The Journey of DIY No-Feedback Class D Amplifier (1) Subtitle: The Motivation and Story Behind It

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#241
Normally i use the 96k
My setting at the fistwatt f6 at 1 watt with a signal gen of 1khz.

Used this setting based on the manual of QuatAsylum.
Because i use the load from QuatAsylum I needed to set the input gain setting to -12 db
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The sample rate can only be changed while acquisitions are stopped. Changing sample rates will result in a
reset of the converters which might cause a slightly pop on the outputs. If you are working on high-gain circuits,
this might manifest as large output changes during sample rate changes. In general, the performance of the
converters is better at lower sample rates. If your measurement task doesn’t require frequencies beyond 20
kHz or so, stick to the 48K sample rate. https://cdn.shopify.com/s/files/1/1631/5609/files/QA40x_Users_Manual.pdf?v=1644257226
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#243
flmhhh said:
The power supply used for the test was a 600W switching power supply, which may have larger ripple compared to a typical linear power supply. If the transient response of the switching power supply is suboptimal, it can indeed increase harmonics during high-power output.
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In this case you would need power supply that is absolutely stable and low output impedance even at the switching rate you use. Quite a challenge.
 
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#246
mvs0 said:
In this case you would need power supply that is absolutely stable and low output impedance even at the switching rate you use. Quite a challenge.
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I measured the power noise today, found that the power supply I used is very terrible. please see the figure below:

Here is the test case, 1kHz, output RMS power 96W.
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Here is the power rail measurement.

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zooming in
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when applying test with 1kHz, the power rail second harmonic (2kHz) is as high as -27.8dBV. comparing to the second harmonic of the AMP output, the Power reject ratio should be larger than 40dB@1kHz.
Unfortunately, I don't have better power supply right now. but believe the THD+N is better than what we measured with better power supply.
thanks @mvs0 to reminder me to consider the power issue.
 

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#249
How about the PSSR of the amp module - can it be improved? Either you go for a bit wide band noise (SMPS) or the 60/120 Hz hum (linear)...

How can you be sure its is PS limited when you haven't tested with something else?

I think it is the amp intrinsic distortion we see - no? Thats the cost of no global feedback - or?

//
 
#251
TNT said:
How about the PSSR of the amp module - can it be improved? Either you go for a bit wide band noise (SMPS) or the 60/120 Hz hum (linear)...

How can you be sure its is PS limited when you haven't tested with something else?

I think it is the amp intrinsic distortion we see - no? Thats the cost of no global feedback - or?
Click to expand...
for the PSSR, obviously, as I said, it is at least 40dB@1kHz. please see the power noise at 2kHz (-27dBV), however, AMP output at 2kHz is -67dBV. if the AMP is distortion free, the PSSR is 40dB. actually, the PSRR is 40dB + distortion contribution.

if you looked at the distortion measurements I posted couple days ago, you will find that the THD+N distortion is suddenly jump from 0.00x% to 0.0x% when the output power exceeds some Watt (5W). this is not the typic behavior of AMP. So I guess it is from the Power, of course, need further verification.
 
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