Sorry to hear your brother is not able to tell us what he did.nigel pearson said:
Feedback amps need to be designed. That means feedback can't just be added or removed or changed in amount. Any decent feedback amp is likely to sound bad without feedback; at the very least it will have relatively poor frequency response. An amp designed for no feedback could develop problems when feedback is applied, such as response peaks (and consequent poor phase behaviour) just on the edge of the passband. This elementary point seems to be lost on many DIYers.
I'm not sure what you are asking. If an amp uses feedback to provide a low output impedance (among other virtues) then no change is needed when feeding different speakers of similar nominal impedance. If changing the OPT secondary to provide a different impedance then a change in feedback may be needed. This is standard engineering.a.wayne said:
DVV, there is very little basis for this. Phase sensitity of hearing runs out at around 2KHz. Blind AB switching between a flat signal and the same with a 180 degree phase shift at 2KHz (if I remember correctly) could be detected only by some as 'different', without any clear preference for one or the other.
For that reason, I don't see why bandwidth limiting (at the input, not the amplification stage itself) would be a problem, even with a low corner frequency of 50KHz or so.
Most believe 40 v/uS to be sufficient ....
Is that First law or second .....
yes, you're right, hi-fi is about as little deterioration as possible so if you can make an amp having low phase shift in the band of interest, why not.
but at the same time I can't help but think of the accumulated phase shift, starting from the air produced by say the singer's mouth and ending in our living rooms
and, again, how about the low frequency phase shift, caused by the decoupling caps? I did some calculations a while back and it looks like it can be a real problem, with objectionable group delay extending to the low-mid region.
do you have any explanation/speculation about why 80V/us would correlate to good sound?
When talking slewing it is seldom stated what the source and sink ability is . I suspect if symmetrical slower is fine . It also usually says enough current was available to drive the VAS if high ( and that is the big deal ) . My guess is if sensible bandwidth is used ( 70 kHz - 3 dB ) , sensible impedance ratios between input stage and VAS ( > 3 : 1 if you can ) and symmetry, then 6V / uS would be OK for 100 W . I very much doubt if SA CD nor MC pick up really justifies more ? Even though there is significant 50 to 100 kHz output from a MC pick up if has to get past 75 uS filter . If it is at a significant level then the tweeters better look out . Make it high because you can is no bad reason . As for SA CD , surely it is to a sensible standard ? It can not demand a > 80 V / us amp ? My feeling is the only question is this . If we avoid feeding an input from a high impedance then why would we do it with the VAS ? The analogy is the big V8 that is so nice to drive slow . The 140 MPH bit is not the guarantee . You wouldn't say to someone your granny must have a 140 MPH car as she is a slow driver would you ? The 100 watt amp I often use is 35 V / uS and symmetrical .
If slewing is connected with CCRR then I will accept it matters . Seems it would be . Some say that transistor amps are badly troubled by RFi , valves less so . CCRR would say the reverse . The high voltages of valves must help ?
From what I said earlier don't infer I dislike pentodes . I disliked them for that amp . I recently used them as the best choice . Friends thought I must be wrong ( mad ) until they were played them . The spectrum analyzer said pentodes and the spectrum analyzer was right . Triodes whilst OK were not the right solution . What I was trying to say previously was the amp with least measured high frequency was subjectively the one with most to offer in the HF . I doubt it was overshoot or HF resonance as no loop feedback was the preferred option .
Fascinating what you say about decoupling . Say more if you can .
somehow that doesn't surprise me.
I haven't done actual tests but, as I said, intuition tells me that evolution took care of this
(and all the studies so far confirm it) our brain manages to do a lot with little and it doesn't have many resources to waste.and I would speculate that evolution made it not spend too much effort dissecting a signal that doesn't contain much useful data to begin with.
on to slew-rate. being a computer science engineer and not an EE I tend to view everything from a system perspective. output vs input. I leave the VAS and input stage stuff to the EEs. all I care about is what the output adds to the input and so should everyone. so, from an user's (buyer's) perspective...
Douglas Self sais in his "Audio Power Amplifier Design Handbook":
The most meaningful definition of an amplifier’s speed is its maximal slewrate. The minimum slew-rate for a 100 W/8 amplifier to cleanly reproduce a 20 kHz sinewave is easily calculated as 5.0 V/μsec; so 10 V/μsec is adequate for 400 W/8, a power level that takes us somewhat out of the realms of domestic hi-fi. A safety-margin is desirable, and if we make this a bare factor of two then it could be logically argued that 20 V/μsec is enough for any hi-fi application; there is in fact a less obvious but substantial safety-margin already built in, as 20 kHz signals at maximum level are mercifully rare in music; the amplitude distribution falls off rapidly at higher frequencies.
Firm recommendations on slew-rate are not common; Peter Baxandall made measurements of the slew-rate produced by vinyl disc signals, and concluded that they could be reproduced by an amplifier with a slew limit corresponding to maximum output at 2.2 kHz. For the 100 W amplifier this corresponds to 0.55 V/μsec[18].
Nelson Pass made similar tests, with a moving-magnet (MM) cartridge, and quoted a not dissimilar maximum of 1 V/μsec at 100W. A moving-coil (MC) cartridge doubled this to 2 V/μsec, and Pass reported[19] that the absolute maximum possible with a combination of direct-cut discs and MC cartridges was 5 V/μsec at 100W. This is comfortably below the 20 V/μsec figure arrived at theoretically above; Pass concluded that even if a generous 10:1 factor of safety was adopted, 50 V/μsec would be the highest speed ever required from a 100 W amplifier.
I'm not sure where those 5V/usec come from (my calculations give less) but at any rate it's way less than 80V/us.
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Perhaps so, but over the years I havecome to realize that to me, an amp with a smaller phase error will most likely be more pleasing to me than another with a greater phase error.
While this is certainly hardly scientific, I am not in the business of advancing science, and am well aware it's all about my taste (for me). For example, I dislike the sound of tubes in general, though I have heard a few impressive devices in my time, or, oh blasphemy!, the older Mark Levinson electronics sounded a little too dry for my taste, etc. Krell was more brash, perhaps even more brutal, but gave me a warmer sound than ML.
As for bandwidth limiting, you are free to do as you please, but in my view, 20 kHz is WAY too low. I don't see how it can fail to impact the sound. Initially, many will say it's a nice, warm sound, cuddly, but givem a few days and something good to compare with and they will realize what a price they are paying for the curtailed sound.
Personally, I took a hint from reVox and Sony and first get them to 350 kHz or so at full nominal blast, then cut the input signal at around 200 kHz. It never failed me, I never had oscillations or some such problem. Of course, there's always a first time.
As do I, so long as it slews at a rate of 1 V/uS per every peak volt of output. Since 100W/8 Ohms is 40V p-p, a slew rate of 40 V/uS is all I need.
I've never done a more powerful amp than that, so I can't comment, perhaps it would also be just fine for say 200W/8 Ohms. National Semi's famed and now reprinted book on audio states 0.5V/uS is quite enough per every peak volt of output, which is half of what I think, and they are probably right. I just like to be double sure.
I reckon it's as simple as that the amp with a high bandwidth will automatically have better distortion numbers, where it counts, in the treble end of the spectrum. Better behaviour, better sound.
dvv, your preferences for the 'sounds' of various types of electronics matches my own - I think we would be in agreement when assessing some new piece of kit, in situ ...
Very possibly so, Frank.
Let me make myself as clear as possible - while certain colorations can make an amp even seductive, the instant I realize it's coloration at work, I am put off. Nice, sexy, warm, whatever, but it's not for me.
My ideal is an amp which simply is not there, it disappears and music appears. That simple, but I realize what I'm asking for - a hell of a tall order.
The trouble is, my Karan very nearly manages to pull that one off. I can still catch it here and there, but those are very minor points, and I have had it for 10 years now. And it does cost a pretty penny indeed, I would expect its US price to be at least $ 7k. As it's readily available, it will of necessity serve as my reference point.
Let me make myself as clear as possible - while certain colorations can make an amp even seductive, the instant I realize it's coloration at work, I am put off. Nice, sexy, warm, whatever, but it's not for me.
My ideal is an amp which simply is not there, it disappears and music appears. That simple, but I realize what I'm asking for - a hell of a tall order.
The trouble is, my Karan very nearly manages to pull that one off. I can still catch it here and there, but those are very minor points, and I have had it for 10 years now. And it does cost a pretty penny indeed, I would expect its US price to be at least $ 7k. As it's readily available, it will of necessity serve as my reference point.
Phase Error .
Dave Mate of SSL said to me . " The problem is humans cope with phase shift very well , problem also is it causes fatigue " . He admitted this was conjecture . His speakers could produce good square waves . This required fill in drive units to give back the bits lost in the crossovers . His speakers were some of the brightest I ever heard and caused me to have many years of back pain treatment ( and the others of other makes , his significantly finished the job ) . I never seemed to get listening fatigue with them . Also the Quad 306 worked well with them . The Quad worked well with nothing else I can remember . The speaker should have been a beast , it wasn't .
David said phase shift up to 7 kHz is the focus . Obviously we have no control over the recordings . The BBC twin mics must help I guess ? I am tempted to say the wrongs of an amp seem minor compared with recordings and speakers . That is assuming the room works OK . I doubt it is the same problem and outcomes are unigue for each ?
David if he doesn't mind never was able to enter our world . As a Professional Audio engineer he possibly didn't know how to talk about his product ? Incredibly intelligent people do sometimes make enemies . Doubtless you have never heard of him ?
Orchid Audio PLL1 [English]
Dave Mate of SSL said to me . " The problem is humans cope with phase shift very well , problem also is it causes fatigue " . He admitted this was conjecture . His speakers could produce good square waves . This required fill in drive units to give back the bits lost in the crossovers . His speakers were some of the brightest I ever heard and caused me to have many years of back pain treatment ( and the others of other makes , his significantly finished the job ) . I never seemed to get listening fatigue with them . Also the Quad 306 worked well with them . The Quad worked well with nothing else I can remember . The speaker should have been a beast , it wasn't .
David said phase shift up to 7 kHz is the focus . Obviously we have no control over the recordings . The BBC twin mics must help I guess ? I am tempted to say the wrongs of an amp seem minor compared with recordings and speakers . That is assuming the room works OK . I doubt it is the same problem and outcomes are unigue for each ?
David if he doesn't mind never was able to enter our world . As a Professional Audio engineer he possibly didn't know how to talk about his product ? Incredibly intelligent people do sometimes make enemies . Doubtless you have never heard of him ?
Orchid Audio PLL1 [English]
Why 70 kHz, Nige? Any particular reason?
Assuming a 6 dB/oct faloff, that would put about -0.6-0.7 dB drop at 20 kHz. Obviously much better than -2.5 dB, but still a bit too much.
Why not work in the opposite direction? Say I want -0.1 dB at 20 kHz, which should be insignificant. That's -0.25 dB at 40 kHz, -0.5 dB at 80 kHz, -1 dB at around 150 kHz and lastly -3 dB at around 270 kHz or so.
I believe this is what H/K had in mind when designing the Citation series.
If you are still happy with -0.25 dB at 20 kHz, that would be around -0.5/-1/-3 dB at 50/100/200 kHz, which is what I try to do.
That is correct . It is acceptable .
I would say feedabck amps sound more closed in than no loop feedback amps with the same factor . That is a sweeping statement to make I know . Some amps like your own Dvv are mid way .
As someone said a speaker might drop 3 db at 20 kHz and still sound bright . An amplifier otherwise that is - 0.7 dB . It is all highly confusing and means we must listen to each thing we choose to do . We might use a multi-pole filter ?
I should have said input filtering is probably the best . If my theory of slewing is right it won't be . That is because slewing is a bi product of reducing IM distortion . Thus high slew rate amps sound good because they have enough current to drive the next stage and do not attempt trans-conductance . Also when symmetrical they do not alter the nature of the natural asymmetry of the signal ( highly asymmetric ) . Valve amps have higher IM problems yet sound OK . I guess the TID is the right label ?
I would say feedabck amps sound more closed in than no loop feedback amps with the same factor . That is a sweeping statement to make I know . Some amps like your own Dvv are mid way .
As someone said a speaker might drop 3 db at 20 kHz and still sound bright . An amplifier otherwise that is - 0.7 dB . It is all highly confusing and means we must listen to each thing we choose to do . We might use a multi-pole filter ?
I should have said input filtering is probably the best . If my theory of slewing is right it won't be . That is because slewing is a bi product of reducing IM distortion . Thus high slew rate amps sound good because they have enough current to drive the next stage and do not attempt trans-conductance . Also when symmetrical they do not alter the nature of the natural asymmetry of the signal ( highly asymmetric ) . Valve amps have higher IM problems yet sound OK . I guess the TID is the right label ?
However, in my experince, ALL zero global NFB amps, of any power or any technology, sound a little loose, a bit unstuck with zero global NFB. As if not quite finished.
SOME global NFB is not only good, but I believe, quite esential. In my book, no more than 26 dB, preferably 20 dB, assuming the open loop full power bandwidth is at least 40 kHz and that the input signal is limited at no higher than say 250 kHz, preferably less, but no lower than say 150 kHz.
SOME global NFB is not only good, but I believe, quite esential. In my book, no more than 26 dB, preferably 20 dB, assuming the open loop full power bandwidth is at least 40 kHz and that the input signal is limited at no higher than say 250 kHz, preferably less, but no lower than say 150 kHz.
That is right . If you take the impedance curve it becomes the frequency response more or less . Some speakers are made knowing that and sound very good . In fact it says that all of this is a bit mysterious . The impedance curves says it should be awful , usually not at all ( room effects also ) . There is simple frequency response and then the effects of mechanisms which limit frequency response . Generally the two things are the same . There is a difference subtly which I feel is the brakes have been applied . My instinct is that your amps Dvv live in a very happy world between disciplines . Nice relaxed loop with lots of local feedback . Someone said if an amp has a damping factor > 16 it is to all intents and purposes a constant voltage source . I think that is worth saying . I would say all amps have an optimum bandwidth and that the majority have not found it ? This is why fake transistors are such a problem . It needs to be that all amps are the same . Fake transistors take away certainty .
One way the zero feedback people could have their cake and eat it is to have volume by some type of change to the transformer . Make the impedance ratio greater when at low volume . It would have to be a push pull amp as 50 watts is the sort of level required to make it possible . 4 x EL34 should be OK in UL . UL without loop feedback is rather good . Very low distortion is possible if care is taken . UL with feedback needs extra care , Bravo Dynaco . I could write a book on that so better stop . All I will say is making a 7199 do all of that is a neat trick . The DC couple into the phase splitter is excellent . I think RCA showed it first ? Dynaco only has 3 valves in the signal path . The input pentode does the work . Feedback restores the linearity and lowers the output impedance .
Equally speakers can be made which suit high impedance amps .
I do like so called zero feedback amps . They often show layers in the music . Listening to the Dire Straits Original album the other day I was for the first time aware of multiple overlaying of the same voice on an zero feedback amp . The amp in question about 1% distortion at 5 watts with suspension bridge harmonics which disappear at 1 watt to be all second harmonic . I do have a feeling the voices congeal on a feedback amp which is as intended by the recording engineer who was listening as 99% will . Intended or not it is there . I have a feeling high end is about hearing what wasn't intended . It is about being real and real shows the welds and rivits if you like .
No insult to conventional amps . Like real motorcars they do a good job . Sports cars are a pain in the bum .
One way the zero feedback people could have their cake and eat it is to have volume by some type of change to the transformer . Make the impedance ratio greater when at low volume . It would have to be a push pull amp as 50 watts is the sort of level required to make it possible . 4 x EL34 should be OK in UL . UL without loop feedback is rather good . Very low distortion is possible if care is taken . UL with feedback needs extra care , Bravo Dynaco . I could write a book on that so better stop . All I will say is making a 7199 do all of that is a neat trick . The DC couple into the phase splitter is excellent . I think RCA showed it first ? Dynaco only has 3 valves in the signal path . The input pentode does the work . Feedback restores the linearity and lowers the output impedance .
Equally speakers can be made which suit high impedance amps .
I do like so called zero feedback amps . They often show layers in the music . Listening to the Dire Straits Original album the other day I was for the first time aware of multiple overlaying of the same voice on an zero feedback amp . The amp in question about 1% distortion at 5 watts with suspension bridge harmonics which disappear at 1 watt to be all second harmonic . I do have a feeling the voices congeal on a feedback amp which is as intended by the recording engineer who was listening as 99% will . Intended or not it is there . I have a feeling high end is about hearing what wasn't intended . It is about being real and real shows the welds and rivits if you like .
No insult to conventional amps . Like real motorcars they do a good job . Sports cars are a pain in the bum .
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