Mullard amplifier load pop on turnoff - UK Vintage Radio Repair and Restoration Discussion Forum

mpegjohn · 2nd Dec 2025, 6:45 pm

Hi,
I have built a Mullard 33 amplifier.
When I go to turn it off I get a load pop in the speakers.
To prevent this I straddled the mains switch with an X capacitor. That fixed it. However I think that is a bad solution. If the cap becomes leaky, one might get a shock still when the switch is off.
Any ideas how I could solve this please?

John

Trevor · 2nd Dec 2025, 7:07 pm

Its not a real problem loads of amplifiers have a suppressor capacitor across the switch ! You have to unplug while working on the amp
Trev

Sideband · 2nd Dec 2025, 9:03 pm

X capacitors don't go leaky! They are designed to lose capacitance slowly as they absorb spikes on the mains and eventually go open circuit. If you have used a proper X capacitor , you won't have any problems. Most of the current manufacturers make decent X capacitors, the likes of Epcos and Wima.

jamesperrett · 4th Dec 2025, 9:46 pm

Though I have seen a piece of gear switch itself on for a couple of seconds before smelling that familiar smell of a smoking Rifa capacitor.

Sideband · 4th Dec 2025, 10:48 pm

I think modern Rifa's are fine. The older translucent types absorbed moisture over time through microcracks that appeared in the plastic. I'd still avoid them though....

Radio Wrangler · 5th Dec 2025, 1:31 am

Suppressor capacitors can have longer lives with a bit of series resistance to limit transient currents. Acts as an R-C snubber. Trying to tame transients with pure capacitance just reflects most of the energy to bounce around until it finds something unplsnned to dissipate in.

Decooupling and suppression is an art!

David

Silicon · 5th Dec 2025, 11:55 am

Wiring that capacitor in series with a 100 Ohm resistor, across the mains transformer primary would be a better option.

Ed_Dinning · 5th Dec 2025, 3:41 pm

Rifa used to supply them with an inbuilt series R many years ago

Ed

Vintage Engr · 5th Dec 2025, 4:55 pm

Correct, they were known as 'Snubber' capacitors.
I still use them, & have no problems.

David,

mpegjohn · 10th Dec 2025, 1:36 pm

Thank you all.
I feel a bit better now. I will also try the 100 Ohm snubber across the transformer primary.

John

Sideband · 10th Dec 2025, 1:53 pm

Quote:

Originally Posted by Radio Wrangler

Suppressor capacitors can have longer lives with a bit of series resistance to limit transient currents. Acts as an R-C snubber.

Exactly what was used in my old Akai hi fi amp. The capacitor did fail however (microcracks again) and I replaced it with a standard X cap and a 100 ohm resistor. 40 years later, it's still OK

2nd Dec 2025, 6:45 pm	#1
mpegjohn Hexode Join Date: Aug 2013 Location: Chichester, West Sussex, UK. Posts: 277	Mullard amplifier load pop on turnoff Hi, I have built a Mullard 33 amplifier. When I go to turn it off I get a load pop in the speakers. To prevent this I straddled the mains switch with an X capacitor. That fixed it. However I think that is a bad solution. If the cap becomes leaky, one might get a shock still when the switch is off. Any ideas how I could solve this please? John

2nd Dec 2025, 7:07 pm	#2
Trevor Octode Join Date: Aug 2006 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 1,174	Re: Mullard amplifier load pop on turnoff Its not a real problem loads of amplifiers have a suppressor capacitor across the switch ! You have to unplug while working on the amp Trev

2nd Dec 2025, 9:03 pm	#3
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,311	Re: Mullard amplifier load pop on turnoff X capacitors don't go leaky! They are designed to lose capacitance slowly as they absorb spikes on the mains and eventually go open circuit. If you have used a proper X capacitor , you won't have any problems. Most of the current manufacturers make decent X capacitors, the likes of Epcos and Wima. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

4th Dec 2025, 9:46 pm	#4
jamesperrett Nonode Join Date: May 2007 Location: Liss, Hampshire, UK. Posts: 2,012	Re: Mullard amplifier load pop on turnoff Though I have seen a piece of gear switch itself on for a couple of seconds before smelling that familiar smell of a smoking Rifa capacitor.

4th Dec 2025, 10:48 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 8,311	Re: Mullard amplifier load pop on turnoff I think modern Rifa's are fine. The older translucent types absorbed moisture over time through microcracks that appeared in the plastic. I'd still avoid them though.... __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

5th Dec 2025, 1:31 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 24,743	Re: Mullard amplifier load pop on turnoff Suppressor capacitors can have longer lives with a bit of series resistance to limit transient currents. Acts as an R-C snubber. Trying to tame transients with pure capacitance just reflects most of the energy to bounce around until it finds something unplsnned to dissipate in. Decooupling and suppression is an art! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Dec 2025, 11:55 am	#7
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,564	Re: Mullard amplifier load pop on turnoff Wiring that capacitor in series with a 100 Ohm resistor, across the mains transformer primary would be a better option.

5th Dec 2025, 3:41 pm	#8
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,887	Re: Mullard amplifier load pop on turnoff Rifa used to supply them with an inbuilt series R many years ago Ed

5th Dec 2025, 4:55 pm	#9
Vintage Engr Octode Join Date: Jan 2008 Location: Shrewsbury, Shropshire, UK. Posts: 1,129	Re: Mullard amplifier load pop on turnoff Correct, they were known as 'Snubber' capacitors. I still use them, & have no problems. David,

10th Dec 2025, 1:36 pm	#10
mpegjohn Hexode Join Date: Aug 2013 Location: Chichester, West Sussex, UK. Posts: 277	Re: Mullard amplifier load pop on turnoff Thank you all. I feel a bit better now. I will also try the 100 Ohm snubber across the transformer primary. John