, absolutely no standard way of being declared. It's all left to the manufacturers. I remember in the late 70ies, Sansui had a few papers published by AES proclaiming that we only need to measure the input stage slew rate, as if the rest of the circuit simply didn't exist. AND with the input low pass filter disabled.

Whereas people like SAE, for example, quoted their slew rates of the device "as is", with everything in place, with everything connected just as the customer will use it. Consequently, their slew rates were far more modest, "just" 40 V/uS, whereas Sansui and Kenwood were quoting 200 V/uS as a minimum, and I did see some specs of 350 V/uS.

As for your mentioning of the damping factor, I agree completely. The wild claims made are simply the reminants of the Great Specs Game, THE most popular sport in audio manufacturing since 1975 to this day, in some cases. The key idea being - what has truth got to do with sales figures?

Quote:
Here is my question: with a given peak signal voltage followed by a first-order low pass filter, what signal before the filter will give the highest slew rate after the filter? (i.e. input waveform shape, frequency?)
You tell me.
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Old 22nd September 2012, 12:58 PM   #7209
DF96 is offline DF96  England
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Originally Posted by dvv
Well, good news here, I do not forget to keep the differential circuits in full balance as if the NFB loop was attached. Seems logical to me to measure things under actual operating conditions, or as near as possible. Otherwise I am likely to get odd results.
The issue arises with single-ended circuits too. The reason I raised it is that it is often omitted from open-loop analysis. You asked "Can we omit some components?"; the true answer is "No, you have to add some components". Fortunately, in many cases it only affects the results by a small amount.
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Old 22nd September 2012, 02:12 PM   #7210
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dvv

My two cents;
For loop gain you have to include all compensation around the entire loop (including loading effects from those which may be considered as outside the loop, for example the input filter, though this should have a very small effect). Not having the compensation in place would be a starting point from which many different final closed loop performances can be achieved, thus it really tells one nothing of the final transfer function.

Slew rate at least in my view is number related to the overall loop or gain bandwidth of the amp. In order to correlate the two you have to bypass the input filter in order to stimulate the amp with frequencies just beyond its bandwidth. The same would be true for pulsed transient testing at least for purposes of showing stability. You would have to pulse the amplifier past the input filter to see the damping factor, similarly for pulsed load testing you would pulse the amplifier output before any RL but you would leave the RL and speaker load in place.

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