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Old 23rd March 2012, 06:51 PM   #2171
MiiB is offline MiiB  Denmark
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Let it be.. this discussion does not belong i this SSA thread...pl make a new thread on frequency demand....no sense in cluttering this fine thread up
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Old 23rd March 2012, 06:53 PM   #2172
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by Lazy Cat View Post
Maybe the thermal track diode causes this distortions, at 1 mA first stage bias, also diode bias current, dV/dI curve is higly nonlinear. Global feedback can compensate this behaviour to some extent but not to ppm level.
Its a shame...the circuit at least in simulation can be tweaked to yield a constant idle current from junction temp 20 degree to 130 degrees...
wow.
But the distortion level is outright too much.
That alone is not bad, but the harmonics spectrum is poor as odd harmonics dominate . That will not sound good.
Thus next attempt coming soon.
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Old 23rd March 2012, 06:56 PM   #2173
mikelm is offline mikelm  England
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it's not to do the signal slew rates . . . .

It's to do with the feedback being able to keep up with the signal thus enabling having a stable amp without having to compensate all of the life out of it.
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Old 23rd March 2012, 07:54 PM   #2174
hahfran is offline hahfran  Germany
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Originally Posted by mikelm View Post
it's not to do the signal slew rates . . . .

It's to do with the feedback being able to keep up with the signal thus enabling having a stable amp without having to compensate all of the life out of it.
Yes that is open loop bandwidth. A technical aspect. But for the aspect of audible quality what is not recorded cannot be amplified. And should not. Thus for the audible impression the slew rate of the amp is sufficient if it is 10 V/µsec at full output. Btw. which speaker can reproduce 10 Volts/µsec?
You find that in step response. The fastest tweeters need 18 µsec for max excursion.
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Old 23rd March 2012, 08:05 PM   #2175
mikelm is offline mikelm  England
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If you build an amplifier without loop feedback I think your augument may have some truth.

For an amp with loop feedback speed is absolutely essential to achieve a good sound - if you don't quite get that yet you should try building and listening to a few fast & slow loop FB amps and then making comments based on ur own experience.

For a fast amp - the SSA is a very very good contender but as stated earlier perhaps not ideal for a beginner - Fetzilla may be easier even if not quite as fast.

good luck !
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Old 23rd March 2012, 09:07 PM   #2176
sonnya is offline sonnya  Denmark
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Topic for a new thread:

Basicly, a no feedback performance depends of of the linearity of the components involved as the dv/di is much lower the in an Open loop design.

The bandwidth and gain of closedloop design does not evolve a lot more, only of it uses several gain stages or take the feedback before the output stage.

So: to lower thd we need to use the design principles from Open loop design, or in other words improve the linearity of each stage. Basicly a proberly design closed loop design should on paper performe better than a Open loop design if there is taken Care of the stability.
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Old 23rd March 2012, 10:16 PM   #2177
hahfran is offline hahfran  Germany
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A side effect:
in this amp the power BJTs are always conducting thus no crossover. But it is not class A rather the idle current is set to 50 mA. The switching is done by Q3 Q11. Perhaps nice to experiment with.
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Old 23rd March 2012, 10:33 PM   #2178
hahfran is offline hahfran  Germany
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Quote:
Originally Posted by mikelm View Post
If you build an amplifier without loop feedback I think your augument may have some truth.

For an amp with loop feedback speed is absolutely essential to achieve a good sound - if you don't quite get that yet you should try building and listening to a few fast & slow loop FB amps and then making comments based on ur own experience.

For a fast amp - the SSA is a very very good contender but as stated earlier perhaps not ideal for a beginner - Fetzilla may be easier even if not quite as fast.

good luck !
The problem that most older amps had a very low open loop cutoff frequency In closed loop an upper cutoff of some 100 kHz is achieved because the feedback factor is very high.
That is the open loop amplification factor is very high and so is the gain bandwidth product. Assume the amplification factor is 1000 and the upper cutoff is 1000 Hz then the gainbandwidth is 1 mHz. The feedback reduces amplification to 10 but does not affect the gain*bandwidth so the upper cutoff becomes 100 kHz. But the feedback speed is still determined by 1 kHz open loop, thus such amps produce TIM and other nasty stuff.
Bode diagrams become important lead lag compensation and so on.
Thus what matters is not the slew rate rather it is the open loop cutoff frequency. This must be at least as high as the cutoff of the source.
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Old 24th March 2012, 12:13 AM   #2179
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Quote:
Originally Posted by MiiB View Post
Let it be.. this discussion does not belong i this SSA thread...pl make a new thread on frequency demand....no sense in cluttering this fine thread up
IT is the heart of the topic, on the contrary. Why did this SSA topography sound so good ? Because it use CF ( and the elegant way this feed back is mixed with the input signal).
It means higher slewrate,= less distortion, and no active stage in the feedback path = less distortion.
Quote:
Originally Posted by hahfran View Post
YBtw. which speaker can reproduce 10 Volts/µsec?
Again it is not about the bandwitch we need for your ears, it is about transient distortion in closed loop systems.
To begin to experiment that, just compare (listen) a strait line with a 10V/µs OP amp ( gain 10 + 11/10 bridge), inside a high end hifi system on one side, and do the same with some 500 or 1000V/µs video OP amp, on an other, you will be surprised.

To make your mind about things like that, I suggest you use your ears. Then to try to correlate your experiences with mathematics. Because there is a lot of factors we don't think about at first sight, or we do not know how to measure.
My assertions about slew rates lie on >40 years of experience and experiments, both as an electronic designer and as a sound engineer. And this is well known nowadays, even in AES; Ask Nelson Pass or Douglas Self.
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Last edited by Esperado; 24th March 2012 at 12:17 AM.
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Old 24th March 2012, 02:25 AM   #2180
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Or Bob Cordell: CordellAudio.com - Another View of TIM
or Walt Jung: http://waltjung.org/PDFs/SID_TIM_TAA77_P1.pdf
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