Foreplay Preamp volume popping

[arc2v]

I have a Bottlehead preamp feeding two ASL wave8 amps and then going to Adire HE10.1 speakers.

Everything was working fine up until last week when I noticed something.

Every time I adjusted the left volume knob, or even pushed it a bit without turning it I got a DC pop in the left speaker. The woofer jumped about half an inch and I shudder to think what might have happened to the tweeter (so far everything sounds okay).

The volume knobs are separate on the channels and the right one does not exhibit this behavior.

The knobs are stepped switches with different resistor values wired in at each junction (i.e. not a potentiometer). Bottlehead referred to this as their Sweet Whispers upgrade.

Any idea what might be causing this? No solder joints look broken and nothing else has changed (tubes, source, etc).

Thanks,
AC

 

[Positron]

popping

Hi ARC,

Could be several things, such as the switch having contact problems or a cold solder connection, even though the connections look ok.

If the contacts lose connection for just an instant, the grid may 'float', causing a thump. Is the switch a high quality one, or a basic open type kind that could get dirty?

Is it possible that something inside could be slightly moved when 'working' the switch, indicating a loose connection at another location?

Hope this helps.
Positron

 

[arc2v]

Thanks for the help.

It's a high quality "open" design. Very firm contact (basically a ball bearing between two plates with detents). I have some contact cleaner, so I'll hit the switch with it tonight.

I'll also check for cold solder joints.

Worst case, I can always go back to the pots, but this really did improve the sound quality when I installed it.

Thanks again. I'll report back tomorrow.

A

 

[CharleyW]

VoltSecond addressed this in his website, which is here: http://www.siteswithstyle.com/VoltSecond/index.html

You might also try this: http://www.audioasylum.com/forums/bottlehead/messages/105505.html

 

[arc2v]

Thanks. I had been spending a lot more time here than over at the Asylum recently, which is why I posted here.

I searched over there and found a variety of problems, most stemming from the break-before-make problem.

I'll look into a large value shunt resistor on the wiper or just cleaning the contacts thoroughly.

I also need to lower the volume substantially. Right now the lowest detent above mute is putting out about 80dB, which is fine for normal listening, but a bit loud late at night, or distracting if I'm trying to read. Ideally that volume should be at the 2 or 3 position, giving me enough room for "background noise" levels. So I'm gonna be soldering away anyhow, so might as well try the Voltsecond trick.

Thanks again. I'll post results once I try a few of these out. I never want to see that woofer jump like that again. Ouch.

 

[CharleyW]

I "upgraded" to the 100K Alps pots that Radio Shack (used to?) sells for $2.99. I got two to preserve the left-rightness (I am one of those whacko's who likes that feature).

I found them a considerable improvement over the originals.

 

[Positron]

popping

Yes, a break before make is definitely not a good idea. Installing a high ohmage resistor from wiper to gnd may still cause some popping.

The best solution is either a make before make rotary, or continuously variable volume control, if this is causing a problem.

Another possible cause is a tube. Might tube roll just to be sure, before spending on new parts.

Just a thought.

 

[arc2v]

Fixed it without firing up the soldering iron.

The rotary switch had worked itself loose. While I had the lid off, I tighened them down and noticed it made the knob harder to turn. So on a hunch I figured the connection would be better too.

Fired it up and the pop is gone. Who would have though it would be that simple?

Thanks for the help.