Some radios such as the JVC Nivico 9425W sound great with balanced sound even in the bass. When connected to an Auratone 5C type cube speaker such as the Pleiades linoleum they sing.
Connecting such a speaker to a Uher 4200 report gives very nice results but there is some loss of smoothness due to HF emphasis compared to bass. This is expected as the cube is a small speaker. The difference is balance or smoothness was yesterday confirmed by a male and a female listener.
Similarly connecting UHER to a Heathkit AA-32 2x10W electron tube amplifier. There was lack in bass.
How is this nice subtle bass boost achieved on JVC 9425W?
After the volume potentiometer there is a transistor stage feeding a transistor stage feeding 2 transistor emitter followers in push pull. All this is direct coupled except the emitters sending to a capacitor sending to the 4Ω speaker.
From out transistors emitters to emitter of first stage there is a resistor in parallel with a capacitor apparently sending frequency selective negative feedback. The capacitor sends more signal at HF so more HF is attenuated compared to bass. The resistor making this a shelving filter.
How would such a filter sound from collector to bass of a single ended Pleiades BD139 amplifier for example with an output transformer?
Or a mod to the UHER power amplifier? Not recommended as it is not nice to change what designers have in mind.
Perhaps another solution might be to mount a 5in fullrange speaker to a Γ shape baffle of large size for increased bass compared to the cube.
Reference:
JVC 9425LS schematic from the service manual
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