Replaced the input base resistors with 68k ohm instead of 100k ohm and... 6mV and 4mV. Super stoked at the result.
I got some help with modelling and it looks like the 68k resistor to the negative rail on one side of the current mirror was likely done to broadband some behavior at high frequency (5-10mhz range) that could cause oscillation.
The ideal solution (keeping the same basic topology as the existing design) would be to add 68k to the other leg of rue current mirror, also to the negative rail. I'm still deciding whether I can accomplish that cleanly.
Cheers and thanks for all the help!
Nathan
I got some help with modelling and it looks like the 68k resistor to the negative rail on one side of the current mirror was likely done to broadband some behavior at high frequency (5-10mhz range) that could cause oscillation.
The ideal solution (keeping the same basic topology as the existing design) would be to add 68k to the other leg of rue current mirror, also to the negative rail. I'm still deciding whether I can accomplish that cleanly.
Cheers and thanks for all the help!
Nathan
Alright! The power amp board is done. In addition to the base resistor change, I replaced R135 and R136 with Vishay Dale films. In addition to this change, I added two new resistors of equal value (68k ohms) joining the collectors of TR119 and TR120 to the negative rail.
Schematic as a reminder:
These were suggested by a friend with amp design knowledge as a way to further balance the diff pair. The results speak for themselves:
The install itself didn't turn out that bad! Board had a lot of flux on it from the factory, I try to get it clean but it's a challenge to not just smear it around without splashing acetone everywhere.
Now all that's left is the phono mods @Leestereo helped me come up with and reassembly! Super excited to get this thing setup in the new house. I'm listening now through headphones and it's pretty damn good. Bass response is radical.
Cheers!
Nathan
Schematic as a reminder:
These were suggested by a friend with amp design knowledge as a way to further balance the diff pair. The results speak for themselves:
The install itself didn't turn out that bad! Board had a lot of flux on it from the factory, I try to get it clean but it's a challenge to not just smear it around without splashing acetone everywhere.
Now all that's left is the phono mods @Leestereo helped me come up with and reassembly! Super excited to get this thing setup in the new house. I'm listening now through headphones and it's pretty damn good. Bass response is radical.
Cheers!
Nathan
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