Sennheiser ew300 & phantom power

[johnhogg]

Hi guys,

 

I'm wondering if any one has ever had a problem when using phantom with Sennheiser ew300's (or any other Sennheiser models)

 

I have a A&H mixwizard with global 48v. When used with ew300 and 48v turned on I get a buzzing, not that loud, but enough to make it unusable.

 

I am about to ring Sennheiser but thought that someone might have experienced the same problem.

 

Thanks in advance.

 

 

JH

[emj]

Hi John

 

 

I have used A&H mix-wizard with global 48v with ew300 and not had a problem so sounds like you have a fault some where

 

 

Regards

Em

[johnhogg]

Thanks emj for the reply.

I have since spoken to Sennheiser and it appears that some circuit boards have had the same problem so it sounds like a dodgy batch. issapointing as I've used Senns for years with few problems.

 

Might have to try using the jack until I can send faulty units back.

 

JH

 

edit: S&G

[Ynot]

Erm, so you're feeding 48v phantom power down the mic line to an EW300 rad mic receiver...?

 

Well, to be honest I am not surprised that you're getting a hum!

These receivers DO NOT require phantom power, and shouldn't be connected to inputs that are sending 48v.

 

If your mixer can only do phantom on all or none of the balanced inputs, what I'd suggest is that you use the UNbalanced output (1.25" jack) output from the receiver into the LINE input.

EW312 manual.

[MarkPAman]

You could try a mic lead with pin 1 disconnected. I usually have an adaptor or two in my tool box do do this.

 

Tony, although the unit does not need phantom power, it really should be able to cope with it without any problems. Sennheiser seem to be saying that it's a fault.

[Shez]

These receivers DO NOT require phantom power, and shouldn't be connected to inputs that are sending 48v.

Very true, however, any well designed piece of kit that can output at mic level on an XLR should be quite capable of ignoring the presence of phantom. There are occasions (such as with desks like that one) where it's unavoidable without faffing around with adapter cables. I'm sure I've used my 300 series kit alongside condenser mics on a baby desk with global phantom without any issues before now. I'd be very surprised if Sennheiser didn't design it to cope with that.

[Ynot]

In which case I bow to the more experienced on the BR and withdraw my earlier statement.

[Solstace]

I too have seen this problem a couple of years ago I think. A phone call and return to Sennheiser saw a warranty repair done (the receivers were barely a year old) but I never did get confirmation of what went wrong.

 

In the end I decided for the sake of long-term reliability to isolate the receivers by placing each behind a 1:1 audio transformer, which should give some DC isolation in normal conditions. Not got my head around the physics of what happens if the link to the mixer loses a leg for whatever reason. Also not sure I picked the correct impedance (looks like these claim 10K-Ohm either side) but audio performance is good without any apparent level or frequency loss. Is there a better way to deal with this than I've found?

 

Philosophically I agree that ideally one would turn off phantom power for channels that don't need it - but with some mixing desks this just isn't possible on a per-channel basis. Also, I agree that well-designed electronics that will interface with mic inputs ought to be able to cope with phantom power even if they don't actually need it...

[SoLiEn]

agreed with everyone who says they should cope with phantom power. you have just checked its not the cable right?

simple solutions that are relatively free are always the best!

[Stuart91]

John - are you 100% sure that you have an original Sennheiser system? There are a lot of very convincing fakes around (sold on eBay amongst other places). I was fooled by one that a customer brought in for repair, I didn't realise it was a fake until Sennheiser told me, when it was sent back to them.

 

This kind of problem sounds like the kind of thing that lower quality circuitry in a fake unit might have issues with.

[johnhogg]

Thanks for all your replies.

 

I am certain they are not fakes, and I'v tested cables etc.

I was going to just connect the effected units using balanced jack outputs but it appears the circuit boards have actually been damaged by the 48v so they have now been removed from the racks and shipped off to Sennheiser.

 

Hopefully an explanation of the cause will follow as we've 12 other units and I've lost a bit of confidence in the build quality of these units.

 

These replaced a load of Sony's we had which had irritating problems. But hopefully this will be sorted in the near future.

Might try disconnecting pin 1 for the rest o our units and see if we have any more problems.

 

Thanks again

 

JH

[Stan Hope-Streeter]

<br />Thanks for all your replies. <br /><br />I am certain they are not fakes, and I'v tested cables etc. <br />I was going to just connect the effected units using balanced jack outputs but it appears the circuit boards have actually been damaged by the 48v so they have now been removed from the racks and shipped off to Sennheiser. <br /><br />Hopefully an explanation of the cause will follow as we've 12 other units and I've lost a bit of confidence in the build quality of these units. <br /><br />These replaced a load of Sony's we had which had irritating problems. But hopefully this will be sorted in the near future. <br />Might try disconnecting pin 1 for the rest o our units and see if we have any more problems.<br /><br />Thanks again<br /><br />JH<br />

<br /><br /><br />

 

Most modern line output stages use a push-pull or balanced drive circuit instead of an expensive, old-fashioned transformer. Usually this is a 5532 IC, sometimes (and preferably) followed with a complementary transistor pair booster. The two legs of the output, driving XLR pins 2 & 3, have electrolytic capacitors to block any dc in the drive amplifier from the output to line.

These electrolytics, to save space and (er, much more importantly) a few pence in cost, are often rated at only 25VDC. Frequent or long-term connection to 48V will eventually damage them. In better designs, 63V capacitors are used. Being larger they take up a bit more space on the circuit board, but are more reliable.

 

Furthermore, these capacitors might be wired with either the + or - terminal towards the output, since under normal conditions there is no DC across them. Electrolytic capacitors definitely don't like reverse polarity. (Although non-polarised types are available they are expensive and much bigger than standard electrolytics) So given the prevalence of cheap desks with Global Phantom, ideally they need the + terminal to the line. This may have been the problem somebody referred to "an error on the circuit board".

 

As has been mentioned, you should avoid connecting active balanced line outputs to phantom powered mic inputs.

Either use the console's line inputs, or 1:1 isolating transformers.

 

There is also the likelihood of overloading a mic input with a line level signal, even with the pad in.