Pop when amp is turned off

[boostin1989]

Hey guys, I recently purchased a Dynaco ST-400. The amp sounds good and functions normally, but sometimes when I turn it off it sends a very loud pop through the speakers. I would say it happens about 50% of the time and is sometimes worse than others. Also the power switch has 2 on positions and it doesn't matter which side it is switched to, it does it in both directions. Is this something that I can fix?

Thanks for any advice.

 

[Elvee]

Hi boostin,

Does the pop happen at very instant you maneuver the switch or is there any delay? And is the sound rather muffled, or sharp and scratchy? Does it happen with any source? Is the level dependent on the volume setting or other factor?

 

[boostin1989]

It does it as soon as you turn the switch. It sounds like the amp has an internal relay or something that clicks when you turn the amp on and off. When you turn the switch off, it will send the popping sound through the speakers, then continue playing music for a few seconds. Then the relay in the amp will click and the music stops. I initially thought it might be the relay causing the noise but after you asked the question I realized that it the popping sound occurs as soon as you turn the power switch.

The sound is not muffled. It is a sharp popping sound that only lasts a split second. It doesn't matter where the volume knob is positioned. Sometimes it is very faint, other times it sounds as though it could damage the speakers.

I only have a CD player so I'm not sure if it matters what source is being used. My previous amp didn't do this and it was running with the same CD player and preamp.

 

[anatech]

Hi boostin1989,
Try installing an MOV directly across the power transformer primary windings. 130 VAC, 10A should do the trick.

-Chris

 

[Elvee]

Hi boostin1989,
Try installing an MOV directly across the power transformer primary windings. 130 VAC, 10A should do the trick.

-Chris

You could also try a snubber network (47ohm/1W in series with 0.1µF/250VAC) or both.
LV

 

[boostin1989]

anatech (or anyone else who knows),

My knowledge of electrical components is quite limited. I looked up the MOV and am going to try and see if it works. There is one thing I'd like to clarify. When you say to put the MOV across the power transformer primary windings, does this mean to put it across the 'black' and 'black and gray-dotted' wires that feed the transformer that are in this picture (it's quite large)?

http://home.insightbb.com/~dunn.greg/ST400/pictorial.jpg

Sorry if this is a noobie question but I'd like to be sure that what I am thinking is correct.

Thanks

 

[anatech]

Hi boostin1989,

does this mean to put it across the 'black' and 'black and gray-dotted' wires that feed the transformer that are in this picture (it's quite large)?

That would be my guess. You want the MOV across the primary, so you should measure 110~120 VAC across those points. The idea is that when you turn off the unit, the secondary inductive pulse breaks down the MOV and it clamps the voltage. This should prevent the arc in your power switch (lasts longer) and reduce the "pop" sound.

As a side benefit, if you get a surge while the amplifier is on, this part will clamp it. This should blow the fuse and possibly short the MOV. However, these are much cheaper than fixing the amp.

-Chris