stepped aattenuator - stop pops

[lordearl]

Anyone know the best way to stop popping noises when turning up volume?

I recently installed this in my linestage:
Balance XLR 23 Stepped Attenuator Potentiometer 50K Log | eBay

However the "thud" each time I turn up the volume is rather annoying - there is no dc offset at the source. Can I install some kind of capacitor to stop the noise?

[SoNic_real_one]

The noise is because of the voltage across the input capacitor. Try installing an 100-150K resistor from the potentiometer output to the ground, so the capacitor does not have a terminal "in air" during switch-over.

Personally I hate any potentiometer over 10kohms - too much noise pick-up.

[wintermute]

There may be some dc present, but the real caused of the popping is probably that it is not a make before break switch. I believe that the pop occurs because when the contact breaks the dc has a chance to build up momentarily (even if you have caps,) a make before break switch ensures there is no break in continuity when switching from one resistor to the next.. Since they say "no noise or bopping when switching" you might want to take them to task over it.

Tony.

[lordearl]

Thanks for the assistance, I’ve tried a few difference sources and the problem remains – might email the seller and try the complaint path!

In order to try the resistor suggestion – I’m using a pcb which has both balanced and unbalanced in.

See here:
6DJ8 preamp w/balanced input - strange schematic

Where is the best position to install the resistors – on the board between +,- and gnd for the balanced section?

[lordearl]

update - the seller has replied:

"Dear Sir:
This potentiometer is 'contact before break' type, so it should not generate pop sound.
What you hear my induced by impedance change during potentiometer shifting. This type of potentiometer, amplifier will not see constant input impedance. Some circuit will generate noise from impedance shifting. It seems no simple method to solve this problem.

Sincerely"

any ideas?

[triplej]

If its a "crack" instead of a "thud" you could try cleaining it with an electronic solvent like "CO contact cleaner".

I had a DACT stepped attenuator that was noisy, cleaned it one and it has not faulted in 5 years.

[DF96]

To get minimum clicks you need no DC coming in and no DC going out. That means coupling caps both ends, with a ground leak on the switchd side (probably output). Your preamp appears to connect straight to a valve grid. That may be the problem - you are switching a little grid current. With a continuously variable pot you can sometimes get away with dropping a coupling cap (although you get noise long term) but you can't do this with a switch.

[lordearl]

thanks again guys - the source runs straight to the pot, which connects to the preamp. Appears I may need to install a "ground leak"? Is this the 100k to 150k resistors that sonic_real_one mentions above? If so, what is the simplest way of doing this?

[DF96]

You need caps plus a ground leak, unless your source already has caps. I'm not sure what you mean by "simplest".

[lordearl]

my source doesn't have caps, - it's just an ad1865 dac chip with 220R i/v resistors. there is a few mV of dc offset (but I tried other sources which do have caps and the popping noise on the attenuator is there nevertheless)

I'll try the resistors, test, then try some caps at the output of the DAC. How do we work out the values of; (a) the resistors (someone suggested 100-150k), and (b) the caps at the output.