DIY Precision fidelity C7A

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reeman

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This phono left me with no choice. I had to build it. Agaton lent me his DIY Precision Fideleity and I used to for about 2 months. Unfortunately, it had to go back so I had to build one. It was just that good.

So off to our local guru here in EL who built that one and I asked him about it and he said he will give me all the info he has on it and how he built it but good luck its a mission and it's a double-sided board that's painted by hand then etched.

Thinking I have all the info how hard can this really be? Well, I think I created a new language I was swearing so much. Anyways I drilled the 1mm holes and painted the boards and after the 3rd time, I got it right.

I assembled the psu boards which are single-sided and I thought I'm on a roll its gonna be smooth sailing from here. Boy was I wrong.

I eventually assembled all the boards and mounted them in the chassis wired it all up turned it on and viola..........absolutely nothing is working! Found a short that I had to fix and turned it on again. Hey, look bottles are glowing but my voltages are nowhere near where they should be. Off to Garth. He says oh he forgot to tell me there are 2 jumpers he added as an afterthought. I added them. Turned it on and poof.....1 of the TIP50 regulators dies. turn off to find the 1 track I painted is slightly thick and was touching the bolt through the board that goes to ground.

Fixed that, replaced the TIP50 turned it on and everything is stable and voltages are good.

Go home wire the audio inputs and outputs fire it up and nothing but hum and an open-ended gain noise. Immediately I know I have a ground issue somewhere. Check to ground everywhere I can't find the issue. I then thought maybe it is a ground loop from other devices. I lifted the ground from mains and that didn't help a thing.

Back to Garth. Fired it up at his place and he started checking things and he was also stumped. So I raised the question. Since we have this screen on top of the board that's an earth and there is a negative/earth section at the bottom of the board should they not be linked. Light bulb moment! Again I got the answer. Oh, I forgot to tell you to put small straps through to link the grounds. What was happening is that the audio signal ground was floating above the circuit's ground so technically it wasn't grounded. Anyways. Put the straps in and it's quiet.

Brought it home and plugged it in and it really is an impressive phono stage.

Still to do is upgrade the output caps to Mundorf Supremes and to wrap the chassis in kiaat wood to match my valve amps and to sand the top and get rid of that galvanizing and respray. Here are some of the pics as the project was being built.
























 

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