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Old 7th May 2012, 02:55 PM   #1
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Join Date: Dec 2011
Default Phono pre-amp, TT PSU, mic pre-amp build log

A few months ago some of you helped me out with thoughts on RF/noise management in this thread: PCB vs matrix vs strip board for phono pre-amp

Had some free time recently so at long last I've made a start. I am building a combined phono pre-amp, PSU for my turntable and a microphone pre-amp. I will also leave enough space for an eventual pair of chip amps to drive my tweeters. All of this will be housed in a single full size enclosure. I liked the idea of a one-box solution for everything to do with phono playback which is why I am putting the TT PSU (80VAC) and pre-amp in a single box. As I will be building a really stable PSU for the signal circuits (15-0-15) and being really careful with RF and noise management I decided I may as well include a mic pre-amp at the same time. This is a bit of an afterthought and I won't be going over the top with that as I only use the mic for recording accompaniments for solo instruments and the occasional play along with family and friends. The priority is to get a good phono pre-amp. If the TT PSU adds too much noise I will eventually re-house it although I think it should be OK. I'll find out when it's done.

So, I have rescued an old AV receiver from a skip (CA 540R) and recycled what I can. It includes a really decent enclosure, torroidal transformer with more windings than you could ever use and a massive heatsink.

I am reusing:
enclosure
transformer (40-0-40 and 17-0-17 windings)
mains switching PCB
heatsink (without fan)
PSU capacitors
rear panel connectors

I am adding:
large unregulated PSU (for chip amps when I get round to them and input to the regulated supplies)
2x 15-0-15 regulated supplies one each channel (to drive phono and mic pre-amps)
2x phono pre-amp PCB (L + R)
1x mic pre-amp PCB (stereo)
2x LM3886 chip amps (eventually)

I've mapped out the real estate, decided where to put the rear panel connections and built and tested the "mains" part and the TT PSU which at the moment is a simple 80VAC straight from the 40-0-40 winding and out through an XLR socket.

Some pics at: https://picasaweb.google.com/1180334...7258/PhonoAmp#
if anyone is interested. [internal layout (amps on the RHS), rear panel, my crappy mains and an example of a "dead bug" phono pre-amp by someone else.]

Next stages:
1) Assemble the unreg PSU (in the picture it is loose in the box while I was planning the internal layout)
2) Build the 15-0-15 reg supplies
3) Build the phono pre-amp boards

The reg supplies and the phono pre-amps will be build on continuous copper-clad board using a "dead bug"construction technique to provide a continuous ground plane. This is one of the RF and noise management measures.

Comments, advice, suggestions for phono pre-amp circuits (yet to finalise schematic) are more than welcome.

Oh yes, although the 540R was a class II device I will be building this as a class I device but with a floating signal ground. I will use class II insulation throughout because 1) I never quite trust myself 100% when it comes to mains powered equipment so I like to minimise risks to myself and others and 2) I will be putting my hands into it a lot during testing and I am quite good at dropping screws, probes, tools etc. which never fall where you want them to.

That's enough for now.
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