Presumably you have a design that does this and is unconditionally stable with load 😱 We grovel at your feet or Guru MagicMagicBox said:
That's easy. A 50W 8R amp used in domestic conditions will clip often. It's likely that overload & recovery behaviour will take priority over 1ppm THD. Bob Cordell is of this opinion too.
With 'normal' speakers in domestic conditions, 200W 8R gets you to the point when overload is too short & rare to be picked out reliably
But a 500W 8R amp will still clip on certain types of music. eg well recorded Beethoven piano played at the correct level. It's just less noticeable. Also very few speakers actually sound 'better' with more than 200W 8R.
All the above assumes the amps are 'good' so nearly 1000W 4R .. and realistic levels. A well played piano in your front room is LOUD.
Modern compressed Pop (music??) can get away with 50W in domestic conditions. There are pieces of music (??) where the best ears in the business can't tell if the amp is clipping 50% of the time.
Not a guru lol, just a hobbyist 🙂 The design I'm working on does need an evil output inductor. It's stable with any kind of C load. TPC can't seem to get away without an output inductor.
Yeah when one deals with clipping the story becomes much different. I was sort of hoping you could say something about what rated power level amp generally sounds better when neither test amp would clip. I was just curious whether there was any correlation between rated power and 'niceness' (of sound).
Personally I'm after this wire with gain and as such low enough THD to reduce distortion below the "layout floor" then I'm fine 🙂 It's more of a challenge thing though. It's going to cost both time and money so I want my proto to be as good as I can get it. Anyways, sorry for the off-topic.
It doesn't make sense to me why a 25w amp would sound better at lower levels than a 150w amp as long as both are well designed.
While we are slightly off topic. I'm still trying to work out why some amps sound "powerful" while others just sound weak. This effect is also audible with heavily compressed music (<6dB dynamic range). I do know this - its not a CFA vs VFA thing. Have experience of both VFA and CFA which exhibit these attributes. The only explanation I've had so far is that the powerful / effortless amps are exhibiting some sort of distortion.
While we are slightly off topic. I'm still trying to work out why some amps sound "powerful" while others just sound weak. This effect is also audible with heavily compressed music (<6dB dynamic range). I do know this - its not a CFA vs VFA thing. Have experience of both VFA and CFA which exhibit these attributes. The only explanation I've had so far is that the powerful / effortless amps are exhibiting some sort of distortion.
With my CFA'ied amp as a template I can get out of it about 20ppm THD 20K @ 50W/8Ohm. That's with a MOSFET output stage. It's just lacking gain and when using TPC/Cherry on just one VAS transistor trying to get phase under control isn't much different from taming that kind of compensation on a VFA. At least, that's what my simming tells me.
Attached: My CFA version.
Attached: My CFA version.
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If your 50W amp dun clip, then your operating conditions are unrealistic. If you build a clip detector which holds its display for a second or so, you'll be VERY surprised how often a 50W amp clips at what you would consider sensible & modest levels.
So how a 50W amp sounds when it clips on music into real speakers is a VERY important part of its sound.
Of course there is music (??) which is unbearably loud well before a 50W amp clips. Much modern Pop is like that.
Though there are notable exceptions, most things which can be picked out in Double Blind Listening Tests bla bla on electronics are quite blatant and easy to find if you know where to look.
eg most (all?) Golden Pinnae amps (try those without output Inductors) show obvious signs of instability on real speakers on overload and coming out of it .. dis being dependent on their thermal & signal history too. No wonder they sound different and are fussy about speakers.
This is why I think its entirely possible a simple CFA amp like VSSA with decent but not spectacular measured performance can sound better than more complex amps. For me, the proof would be Double Blind Listening bla bla but some listening tests are better than none.
In my comparative simulations, VSA win on PSRR. But, if you use the two extra transistors of the LTP to build a little cap multiplier (12ma) to filter the input and VAS rails, the CFA will win on 100 Hz PSRR too.
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Then the LTP may use 2 extra transistors too and be loaded with a mirror. At least then your LTP version is less crippled as JCX mentioned.
For 90% of listeners, this is not the case :
diyAudio - View Poll Results
As digital formats have well known Full Scale Deviations, it is easy to predict if an amplifier, knowing its voltage gain (closed loop), is ever going to clip or not.
Everybody mastering basic notions about voltage audio levels in his system should dimension his amps such as they never clip.
Here, a better friend than a clip detector is a peak voltage meter.
kgrlee - ok I see what you are comparing.
I tend not to worry too much about noise in a power amp, unless you are talking about connecting some really efficient speakers to your amp. The trade off between the conventional CFA front end and the VSSA is that the conventional CFA front end can be made all DC coupled and with a single set of resistors setting the gain - some people would see this as a distinct advantage.
Distortion is no indicator of 'sonic' quality - but I don't think we should start that discussion here. 🙂
I tend not to worry too much about noise in a power amp, unless you are talking about connecting some really efficient speakers to your amp. The trade off between the conventional CFA front end and the VSSA is that the conventional CFA front end can be made all DC coupled and with a single set of resistors setting the gain - some people would see this as a distinct advantage.
Distortion is no indicator of 'sonic' quality - but I don't think we should start that discussion here. 🙂
You mean 2 more active devices in one than in the other is fair ?
It seems some (you and JCX ?) have difficulties to understand the purpose or continue their crusade to auto convince themselves their amp (VFA ?) is the best ? My purpose was not to fight for the biggest. but to lighten the differences between the two topologies and discover few things about some urban legends.
Identical VAS and power stages... same currents and gain in input stage...two more active devices in VFA than CFA. How to be less"crippled" or "flawed" ?
Please, when you assert something is such a disagreeable way, i have spend hours to this work and you not a second- the minimum is to understand the purpose, precise and prove your assertions, not just blabla and criticize.
Your less crippled VFA version is welcomed. (no extra transistors, please, because, then, we can use them for a diamond buffer in the CFA version too -as an example- and it will be endless).
Apple VS apples.
Good fights 😀
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kgrlee, 15W of class A sounds very good. If you are trying to reproduce realistic concert hall levels in a domenstic environment, well then you may need something substantial, but then I fear the neighbors will be complaining rather a lot . . .
It depends of what kind of music.
Realistic classical concert levels at a good seating place are easily achievable in domestic conditions.
Yes, I agree in principle forr. I listen to my 15 watt amp a lot to classical and light jazz and enjoy it - but I am not trying for high SPLs (B&W 703's)
Why designing to handle step functions should lead to a better reproduction of bandwidth limited audio signals ?
If a CFA input stage is more linear than a VFA input stage
but if a CFA has less loop gain than a VFA,
the VFA input stage has less error to handle and
then may introduce less non linearity than the CFA input stage,
meanwhile the output is submitted to more feedback.
So the VFA has less HD and IMD over the whole audio band.
forr, as I already said, the differences are single digit ppm . . . I rather spend my time exploring other ideas than an endless focus on THD where differences are effectively zero. Think Pass amplifiers, JLH, VSSA . . . Even Krell never chased ppm distortion.
Yes. I know all this and in fact could do the calculations even in vinyl days.
But why don't you build a clipping detector and live with it for a while? It will open your eyes if you have never done this before. I think Rod Elliot has a good design.
If nothing, at least it will tell you which type of music is the most demanding.
I also accept that there is loadsa modern music (??) where it doesn't matter if the amp clips.
I've lived with both a clip detector and various peak voltage meters and can tell you the clip detector is more useful.
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But forr, what did you measure on YOUR system? What size amp?
According to that survey, 85 out of 310 measured more than 5V so would clip a 50W 8R amp (20V rms) some (??) of the time. That's 28% of the sample.
And it doesn't say what sort of music either which, as I've said, has a major influence .. and also the speakers.
http://www.aes.org/e-lib/browse.cfm?elib=3793
I was involved in these listening tests but it was just the start of my interest. Our own little survey of friends and people working at Wharfedale came up with about 50% circa 1980
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IMHO the classic CFA is only one stage, the folded cascode only acting as a level shifter.
As for the low frequency distortion, if you do the analysis it's the CURRENT into the inverting input through the feedback resistor (the one from the output to -input) needed to compensate any CURRENT errors at the high impedance node that is important. This resistor is usually large 1K or so larger than 1/gm of a VFA so the net voltage error is larger. Same with the current noise contribution.
If you add 2 for better PSRR, I will add 2 for a current mirror 🙂
I'm by no means trying to say my VFA amp is the best. On the contrary, I've been trying to provoke the CFA folks to detail the workings of, and the designing of CFA's.
A CFA like the VSSA is already "most optimized" for its topology. A resistor loaded LTP is not "most optimized" for its topology. A mirror loaded LTP is.
I didn't mean to downplay your work. I put some hours into my CFA version as well. Not to mention the tons of hours I put into my VFA with its new VAS.
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Thanks Andrew.
At last! A definite technical win for Diamond Input over simple VSSA type 🙂
Not sure if it tips the balance against the sound, simplicity & THD of simple CFA though 😀
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forr, I'm really a speaker & microphone man. I used to do a lot of recording of live concerts with a Soundfield Microphone placed in the audience. The mike has exact calibration for spl. So I can't agree with your statement.forr said:
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Magic, my #502 is your 'optimised' VFA.Magicbox said:
If we use the extra 2 bits to enhance the VASs in my #500, it beats the #502 design in all respects. It may be that I'm incompetent to design simple VFAs in which case, I'd be pleased to learn how to improve my simple #502 VFA without adding complexity.
I certainly don't think I'm competent at this stage to design the 'best' CFA but even then, at this very simple level and with equivalent complexity, I think the simple CFA wins.
But as I said, this is not about VFA vs CFA but about how to do better CFAs. I'm disappointed that I'm so incompetent that adding an extra small signal stage improves THD by less than 6dB 😡
Anyone with suggestions on how to better use the 2 devices to improve my #500 offering?
Output triples are better in some respects, worse in others than the enhanced VAS but overall not as pleasing.
Diamond IPS quite disastrous.
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