Measurements of JFET-BJT Headamp with Niceness Knob
Aksa,
I know you have been waiting for this one... I hooked up the amp to the Focusrite Solo Gen2 audio interface and took some data. Initially with the SMPS and DC-DC converter. When using the DC-DC converter, too many HF artifacts showed up in the FFT so I removed it and connected a pair of fresh Li-ion 9v batteries (measure 16.0v) after the cap multiplier. The HF components were fairly low in the -100dB to -110dB range but I wanted to give this amp a fair test.
Here is a typical FFT with the DC-DC converter and 12v SMPS as power source:
So with batteries, I took data with R6 set from 0ohms to 100ohms in 25ohm steps. The trend is that at lower degeneration resistance values, the THD is lower, and increases pretty much linearly with increasing resistance. Note that there is a 60Hz peak but that is probably EMI pickup due to fact that amp is not in a metal box as everything is battery powered. The -120dB noise floor is quite flat otherwise.
R6=0ohms:
R6=25ohms:
R6=50ohms:
R6=75ohms:
R6=100ohms:
Here is a typical frequency response plot (multi-tone and not swept), you can see that this amp measures exceedingly well with a -0.5dB point of about 17Hz and upper limit is really just the sampling rate (set at 96kHz and 24bit):
So we can see the trend of the H2 and H3 increasing with R6. The stated THD was as follows:
R6=0ohms, THD=0.033%
R6=25ohms, THD=0.046%
R6=50ohms, THD=0.068%
R6-75ohms, THD=0.101%
R6=100ohms, THD=0.123%
I also hooked up the left and right headphone channels to be driven from one amp in mono mode so I can hear a better representation of the full impact (without stereo or imaging). I hve listened extnsively to my collection of test tracks. I have to say this amp sounds very very good - nice deep bass that is articulate and powerful, very smooth mids, and an excellent presentation of treble detail without being harsh.
I even listened to it while adjusting the Niceness knob in real time. Most music (90%) I cannot detect an audible effect of the knob over the 0ohm to 100ohm range. However, on certain tracks in certain portions of the song, I can hear what I would describe as better detail and a more forward presentation when the Niceness knob is set at 0ohms and a more relaxed presentation (less analytical) when at 100ohms. It is most noticeable on the hard rock guitar intro by AC/DC for Highway to Hell. That track was recorded in a studio with extensive sound treatment as it is very dry and free of room ambiance. The strumming of the guitar can be described as a "duh duh duh" sound at R6=100ohms vs a "dah dah dah" sound at R6=0ohms. The detail is more crisp and vivid. So not necessarily better but very sublte and noticeable if one really concentrates. I am quite good at hearing distortion, but I think what we are hearing here may be how transient attacks are modified by the harmonic profile. The biggest thing we can hear are changes to the frequency response profile - but they are essentially identical.
Overall impression of this amp so far in mono is that it is excellent sounding and very low noise when operated by batteries. The detail and clarity of the BJT output is a bit different than a MOSFET. I can't pin it down but it sound different - not better or worse, just different.
You may ask me, what Niceness setting do I like to listen at? It's hard to say, but I think about the midpoint is a nice mix.