P06 buzz

[elco]

Since I had some OPA2132 and a small radioshack pcb lying around, I decided to give a try Rod's p06 phono preamp.

It's working pretty well despite the fact that I did not use Rod's pcb, but I do get a mid-high frequency buzz. Something like a "sawthooth" 60 hz sound. I'm poewering it with two 9V batteries and I tested it with a P05 powersupply (Rod's PCB) — same buzz.

I checked quickly with my computer soundcard and it's down about -60dB (this is a bit of an aproxmation tho...). Is this a normal noise figure for such a design? Comparing to an old receiver's phono preamp, the noise is perceptibly higher in the P06, but the receiver does have a low frequency 60hz hum which measure as high in level.

Note that I used standard polarised electros for the C2 and C3.

See the attached picture for layout. Bypass caps are in red, others in blue. Dashed lines are links under the PCB. If anyone has suggestions to get rid of that buzz...

P06: http://sound.westhost.com/project06.htm

[Hugh M]

Hi Elco
I may be all wet but try putting your GND bus up the center and eliminate it from the outside. Then jumper the 9v feeds.
Your gnd looks very long. You'll get potential differance between each gnd point.

Hugh

[elco]

well, a few days ago I discovered that it had a bad solder joint on a shunt resistor. Running off batteries it's now total hum free. Noise floor (white noise) seems to be at about -72db peak (digital, ref signal at 0db).

I'm wondering if the noise floor is as low as phono preamps can go. It's silent at low listening levels, slitghly perceptible at moderate levels (~70db) and very present at loud levels. What is the noisiest component on such a design? OPA2132 opamps? Should it make a difference if I try with OPA627/637 ?


I'll try to shorten the ground line and try it back on my regulated PSU. Thanx for the suggeston. 9V alkaline batteries don't last long enough for my taste...