The "As the original parts" bit seems illogical, as I was under the impression you wanted to improve the performance somewhat compared to the original design.
The original poster's new FETs behave like the originals, except for a 400 mV shift in gate-source voltage that will be compensated out when you change both FETs. If you don't want to improve or degrade anything, it suffices to just replace those FETs.
Thank you, In the spirit of learning I would love to look at improving the performance of the stage. Is that even possible by simple adjustment of the source and drain resistors? It sounds like at minimum there is a tradeoff between losing open loop gain and increased current for less distortion.
Thank you, this is really helpful. Where I struggle is with the confidence in my assumptions. Now, how about calculating this loss in open loop gain and it's impact on performance?
Relative.
Notice my first post (<- this is a link!) on this matter.
Original first stage open loop gain:
Vpo=-0.45, Idss=2.2mA, setpoint is 0.13V/0.75mA -> S=6.95mA/V -> rs=144Ω => Ao =22k / 144Ω = 153x (43.7dB) {per leg}
Modified first stage open loop gain:
Vpo=-0.80, Idss=5.5mA, setpoint is 0.42V/1.1mA -> S=6.53mA/V -> rs=153Ω => Ao =15k / 153Ω = 98x (39.8dB) {per leg}
Open loop gain loss is 1.56x (3.86dB).
It does not looks like a great deal, but it may have effect on the 1/f noise, what will be amplified mostly in a RIAA pre stage.
2dB level difference is audiable, for experienced ears even less (I've been a (live) stage engineer, both sound, lights, props and what not.).
Keep in mind that the given values in the service manual are a 'median production line' value, as well as the known variation of individual parts within a grade, can cause small deviations for every single produced/sold unit.
Notice my first post (<- this is a link!) on this matter.
Original first stage open loop gain:
Vpo=-0.45, Idss=2.2mA, setpoint is 0.13V/0.75mA -> S=6.95mA/V -> rs=144Ω => Ao =22k / 144Ω = 153x (43.7dB) {per leg}
Modified first stage open loop gain:
Vpo=-0.80, Idss=5.5mA, setpoint is 0.42V/1.1mA -> S=6.53mA/V -> rs=153Ω => Ao =15k / 153Ω = 98x (39.8dB) {per leg}
Open loop gain loss is 1.56x (3.86dB).
It does not looks like a great deal, but it may have effect on the 1/f noise, what will be amplified mostly in a RIAA pre stage.
2dB level difference is audiable, for experienced ears even less (I've been a (live) stage engineer, both sound, lights, props and what not.).
Keep in mind that the given values in the service manual are a 'median production line' value, as well as the known variation of individual parts within a grade, can cause small deviations for every single produced/sold unit.
