25-11-2016, 08:56 PM
During the 1950s, a single pentode, often an EF86, with shunt feedback was the common form disc input amplifier for valve amplifiers. In those days, magnetic cartridge specific outputs were higher, and the pentode, when well-designed, was good, in a noise sense for sensitivities to 4 or 5 mV. Input sensitivity would have corresponded to nominal cartridge outputs at 1 kHz and at 3.54 cm/s (later 5 cm/s) record modulation velocity.
Into the 1960s, magnetic cartridge specific outputs followed a downward trend, the nadir probably being the 0.7 mV/cm/s of the Shure V15. Thus more sensitive input amplifiers with lower noise floors were required, and one answer was the triode pair (e.g. ECC83) with series feedback, although for best results in respect of hum level, that required DC heating, at least for the input triode. Sometimes a single triode with shunt feedback was used, perhaps followed by another flat gain stage. Or a triode as a flat input gain stage followed by a shunt feedback equalized stage. Notably Radford, in its SC22 control unit, used a transistor (BC107)/triode (½ ECC83) combination with series feedback to avoid the need to provide DC heating. The problem was effectively solved in the solid state era with the transistor pair (or triple) with series feedback.
The Shure M75 as I recall has an output of around 1 mV/cm/s, so I think would be borderline for use with a good pentode-based input. I recall from around 45 years ago using a Shure V15 Type II with a Quad 22; that required quite high volume settings and background hiss was quite perceptible between tracks. The M75 would have been somewhat better, I think. Still, for a pentode disc input amplifier, that of the Quad 22 might be a good prototype; specifically that in the “blue” channel, which lacked the multiple-equalization-curve switching found in the “yellow” channel.
Amongst discrete transistor RIAA-equalized amplifiers, the H.P. Walker design shown in Wireless World 1971 May, page 222, was generally reckoned to be a very good example.
Back in the 1950s and 1960s, moving coil cartridges were usually supplied with stepup transformers enabling their use with inputs designed for conventional magnetic cartridges. The idea of directly connecting moving coil cartridges to very low noise head amplifiers seems to have been developed in the 1970s, although I vaguely recall that Kenwood/Trio had tried it using a single-transistor grounded base stage at the end of the 1960s.
Cheers,
Steve
Into the 1960s, magnetic cartridge specific outputs followed a downward trend, the nadir probably being the 0.7 mV/cm/s of the Shure V15. Thus more sensitive input amplifiers with lower noise floors were required, and one answer was the triode pair (e.g. ECC83) with series feedback, although for best results in respect of hum level, that required DC heating, at least for the input triode. Sometimes a single triode with shunt feedback was used, perhaps followed by another flat gain stage. Or a triode as a flat input gain stage followed by a shunt feedback equalized stage. Notably Radford, in its SC22 control unit, used a transistor (BC107)/triode (½ ECC83) combination with series feedback to avoid the need to provide DC heating. The problem was effectively solved in the solid state era with the transistor pair (or triple) with series feedback.
The Shure M75 as I recall has an output of around 1 mV/cm/s, so I think would be borderline for use with a good pentode-based input. I recall from around 45 years ago using a Shure V15 Type II with a Quad 22; that required quite high volume settings and background hiss was quite perceptible between tracks. The M75 would have been somewhat better, I think. Still, for a pentode disc input amplifier, that of the Quad 22 might be a good prototype; specifically that in the “blue” channel, which lacked the multiple-equalization-curve switching found in the “yellow” channel.
Amongst discrete transistor RIAA-equalized amplifiers, the H.P. Walker design shown in Wireless World 1971 May, page 222, was generally reckoned to be a very good example.
Back in the 1950s and 1960s, moving coil cartridges were usually supplied with stepup transformers enabling their use with inputs designed for conventional magnetic cartridges. The idea of directly connecting moving coil cartridges to very low noise head amplifiers seems to have been developed in the 1970s, although I vaguely recall that Kenwood/Trio had tried it using a single-transistor grounded base stage at the end of the 1960s.
Cheers,
Steve
