(there is a design note about AMP15 available elsewhere on the site)
We have previously mostly given an overall number of 0.008%THD+N.
Now here are some more real measurement details, but the short summary for the less technically oriented is that the standard AMP-15-PS, 2x150W performs very well. Noise and THD numbers outperform the Truepath data sheet numbers for the TA3020 chip, with margin!
The measurements were done for an AMP15 set up for +/-40V which was also used in this case. The load was a wire wound 8ohm resistor. Measurements where done with one channel driven and each data point is an average of about 4 seconds.
The measurements are given without any filtering. No A-weighting or other filtering was done except that the measurements where done at 48kHz i.e. 24kHz bandwidth.
First the background noise, if you can talk about noise when it is below the -110dBV (3uVrms) level. The scale is dbV. Our amps typically display -100dBV noise, ie. are VERY quiet, and AMP15 is even better!
The noise floor relative to a 12kHz signal at 10W. The noise is lower than -120dB-FS! No filtering, no A-weighting. The scale is dbFS; main signal is 0dB, even if the top does not look like it reaches zero on the diagram. An image resolution problem.
The THD+N diagram over the frequency range for 1W and 10W power. Most datapoints are in the 0.007-0.008% range.
Measurements over the power range, from 1W and up. THD+N is also here around 0.008% up until the clipping starts. With 40V supply, clipping starts to show at 74Wrms (24.5Vrms).
With AMP15 set up for +/-60Vdc, and an 8 ohm load, the start of clipping point should be at around +/-36Vrms or 170Wrms.
THD+N FFT for a 1kHz signal, no A-weighting or other filtering. Scale; dbFS. The harmonic is at a bit better than -80dB or in this case 0.008%. You can also see the mains 50Hz picked up somewhere in the chain. For these measurements, there was no shielding at all used, except the measurement cables.
Just as a comparison, the THD+N FFT for the same 1kHz signal, but with A-weighting filter.
For reference: the output from the signal generator (Black Star LDO100). The harmonics are a bit better than -90dB and the noise floor at around -130dB
... and the measurement system, a NI PXI-4461, with no input signal. The scale is dbv and is for the 1V measurement range. The small spike at around -140 dbv / 7.5kHz is from a power supply. The noise floor is around -150dBv
Added Sept. 2010:
Damping factor and output impedance
The damping factor was measured at 33Hz, 50Hz and 100Hz, output level between 2.5 and 10Vrms. With 4 ohm load the damping factor was always better than 500, usually around 600. With 8 ohm load, the damping factor was well above 1000 for all measurements. The damping factor translates to less than 10m ohm output impedance, or low enough to be neglectable in just about all applications.
The very good results can partly be explained by the layout and component selection, especially the inductors, which thanks to being two in parallel per channel, with wound flat wire, minimizes output impedance down to a level which is unmeasurable with anything but high quality instrumentation.
The measurements where done with a high end 6,5 digit dsp based true rms voltmeter (NI-PXI-4070) which has been calibrated recently.
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