Frequently Asked Questions - Wireless

FAQs - Wireless

The IM has an extended low-frequency response (-3dB at 35Hz vs. -3dB at 70Hz) when compared to the LM bodypack transmitter. Otherwise, they are the same.

This test is a matter of setting up two identical microphones, into a mixer, one connected with an audio cable and the other with a wireless system, to perform a listening test. An even better way to perform the test is to split a single mic signal so that one path is through the wireless mic system to the mixer and the other is hardwired to the mixer. Though seemingly simple, there are several mistakes that can really mess up the results:

  1. The listening levels of both paths must be EXACTLY the same. Even a very slight difference in level will “fool” the ears into hearing differences that may not actually exist.
  2. The test absolutely requires two people. One person speaking and the other person listening. A single person hears the sound coming from their own mouth and the sound coming from the sound system and the sounds combine at their ears to give very misleading results. This particularly true with systems that have audio delays (digital wireless systems or mixers). Even having the two people in the same room is enough to mess up the results. The only way to do the test with one person is to use a very high quality, pre-recorded signal as the source.
  3. Keep in mind that the truer system is not the one that sounds brighter or warmer or bassier but the one that sounds most like the hardwired signal. You'd be surprised how many people ignore this fact.
  4. If you have to use two microphones, it is always a good idea to swap the microphones and listen to them a second time to see if there are slight differences in the microphones themselves that may have been detected in the first comparison.
  5. If you have to use two microphones make sure they are positioned exactly the same from a sound source or someone’s mouth so the the same audio signal enters both microphones.
  6. Slight amounts of background hiss will make the overall sound of that signal seem brighter. It is a known pre-sensitization effect in the ear. Make sure that the gain of the wireless system is set properly so that background hiss is minimized.
  7. If at all possible, use headphones. Again, it's not whether you like the sound it is how close is the wireless system path to the direct wired path. Headphones are much more analytical and make it many times easier to hear small errors in sound reproduction.


Switch back and forth between the cabled and the wireless setup as the listener compares the sound of each setup. This, of course, is best done in a “blindfold” manner where the listener has no way of telling which setup is being monitored, and by writing down a few notes about the results.

The previous limit LED indicator on the UM200B was wrong (misleading) and led to many users setting the transmitter gain wrong (too low). After 5 years of confused users calling the sales crew, the service troops and yours truly, I decided to change The Damned Thing. The reason I think it was the right thing to do, is that the number of calls about level problems has declined dramatically. 

I didn't do the change casually for the very reason some users have brought up; mixing B's and C's is confusing. If you set the LED's the same, the modulation of the UM200C will be about 10 dB (!) hotter than the B. 

We told users in the UM200B manual to set the limit LED so it was on 10% of the time. The users didn't want to do that since the red LED was worrying them so they were setting it for no limit LED or occasional flashing. The new recommendation on the UM200C is for occasional flashing which is the way most users wanted to set it.

Here's what can be done:
Just use the receiver monitoring. The receiver metering has not changed and the basic audio in the transmitter didn't change either, just the transmitter readout. If you use the metering on the receivers, then the B or C transmitters will be set the same. Also, just remember to set the B's for more LED flashing than the C's.

(See FAQ#027-WIRELESS for UM200B and C Differences)

Antenna gain is specified in some different ways that are confusing. The first is Gain referenced to an isotropic radiator (G subscript i) which is an antenna that radiates omni directionally and equally in all directions. A dipole antenna in this specification has a gain of about 3 dB Gi. However,an isotropic radiator doesn't exist in the real world. Any efficient antenna has more gain than an isotropic radiator. Even a simple 1/4 wave whip antenna has a gain of 3dB Gi. Since it radiates its power only above the ground plane, or into half free space it has a Gi of 3 dB. 

The other way of specifying the gain of an antenna is referenced to a dipole (or G sub d ). Obviously a dipole has a gain of 0 dB or unity referenced to Gd. The dipole is just referenced to a dipole. 

As an example, our ALP600 log periodic antenna has a gain of 4 dB Gd or 4 dB better than a dipole. If we wanted bigger, more impressive numbers, we would just rate it at 7db Gi. 

The ALP600 is more directional than a dipole of course, in fact 4 dB more directional. Gain is always proportional to directionality. Most of the directionality is in the vertical plane or up and down. The horizontal plane, left to right, which concerns you, is +- 60 degrees or 120 degrees total for a gain equal or better than a dipole.

The RG-59 will work for even longer runs than 3 feet. Assume operation at 600 MHz and 70% velocity factor for the RG-59. You do get some mismatch, but you will get it at any point beyond a 1/8 wavelength which is only 1.7 inches (!). At a 1/4 wavelength, 3.4", you get the worst mismatch but that will still give you 88% of your power. At a 1/2 wavelength, the 75 Ohm cable will look like a perfect 50 Ohm match (!), since whatever impedance is at one end is transformed to the exact same impedance at the other end. At 3/4 wavelength it looks like the 1/4 wavelength case, 88%, and at 1 wavelength it looks like the 1/2 wavelength case or a perfect match again. This just repeats for each additional 1/4 or half wavelength of cable. Added to this small mismatch, you will have the cable loss which is actually lower for RG-59 than for RG-58, since RG-59 is physically larger. You can use RG-59 for up to 15 feet or so where I wouldn't recommend RG-58 beyond 10 feet. 

You will see some mismatch to input or output filters but it isn't severe. Basically under the worst mismatch 1/4 wavelength, the 50 Ohm source or load is transformed into 100 Ohms. Antennas and filters will shift some but it isn't horrible. (RF is weird stuff.)

UCR201 receivers built between the beginning of 2004 and January of 2005 have a voltage divider problem that we became aware of in early February of 2005. External supply voltages greater than 12 Volts overload the analog inputs of the microprocessor and cause all analog readings such as audio level and RF level to be incorrect. This also causes the scan function to work incorrectly and display "interference" in frequency bands where there actually is no interference. 

The quick solution is to run the 201 from 9 Volt batteries or power it from voltages below 12 Volts. The permanent and better solution is to let us modify the unit at no charge, in or out of warranty. This can be done by us at the factory, by one of our servicing dealers or by you if you are moderately handy with a soldering pencil. We will send the surface mount diode, layout drawing and "fix it" instructions to anyone who needs the fix. Check with service to get the package. We include an extra diode because if you drop one on the rug, you'll never find it.

All units shipped after 9 Feb 2005 will have the same fix as described below and the next board rev will incorporate the exact same fix. The fix consists of a small SOT23 surface mount diode from the offending voltage divider to the 5 Volt supply at the micro-processor. The 3 terminal diode is soldered to a copper trace and to two vias on the board. It is necessary to scrape a little bit of solder mask (green screen) from the trace so the solder can bond. This fix, or attempt at a fix, does not affect the warranty. Using a Black Beauty soldering iron and blow torch will, however, result in nasty comments from the repair crew.

  • Here's the S/N for the affected 201's.
  • 4895 to 5649 need to be modified.
  • 4795 to 4894 are in a grey area and MAY need to be modified. This can be checked by operating from a 9 Volt battery, finding a clear RF frequency on the display and then plugging in the CH20 supply and seeing if the RF level goes up 3 or four steps. If the CH20 level is different than the 9 Volt level, the unit needs the modification.
  • Units before 4795 are OK.
  • Units after 5649 are OK

The following units were in our finished gods and have been modified already:
5269, 5304, 5321, 5389-5394, 5417-5419, 5458-5459, 5463, 5465-5469, 5471-5479, 5484-5504, 5508-5515, 5517. Dashes indicate anything between these number are OK.

Another rule of thumb. Anything shipped from Lectro after 9 Feb 05 is modified and any units in for repair after that date will be modified.

Yes and no. If you charge the battery just until the charger indicates that it is done with the fast charge, you will have charged for 15 minutes or less. A reasonable person that hadn't read the manual, (like you and me), would think that it is completely done. In fact, it is only changing from the high charge rate to a trickle charge. If you stop charging at this point, you will get about 90% of full performance out of the battery. If that is all you need, then there is no reason not to just do a 15 minute charge. To get the last bit of charge into the battery, however, you will have to let it remain on the charger for another hour or a little more. It is still pretty remarkable that you can get a nearly fully charged battery in only 15 minutes.

The reason for the slight undercharge is that the battery gets hot from having charge crammed into it in such a short period of time. The charging current is over 8 Amps. That's why the 15 minute chargers actually blow cooling air over the battery and the charger's electronics.

Hot NiMh batteries do not hold as much charge as a room temperature battery. So it is necessary to allow the battery to cool down a bit before the last bit of charge can be put into the battery using a trickle charge of a fraction of an Amp.

The battery drain increases by a third if you use a high current mic at 48 Volts. Fortunately, the most common professional mics are relatively low current (such as Sennheisers). Approximate battery life will go from 4.5 hours at no phantom, to 3.7 hours with a low current mic, and to 3 hours with a high current mic. Battery life can be improved for some low voltage, high current Schoeps by running them in the 18 Volt position. A number of popular performing microphones are just as happy at 11 Volts as they are at 48 Volts. There is absolutely no advantage to running them at 48 Volts; it is just wasted battery power. Switch the UH transmitter to 18 Volts with these mics and reduction in battery life will be reduced by more than half. 

So the overall answer here is, check the specs of the microphone to see what minimum voltage it really requires and then set the UH to 18 Volts if possible. If you do that you should lose even less than a half hour of battery life. If the specs are not at hand, try running the mic at 18 Volts and see if you are happy with the results. In fact, have someone else switch the voltage and see if you can tell the difference, no matter what the specs say. You may be able to save some battery money.

The UH series has always had an unbalanced input. Since a mic is usually plugged right into the unit, a balanced input was unnecessary. Recently we have gone to a pseudo-balanced input to make some professional mics happier with our input. The current arrangement for the UH400 has a 500 Ohm input load on pin 2 and a 500 Ohm resistor load to a 20 uF capacitor to ground. As far as what the mic sees it is 500 Ohms to ground on both pins 2 and 3 or equivalent to a center tapped 1000 Ohm load. If the mic has emitter follower outputs for pins 2 and 3, this keeps the UH400 from "shorting" the audio on pin 3 and raising the distortion level of the mic. Mics with a floating and balanced output (a transformer or equivalent) will work into this input with the additional benefit of standard common mode rejection. The advantage to us is backwards compatibility with older transmitters and our unbalanced lavaliere mic adapter.

See MCA5X Adapter

This wiring also makes it possible to use a two wire lavaliere mic with the UH400. Simply set the phantom power to 5 Volts and wire the two wire lavaliere to pin 1 shield and pin 2 hot.

It is a gentle statement to say that our phantom wiring is merely unconventional. The UH series has always had an unbalanced input. Mics used with the plug-on units usually go right into the unit and a balanced input was unnecessary. Given that, a balanced phantom feed would be a waste of effort. So pin 3 has phantom power on it but audio is tied to ground with a series 500 Ohm resistor in series with a 20 uF capacitor. Pin 2 is audio in (hot) and is tied to the phantom voltage with a 1k resistor and goes to an amplifier with a 1k input impedance. (See FAQ#049-WIRELESS for full impedance details.)

If that isn't enough, the 48 Volts is really a regulated 42 Volts to compensate for our lower impedance feed resistors. Forty two Volts is an approximation of what the usual 2 mA, 48 Volt mic "sees" from a 48 Volt supply due to voltage drop in the DIN specified 6.8k feed resistors. One of our goals was to be able to operate higher current pro mics. With the 1k feed resistors and 42 Volts , we can provide 7 mA with reasonable voltage drop. Just don't think we cheated you on Volts if you measure the phantom voltage with a meter without a mic load. 

There is yet another facet to our madness. The lower voltage and reduced resistor loss means less power has to be supplied by that overworked 9 Volt battery that you have to buy. By using constant current diodes instead of larger lossy resistors the supply noise is still well filtered but we don't have big power losses. This arrangement also meant we didn't have to use huge 63 Volt capacitors to filter out supply noise on a 48 Volt supply. We can use 50 Volt parts on the 42 Volt lines. The inside of the UH400 is really packed and figuring out how to get around those large caps is one of the things that kept us from doing phantom power years earlier.

Finally, by using lower value feed resistors, we can accommodate some older pro mics that want 12 to 15 Volts at about 10 mA . Doing this much current with 6.8k resistors would waste a lot of valuable battery power. In fact we make a T-power adapter that takes advantage of our capability of delivering relatively large currents at low voltage