It’s always great to come back and revisit a great design. The 4P1L preamp performs flawlessly so I tweaked the gyrator board to see how it worked with the BF862 FET. The result is great, it sounds as good as it measures:
The 4P1L is biased to 150V/25mA which is the maximum current that the BF862 can do (IDSS max). You can see that the frequency response is flat up to 1.5MHz. The LF response of my test mule is affected by the AC coupling of the measuring gear. However it should be around 5-10Hz.
The distortion of low-level signals is really good:
Predominantly H2, it’s very nice to see THD<0.015% for a 4Vrms output. The load is 100KΩ which is the typical input impedance of an amplifier (with exception of solid state gear)
This low distortion manifests across the entire audio band (ignore the THD below 20Hz which is a byproduct of my testing gear):
The nice thing to see also, it’s how well the 4P1L can drive larger voltage swings:
We can see H4 popping up, however odd harmonics are lower (H5 in fact is higher than H3). THD at 10Vrms is still below 0.03%!
Nice!
Which soft/hardware did you use to measure the frequency response?
Wish you a nice weekend,
Erik
I used the setup described here: http://www.bartola.co.uk/valves/2016/04/17/differential-buffer-and-preamp/
Hi Ale,
now it is clear, you have a USB scope for this! I have been looking into one as well, a lot of options out there.
Cheers! Erik
Yes, the Instrustar is ok. You need a floating PC (e.g. laptop) and an output buffer. The oscillator was a bit unpredictable at HF with loads. Buffer sorted things out
Hi Ale,
Your test result is really encouraging!
What value of R7(RMU) and filament current had you applied for the test?
Secondly, considering safety measure (j310 up to 30mA), sound quality and performance, which jfet (j310 or BF862) would you prefer more, for 4P1L preamp/driver?
Thanks & Cheers! Paul
Hi Paul
Rmu should be around 470-510R. I tried the 4P1L above 30mA with a J310. I couldn’t test same operating point with BF862. Both sound really good. If you are not used to SMD, stay away from the BF862!
cheers
Ale