Lower AC ripple on one side of coupling cap than the other side?

[deicide67]

I just finished wiring up a preamp. Going through some of the standard things I usually do, I started probing around the power supply with my meter. I have 1.1mV AC ripple at the last stage. Not great, but not horrible. It is tube rectified.

Anyway, I probed around the output coupling caps, and in the tube socket side, I get ~1mV AC ripple, but on the other side, I get around 3.5mV AC ripple. Any reason for this? Seems kind of odd to me. It would obviously be audible through the amplifier if it were hooked up.

I ran this test with a 15K resistor strapped across the outputs to load it a bit.

Thanks!

Blair

 

[DF96]

Circuit? Same ground point used for both measurements? Where? Value of output cap? Type of probe used?

I ask because you might just be seeing the effect of output impedance on the hum picked up by the probe itself.

 

[sreten]

Hi,

Ripple on coupling caps is very value and gain dependent.

rgds, sreten.

 

[deicide67]

I think DF96 nailed it. I took it off the passive load, and hooked it up to an old receiver. Grabbed the ground be the RCA, and now I measure .2mV AC ripple on the output. The hum is still audible and increases with the volume, but it could also be amplified by the gain stages in the receiver.

Thanks,

Blair