sound practices reading club /44

We kick off our phono preamps topic with simply the first article on it in sound practices, but boy, what an article it is. We are reading Siren Song: A Preamplifier for Hedonists by jc morrison, from sound practices issue 3. In it, jc starts of by describing the problem of the Puritans ruling the audio world, multiplied by the challenges of designing a phono preamp. This is followed by a programme for the preamp design and the technical architecture that will implement it. Three types of RIAA equalisation architectures are discussed, followed by guidelines for part choices. Then the design choices, i.e. tube choices for each stage, are explained. jc wraps up the article by encouraging us to gut a commercial chassis and build ‘a platform for our obsession with music and art’ in it.

teaser quote: ‘To my mind, nothing seems more impossible than a preamp, especially one which would satisfy the truly hedonistic listener. LP records contain an amazing quantity of subtle timbral information. This same information is subject to bombastic dynamic shifts, both upward and toward silence. […] However, dragging both extremes out of the the grooves is a real challenge. […] Compromise is always the case with preamps.’

a gift from Joe Roberts, SP editor: from the sound practices CD, here is the original article.

my take
What a good point to get started with, miles away form the standard solution—two stages of ECC83/12AX7 wrapped in feedback RIAA eq. Instead we get deeply cultural reasons to not go there, and are introduced to octal tubes and split RIAA eq. Also we are now aware of a whole load of challenges that faces the phono preamp designer. Designing a single ended power amp (of a performance level that a store-bought amp can hardly match) is a cakewalk compared to designing a phono preamp. That is what I took away from the article so many years ago.

The power supply has got some tricks. The 6AU4 damper diode is in the common ground leg. As I found out working on the Coppatone power supply, it is that fact that all DC current runs through this leg—i.e. no DC current can run between the 250–300V and 300–350V legs—that make this scheme work. Now tricky bit: the 300–350V leg gets the full voltage from the bridge rectifier, and being a choke input supply ends up with up to 350V; the 250–300V leg gets half the rectifier voltage, but is a cap input filter and makes up to 300V out of that. Yes, choke input and cap input from the same rectifiers. It could have been more tricky: because of the (guaranteed) common ground leg, one 10H choke could have been put in that leg, instead of the two shown in the schematic. But that would have been even opaquer than this supply.

bonus tracks
See also sound practices issue 4 for mods & tweaks of the Siren Song. Also on that page is the Morrison Micro that we encountered before, the one that ‘kicks butt.’ In issue 3 itself there is more information on the RCA Special Red Tubes that jc mentions. Also check again the No Stone Unturned article in the same issue, with the famous Loesch preamp.

Now go and read the article, see you next week.