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Thomas Scherer

Supra 2.0 - High End Preamp for Record Player [150616-I]

Status: Finished
August 27, 2015
in Elektor magazine | July 2016 | Find it here
Supra 2.0

More than 2^5 years ago a circuit was published called SUPRA, a "superrauscharmer" MC/MM-Vorverstärker" for record players. The low noise behaviour comes from paralleling 8 cheap low noise transistors (BC550/BC560) which reduces noise by √8 = 2.82 resulting in a big board with 20 transistors (per channel!!!).

Today there are integrated circuits which are so noise optimized that a discrete aproach would make no more sense. LT1028 for example has a noise figure of 0.9 nV/√Hz. If you use four of these chips in parallel noise will be halve of this! This is so low that a lower noise behaviour is nearly impossible, because the resistors - even the low Ohm - have a higher thermal noise. One of this Opamps costs between 5 an 10 € which results in about 100..150 € for a preamp in stereo. Seems not so much because people who hear still records instead of MP3s or CDs are prepared to pay a bit more for their hobby.

But combining this 4 Opamps per channel is not all. If you build such expensive hardware it makes sense to optimise the remaining circuit too. Why not integrating a true passiv RIAA correction? Most manufacturers only speak about it. Here they are: buffered RC combinations. Have a look at the design. I made layout files (EAGLE 7) if somebody wants to test this preamp in reality. The values are for a MM preamp. A MC preamp needs some higher gain and lower impedances: R1 = 470Ω, R2...R5 = 10Ω, R14 = 3k9

And just for the ones who would prefer a concept with "really true" passive filters (not in the feedback loop of an opamp) I made a version with two RC low-pass and one RC high-pass filters in series (each buffered). But without the 10 Hz subsonic filter of the previous solution. Have fun...  

NOTE: the kit is back in stock ! Buy the kit now and make your own Supra !




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