Noise in MC Phono-Amps
Because of its tiny output signal the (low-output) Moving Coil cartridge (MC) forces the design engineer to develop extremely low-noise input sections of phono-amps. Two very different examples for input sections are discussed in this chapter: the transfo
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Noise in MC Phono-Amps1
Intro Despite the fast growing digitisation of the electronic world the vinyl and analogue aficionado is still alive. Moreover, it’s a world-wide growing market with increasing demand for excellent phono amplifiers for moving coil (MC) cartridges. Unfortunately, many so-called high-end (and high-price) products show lousy signal-tonoise (SN) figures, far away from what is theoretically achievable. Table 3.12 gives an overview. But there are solutions for lowest-noise phono-amplifiers with a specific MCcartridge as their input load. There are two alternatives to connect a MC cartridge with an output voltage lower than 1 mVrms to an amplifier chain (see Figs. 1.4 . . . 1.5). The two-stage solution consists of a step-up transformer or a solidstate/valve pre-pre-amp (stage 1) attached to a moving magnet (MM) RIAA phonoamp (= stage 2) with an input sensitivity of app. 5 mVrms . As such, it creates enough output voltage to drive a pre-amp. The other option is the full solid-state or valve solution in one stage, including the two different stages. Although the valve driven pre-pre-amp or one-stage solution might be possible to design, inherent obstacles make it impossible to meet the high SN ratios that can be achieved with other approaches. For low output voltage producing MC cartridges it’s extremely hard to find the right lowest-noise input section. An additional noise-driving factor is the cartridge’s source resistance, while its inductance doesn’t play an influential role. As a typical representative of phono-amp challenging MC cartridges, I’ve chosen a cartridge with output voltage of only 0.35 mVrms at 1 kHz and 5 cm/s velocity, with source resistance = 43 R0 and coil inductance = 56 µH: the Denon DL-103 (no. 2798). It’s not too expensive and it sounds excellent.
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Most parts of this chapter were published in EW 10-2006 under the title: “The sound of silence: transformer or solid-state? In this article, the author analyses different approaches to solutions for lowest-noise RIAA pre-amplifiers for use with a specific MC-cartridge” B. Vogel, The Sound of Silence DOI: 10.1007/978-3-540-76884-5, ©Springer 2008
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6 Noise in MC Phono-Amps
Benchmarks and Related MC Phono-Amp Problems Before I started developing a suitable amplifier, I tried to get a benchmark on SNs of excellent MC pre-pre-amps and phono-amps. I’ve checked several test magazines about test reports on MC pre-pre-amps (two-stage approach) and MC phono-amps (one-stage-approach). This survey ended up in very mixed messages: it is nearly impossible to compare SN measurement results from one magazine with those of another. In one case they’ve measured with the input shorted, in another with an input load of 20 R, in other with 25 R and so forth. The next obstacle is measurements with A-weighting filters: with or without IEC roll-off at 20 Hz. Standard A-weighting filters are allowed to have enormous tolerances. They are developed for sound measurement purposes only and not for amp noise measurements. So, if you have two res
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