I have removed C44, C45, C46, C48 and CR7. I installed an LT1468 in for U1 and replaced the most of the electrolytics in the power supply and oscillator and notch filter board. I installed a 4 turn pot at r51 and r30 and replaced the output level pot. I still cannot get -100db of THD+N at 1khz, getting -98db at 1khz and -95db at 20khz. I am using a 680 ohm load on the loopback cable.
input balance and high frequency adjust have not been calibrated but the oscillator and notch filter has been.
input balance and high frequency adjust have not been calibrated but the oscillator and notch filter has been.
Try removing 680 ohm load resistor to see if there's any impact though there shouldn't be.
I believe you indicated better performance before oscillator mods--- would you please refresh my memory?
Any insight re distortion residual at the monitor output?
Look carefully for high frequency oscillation in both oscillator and buffer stages.
Good luck.
I believe you indicated better performance before oscillator mods--- would you please refresh my memory?
Any insight re distortion residual at the monitor output?
Look carefully for high frequency oscillation in both oscillator and buffer stages.
Good luck.
I get slightly higher THD in the upper band but lower THD in the mid and low band with the LT1468, vs the HA2625
I get some 33Khz on the monitor output and with the 680 ohn resistor removed the THD change is minimal
Advice from other members who've actually made these mods would be welcome.
Presence of CR8 and R29 shouldn't matter since other components are removed but double check in this area.
Output buffer (U2) is unchanged?
I'm curious about 33kHz in monitor output. Can you find anything that changes its amplitude? Are all covers in place? This could make a difference in residual THD.
Does bringing the output filters into play have any effect?
Presence of CR8 and R29 shouldn't matter since other components are removed but double check in this area.
Output buffer (U2) is unchanged?
I'm curious about 33kHz in monitor output. Can you find anything that changes its amplitude? Are all covers in place? This could make a difference in residual THD.
Does bringing the output filters into play have any effect?
I tracked down your earlier mention of THD--- 0.0017% or about -95dBc. So if you can discern 33kHz overlaying random noise in the monitor output using your scope then 33kHz is definitely suspect. Any nearby equipment that might be coupling in--- maybe a computer display?
If 33kHz coming from within the instrument look carefully at power supplies for 33kHz ripple both in oscillator and analyzer sections. Obviously that spurious signal originates somewhere and shouldn't be there. Once found and resolved be sure to go through the adjustment section that trims amplitude and phase nulling in the analyzer before returning to suspicion of oscillator distortion.
Good luck!
If 33kHz coming from within the instrument look carefully at power supplies for 33kHz ripple both in oscillator and analyzer sections. Obviously that spurious signal originates somewhere and shouldn't be there. Once found and resolved be sure to go through the adjustment section that trims amplitude and phase nulling in the analyzer before returning to suspicion of oscillator distortion.
Good luck!
