Nelson updated Forte with more of the audiophile tweaks in later versions. 1a, while operating in pure class A mode, has a significant amount of negative feedback compared to its younger siblings. I had the amp on the bench to try to measure ultrasonic and RF interference from DACs into the amp, and since it was already hooked up, decided to take some quick measurements. All measurements were made at 1w into 8Ω.
PLEASE NOTE: An updated set of measurements with better power isolation, bias adjustment, and a different DAC were performed here (link). This resulted is a much better set of amp measurements. Some of the original measurements were updated in this post.
1kHz and harmonic distortion measurement using REW and Apogee Element24 as ADC @ 192kHz:
I couldn't find a way to get rid of the 120Hz frequency.
Here's no signal going into the amp, but the amp is on (nearly the same as amp being off!)
Frequency response is pretty good, about 4dB down at 40kHz, 3dB down at 10Hz:
Multi-tone test shows increased noise due to IMD towards higher frequencies:
And for good measure here's the 1k square wave:
I didn't do any stress tests (at least not intentionally!) this time, and don't have the load simulator to measure output at greater powers.
I did take some Picoscope measurements (also posted in another thread) to see what it look like above the audible range. This is playing a 10-22k sweep in REW, also at 192kHz:
PLEASE NOTE: An updated set of measurements with better power isolation, bias adjustment, and a different DAC were performed here (link). This resulted is a much better set of amp measurements. Some of the original measurements were updated in this post.
1kHz and harmonic distortion measurement using REW and Apogee Element24 as ADC @ 192kHz:
I couldn't find a way to get rid of the 120Hz frequency.
Here's no signal going into the amp, but the amp is on (nearly the same as amp being off!)
Frequency response is pretty good, about 4dB down at 40kHz, 3dB down at 10Hz:
Multi-tone test shows increased noise due to IMD towards higher frequencies:
And for good measure here's the 1k square wave:
I didn't do any stress tests (at least not intentionally!) this time, and don't have the load simulator to measure output at greater powers.
I did take some Picoscope measurements (also posted in another thread) to see what it look like above the audible range. This is playing a 10-22k sweep in REW, also at 192kHz:
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