Morse
New Head-Fier
- Joined
- Nov 17, 2001
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Well Domer2004,
to answer your questions, even harmonics are even numbered multiples of the fundamental tone; in other words, amplify a 1 kHz test tone to 100 dB (loud) through your speakers. Now, if you can hear a 90 dB 2 kHz tone in addition to the 100 dB tone, you have 10% "harmonic distortion" of the second order (which, because 2 is an even number, is an even harmonic). If you have an 80 dB 3 kHz tone that is also showing up, then you have 1% 3rd order harmonic distortion (odd). Some amps will produce harmonics out to the 20th or higher order!
Manufacturers only tell you what the total amount of harmonic distortion is; they don't tell you which harmonics and to what amount they are individually present. As Joe Beilin correctly pointed out, even harmonics sound different to the human ear than odd harmonics do. Different amps have a different amounts of each in their output. This accounts for the difference between a "clean, smooth full sound" and "gritty, grainy hollow sound" in an amp.
Something else to consider is the distortion caused when the "up" part of the waveform crosses the zero amplitude line to become a "down" waveform. In a Push Pull amp, there is one device (valve or transistor) that handles the "up" part of the wave and another device (an identical valve or transistor) that handles the "down" part of the wave. When the signal "crosses over" from one device to the other, there's a blast of distortion for a very brief moment in time. This effect is minimised when an amp is running at high power. My own speakers are 89 dB efficient; I personally seldom listen to music at that volume, let alone much louder than that, so I usually average something like 1 watt rms (with some peaks that are louder, of course). In other words, I'm very concerned about things like what the amp sounds like when it's played at moderate volume settings.
Thus, in my opinion, it's possible for an amp with .01% THD to sound worse to the human ear than an amp with 1% or 2% rated THD. There are some who maintain that specs are meaningless, and others who maintain that they are the only measure with meaning; I am not in either camp. All that I maintain is that for distortion specs to be truly meaningful, it's necessary to supply more information than the total amount of distortion present at clipping*. That's my opinion, anyway, and you're certainly free to disagree with it! Pax Vobiscum everyone! We're all friends here, right?
So, does all of that mean that it's a great sounding little amp? Maybe, maybe not. There are good and bad valve amps, and everything in between. Moreover, not everyone likes the sound of valve amps; some people genuinely loathe them, citing sloppy bass, high levels of measured distortion and the like. You may well hear a tube amp that I like a lot and be thoroughly underwhelmed. Last year I slowly fell back in love with valves though, listening to a friend's vintage high fidelity equipment, which prompted me to do some studying on why they sound different. Now I've taken the plunge myself.....
Personally, the thing that impressed me the most about the TM4 amp was it's extremely compact size. I've been planning to toss together a portable solid state headphone amp in a few weeks if I get time. Now I can think about a portable valve amp instead....
As far as a place to go to look up info about valves, amplifier types (Push Pull vs Single Ended, etc), even and odd harmonics and such, try:
http://www.spectrum.ieee.org/select/0898/tube.html
http://www.trueaudio.com/at_eetjlm.htm
http://www.audioasylum.com/audio/scr...ubeprimer.html
http://www.svetlana.com/docs/tubeworks.html
There are others, but if you read these, you'll have some food for thought. Remember that the TM4 is going to be Single Ended, due to it's valve complement (it would need to have 4 valves in the output stage to be a Push Pull stereo amp).
I hope that this helps!
Enjoy your music!
Morse
*Clipping occurs when an amp is called upon to deliver a larger amplitude waveform than it's power supply has the current reserves to provide. Ohm's law is: Potential (in Volts) = Current (in Amps) times resistance (in Ohms). Here's an example: your amp needs to supply 32 volts to make a waveform (it's playing a really loud cannon shot on your Telarc 1812 LP) at the present volume setting, and your speakers are a "perfect" 8 ohm load (not likely, but that's another story). Thus, your amp momentarily needs to supply 4 amps of current to the speakers; if your amp's power supply does not have the capacity to provide it, the top of the note is 'sheared off'. Since the waveform is sharply 'clipped', it resembles a square wave. Square waves can be shown mathematically to be the sum of a lot of waves that include very high order odd harmonics of the fundamental tone, which is why so many tweeters have been fried by an amp clipping a bass note....
to answer your questions, even harmonics are even numbered multiples of the fundamental tone; in other words, amplify a 1 kHz test tone to 100 dB (loud) through your speakers. Now, if you can hear a 90 dB 2 kHz tone in addition to the 100 dB tone, you have 10% "harmonic distortion" of the second order (which, because 2 is an even number, is an even harmonic). If you have an 80 dB 3 kHz tone that is also showing up, then you have 1% 3rd order harmonic distortion (odd). Some amps will produce harmonics out to the 20th or higher order!
Manufacturers only tell you what the total amount of harmonic distortion is; they don't tell you which harmonics and to what amount they are individually present. As Joe Beilin correctly pointed out, even harmonics sound different to the human ear than odd harmonics do. Different amps have a different amounts of each in their output. This accounts for the difference between a "clean, smooth full sound" and "gritty, grainy hollow sound" in an amp.
Something else to consider is the distortion caused when the "up" part of the waveform crosses the zero amplitude line to become a "down" waveform. In a Push Pull amp, there is one device (valve or transistor) that handles the "up" part of the wave and another device (an identical valve or transistor) that handles the "down" part of the wave. When the signal "crosses over" from one device to the other, there's a blast of distortion for a very brief moment in time. This effect is minimised when an amp is running at high power. My own speakers are 89 dB efficient; I personally seldom listen to music at that volume, let alone much louder than that, so I usually average something like 1 watt rms (with some peaks that are louder, of course). In other words, I'm very concerned about things like what the amp sounds like when it's played at moderate volume settings.
Thus, in my opinion, it's possible for an amp with .01% THD to sound worse to the human ear than an amp with 1% or 2% rated THD. There are some who maintain that specs are meaningless, and others who maintain that they are the only measure with meaning; I am not in either camp. All that I maintain is that for distortion specs to be truly meaningful, it's necessary to supply more information than the total amount of distortion present at clipping*. That's my opinion, anyway, and you're certainly free to disagree with it! Pax Vobiscum everyone! We're all friends here, right?
So, does all of that mean that it's a great sounding little amp? Maybe, maybe not. There are good and bad valve amps, and everything in between. Moreover, not everyone likes the sound of valve amps; some people genuinely loathe them, citing sloppy bass, high levels of measured distortion and the like. You may well hear a tube amp that I like a lot and be thoroughly underwhelmed. Last year I slowly fell back in love with valves though, listening to a friend's vintage high fidelity equipment, which prompted me to do some studying on why they sound different. Now I've taken the plunge myself.....
Personally, the thing that impressed me the most about the TM4 amp was it's extremely compact size. I've been planning to toss together a portable solid state headphone amp in a few weeks if I get time. Now I can think about a portable valve amp instead....
As far as a place to go to look up info about valves, amplifier types (Push Pull vs Single Ended, etc), even and odd harmonics and such, try:
http://www.spectrum.ieee.org/select/0898/tube.html
http://www.trueaudio.com/at_eetjlm.htm
http://www.audioasylum.com/audio/scr...ubeprimer.html
http://www.svetlana.com/docs/tubeworks.html
There are others, but if you read these, you'll have some food for thought. Remember that the TM4 is going to be Single Ended, due to it's valve complement (it would need to have 4 valves in the output stage to be a Push Pull stereo amp).
I hope that this helps!
Enjoy your music!
Morse
*Clipping occurs when an amp is called upon to deliver a larger amplitude waveform than it's power supply has the current reserves to provide. Ohm's law is: Potential (in Volts) = Current (in Amps) times resistance (in Ohms). Here's an example: your amp needs to supply 32 volts to make a waveform (it's playing a really loud cannon shot on your Telarc 1812 LP) at the present volume setting, and your speakers are a "perfect" 8 ohm load (not likely, but that's another story). Thus, your amp momentarily needs to supply 4 amps of current to the speakers; if your amp's power supply does not have the capacity to provide it, the top of the note is 'sheared off'. Since the waveform is sharply 'clipped', it resembles a square wave. Square waves can be shown mathematically to be the sum of a lot of waves that include very high order odd harmonics of the fundamental tone, which is why so many tweeters have been fried by an amp clipping a bass note....
Thanks!