LM833 based phono-preamp

I wanted to build the phono-preamplifier from the LM833 application notes, but I made a stupid mistake when I ordered the parts: I ordered a walwart with only +15V, forgetting I also needed -15V…

Am I right assuming I cannot use it as it is, and I will either have to get a second one of these to have a symmetrical supply, or make a PSU myself instead of using a walwart?

regular wallwart isn't a good idea cause phono pres are very sensitive for power supply hum. try running it on 9V batteries and if you get everything right, start thinking about power supply.
that's how i'd do this.

michał

The easiest approach for now is to change the preamp to operate from single +15V supply.

You will have slightly less headroom, so consider changinging 390 ohm to larger (like 750 ohm or 820 ohm).

Use polar capacitors for input, output, and gain leg, with + toward the 833.  I don't see why they were compeeled to use three different values there other than to make your life more difficult. Perhaps standardize on 33uF all around.

Connect + input 47k(s) to filtered V/2 reference instead of ground. Make v/2 reference made from another pair of equal value resistors and cap to ground. These could also be 47Ks and another 33 uF cap.

Don't worry about wall wart or lump.. I used a wall wart for my third phono preamp design and hum levels were quieter than first two using internal transformers. 9v batteries are 200 mAhours or so, and will run down before you get through Phillip Glass's "Music in twelve parts" a couple times.

JR

I found a 9V or 12V AC wall wart ideal for generating split power supply rails and used one for precisely this application.
Use a half-wave rectifier and good caps to reduce the ripple. Since it draws only a miniscule amount of power you'll find that you can get a +/-9V or +/-12V regulated DC rail out of it (the 12VAC wall wart gave out about 20VACRMS no-load).

Thanks for all your answers!

I was just getting back to bump the topic and saw I had once more forgotten to turn automatic notification on.

JohnRoberts said:

Use polar capacitors for input, output, and gain leg, with + toward the 833.  I don't see why they were compeeled to use three different values there other than to make your life more difficult. Perhaps standardize on 33uF all around.

Click to expand...

I see my idea of trying and doing it all by myself wasn't such a good one: The only polar I have is the 100µF (that's the gain leg, right?), Tantal for the 33µF and the 10µF, and I guess the rest isn't that important. Could you give me a hint what to search for if I want to understand why they should be polar?

EDIT:
Obviously that was wrong, Tantal capacitors are polar…

Now I managed to get the preamp to work on breadboard with a ±15V supply (which will stay here while the preamp is for a friend).

As for some reason the version I soldered on Veroboard does not work, I guess I will try doing it again or etching or cutting a little board.

But there is another problem I do not understand: I followed the advice of dividing the +15V from the wallwart, but when I applied that voltage, I cannot hear anything on the output of the preamp, whereas it is pretty loud with the ±15V. Is that normal?

bump

Did you really get +/-15V from the wall wart? Or only +15V?

I only got +15V, which I divided to ±7.5 with the ground in the middle of the divider.

When the +/- 7.5V was applied to the circuit, was it really +/- 7.5V or were the rails unbalanced by the loads?
The LM833 is supposed to work down to +/-5V. Did you have a capacitor (about 10uF should do) from each rail to the pseudo ground point?

I only had one capacitor from between both rails to the virtual ground, and that way it was balanced equally (7.41 per rail). I just changed it to one capacitor per rail, and then it was unbalanced. Does it make sense for me to try other resistor values to get it to be balanced or is that value variable?

Dear An:  How are you forcing the power supplies to share?

Is the ground on the 15V supply floating, so it can range negative?

There are specialized ICs designed to force sharing but it can be pretty easily done with an opamp. I've even done it with a single transistor when there is an obvious imbalance in one direction or the other.

JR

Dear John,
I start to think once again that I should not have started this project. I assumed the ground was floating, but I am not really sure. As for the first question, I have not really thought about what I was doing.

Could you give me a hint what to search for to force sharing with a transistor or an opamp? (I tried with the italic words, but without success)