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Old 19th January 2013, 03:35 PM   #32961
SY is offline SY  United States
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Possibly for some situations, but not necessary for a line stage meant to drive 10k, especially at very moderate voltage swings. So... what's the gain?
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Old 19th January 2013, 03:35 PM   #32962
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Talk about 'babying' the 797. Now, all we have to add is a high quality inverter to give us balanced output. Maybe another AD811.
I bet the combination will measure pretty darn well.
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Old 19th January 2013, 03:36 PM   #32963
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The usual line amp gain is between 4 and 10. You choose.
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Old 19th January 2013, 03:42 PM   #32964
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John, i was curious to know the Ampex recorders you had designed some audio parts in. Would-you be nice enough to satisfy my curiosity ?
Quote:
Originally Posted by john curl View Post
Talk about 'babying' the 797. Now, all we have to add is a high quality inverter to give us balanced output. Maybe another AD811.
I bet the combination will measure pretty darn well.
Why bother ?
AD: SSM2141, SSM2142,
or better: TI: DRV134, DRV135
THAT:1606, 1646 ...etc
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Last edited by Esperado; 19th January 2013 at 03:50 PM.
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Old 19th January 2013, 03:46 PM   #32965
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The values in Table 6 or Table 8 of the datasheet would be a good starting point- and likely a good ending point.

There's also likely a lot of other candidates in the IC world, I chose the 797 for illustration because I have some and I'm familiar with them.

So, where's that all-important 7th and 9th harmonic distortion data? Lower than -200dB is not good enough?
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Old 19th January 2013, 04:06 PM   #32966
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I worked with the AG-440, MM1000, and the AG500, and 600.
45 years ago, I was testing the AG-440 electronics virtually every day, with an HP wave-analyzer, sometimes for distortion, sometimes for noise.
At the same time I developed the first phase locked capstan servo for the AG-440 back in 1968.
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Old 19th January 2013, 04:16 PM   #32967
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Part of an Ampex Audio report written in 1968, and presented 44 years ago.
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File Type: jpg ampex68report.jpg (475.7 KB, 147 views)
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Old 19th January 2013, 04:16 PM   #32968
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I designed the signal recording head amplifier for my 16/35mm machines. As i wanted a max headroom, and the habit was to set a CS with a serial resistance in the head path, i used a (shielded) spare recording head in feedback loop instead. With incredibly good results.
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Old 19th January 2013, 04:29 PM   #32969
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Quote:
Originally Posted by john curl View Post
Then: For 2'nd harmonic, estimate that its value drops directly with voltage level.
For 3'rd harmonic, estimate that its value drops directly with the SQUARE of the voltage level.
For 4'th harmonic, estimate that it drops to the CUBE of the voltage level.
Etc, etc.
Now what about 7th harmonic? It should drop as the 6th power, which implies that if you drop the voltage level in 1/2, then the 7th harmonic should drop by a factor of 64.
.
Really, I think you need to review the concept of second and third order intercept.
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Old 19th January 2013, 04:36 PM   #32970
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OK, time for clarification: 7th harmonic is one of the most significant harmonics that the ear can detect with music. It is fairly easy to generate, as well, with crossover distortion and diode nonlinearities in electronic circuitry.
A dozen years ago, my main assumption was that if I could reduce 7th harmonic at listening levels to, let's say: -120dB or one part in 1 million, then the circuitry should be OK. I used this standard to compare my Parasound power amps, testing them invariably at the Xover region between Class A and Class A-B, where the transfer function is the weakest. This is NOT a classical Class A situation, and like IC's, can suffer Xover distortion, of some magnitude, during the transition between Class A and Class B.
Now, this transition may happen between 1W and 30W on my power amps, but where does it come in from a typical IC?
Let us use the approximation that the output stage is biased at 1/2ma. That means that the A-B transition will happen after about 1ma peak output, either + or - .
What voltage level does that translate to?
Well if we did what SY directs, then we would have a 10K feedback resistor with a 1.1K to ground, and a 50K amplifier load. This would be an effective load of about 10K, so 1ma and 10K load, gives us 10V peak output. Pretty good, but is it practical? No, because in the case of the AD797, the noise has risen 5 times over the design's inherent capability, due to the 1K of feedback resistance of 4nV/rt Hz. So, IF you want to use the capability of the AD797 to its advantage, we MUST reduce the feedback resistor. I recommend 50 ohms, that only throws away about 3dB from the best noise possible. Now for a gain of 10, what is the main feedback resistor value? It is 450 ohms, giving the op amp a 500 ohm load. Too much? What to do? How about 1500 ohms for the output, and 165 ohms for the feedback resistor? Not bad, BUT you could not get any more than 1.6V PEAK from the IC, before it went into Class A-B transition. Now, is that a problem? To me, yes, to you, perhaps not.
Well the only thing I get from this is that feedback resistors determine noise performance, not the 797.
Scott must be happy for you to endorse his baby like that!

jan
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