Dynaco Pas Thud

When I turn the selector switch from phono to anything else, there is a thud or pop heard through the speakers. This is only when the volume control is turned up. I've cleaned the switch. Any way to resolve this? Thx

rudes wrote:When I turn the selector switch from phono to anything else, there is a thud or pop heard through the speakers. This is only when the volume control is turned up. I've cleaned the switch. Any way to resolve this? Thx

That is pretty much the nature of the beast. Keep in mind that the phono head-amp- makes millivolts into volts - so even going through that position will cause a bit of a thump.

You can mitigate it, however, by shorting the phono and tape inputs. Ideally, all unused inputs should be shorted.

Enjoy!

rudes wrote:When I turn the selector switch from phono to anything else, there is a thud or pop heard through the speakers. This is only when the volume control is turned up. I've cleaned the switch. Any way to resolve this? Thx

You have probably leaking .1uF caps ( the final ones at the riaa board, connected to "1" and "7"
on the board. Replace them and you are fine!

peterh wrote:

rudes wrote:When I turn the selector switch from phono to anything else, there is a thud or pop heard through the speakers. This is only when the volume control is turned up. I've cleaned the switch. Any way to resolve this? Thx

You have probably leaking .1uF caps ( the final ones at the riaa board, connected to "1" and "7"
on the board.  Replace them and you are fine!

Yes and no. If these caps are leaking, the thud will be louder that not. But it will still be there. A combination of shorting inputs and (possibly) new caps should fix most of the thump - but it will never go away completely.

So. What did you do? What are the results?

Peter W. wrote:So. What did you do? What are the results?

If you asked me, i replaced the caps and the thud was gone.

I had done the Phono Curcio mode and had the .1uf caps shorted. Replacing the caps. Thx!

peterh wrote:

rudes wrote:When I turn the selector switch from phono to anything else, there is a thud or pop heard through the speakers. This is only when the volume control is turned up. I've cleaned the switch. Any way to resolve this? Thx

You have probably leaking .1uF caps ( the final ones at the riaa board, connected to "1" and "7"
on the board.  Replace them and you are fine!

+10

a STOCK Dynaco PAS preamp has a 3 pole wafer switch, that grounds ALL inputs except the selected one, to ground.
If you've replaced the switch it's more likely that you will get a glitch when switching inputs, but those capacitors should eliminate most of it.

Did not replace the wafer switch. This is weird, because I have another of these Pas preamps and it does NOT make the thud. Both have the same power supply and both have the tone delete mod. The only thing that I can see that's different is where the grounding wires were soldered on the boards.

rudes wrote:Did not replace the wafer switch. This is weird, because I have another of these Pas preamps and it does NOT make the thud. Both have the same power supply and both have the tone delete mod. The only thing that I can see that's different is where the grounding wires were soldered on the boards.

Grounding is essential on PAS ( and other amps).
Is the 0.1 uF caps replaced ? You mentioned the curicio mod, and some tonecontrol "delete", thus
i suppose the PAS is severly munged. The best you can do is to revert it to stock config as per
the manual.

I keep two of the 3X version, that are very nearly thud-free simply by shorting the inputs on the Phono and Tape-Head inputs. One is as close to 'factory' as is safe, the other is fairly heavily tweaked. Both retain their tone controls.

Still can not figure out why I have this thud when turning selector out of phono. I have all new caps and all grounds check. I even tried grounding the caps by 1 & 7 on riaa board through a 2 meg resistor. Audio out also has 1uf cap. I have an identical Pas that does not have this problem. Could it be a poor contact on the selector switch?

Again, a phono amp turns millivolts into volts - which if you go through the "tape head" (which does exactly the same thing) may result in a transient - which you perceive as a "thud".

Short all, repeat ALL the unused inputs - and please report back.

Pete, The post before last you said to short phono and tape head inputs. Now you're saying to short all but those?

I must be having trouble with basic English today.

If an input is being used, keep using it.
If an input is not being used, short it.

Use shorting plugs for best results.

All Inputs shorted except for phono. When selector switch is turned from phono still makes the thud. As stated, my other preamp is wired the same exact way and this does not occur.

Just to be clear - you are using the phono input?

And, what happens if you remove the phono leads and install shorting plugs?

Solved! Being that both of my preamps are identical except for the tubes, I swapped the tubes from the phono stage of the noise free one and no more thud!