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In Reply to: RE: More posted by Tre' on September 02, 2014 at 12:12:33
I don't think I've ever seen a definition of Class A that was complete in the context of modern hi-fi. Some say simply that the tube never draws grid current, others that it conducts over 360 degrees. These definitions lead to the often-heard myth that Class AB amplifiers operate in Class A for small signals. It's unfortunate that the majority of engineering texts fail to expand on these simple definitions, but most engineers probably assume that Class A operation will position the tube at the mid-point of conduction. Considering that this not only allows maximum +/- current swing, but also the most linear operation, it seems superfluous to note this in even a formal definition.
Getting back to your statement, when you say the "non-linear region of operation," are you referring to driving the tube into grid current? Or do you mean the general areas just prior to cutoff and saturation, areas through which the tube continues to conduct well, but nevertheless responds in a degraded manner with respect to linearity?
On the subject of designing for headroom (or more correctly, minimal recovery time) is your CD player actually incapable of producing more than the 5.6V PP that represents 2V RMS? That would seem to place the allowable average output level at a rather low number, voltage-wise. Not saying you're wrong, just asking. I don't remember what peak levels I saw the last time I put a scope on my CD player...
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Buy Chinese. Bury freedom.
Follow Ups:
The general areas just prior to cutoff and saturation, areas through which the tube continues to conduct well, but nevertheless responds in a degraded manner with respect to linearity.
The manual says 2vrms and that's what I remember measuring when playing a 60Hz CD I made in Sound Forge normalized to 0db.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"The manual says 2vrms and that's what I remember measuring when playing a 60Hz CD I made in Sound Forge normalized to 0db."
Have you looked at the output with a scope to see whether signals created from a standard music CD can exceed that level? I'll do it on mine if you don't have time, they're probably similar in terms of levels...
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Buy Chinese. Bury freedom.
Here a screen shot of the wave form in Sound Forge and a close up showing the peak sample sitting right on the digital "0".
TK, here's the part that maybe, somehow I'm not understanding.
A digital "0" is a digital "0".
There can be no level above that without digital clipping.
If my 60Hz wave form is peaking at a digital "0" and I measure 2vrms with a multimeter, then I think I'm done.
No other non-clipped signal can be higher than that. Sine wave or music waveform.
That's the thing about digital. It has an upper limit that can't be exceeded.
I can look at it with a scope but I don't see the need. If a wave form that is peaking at digital "0" measures 2vrms at the output of the CD player with a volt meter then how can there be any level above that?
Maybe I should measure it again. I seem to remember that it measured just a little over 2vrms and at the time I thought "close enough".
I posted/hosted the wav file at the link below.
Edit, a digital 0 is represented as all 1's for the word for that sample. The D to A converter will output full voltage for a sample with the word written with all 1's.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/03/14
Tre', I'm not familiar with the system or software you're using, so I have to ask - are you certain the signal in the upper screenshot represents "all ones"? If it does, then I certainly agree, the amplifier won't saturate. I don't think this inability to clip is representative of most systems, but it's a smart thing to do. The only problem might come about when a CD with a very wide dynamic range is in use, especially if much of the music is at a rather low output level. I often find myself turning up the volume so I can hear the quieter passages in more detail, but that makes the system prone to clipping on peaks. If the amplifier is designed for quick recovery, those brief periods of saturation are much less offensive. Anyway, it sounds as though you've "calibrated" your system so that none of this is a problem. If your speakers have the sensitivity to raise you off the ambient noise floor of your listening environment even at low source levels, it's a good approach. Wish I was that lucky! :)
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Buy Chinese. Bury freedom.
"...are you certain the signal in the upper screenshot represents "all ones"?"
No, just the one sample that touches digital O. The peak, the apex of the wave form.
And as such, that's as much voltage as the D to A converter can output.
I live in a rural area and my house is back off the street. It's pretty quiet here most of the time. People from the city are always amazed.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
and to expand on one of them a common misconception is that tubes are inherently non-linear at low current because that's the way curves appear on data sheets. If we magnify that region they look pretty much the same as they do at higher current.
Higher current does improve the overall performance parameters of a tube, lower rp, higher gm etc etc but properly designed any tube is capable of great performance and sound at low current. Unfortunately OP tubes need to deliver power into the load so we are limited to how far we can go.
Naz
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