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Wiring problems with Voice Technolgies lavaliers


kosty

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I'm having troubles getting my Voice Technologies VT402 lavs to work with my Lectro SMDB and Sennheiser SK 2000 transmitters.

I used the servo bias wiring with a bridge between pins 2+4 for 4VDC positive bias for the Lectrosonics SMDB.

The lav needs a crazy amount of gain to get any decent amount of level and then the signal is intermittent every few seconds. It's not a hard on/off signal loss, but the signal fades in slowly (~2seconds) before disappearing again. The transmitter is not showing any overload or not enough signal, I think it's overloading something (zener diode, FET?) in the input of the transmitter.

I wired a second lav the same way to make sure it isn't a bad soldering job, but the result is the same. I also tried to ground the strain relief in one of the lav as suggested for other VT lavs.

I then ordered 2 more VT402 from Canford UK and asked them to terminate them for my Lectro SM. I also ordered another one terminated with LEMO 3 for Sennheiser 2000.

The Lemo 3 one is behaving exactly the same with my SK 2000 transmitters. Not enough gain and intermittent signal with slow fade ins. This confuses me because it's a different connector with a different transmitter and a different wiring. What can cause this behavior ?

The other 2 lavs for the Lectros have been wired the same way as I did previously (1-ground, 3-NF, 5-VDC and bridge between 2 and 4.) These are acting differently now. The signal is not intermittent and the level is good (around were my COS-11 is usually set) but the noise floor is very high. If I switch to my Sanken they are almost dead silent.

A technician from Voice Technologies couldn't explain what was wrong with my first 2 lavs I wired myself but suggested I can try removing the bridge between pins 2-4 to lower the bias voltage to 2V instead of 4V. According to VT, the lav accepts a bias voltage between 0.8 and 5V.

 

So my questions now are :

1.) What is causing this intermittent behavior with my Lectros and also with my Sennheiser 2000 ?

2.) What can cause the high noise floor on the one's that are more or less working ? And why are they behaving differently then my others with the same wiring (the only difference I see are the Switchcraft connectors I used instead of the Rean from Canford) ?

 

I'm going to try to reduce the bias voltage to 2V and if this doesn't help, I will resolder one to universal wiring to see if that works.

Thanks for any input !

 

 

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Hi Kosty,
Given what you have posted, I see no reason for this weird behavior. The fact that a Sennheiser with Lemos is acting the same way almost says it is after the wireless receivers, such as a headphone amp or mixer. Another wild ass possibility is high level ultrasonics in your testing area but changing locations should put that to rest. This has me stumped. By the way what is "pin 3-NF"? NF is a new one on me.

Best Regards,
Larry Fisher

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Hi Larry,

Apparently VT is referring to "Mic Output" as NF, but I don't know what it stands for.

I tried to rule out every possible fault after the wireless receiver. Changed channels, frequencies, cables and also transmitters. With the same settings other lavs like the Sanken, Lectrosonics or DPAs are working fine. On the intermittent ones I can also see the metering on the SMDB dropping when the signal cuts off, so I'm quite sure it's not after the receiver. On the Sennheiser I also connected my headphone to the EK 2000 receiver itself without my recorder connected to the receiver.

I'm going to test the setup at my office tomorrow to see if it has something to do with the location and also shoot a quick video of the issues.

Best regards

Pierre

vt402_specs.jpg

vt_lectro_wiring.png

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Update: I rewired one of the black lavs to universal wiring and it's working now !

Since my COS-11 are working with servo bias, I don't think it's the transmitter. Is it possible that the VT402 requires a 1k or 1k8 resistor like the Sanken in order to work with servo bias?

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56 minutes ago, Patrick Farrell said:

This appears to have NF and VDC reversed compared to the chart you posted. Maybe double check that wiring?

Thanks for the hint ! Unfortunately it was only a typo and the wiring was correct (pin 3 VDC and pin 5 NF).

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