Sennheiser xsw radio mic distorting when the transmitter pack gets warm

[Mike Shand]

I apologise for this long rambling post, but I wanted to pre-empt at least some of the inevitable "have you tried..." questions

We have 3 Sennheiser XSW audio mics operating in the licensed GB band (and we have licenses!) which we have been using with no problems for the last 10 years.

Over the last few months we have observed occasional distortion on one of the radio mics. This seems to build up over the time that the mic is switched on and is a distortion related to the voice. I.e. there is no noise/distortion when the speaker is silent, but we get a rasping sound rather like clipping when they speak.

This only occurs on one transmitter and never on the others.

We have checked that the batteries are good.

We have swapped the receivers to different input channels on the mixer and the problem stays with the offending transmitter

Swapping the microphone attached to the transmitter pack makes no difference. I.e it is not related to the microphone or its placement. We still get distortion with a different microphone attached.

The gains of all the transmitters are set to -30dB and the distortion doesn't seem to be related to how loud the speaker is speaking. And we haven't changed the gain settings recently.

Crucially, it appears that the distortion gets worse as the transmitter pack warms up from being in a trouser pocket.

Thinking this was a faulty transmitter, we bought a new one and set it to the same frequency. This too suffers from the same problem!

If when the news transmitter is distorting we swap back to the old transmitter pack which has not warmed up, there is no distortion on that.

When the warm distorting pack is left uncovered for a while and it cools down, the distortion gradually goes away.

So it definitely seems to be related to heat in both the old and new pack cases.

But why, after 10 years, has this only recently started happening?

I have run out of options to troubleshoot this!!!!

Any help gratefully received.

[Hugh Robjohns]

I wonder if the transmitters tuning or modulator is drifting as it gets hot?

Have you raised this with Sennheiser?

[Wonks]

Can we assume that the transmitters aren’t being insulated at all, by say clothing or a protective pouch, that could stop internal heat escaping?

OK, just read the trouser pocket thing. Can you keep it exposed, say attached to a belt?

[Wonks]

I’m just wondering if higher temperatures are affecting the battery output, so you aren’t getting full voltage once they warm up?

[Mike Shand]

I wonder if the transmitters tuning or modulator is drifting as it gets hot?

Have you raised this with Sennheiser?

Yes, but it is happening with two instances of the transmitter, so unlikely to be a faulut per se.

I haven't raised it with Sennheiser (yet).

Wonks.

Could be a warming battery thing, but we haven't had any problems over the past 10 years!
Yes it does seem to be only when in a trouser pocket. We will try keeping it on a belt.

Example audio

Here is a particularly bad case that happened this morning.
https://drive.google.com/file/d/1VZpJN0 ... sp=sharing

[James Perrett]

Are both the old and new transmitters working on the same RF channel? If so, could you try setting them to a different channel?

I'm wondering if the alignment of the receiver on that channel has drifted off slightly so that it can no longer compensate for the drift of the transmitter due to temperature.

[Mike Shand]

Are both the old and new transmitters working on the same RF channel? If so, could you try setting them to a different channel?

I'm wondering if the alignment of the receiver on that channel has drifted off slightly so that it can no longer compensate for the drift of the transmitter due to temperature.

Yes, but I only have the 3 licenced frequencies. I suppose I could swap the frequencies of this one and the one that works fine and see if it follows the swap or not.

That's an excellent thought about the receiver drifting. So essentially the problem might lie with the receiver not the transmitter as such.

As I mentioned the receivers are over 10 years old, so I guess its possible that one has drifted and the others havn't (yet!).

Definitely worth investigating.

Thanks.

[James Perrett]

Yes, but I only have the 3 licenced frequencies. I suppose I could swap the frequencies of this one and the one that works fine and see if it follows the swap or not.

That's an excellent thought about the receiver drifting. So essentially the problem might lie with the receiver not the transmitter as such.

If you are lucky you might find a transmitter/receiver combination which works - although you may be living on borrowed time.

[cashhewn]

Bit of a long shot here but…have you changed AA battery types by any chance over the last ten years, from old style to rechargeable? Sometimes rechargeable AAs can behave a bit unexpectedly in terms of voltage compared to the traditional style. Just a thought.

[Mike Shand]

Bit of a long shot here but…have you changed AA battery types by any chance over the last ten years, from old style to rechargeable? Sometimes rechargeable AAs can behave a bit unexpectedly in terms of voltage compared to the traditional style. Just a thought.

Good thought, but no. We have always used Duracell pro cell AAs and the ones we are using now are from the same batch we bought about 18 months ago.

[AlecSp]

I suppose I could swap the frequencies of this one and the one that works fine and see if it follows the swap or not.

Yes, that's an obvious thing to try.

You're already guilty of jumping to conclusions, which is the enemy of good problem solving technique (and has bitten me in the past).

There's always the possibility it's interference from an external source.

If it appears to be exacerbated by the Tx heating up, what happens if you cool it. Try applying a bag of frozen peas wrapped in a tea towel for, say, a minute - does the problem disappear.

As others have said, it could be down to the use of the pack. If this one is put in a pocket and the other, happy, ones aren't, then try using one of the "good" ones in a pocket and see what happens.

[Mike Shand]

I suppose I could swap the frequencies of this one and the one that works fine and see if it follows the swap or not.

Yes, that's an obvious thing to try.

You're already guilty of jumping to conclusions, which is the enemy of good problem solving technique (and has bitten me in the past).

There's always the possibility it's interference from an external source.

If it appears to be exacerbated by the Tx heating up, what happens if you cool it. Try applying a bag of frozen peas wrapped in a tea towel for, say, a minute - does the problem disappear.

As others have said, it could be down to the use of the pack. If this one is put in a pocket and the other, happy, ones aren't, then try using one of the "good" ones in a pocket and see what happens.

Cooling by just leaving it in the open air does make the problem go away after a while.

A "good one" on a different frequency and a different receiver in a pocket shows no problems.

It is definitely exacerbated by heating, whichever transmitter you use on this frequency, but only with this receiver.

[AlecSp]

It is definitely exacerbated by heating, whichever transmitter you use on this frequency, but only with this receiver.

Another thing to test is to swap two receivers, retuning each, to see if the problem follows the receiver or the frequency.

Your recording certainly sounds more like RF than internal distortion (but rule nothing out until a root cause is finally identified).

I'm no expert, but it's always possible that frequency is more succeptable to drift (if that's what's happening) owing to intermod with other channels.

It's just possible that there's been no change to your kit, but that an external factor is causing issues with the normal operational tolerances of your kit.