Hum in 'Operate'

Stage & Studio Power Station PS-2
23

I’m just going to chime in to address a few issues I see here, starting with a question:

When listening to this hum in your video, where is the front panel volume control set? Ok, on to the issues.

  1. There have been no significant changes in a few years to the design or transformer specs of the PS-2 except the addition of the XLR connector making it the 2A.

  2. A preamp tube with excess cathode leakage can cause noticeable hum with the volume on zero. So, that’s worth having a look at.

  3. The bias is factory set, so bias is not adjustable. That said, higher IP rated power tubes will pull more current on the power supply, which in extreme cases, could cause a detectable increase in hum. A simple way to measure this is to read the AC voltage at the speaker out with an 8 ohm static load connected, volume set to zero and unit set to Operate. A typical reading is 1mVAC or less. Maximum acceptable is 3mV, but typical is 1 to 2mV in production.

  4. Power tubes that are not well matched will cause a bit more hum at the output jack, so to get it as low as possible, make sure your set is well matched.

  5. With no amp connected to the Amp In jack, the noise will be within spec. If you are turning the volume all the way up, you will hear an increase of residual hum, but since this is not a practical level for any purpose, there is no reason to set it that way. If you are evaluating with the volume full up, This may be why the repair shop is saying it is within spec.

  6. You may experience a more noticeable level of hum with a UK or EU model due to the lower line frequency. Still that would only be noticeable with the volume up high. That said, we test those units at 50HZ line frequency, so they are capable of passing the 1-2mVAC noise spec.

  7. This is a compact assembly and the power transformer is very close to the top cover. Therefore, it is normal that the transformer may cause the top cover to resonate a small amount. We test for that here too. This is not considered a design defect. It’s simply a lot of performance packed in a very small package.

  8. Finally, if you are in the UK, buy from a dealer in the UK. The Thomann units are specified for 230V operation. UK units are specified for 240V. Using an EU model in the UK will push the plate and filament voltage up around 5% . That could possibly aggravate noise issues.

Hope you find this useful.

Dave