All about Tubes, Tube Circuits, Tube Gear

Friday, June 22, 2012

RIAA Preamp Build (Part 1)

Well, someone asked me to design a low-noise stereo preamp for RIAA, (record phono input Equalization), so Challenge Accepted!

I made a preliminary design effort using a Mu-Follower type layout for dead-quiet PSRR and low noise, and also chose a low-voltage tube.  I will be using either 6922s or a 6GM8.

Decided on separate power supply and preamp chassis, for even lower noise, because this is a really low-voltage input signal (old magnetic cartridges etc.).

I enlisted the boy to help me with the power supply.  I took a standard but nice-looking metal box as a base.

The boy loves these special wire-strippers.  He thinks they look like Oscar the Grouch, and calls them the "Eyebrow tool":

I happened to have a ready-made 60 volt filtered supply PCB, loaded and tested. All that was needed was a 12 vAC transformer.  Well, I ordered one on Ebay (notice it says right on it TRANSFORMER).  I assumed the housing was just shielding and a convenient mounting.  This turned out not to be a transformer at all, but a DC power supply containing a transformer!  Who knew!  Idiots.

Well, pressing on, I got some four-wire shielded cable and attached a nice solid connector to it.  This will go to the preamp chassis proper.   I improved the HV insulation inside the plug by weaving electrical tape between the pins.

For tagging purposes, (and additional isolation), I re-paired (no pun) the wires so that the heater (which doesn't matter is two whites, and the HV (60v) is blue/orange (-/+).  Keeps things simple to hook up correctly.

Here's a sneak preview of the other half of the project: My part collection for the preamp itself.  I'm using this nice knick knack box as a base, it looks really nice.  It will have two tubes on it, and some RCA jacks, and a pwr connector.

Not a lot to remark on schematic-wise so far.  Will discuss circuits in following posts.

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