gslikker
Super Member
I put it more or less together (front plate and knobs).
Several observations:
1. There is a need to clean the input switch. Because it is also in the feedback loop of the input amp, moving the switch selector from Tuner towards Phono and in between Phono and Mike selections, gives a rather loud switching effect. I guess I also need a high ohms resistor to ground at any place where a capacitor is switched (to have 0V DC level at it beforehand) Those places are the F1228 input amp output cap and the RIAA filter.
2. The amp is sensitive to hum from the outside, when getting close to the input amp with my hand I hear hum getting more. So it really NEEDS the metal shielding plate to get rid of the hum, as it has a wooden case.
The hum is gone having the metal shield attached.
3. The main output amps slowly hunts a bit within a millivolt. Also, from cold to warm, there is close to a millivolt difference. So, offset definately stays within 2 millivolts from zero, whether warm or cold. The decision to be a bit paranoid for component selection and modifying the circuit pays off for DC offset measurement.
4. Still, a "baxandall diode"is to be considered for the main amps.
Several observations:
1. There is a need to clean the input switch. Because it is also in the feedback loop of the input amp, moving the switch selector from Tuner towards Phono and in between Phono and Mike selections, gives a rather loud switching effect. I guess I also need a high ohms resistor to ground at any place where a capacitor is switched (to have 0V DC level at it beforehand) Those places are the F1228 input amp output cap and the RIAA filter.
2. The amp is sensitive to hum from the outside, when getting close to the input amp with my hand I hear hum getting more. So it really NEEDS the metal shielding plate to get rid of the hum, as it has a wooden case.
The hum is gone having the metal shield attached.
3. The main output amps slowly hunts a bit within a millivolt. Also, from cold to warm, there is close to a millivolt difference. So, offset definately stays within 2 millivolts from zero, whether warm or cold. The decision to be a bit paranoid for component selection and modifying the circuit pays off for DC offset measurement.
4. Still, a "baxandall diode"is to be considered for the main amps.