Audio attenuation

[sunray]

I've been out today helping set up for a conference tomorrow. The kit includes a 6 channel passive attenuater in a 1U unit. Make something liike Tukaci so I assume oriental

In this situation being used to remotely adjust the volume of 4 active speakers. Took me a while to workout what seemed to be the problem with it.

Each channel consists of XLRf - dual gang 10k log pot - XLRm wired:-

  Anyone else see the problem?

[David Morison]

Pot(s) should be wired from 2-1 and 3-1, not between 2 & 3, right?

Presumably, when turned all the way down, the current arrangement is effectively shorting pins 2 & 3 of the input device together, which shouldn't be a problem for decent equipment but could make some devices rather unhappy if they don't have suitable over current protection on their outputs.

Edited February 26 by David Morison

[sunray]

12 minutes ago, David Morison said:

Pot(s) should be wired from 2-1 and 3-1, not between 2 & 3, right?

Presumably, when turned all the way down, the current arrangement is effectively shorting pins 2 & 3 together, which shouldn't be a problem for decent equipment but could make some devices rather unhappy if they don't have suitable over current protection on their outputs.

The 2 dots indicate the anticlockwise end of the track so all the way down (anticlockwise) means female pin 2 is connected directly to male 2 and same for pin 3 ie full on.

I'll also add the symbol above each control is typical of a volume control but reversed, IE:

Edited February 26 by sunray

[David Duffy]

Looks like it would be minimum volume in the middle and loudest at each end with the phase reversed at the CW position.

[Lamplighter]

I would rewire with the two pots joined in series, that would give 0 to 100% attenuation

 

[sandall]

Presumably started off life as a Balance control for a(n unbalanced) stereo source

[sunray]

2 hours ago, David Duffy said:

Looks like it would be minimum volume in the middle and loudest at each end with the phase reversed at the CW position.

This is the main point I wondered if peeps would identify and precisely how it works.

Its very confusing to someone (me) who didn't know the kit. It is also new kit to the guy I was assisting and from the floor he had no idea what I was doing to create sound cancelled areas, for that matter neither did I. It doesn't help that being log curve pots the null is near one end and surprisingly sharp that it's easy to miss when making a big change and overshoot.

2 hours ago, Lamplighter said:

I would rewire with the two pots joined in series, that would give 0 to 100% attenuation

That is very much on the cards. And also ensure wired as: ccw off - cw full.

2 hours ago, sandall said:

Presumably started off life as a Balance control for a(n unbalanced) stereo source

This looks to be purpose built as a balanced attenuator and sign written as such. I see where you're coming from, however as it is it would have a flipping effect in that mode rather than balance.

[sunray]

I've mocked this up and thinking about sandall's balance comment I set it up using a 10K lin pot for unbalanced stereo and played a bit of Pink Floyd Sheep as it has a heavy stereo effect.

Rather than having a balancing effect it narrowed the width of the sound and only cancelled those sounds equal on both channels.

 

Wiring as balanced attenuater (as Lamplighter offered) works a treat except for the kit I was using (Spirit notepad and Canford powered diecast monitor speaker via isolating transformer to ensure it is balanced) there is a very non linear feel with lots happening at each end of the track and not much over the middle half, reducing the 10K down to 2K made much better sense.

I will demonstrate it to the owner of the unit to see if he wants to reconfigure it.

As built the current arrangement is very difficult to alter on the hoof.

[sunray]

All sorted, luckily the double sided pcb (containing 2 pots each and linked with 3 pin jumper leads) are laid out in such a way that cutting 2 tracks by each pot and soldering a link across the pot pins was easy.

Input resistance is 6* 10K+10K in parallel = 3K33

I know it shouldn't but that arrangement seems to work more 'linearly'  than just one such control.

 

This came from AliExpress at a total cost including delivery under £7 with a white stripe to write on and took 20 minutes from the time it came through the door to modify and test. Yes it's cheap but at the moment it seems to be useful, even the XLR's feel OK. The only downside that I can see is it's only 58mm deep so can be very deeply buried when it comes to access for plugging.