Stand alone phono preamp

A thread in another area of the forum reminded me that I had forgotten to post pictures of my phono preamp project.

When I moved my Technics SL-1200 into the surround setup in the living room, I found the built in phono stage of my Sony DA3200ES receiver to be lacking in dynamics and it had a bit of hum and noise as well. The Technics sounded fine on my Marantz 4300, and if I used my 2230 as a preamp that worked well too.

I did a little shopping on our favorite auction site and scored a phono preamp board from a Marantz 2250B.

On the phono board, I replaced the power supply filter cap (C413) with a new one of the same physical size, but with about twice the capacitance. The input coupling caps (C401-2) which were tanatalum types, were swapped for film caps of the same (1 uF) value. The board is made to accept either footprint. I used a 30 VDC wallwart adapter from Jameco for power. The adapter puts out about 38 Vdc with the preamp's light load, so I replaced the power supply feed resistor (R434) with a 1K Ohm 1/2 Watt and added a 30 volt 1 Watt zener diode across the power supply filter capacitor. This keeps the preamp operating at near the level of the original design, but with a little more safety margin for the electrolytic capacitors used (35 volts).

The sound I think is very good. The dynamics are back and the hum and noise are gone!

I'm still trying to figure out a way to put a blue face on it though!

Tom

That's really sharp! What did you use for the chassis? I may have to borrow some ideas from your design...

Redboy said:

That's really sharp! What did you use for the chassis? I may have to borrow some ideas from your design...

Click to expand...

Thanks! Please do borrow the ideas, as I stole them from someone!

The chassis is an aluminum box from, of all places, my local Radio Shack, model number 270-238. About $3. It comes with a fairly nice finish on it, but make sure it didn't get scratched in handling. I buffed mine up with a cloth my wife uses to polish the metal jewelry she makes. The side pieces are just some oak that I finished with walnut oil stain. I want to make some out of actual walnut at some point.


Tom

Fantastic project and really well done!

Good job and great thinking.
Looks really impressive and I like the idea of replacing the old tantalums.

I'm sure it sounds great.

Nice job!

Thanks for the compliments!

I need to build another to give to my Grand-Son.
I fixed up his sister with a vinyl rig, now he wants one!

Tom

Wow! It'd be great to try it against some standalone phono preamps in front of a crowd without telling them what was in the box.

catrafter said:

...The chassis is an aluminum box from, of all places, my local Radio Shack, model number 270-238. About $3...

Click to expand...

:arrow: Finally almost everyone on Karma recognizes the true value of the shack.

Cool! I have a rusty old "too much time by the seaside" 2220B that I've been wondering what to do with.

Very nice project! probably sounds as good as it looks. On getting it blue are you looking for a bluelight or just getting the metal blue?

That is awesome! Very clean and neat!

Thanks for the kind comments!
The blue face thing was my feeble attempt at humor.

Tom

Very Nice!
Regards,
Jim

on the panel you have the wall-wart jack, in's and out's,... what's the black connector? is that a fuse or is it the phono ground connection?

Agree on nicely done. Last month I did something almost exactly similar with a board from an H-K receiver. I didn't think to upgrade any parts and my wall wart is pushing a bit more than 30 volts but seems to be working ok. I put my board in an old candy tin that was about the right size but yours looks so cool I may just need to redo it.

Doesn't 30 volts seem like a lot of juice to be sending to a phono preamp? I mean it probably needs that much to work properly, it just kinda seems like a lot..

The phono stage in the Kenwood 700C uses +/-50V (100V differential). +/- 40V or more is typical of higher-end gear of the 70's.

30V single-supply is pretty tame.