Preamp power question

[craigmorris74]

I'm building a pair of API 312 clones, and ran into an issue when setting the voltage for the discrete opamp (I'm setting it at +-16V for the Seventh Circle Audio SC-25).  I set the voltage where I wanted it for the first preamp, then checked the second preamp.  It initially shows the correct voltage, but then slowly starts to fall.  I've checked for reversed capacitors, shorts, etc, and don't see any problems. 

Any idea what could be causing this issue?

Thanks,
Craig

ps-posted this in the preamp forum, but nobody knew exactly what was causing the problem, so I'm posting here.

 

[Walrus]

Sorry, my crystal ball has let me down this time!
So how about some more info, like what PSU have you used; how are the pre amps connected; what 312 circuit have you used. etc etc

 

[craigmorris74]

I used a Power One supply that is in an external case.  The preamps are connected to the power supply at a barrier strip.  I used the access 312 build.

 

[Harpo]

A Power One supply might have a type number printed on its external case ...

 

[Walrus]

Are you measuring the volts at the pins of the sc25? If so then you have a problem on your card. Diodes back to front?

 

[craigmorris74]

The supply is .8A +-15V.

I'm measuring the voltage at the opamp pins without the SC-25 installed.

 

[sahib]

Check the power supply to see if there are outputs marked +S and -S. These would stand for SENSE. You may need to take a seperate wire from them and connect it to Ground at the 312 DC connection.

 

[craigmorris74]

There are S and S+ wires.  Could this cause the voltage to drop on one pre and not the other?

 

[Walrus]

There are S and S+ wires.  Could this cause the voltage to drop on one pre and not the other?

No.
If the PSU is in the same box as the pre-amps, just connect the S+ to the + and the S- to the - at the PSU terminals.
If you are measuring both of the pre's without the SC-25's in situ, and one is OK and the other isn't....... Well! 
How about a pic of the build??

 

[craigmorris74]

I did a bit more investigation, and found the following:

V+ opamp socket (using ground pad where ground from PSU connect to PCB) +16V
V- opamp socket (same ground as above)-16V
V+ opamp socket (using 0V opamp socket as ground) +14V
V- opamp socket (0V opamp socket as ground) -14V

Also, when I measure the voltage between power supply ground and the 0V opamp socket, I get about 1.5V.

 

[Brian Roth]

Also, when I measure the voltage between power supply ground and the 0V opamp socket, I get about 1.5V.

What is the DC resistance between PSU 0V (common) and the 0V pin on the opamp socket...measured with the PSU powered off and the voltages discharged?

Bri

 

[craigmorris74]

Brain, this is a problem.  The resistance is between PSU 0V and 0V pin on the opamp socket is very high-around 1.2Mohms.  I even tried to run a wire from the socket to a different ground pad and still get the same thing.

 

[CJ]

this might be a protection diode thing...jus sayin...

 

[3nity]

could this be due to psu cable resistance? i have the same problem psu is on external case.
cable is 6 feet long though.
Good luck.

 

[Brian Roth]

Sounds like an open circuit somewhere in the 0V wiring between the Power-One and the preamp.  You should be measuring somewhere very close to 0 Ohms.  Where are you connecting the 0V wire at the power module?  It should connect to the solder turret terminal on the PSU's circuit board....I believe it is marked as COM.

Bri

 

[CJ]

if i see this right, D1 and D2 are protection diodes, not rectification for the transformer,

the transformer and bridge are not shown, they are probably connected via the 4 pin jack,
so if you happen to have your ohm meter backwards while clipped on the outside of the diode,..get me?

this would also explain the 0.6 + 0.6 voltage drop?

the best way to measure rails is from ground to the opamp power pins,

 

[craigmorris74]

Thanks for all the suggestions.  I think the problem was that the opamp ground pin wasn't connecting with the PSU ground.  These boards have separate ground pads beside the PSU V+ and V-.  I only had one of them connected, which worked fine for one of the pres, but not the other for some reason.  Everything looks good now, and I will install the opamps and hopefully put them in the rack tomorrow.

Again, thanks for the help.
Craig