The problem is described
in my Thöress preamplifier instruction manual:
" However, if a single-coil low-output (dedicated mono) MC cartridge is to be used on these (stereophonic) phono inputs and it is intended for dual-speaker-mono listening, precautions must be taken to "double" the mono signal in such way as to avoid hum. Please see the relevant chapter below for more details on using such cartridges. Examples of current production single-coil low-output MC cartridges are the dedicated mono cartridges made in Japan by Miyajima Lab, all models of which employ monoflexible styluses.
When both inputs of a stereophonic phono preamplifier are wired to a single-coil generator cartridge in attempt to create signal doubling to allow for dual-speaker mono reproduction it is impossible to avoid a conductive connection between "left ground" and "right ground" of the (non-symmetric) phono circuit and, as a result, unwanted ground loops are formed.
Single-coil (mono) cartridges will cause residual hum due to ground loops when they are wired to feed both inputs of a stereophonic preamplifier (with non-symmetric circuitry) in an attempt to double the mono signal at the phono input for dual-speaker-mono listening !
To overcome this problem the signal should be doubled at the point of output rather than attempting to accomplish this earlier in the signal path. The paralleled main output jacks can be used to conveniently achieve this aim in the following manner. Only the right channel of the phono section should be fed by the single-coil generator via a customized monophonic tonearm cable, whilst the corresponding left input should be short-circuited directly at the jack with the help of a RCA plug internally soldered to suit this purpose. If a monophonic tonearm cable is not at hand, a conventional stereophonic tonearm cable can be used equally well (with both plugs inserted into the respective jacks), in which case shorting the inactive left input can be conveniently accomplished at the very end of the left channel lead of the tonearm cable by shorting the "left hot" and "left ground" pin of the headshell. If the tonearm is equipped with free leads, the desired conductive connection can be easily formed by taping the lead ends of the respective tonearm leads firmly together. After these adaptations required to create dedicated single-coil cartridge input, signal doubling can be obtained at the output by connecting both power amps to the right channel main output by using the respective paralleled pair of jacks. "
" However, if a single-coil low-output (dedicated mono) MC cartridge is to be used on these (stereophonic) phono inputs and it is intended for dual-speaker-mono listening, precautions must be taken to "double" the mono signal in such way as to avoid hum. Please see the relevant chapter below for more details on using such cartridges. Examples of current production single-coil low-output MC cartridges are the dedicated mono cartridges made in Japan by Miyajima Lab, all models of which employ monoflexible styluses.
When both inputs of a stereophonic phono preamplifier are wired to a single-coil generator cartridge in attempt to create signal doubling to allow for dual-speaker mono reproduction it is impossible to avoid a conductive connection between "left ground" and "right ground" of the (non-symmetric) phono circuit and, as a result, unwanted ground loops are formed.
Single-coil (mono) cartridges will cause residual hum due to ground loops when they are wired to feed both inputs of a stereophonic preamplifier (with non-symmetric circuitry) in an attempt to double the mono signal at the phono input for dual-speaker-mono listening !
To overcome this problem the signal should be doubled at the point of output rather than attempting to accomplish this earlier in the signal path. The paralleled main output jacks can be used to conveniently achieve this aim in the following manner. Only the right channel of the phono section should be fed by the single-coil generator via a customized monophonic tonearm cable, whilst the corresponding left input should be short-circuited directly at the jack with the help of a RCA plug internally soldered to suit this purpose. If a monophonic tonearm cable is not at hand, a conventional stereophonic tonearm cable can be used equally well (with both plugs inserted into the respective jacks), in which case shorting the inactive left input can be conveniently accomplished at the very end of the left channel lead of the tonearm cable by shorting the "left hot" and "left ground" pin of the headshell. If the tonearm is equipped with free leads, the desired conductive connection can be easily formed by taping the lead ends of the respective tonearm leads firmly together. After these adaptations required to create dedicated single-coil cartridge input, signal doubling can be obtained at the output by connecting both power amps to the right channel main output by using the respective paralleled pair of jacks. "