POP with stepped attenuator

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lars on

Well-known member
Joined
Nov 18, 2018
Messages
83
Hi everyone,

I'm building a summing mixer and I put some stepped attenuator and at each step I have
some POP, CLOC.
It is break before make contact...

Do you have any technique to avoid this ?
Or is it a bad component maybe ?

Thank's
 
Pops and clicks are usually cause by dc voltages. Can you post a schematic showing the stepped attenuator and what it is connected to?

Cheers

Ian
 
I didn't chase down the specific active circuit but if the stepped attenuator goes momentarily open circuit between switch positions that can cause a large step in the closed loop gain. This is not unlike wiper bounce in conventional pots and why they work to eliminate wiper bounce (where the wiper is momentarily open circuit).

Depending upon the topology you may be able to mitigate with a capacitor.  Perhaps a make before break switch would eliminate that, but I do not have any experience with that.

JR
 
Hi Ian,

Here is the simple schema of how I plugged the gain rotary switches.
 

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See this other thread all about the v475:
https://groupdiy.com/index.php?topic=13857.msg748754;topicseen#msg748754

I searched for 'gain' and found a post saying Rg from 0 ohms to infinite adjusts the gain from -12dB to +6dB

kante1603 said:
You mean using the mentioned switched pot for RG (Feedback-Rs)?
Could work technically but I'm not sure about the pot travel behaviour (it seems to be a log pot).
With the pot completely closed (= whiper shorted) you will get -12dB,totally in circuit (100k) I guess you might get some +5dB or so,e.g. I have Volkers recommended 48k7 to get +4dB for the RGs.
To get +6dB you need an open condition for RGs (=infinity).

When you use a break before make, your resistance is momentarily infinite and your gain is momentarily +6 dB
Wrong kind of switch. Get a make before break. Or just use a pot.
 
I mistake, the gain pot is a "make before break".
So the seller told me that normally it doesn't make POP sound.
So I think I'm wrong somewhere...
But my knowledge is limited with that...

The seller also told me : "Some people will make their own amplifier in Direct Coupling (i.e. there is no input/output capacitor), this step potentiometer will have POP sound."

Does anyone know something about this coupling capacitor technic ?
 
I would test the switch. Just attach an LED and small power source so the light will change intensity as you flip the switch. If the LED goes out between switch positions, then the switch is opening between positions, and the 'pop' is explained by a momentary surge in gain. 
 
Easier than that perhaps, if you have a VOM with continuity function. Probe between wiper and two adjacent switch positions while slowly rotating the switch shaft... listen for the tone to drop out if break b4 make.

JR 
 
I tested the switch with the contuity test and there is something strange.
This is a 21 steps switch, the first 6 steps are continue but the others Not !??



 
DC makes things worse but even without DC any kind of stepped level control will make clicks when it is turned with audio present. The same can happen with software based level changes. A transient is being created (either positive or negative, it's random) which presents an audible click.
 
A stepped attenuator is the wrong part for switching gain in a V475 summing amp.

dmp describes the problem very well:

dmp said:
When you use a break before make, your resistance is momentarily infinite and your gain is momentarily +6 dB
Wrong kind of switch. Get a make before break. Or just use a pot.

but don't use a pot either! since you need a very tight tolerance between the two RGs in each channel.
 

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