Tag: 4P1L Robustiano

Robustiano (V0.7)

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:

Robustiano v07 bench test

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:

Robustiano v07 THD tests

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:

Robustiano v07 THD versus power

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…

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Cheers

Ale

Robustiano (version 0.6)

It was now the turn to the BJT to show what it can do in this circuit. Here is my quick breadboard with components I had at hand, so there is a slight variation from the simulation:

Robustiano 4P1L v06 BJT testThe Q1 is obviously an MPSA42. C3 was substituted with a 10uF/25V electrolytic (yes you read well). Rf is a 250k carbon pot and R2 is built of a series of 1K pair in parallel plus 100 ohms in parallel, four resistors are wire-wound 1W. C1 is actually a 30uF/450V ASC Oil cap.

I haven’t measured the collector current but it seems to be around 1.3-18mA. Variance is due to tolerance of RE and R3. I need to measure the actual resistance of Rf but is somewhere between 180 and 200K.

As Rod suggested, the circuit is very stable and easy to dial the right feedback with the potentiometer. The 4P1L is biased about -9V and is slightly under the max Pa in this case.

Distortion is as predicted in the simulation (e.g. 0.1% for 1W and about 0.2 – 0.25% for 2W). Just above 2.2W distortion creeps up rapidly given grid current, which is not modelled properly by the 4P1L pentode model:

Robustiano v06 THD versus powerLooking at distortion versus frequency, it’s interesting to compare the BJT performance against the depletion FET. With lower Cob compared to the Coss of the FET, the BJT should be able to drive better the 4P1L. In fact, the BJT is more linear when swinging many volts compared to the depletion FET, so the proof is in the pudding:

Robustiano v06 tests THD The BJT is indeed more linear but if we compare the THD vs frequency of both drivers when providing 2W output power, we can see that the BJT is suffering as much (and even more given poorer slew rate) than the FET at frequencies above 12kHz. Also FET’s THD versus frequency is more linear up to 10kHz, whereas the BJT has a peak around 6kHz and a dip closer to 10-11kHz. Either way, the BJT outperforms the FET in overall THD up to 11kHz.

I need to listen to this circuit now…