Q For Those Who Know about Logic-Controlled Relay Attenuators

[thermionic]

Hi,

A friend of mine has sent me a logic-controlled relay attenuator. It has 128 steps – in 0.5dB increments.

I notice that if I spin the digital encoder quickly I can get clicks – occasionally I get a click regardless of whether I spin it fast or slow.

The only comparison I can make for this click would be to say that it’s akin to when you edit a waveform and don’t cut it at a zero crossing point – and I suspect it’s a similar phenomenon going on here.

Can anyone give me some tips to give my friend regarding reprogramming it? I suspect it’s a software issue. Is this simply a question of reprogramming it to make it MBB? Do all logic-based attenuators that use mechanical relays click if you really push them?

I hope you don’t mind me talking in such vague terms – I flunked logic at college! (I was 17 - please give me a break!). I’ve sent the attenuator back to him before and I’d like to get some pointers to save having to keep the thing travelling to and from Wales all the time.

Many thanks in advance,
Justin

edit - by "clicks" I mean audible "pops" through the speakers. The relays are fed from 470u output caps and go into 4u7 film caps.

 

[thermionic]

Anyone??

 

[ilimzn]

This is normal and has to do with the way the attenuator works, and the time needed for the relay contacts to engage/disengage.
Relay stepped attenuators are normally made with double-throw relays, these work like two position selectors. The thing is, it takes a different amount of time to go from position 1 to 2, compared to going from position 2 to 1. In an attenuator, when you change the attenuation, the particular combination of relay positions changes, and in some cases, because of the unequal times it takes to change positions, temporary intermittent attenuation settings occur. These can be much higher or lower compared to the target value, but they last a very short time, and these are the 'pops' you hear. They can be mostly avoided by some clever sequencing of the relay actualtion, but some transient phenomena due to the time it takes the relays to switch can always be heared. This is the 'penalty' one has to pay for having nearly the best attenuator possible while youa re not moving the volume knob
One other possibility is that the attenuator changes it's impedance depending on the state of the relay contacts (i.e. the attenuation level chosen). In some applications, this can lead to pops because it interacts with various offset voltages and currents of the surrounding circuitry.

 

[myhrrhleine]

Often, stepped attenuators are direct-coupled rather than capacitor coupled.
So, DC can be present allowing occasional ticks or pops.
It's considered by many that switching ticks are better than sound degradation from a capacitor.

 

[benchtester]

In the 90's, I worked on a project with a relay attenuator. It had steps of 1, 2, 4 ... dB stages. We got some loud pops at particular transitions. My memory is that sometimes we had no attenuation during the transition (for about 5 ms). We improved it significantly by splitting the transition in to two stages. First, we applied all the attenuation stages of the new level. After that, we turned off all the unwanted attenuation stages of the old level. This way there was always attenuation on the signal path.

 

[benchtester]

In the 90's, I worked on a project with a relay attenuator. It had steps of 1, 2, 4 ... dB stages. We got some loud pops at particular transitions. My memory is that sometimes we had no attenuation during the transition (for about 5 ms). We improved it significantly by splitting the transition in to two stages. First, we applied all the attenuation stages of the new level. After that, we turned off all the unwanted attenuation stages of the old level. This way there was always attenuation on the signal path.

I wrote this in a hurry. Did it make sense? Does it relate to your problem?

 

[ilimzn]

I wrote this in a hurry. Did it make sense? Does it relate to your problem?

Yes, this is precisely what the problem is and how i is fixed. The trick is that on any major carry (1dB-2dB, 2dB->4dB, 4dB->8dB etc) and back, you get a pop because for a short while there is no attenuation.

A relay attenuator is actually made as a series of binary weighted attenuators. For instance, we could look at it as the simplest example, an attenuator with only 4 steps, this has 2 series conencted attenuators, one for say 1dB, the other for 2dB. The actuall attenuator setting is chosen by switching the attenuation on or off in the series conencted stages. So, 0dB attenuation is done when both stages are switched off. 3dB is when both are switched on (1dB + 2dB = 3dB). The problem arises when one attenuator has to switch on, and the other off. Because the switching times are different, it is possible to generate an intermediate situation where will be swiutched on (so attenuation will be maximum) or both will be switched off (so no attenuation) before the actual desired state is assumed. This transient condition, usually the setting where there is no attenuation, generates the clicks.

The way it is avoided when a new setting is desired, is to first switch on all attenuators in the new setting, while keeping on the ones from the old setting. Then, after a delay at least equal to the switching delay of the relays, the attenuators that do not need to be switched on for the new setting, are switched off. There will still be a transient (a short period of time where the signal is attenuated more than either the new or old setting, or even muted), but no clicks.

All of this assumes that the origin of the clicks is not due to DC offsets that myhrrhleine mentioned. This sort of conenction should be considered bad form ulness measures are taken to minimize DC offset, of course.