All about electronic valves and hi-fi
My adventure with the HT SMPS modules continued. Did further testing with higher power modules which worked ok.
The headphone stage (HP) has a 1MHz bandwidth, impressive. Here is the circuit breadboarded after the modification of the 2P29L HP amp:
Continue reading “6e6p-dr HP amp – part 2”A few weeks back we managed to meet at Jon’s with a few of the London Audio Circle members. I haven’t seen the bunch for a long time due to family and work travel commitments, however it was a great opportunity I didn’t want to miss this time round.
One of the intended purpose of the meeting , which for obvious reasons these meetings never goes as planned due to the random nature of DIY, was to shootout two headphone (HP) amplifiers over modded HD800 headphones.
Jon’s HP amplifier was a simple stage formed by a C3g (triode-strapped) valve loaded with a nice amorphous-core Valab choke. Output cap-coupled to a TVC to his main system when he uses it as preamp. Otherwise cap couple to HD800s headphones.
Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.
The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!
With 15.5% feedback and 125V screen voltage I got the best curves as shown below:
Nice to see the anode resistance coming down to 1kΩ.I tried matching a triode model to these curves with a good result