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Old 26th December 2011, 07:24 PM   #191
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I was referring to driving loudspeakers rather than resistive loads.
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Old 26th December 2011, 07:39 PM   #192
Cassiel is offline Cassiel  Libya
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This is a DIY site, right? People here do their own building because they are not happy with commercial products, in one way or another. So, if engineers do amps by the numbers or by quality of sound it's really something that does not interest me. I don't buy amps from other people - thanks God I'm free of that hassle. They must be doing something wrong anyway because I'm not an engineer and the amps that I build generally sound better to me. The only thing that I miss is the quality of the chassis. In that area I'm miles behind and that's what I most often admire - a well engineered chassis.

No wonder that people interested in these debates either build for other people or buy commercial amps.
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Old 26th December 2011, 07:55 PM   #193
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Is there no standardized calibrated reference speaker that amplifiers are tested with?

if so this seems a problem.

addendum: is such a thing possible?

Last edited by revboden; 26th December 2011 at 08:05 PM.
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Old 26th December 2011, 09:38 PM   #194
sbrads is offline sbrads  United Kingdom
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I've always wondered if the real problem and perceived variability in amps performance assuming the usual very low THD is really in how the speakers perform with different amps, and whether it's for reasons not usually considered during amp design. Like, how do amps cope with back emf from speakers? Does anyone measure output distortion with 1kHz applied to the input with bursts of 100Hz also applied to the output (not the input) to muddle things up a bit and be a bit more like music with back emf from the speaker.

How much back emf is there typically, I don't know how to measure it, but I bet it's time delayed and has a non-flat frequency response. Even worse, passive crossovers behave totally different to normal with back emf applied, i.e. in reverse, creating sharp resonances as the speaker sees a parallel high Q LC network due to the amp looking much like a short circuit at the output. I know I've kept 20 ohm approx resistors across each drive unit in my much modified 3-way speakers because I can hear these muddled high Q effects reduced dramatically, with transparency much increased.
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Old 26th December 2011, 10:01 PM   #195
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Quote:
Originally Posted by sbrads View Post
I've always wondered if the real problem and perceived variability in amps performance assuming the usual very low THD is really in how the speakers perform with different amps, and whether it's for reasons not usually considered during amp design. Like, how do amps cope with back emf from speakers? Does anyone measure output distortion with 1kHz applied to the input with bursts of 100Hz also applied to the output (not the input) to muddle things up a bit and be a bit more like music with back emf from the speaker.

How much back emf is there typically, I don't know how to measure it, but I bet it's time delayed and has a non-flat frequency response. Even worse, passive crossovers behave totally different to normal with back emf applied, i.e. in reverse, creating sharp resonances as the speaker sees a parallel high Q LC network due to the amp looking much like a short circuit at the output. I know I've kept 20 ohm approx resistors across each drive unit in my much modified 3-way speakers because I can hear these muddled high Q effects reduced dramatically, with transparency much increased.
It is possible to check how linear is output resistance of the amp applying current to output. Say, 100W of sine wave through 8 Ohm resistor, when the amp itself is at idle. FFT plot is quite revealing.

Long time ago when our laboratory designed thick film 100W ICs I did something similar, bridging a couple of ICs.
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Old 26th December 2011, 10:03 PM   #196
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Now you are thinking, sbrads. Matti Otala addressed this problem back in 1977. It is called IIM. I am relatively sure that my IIM is as low or lower than virtually any feedback controlled power amp, due to the inherently low open loop output impedance that I design into my power amps. This lowers the potential for IIM. I let the natural output impedance absorb the reverse EMF before the feedback loop has to do anything significant. It 'might' also be another reason why triode amps always have sounded better than pentode amps.
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Old 26th December 2011, 10:13 PM   #197
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Quote:
Originally Posted by john curl View Post
It 'might' also be another reason why triode amps always have sounded better than pentode amps.
I know that it is true, without any doubt. When I apply local feedback across pentode output stages the effect is like with triodes with very low output resistance. However, it requires more power from drivers, but pentode LTP drives nicely pentodes with parallel feedback by voltage across them.
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Old 26th December 2011, 10:14 PM   #198
tvrgeek is offline tvrgeek  United States
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sbrads
You don't have to wonder. It's ugly. That would suggest a self powered design would have the advantage. If we could get a good amp designer to hook up with a good speaker designer.... The ones I have listened to have never really showed off the advantage. Saw a nifty Meridian system today. Way out of my price, but at least they seem to be thinking.
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Old 26th December 2011, 10:30 PM   #199
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Originally Posted by CopperTop View Post
Isn't it a strange coincidence that the point beyond which amplifiers start to behave unacceptably, apparently, is at the upper limit of human hearing? The same electronic components that are used for processing signals in the many megaherz or gigaherz ranges (i.e. nothing to do with audio) can only just be configured to reproduce a complex waveform whose components are all within a bandwidth of 20kHz, or so it seems. It's a good job that the physics of air, and the laws of biology mean that our ears evolved the way they did.

Or is it the case that even if our ears ran out at 10Hz, people would still manage to make a living telling us that our amplifiers are rubbish, and that they can hear huge differences between valves and transistors, and that circuits without feedback are so much better than those with?
A BC547 TO92 Transistor behaves well, and is used in circuits at audio frequencies and RF

An important development that has already had a commercial sale cycle namely companding,especially in the form of DBX Type 1 ( David Blackmer ) needs to be reconsidered as commonplace in audio systems. David had many splendid ideas and was most interested in extending the band of perceived aural frequency much higher
David E. Blackmer - Wikipedia, the free encyclopedia
and article "The World Beyond 20Khz" http://www.drtmastering.com/blackmer.htm


Cheers / Chris

Last edited by Chris Daly; 26th December 2011 at 10:37 PM. Reason: added article reference
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Old 27th December 2011, 01:48 AM   #200
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As were mentioned here several times, distortions that slip out of traditional measurements are of dynamic characted. That means, if you don't hear 10 Hz frequency, changes with such frequency still are audible as dirt.
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