Sweet Whispers Popping Noise Fix

The right channel of my Foreplay was making a popping noise when I changes volume setting on my Sweet Whispers attenuator. This was a loud pop that would make the dogs occasionally jump. My dogs are hyperactive anyway so the pop was not deafening.

I've got a ton of gain in my system with a horn loudspeaker and a push-pull power amp. My wife thinks it is too loud at 3 clicks from mute on the Sweet Whispers attenuator in the -20 to -50 dB mode. Because of all this gain, my system will be extra sensitive to clicks when adjusting the volume. To debug the problem I did the following:

* I swapped left and right inputs and it stayed with the right volume control knob.
* I reflowed all the solder joints, no improvement.
* I checked the DC bias on the cathode follower. If it was changing, I had a cold solder joint. The DC bias of the cathode follower was not changing at the different switch positions and the music was both full range and clear once the switch had been moved. Because the normal operation was good, I knew the basic build quality was OK.
So it thought about it for a while and said, "what if the make before break feature of the right switch wasn't working or had solder flux on it."

I didn't have any flux cleaner I was willing to spray on my painted Foreplay so cleaning the wiper was temporarily not an option.

So, I added a 470K 1/4W metal film from the wiper terminal to ground on the switch to make sure the wiper (grid) always had a path to ground. Viola! The pop was almost completely gone. I bet if I had used a 100K to ground it would have gone away even more.  To keep the attenuation balanced on both right and left channels, I installed a 470K on BOTH attenuators, not just the noisy one.

I'm happy with how the 470K fix is working, so I won't fix it any further.

Another way to Reduce the popping noise when changing volume settings in Shunt Mode
9-Jan-02 First added

Gdaddy did this trick and let us know about it on 9 Jan 02.  Gdaddy on Bottlehead.

The popping when changing volume with a shunt mode volume control comes from the shorting action of the shorting switch not working right. If the switch goes open as you turn it, you get full volume out for a moment because there is no resistor to ground. What the shorting action is supposed to do is tie step 1 to step 2 before you are 100 % step 2. If the wiper bounces, this won't happen.

By using two decks of shorting switches in parallel any popping from changing settings can be eliminated or greatly reduced. (Gdaddy used three decks.) This is because it is unlikely for two or more decks to bounce at the same time.