21-07-2015, 12:07 AM
Now that I have identified the source of the hum I have become pretty sure it is coming from the reservoir capacitor being earthed to chassis too close to the preamp.
The preamp has enough gain to work directly with a moving coil microphone making it suitable for an MP3 player also without additional gain and I suspect that the proximity of the earth for the capacitor to the earth from the preamp stage is causing the problem.
Tomorrow I will create a map of all the earth points in order to verify this is true and to work out if any need to be moved. Reservoir capacitor to the 0V on the HT secondary will be the first for sure.
I have a feeling that the reservoir earth wire might even be sharing a solder tag with the phase splitter but that will have to wait for the morning.
The preamp has enough gain to work directly with a moving coil microphone making it suitable for an MP3 player also without additional gain and I suspect that the proximity of the earth for the capacitor to the earth from the preamp stage is causing the problem.
Tomorrow I will create a map of all the earth points in order to verify this is true and to work out if any need to be moved. Reservoir capacitor to the 0V on the HT secondary will be the first for sure.
I have a feeling that the reservoir earth wire might even be sharing a solder tag with the phase splitter but that will have to wait for the morning.
