Help: Dynaco PAS 3 Hum

[sriram]

Hi,

First time post here and thanks in advance for your time.

I use a PAS3 preamp in a couple of configurations using a SS power amplifier currently and it hums in one of the setups even at zero volume. The preamp has new PC-5, PC-6 and Power supply boards. Other than this the wiring seems stock. The modification was not done by me, but was there when I bought it.

1. PAS3 > Harman Kardon AVR 240 Rcvr (volume control) > Rotel RMB 1075 SS Power amp > JBL spkr
   • There is no hum/noise audible at listening volumes in this setup
   • PAS3 set to high volume (around 3'oclock), the HK rcvr is low volume (around -50dB) and there is no hiss
2. PAS3 > Rotel RMB 1075 SS Power amp > JBL spkr
   • There is a constant annoying hum/distorted hiss even with zero volume
   • PAS3 set to low volume (around 9'oclock) for music listening and no other volume control in this configuration

I read a few forums and it was recommended that the SS input impedance of the Rotel (~27k) is probably too low for the PAS to drive it. To confirm, I connected a Nelson Pass B1 buffer amp (https://www.passdiy.com/pdf/B1 Buffer Preamp.pdf) between the PAS3 and the Rotel in the second configuration. The buffer amplifier has a 470k input impedance that PAS3 likes to see (I guess) and in spite of this the hum persists. So this looks like poor grounding/wiring issue in the PAS3 itself? I also shorted the input of the PAS3 and the hum still persists which seems to again indicate an issue with the preamp?

Eventually want to go to an all tube setup, but looking for any ideas on debugging/fixing the hum issue first. I am posting the top and bottom views of the preamp. Also open to complete rewiring if it will fix the annoying hum.

Thanks for the help.

 

[peterh]

Try grounding the chassies together with a coarse wire. Use a screw at the back of PAS and similar
at the power amp.

 

[sriram]

Thanks for the quick reply. I tried to ground the two chassis together using some speaker wire. No change in the hum/hiss. Also, the ground on the RCA on the PAS is already chassis connected (checked it with a multimeter) to the power amp input RCA through the cable.

 

[peterh]

Grounding via the signal cable is sometimes a receipt for hum.
What else is connected to the PAS ? Try grounding the amps with a course wire and remove everything else
from the pas, have volume at zero. If it still hums you have a ground loop.
Is any of the amps using a grounded outlet ?

 

[sriram]

Thank you for the response. What is a course wire? Can I use a 12AWG speaker wire?

The input to the PAS is coming from a USB DAC connected to the PC. When I connect the DAC directly to the SS amp, there is no hum only a small hiss that sounds like noise but is hardly audible unless I go really close to the speakers.

The Power amp is using two prong power cord, so I guess it is ungrounded? How should I try to ground the PAS and the amp? When I check with a multimeter, I get a short to some of the screws behind the power amp, but the chassis itself has some paint that does not seem to give a short on the meter.

If I do have a ground loop, how do I go about debugging and trying to fix it?

Thanks.

 

[trobbins]

The PAS has no mains protective earth connection, nor an AC input fuse?? Whatever your final configuration, I suggest those aspects are achieved somehow.

 

[sriram]

Yes. That is correct. Is there a modification I can do to achieve this? Currently, I see a the chassis is grounded to the neutral as far as I can figure out going over the connections. No fuse currently either.

 

[peterh]

"Chassies grounded to neutral ???" There should be no connection between any of the mains ( 110V AC ) leads to the PAS chassies.

 

[sriram]

Sorry, you are correct. Chassis is grounded to the center tap on the secondary of the transformer (not to the neutral). My mistake.

I have been trying to track it down, but it looks like the wiring of the B+, GND, PC-5 and PC-6 eyelet 16 are being routed in close proximity and together with the heater wires (circle in the picture below) from the power supply PCB above. Could this be a cause for the hum?