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Old 4th August 2007, 01:23 PM   #21
Progg70 is offline Progg70  Sweden
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suzyj
What kind of analyzing equipment do you use? Software on a Comuter?
It’s looks nice your project. And I’m looking for some analyzing software…
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Old 4th August 2007, 02:42 PM   #22
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SG505 -- my mistake -- I meant the AA501 or DA4084 -- both of which have a 400 Hz high pass filters --
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Old 4th August 2007, 11:36 PM   #23
suzyj is offline suzyj  Australia-Aboriginal
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The FFT shown was done with an output Zobel network and 8 ohm load. When I did the sims for that amp I was using a 6K2 feedback resistor and 15p miller compensation caps on the VAS stage.

From memory, I did have the 400Hz high pass switched in on the AA501 when I measured the noise levels, though it was some time ago

Progg70: I use LTspice, from Linear Tech, for simulations of virtual circuits before building them. It's free and wonderful. For measurements of real amplifiers, I use an old Tektronix SG505 audio oscillator, and an AA501 distortion analyser, as well as a nice new LeCroy digital sampling oscilloscope.

I've been playing around with RightMark Audio Analyser software (which is windows freeware) on my PC as well, though haven't used it to measure any real amplifiers, as my sound card (an M-Audio Delta-44) doesn't have appreciably better specs than the SG505/AA501 combo.
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Old 5th August 2007, 04:01 AM   #24
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Quote:
Originally posted by suzyj


From memory, I did have the 400Hz high pass switched in on the AA501 when I measured the noise levels, though it was some time ago
That's why I was wondering about the 12nV/rt Hz claim -- noise is specified at a specific bandwidth and frequency limit. a 400Hz high pass filter sort of defeats the purpose.
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Old 5th August 2007, 06:32 AM   #25
suzyj is offline suzyj  Australia-Aboriginal
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Quote:
Originally posted by jackinnj


That's why I was wondering about the 12nV/rt Hz claim -- noise is specified at a specific bandwidth and frequency limit. a 400Hz high pass filter sort of defeats the purpose.
How so? Having the 400Hz high pass and 80KHz low pass switched in allows me to measure the noise voltage over that bandwidth without having mains hum and flicker noise affect the measurement.

I measure 52uV RMS over a 79.6KHz bandwidth (80KHz-400Hz). That corresponds to 180nV/sqrtHz at the output, or 12nv/sqrtHz referred back to the input (voltage gain of 16).

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Old 5th August 2007, 09:27 AM   #26
AndrewT is offline AndrewT  Scotland
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Hi,
10Hz to 20kHz would be fairer, but that would push your result up to >20nV/root Hz. Additionally the figure looks good because the amp gain is only set to 16times.

For that spec, 400 to 80kHz & 16x, I would expect 4 to 7nV/root Hz. Seems the FET input has not helped too much.
On that point, does the adoption of the FET input improve the STABILITY of the output offset? or does it simply reduce the level of offset for a particular Zin setting?
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Old 5th August 2007, 02:35 PM   #27
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this is getting outside the realm of the stated purpose of the thread, but the noise is relevant down to tens of Hertz.

If you have access to the AA501/SG505 again I would suggest that the 400Hz filter be switched out, then take an FFT of the residual of a 1kHz signal -- this will tell you a lot about your amplifier and power supply.
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