Hacked a simple PCB to build the follower to drive the 4P1L as suggested by Rod. I had to play with the LND150 setting resistor (R4) to achieve the 2mA of idle current. I ended up biasing the 4P1L rather hot at about 11.5W which exceeds the specs. The Q2 VBE was not possible to measure as the Q2 would oscillate I guess when I place the tester lead on Q1 collector and the voltage seems to drop when I try to measure it. Should have added a ferrite bead:
When measuring distortion against frequency, I was keen to see that the follower provided some impact in reducing the HF distortion. For example at 20kHz, THD reduced from 0.96% to 0.59% @1W output power and from 7.84% to 3.52%, that is close to half the distortion I had before:
What is nice to see now is the effect of the follower providing sufficient source current to the 4P1L grid. Above 2.5W, the grid current kicks in and we can see how “Robustiano” can deliver 3W at less than 1% until starts to clip about 3.5W:
I found that if I were to reduce the Rf further and therefore increasing the collector current but obviously exceeding the 4P1L power dissipation too much as collector current was about 45-48mA, the distortion at 20kHz falls significantly. I suspect I should increase the collector current to enable better drive of Q2 due to its Cib (30pF). To keep the current feedback arrangement this could be done by reducing the negative emitter voltage source (V1). Should try this I guess…
Cheers
Ale
So at higher collector current, slew rate is better which should favour to reduce distortion produced by driving 4P1L input however the BJT distortion is higher given increased base current. This explains now what I found that HF distortion continued to grow exponentially despite the addition of the follower, albeit reduced significantly by the follower. This meant that the BJT distortion was causing it. Now, if I reduce the collector current (and have only the follower input capacitance rather than the miller capacitance of the 4P1L) I should find a point where the distortion of the BJT is lower (as experienced briefly this morning). I will play with R3+RE to get this and retain -9V for VEE.
I also noticed the harmonic cancellation, specially for H2 between 1 – 5kHz, whereas at higher frequencies, it was a clear H2, H3, H4 progressive decay spectrum. I suppose this should have a sonic impact which is not what we all want. Anyhow, need to adjust and listen to.
I listened to the amp earlier this evening. I guess the listening test wasn’t ideal as only tried one channel and played some CD via the Pete Millett’s audio interface to drive Robustiano. But what it was worst of my setup was a simple Monacor full range SP200X driver (92dB) laying barebone on top of my workbench. Despite all this, I liked the sound. Detailed and clear. Very loud indeed, I could definitely live with 2.5-3W on this amp.
I think I need to build this properly and test it with proper speakers
So far so good…