Try using the 21750 phase reverser. URL below.
This will swap pins 2 and 3 on the XLR and the Oktava should now work. You can also switch the wires on the XLR in the microphone. In either case you will need to switch the phase polarity in the Lectrosonics receiver if you are using multiple mics picking up the same audio source to prevent phasing artifacts.
For a typical two wire lavaliere mic that specifies 5 Volts, the manufacturer is actually assuming that the transmitter has a 5 Volt supply in series with a bias resistor of 1k to 5k, depending on the brand of transmitter. The actual voltage at the mic will be 5 Volts minus the drop across the bias resistor. For example, a mic that is listed to draw 500 uAmp would produce a 2.5 Volt drop across a 5k bias resistor. The mic would only see 2.5 Volts (5V minus 2.5V drop). A different mic that pulled only 100 uA would see 4.5 Volts. So for most all transmitters, the voltage to the mic is all over the map. Generally the mics still work, because they actually can handle a wide range of voltages.
All the current Lectro transmitter models, such as the LMa, have a servo input that regulates the bias voltage to exactly 4 Volts under any condition of bias current. The voltage is set to 4 Volts by using the pin 2 to pin 4 wiring. This allows us to handle a wide range of microphones with any current draw with no concern about excessive voltage drop across the bias resistor and is unique to the Lectro transmitters. We chose 4 Volts because this was a typical design voltage and all the professional lavaliere mics we looked at worked very well at that voltage. The one exception is the tiny Countryman B6 and E6 models which require 2 Volts at high current. For the Countryman mics Pin 4 is NOT connected to Pin 2 and this sets the servo input to a regulated 2 Volts which is ideal for those lavaliere mics.
From the Shoeps web site: "The SCHOEPS CMR microphone amplifier allows any SCHOEPS ”Colette” series capsule (except the BLM 03 C active boundary layer capsule) to be used with pocket transmitters."
In the email below is the TA5F wiring that works with our older transmitters such as the UM200 and the newer servo style inputs such as the UM400a or the SM transmitters.
The CMR adapter is on its way back to you. The rewiring was pretty simple. The problem was that the CMR adapter doesn't like any DC voltage applied to the audio line (our pin 3). The SM servo unit applies 2 Volts to pin 3 when pin 4 is not wired to ground. With the voltage switched off, the mic sounded great but had too much gain in my opinion. We added a 1.5 k resistor in series with the audio line (our pin 3 again) and now the mic has about the same gain as a COS-11 when used with a UM400a or an SM. It still works with an older transmitter such as a UM400 or UM200 with a few dB more sensitivity than with the SM. If you should want more gain with an SM, reduce the size of the resistor; 500 Ohms = +6 dB.
So here's the final wiring for full compatibility:
Here are some things we've found that will strengthen and protect the connection:
To wire a positive ground lavaliere mic (some older TRAM's and Sony's) to the new servo input used on the SM series and the new UM400a, LMa and UM450 transmitters, use the following wiring arrangements. (Positive ground lavalieres are also known as negative bias lavalieres.)
This is the simpler "servo only" wiring and is not compatible with older Lectro transmitters (UM200, UM400, etc.)
The following is the compatible wiring and requires an external resistor but this wiring can be used with all Lectrosonics transmitters, old and new.
Other value resistors can be used in a pinch from 2k to 4k, including the 3.32k resistor that we provide in the 5 pin wiring kit.
A customer sent us a SONOSAX SX-BD1 and indeed we found excess noise with this unit when used with a UH400a. We are 99% sure this is due to common mode noise on the Sonosax outputs particularly when the 40 dB output attenuator is selected. (We didn't have a schematic and we didn't want to tear up the customer's unit). When used with a good mixer, the common mode noise was mostly canceled out but that was not true with the UH400a.
We found that grounding the XLR pin 3 output of the SONOSAX cable to pin 1 of the same XLR removed the common mode noise by forcing the output into an unbalanced mode that matched the input of the UH400a. This can be done either in the SONOSAX male XLR itself or inside a barrel adapter available from Lectrosonics P/N 21750. This adapter is normally used to reverse the XLR polarity and is wired as pin 2 to pin 3 and pin 3 to pin 2. You can undo this by switching the wires at just one end of the XLR. To fix the SONOSAX SX-BD1 problem, simply wire pin 3 to pin 1 at the male end.
You can add a RF choke to each lead of the XLR in the Schoeps mic. We have helped several customers with an RF bead placed around all three wires at the XLR connector inside the mic. The Mouser part number is 623-2643000301. The Fair-Rite brand part number is 2643000301. Placing short leaded 100 pF capacitors between all the XLR pins and one to the ground lug might help also, but the chokes are most effective. Schoeps has a factory modification that is more elaborate, that works very well.
The recommendations below are for more "normal" usage.
We recommend wiring the standard level COS-11 in a 2 wire configuration with a 1k resistor in series with the white wire (source wire) to ground. This reduces the output of the standard mic by 6 dB which will prevent overload of the input buffer on the UM400. This isn't necessary for the SM series since it cannot be overloaded by the COS-11 under any circumstances. It does make the microphone compatible with both transmitter series.
The Red Dot Cos-11's are designed to have 9 to 10 dB less output than the standard COS-11's and can therefore be wired with the source lead directly to ground.
Adding a 1k resistor to the source lead of either microphone will drop the 2 wire output by 6 dB compared to the source being wired directly to ground.
Using a 3k source resistor will drop the output of either mic an additonal 6dB compared to using the 1k resistor or 12 dB total. This much gain reduction should not be necessary and is for information only.
We also recommend wiring our pin 2 to pin 4 to make the wiring fully compatible with our UM400 and older Lectro transmitters. A schematic is below.
Here is schematic for muting a two wire microphone. The in's and out's can be swapped. You can treat the arrangement as if it is just a normal two wire mic. That means that it will have to be hooked up to a transmitter or other bias supply in the normal manner. There will be a faint click due to RF in area from the transmitter but it won’t be objectionable for your application. This only requires a single pole, single throw switch. When the switch (or push button) is closed the capacitor is connected to the two wires and shorts out the audio. There will be a little audio bleed-through, mostly at low frequencies. The larger the capacitor, the less bleed there will be. The resistor across the switch is necessary to keep the capacitor lightly connected to the bias supply so there won’t be a large pop when the switch is closed. A better but much more expensive solution is to use a unit designed to be silently muted by a DC control signal.
The Sanken CUB-01 boundary mic does not seem to have the usual FET output stage and also seems to have a large capacitor across the power supply lead (bias lead). This means that it can't be wired as the usual three wire microphone with the SM. The wiring below seems to work well and is fully compatible with our other transmitters.
The SM input is a radically different input system compared to our previous microphone inputs. It is so superior to the old way of doing things that we will eventually introduce this input system on all our UHF transmitters. We realize this causes some confusion for our customers but the advantages are very real. The improvements are audible and make the transmitters easier to use and much harder to overload. It is no longer necessary on some mics to introduce pads to prevent overload of the input stage, divide the bias voltage down for some low voltage mics, or reduce the limiter range at minimum gain settings. For a more detailed technical discussion of the improvements in the SM servo input stage, see FAQ#061-WIRELESS. We have spent many, many hours trying to make the change from the old system to the new system as painless as possible.
For 90% of the microphones in common use, no changes are necessary to the wiring of the 5 pin connector. For some microphones the wiring can be simplified. For line level inputs, our custom musical instrument cables, adapter cables and so forth we have managed to keep the 5 pin wiring the same for old and new transmitters. You can find complete wiring diagrams for the SM transmitter on our web site. The exceptions to this compatibility are all three wire microphones (including our own M150) and a few odd wirings such as the 40 dB attenuator wiring for line level inputs. All this can be found on our web site under the Support tab, but I will list some of the more popular mics here after I discuss some of the headings on the diagrams on the web site.
The first section discusses what each pin of the 5 pin connector does. The most radical change is that pin 4 is now a voltage selector pin. You can skip this technical section if you just want to know how to wire your mic.
The next section is is boxed and labeled "Works with SM only". These wirings are specific to the SM transmitter and make wiring a Countryman B6 or E6 or a three wire microphone such as a COS-11 very quick and easy. However, these wirings won't work with older Lectro transmitters such as the UM400, UM200, etc. If you need the two wire Country B6 or any three wire mic to work with both older transmitters as well as with the SM go to the last section below labeled, "Compatible with SM and other Lectrosonics Transmitters".
Countryman B6 and E6 are shown in the first diagram of the section labeled "Compatible with SM and other Lectrosonics Transmitters". TheB6 and E6 are two wire mics but still need special wiring because they are unhappy if run from more than about 3 Volts. The added 1.5k and 3.3k resistors shown in the diagram make the microphones compatible with any Lectro transmitters. This wiring bypasses the servo section and runs the Countryman from the 5 Volt bias supply directly. If you can use the easy wiring above in the "Works with SM only" section for the B6, it gives a little better control of sub sonics and voltage drift with humidity; otherwise there is no difference in audio response. If you have a Countryman B6 or E6 already wired for attenuation for use with a UM200 or UM400, it should still work fine with the SM. font-family: 'Times New Roman'; font-size: medium; line-height: normal;"
Sanken Cos-11 microphones, the Lectrosonics M-150 and other three wire microphones to be used with the SM will all require new wiring. If the wiring is not changed, they will have much higher output than usual and extra distortion at high levels. The reason is that the source follower wiring used with the UM200 and UM400 series is not compatible with the SM virtual ground input. The second diagram in the "Compatible with SM and other Lectrosonics Transmitters" section shows a compatible wiring that will work with all 5 pin Lectro transmitters. This wiring converts the three wire microphone to a two wire system with no changes in audio quality. The microphone polarity will be reversed so you may want to enable the phase switch on the Lectrosonics receiver. This wiring is electrically equivalent to the easy wiring in the "Works with SM only" section above.
All two wire mics (except the Countryman B6 and E6 as described above) such as the MKE-2 and the Lectro M-152 will work with the SM with no changes. The two wire setup is shown in the third diagram in the "Compatible with SM and other Lectrosonics Transmitters" section.
The fourth diagram and fifth diagrams in the "Compatible with SM and other Lectrosonics Transmitters" for unbalanced and balanced line level inputs are the same as for previous transmitters.
The sixth diagram at the lower right for low z dynamic microphones is changed compared to previous transmitter wirings and has the addition of a jumper wire from pin 4 to pin 1. This tells the servo bias supply to shut down and set the pin 3 input voltage to 0 Volts. This additional jumper will reduce the mic output by less than a decibel when used with older transmitters.
Some phantom powered mics have a balanced and floating output and some have both outputs balanced but referenced to ground. Most outputs are electronic but you can think of the two cases as being a floating transformer winding or a center tapped transformer winding. Either way works fine into a balanced (mixer) input. The fully floating output does have some common mode noise advantages when operating into a less than perfect balanced system.
In the case of the Lectro transmitter, the input is unbalanced and you have to unbalance the mic output. (As far I know, the universal box is an innocent bystander here.) The problem is that the two different balanced systems require different wiring and what is right for one is very wrong for the other.
Fortunately, you can try one way and then the other and pick the one that gives the best results. By high and low, I am referring to plus and minus polarity from the mic. By best results, I mean loudest and clearest. Generally the differences will be dramatic.
For the fully floating balanced output, ground the shield at the TA5F pin 1, the low side wire (from XLR pin 3) at the TA5F pin 1 and the high wire (from XLR pin 2) to TA5F pin 3. This grounds the shield at the transmitter where the RF is the highest and ground references the mic low side at the transmitter.
For the balanced but ground referenced mic output (center tapped) everything is the same but the low side (XLR pin 3) is not connected to anything. If pin 3 were to be grounded, in this case, half the transformer winding is shorted to ground since the winding is grounded both at one end and the center. If it is an electronic output referenced to ground (Schoeps and some others) then that output is shorted and distortion will rise on the other output.
There are several ways of making a universal setup that will work with both types of mics. The first is to use a balanced to unbalanced transformer. The drawback here is that you need a very good transformer. The second way is to put 200 to 500 Ohm resistors in series with the low side signal (pin 3 XLR signal) and tie it to ground. This will not short a ground referenced output to hard ground but to a 200 to 500 Ohm load. A floating output will have one end of the signal referenced to ground through the same 200 to 500 Ohm resistor. The down side is that you have a resistor in the mic line and will lose a little signal but that usually isn't a problem.
After all of this was written, a customer sent in his wiring solution. Though it doesn't have the shield grounded at the transmitter, which bothers me a little, he has had good luck with the following wiring.
" I believe all's well now. I power the mics with a stand alone 48VDC power supply. I have two of them. A Neumann and Sennheiser. Both lost in effect about 10dB or so when the cable adapter to the input of the UM400 transmitter unbalanced the signal by grounding pin 3 to 1 as suggested in your wiring scheme for self powered mic level sources. Lifting pin 3 from ground brought the signal level back to normal. The wiring I am using now is this: The XLR pin 1 is tied to the shield but the shield is open at the transmitter TA5 end. Pin 2 (high) of the XLR goes to the transmitter input (TA5F pin 3) and XLR pin 3 (low) goes to pin 1 of the TA5F. The levels now are fine."
As long as the 48 Volt box ground doesn't get tied to the transmitter ground through something like a common power supply, this should work fine with either variety of mic, floating or ground referenced.
A similar problem in the UH400a transmitter was fixed in the following way.
(See FAQ#049-WIRELESS for UH400a fix)
There are a variety of professional lavaliere microphones that are well liked. Every user has their own opinion about which ones are the best for different situations. The problem is that these mics are radically different in their output levels, bias currents and in some cases the voltages that they will tolerate. In addition some are wired as three wire microphones (bias + audio+ ground) but others are two wire microphones with bias and audio on one lead plus a ground lead. Variations in output levels from different manufacturers can be more than 30 dB and bias currents can range from 20 uA to 800 uA. In the movie industry, the mic may be required to pick up a whisper in one scene and a scream in the next. It is no wonder that microphone and transmitter design is always a series of compromises. The input to the SM transmitter tries to overcome these compromises.
The bias voltage in the SM input is set by a servo loop that regulates the DC voltage at the microphone to a user selectable choice of 2 or 4 Volts. This is in contrast to the typical 5 Volts plus series resistor bias circuit that can result in a mic voltage that can vary from 1 Volt to almost 5 Volts. The lower voltage range can result in reduced headroom and the higher voltage can result in internal Zenering (overload) in some microphones. The SM input can handle mic bias loads from 1uA to 2000uA while still maintaining full bias voltage regulation. The servo loop also incorporates a filter that causes it to servo out frequencies below 20 Hz and rolls off the response of the lavaliere itself to wind noise, thumps and breath pops. These low frequency excursions are stopped right at the mic FET and then do not overload early audio stages in the transmitter.
At audio frequencies, the servo bias looks like an extremely high impedance resistor (constant current source) so that none of the output of the microphone is wasted in a 1k to 4k bias resistor. To prevent large voltage swings, the input to the first amplifier is a virtual ground input. This input is very low impedance so that the current developed by the mic FET is used entirely to drive the virtual ground input. Since the virtual ground input sees a high impedance source made of the mic FET's drain and the servo bias, the virtual ground input has very little loop gain noise. Since the mic's FET is operating into a virtual ground, there is very little voltage swing on the FET drain which reduces distortion on the FET compared to a conventional input.
The new input has the advantages of low noise since the noise is determined by the noise of the mic's FET and not by a bias resistor. It has the advantage of a well defined bias voltage that is not dependent on a compromise choice of transmitter bias resistors and mic current drain, i.e., two different manufacturers trying to guess what the other one is going to do. The input also has the advantage of very low voltage modulation on the FET drain reducing distortion. Finally, the input does not run out of voltage or current headroom since the bias voltage is well defined, DC current is supplied by the servo loop and AC current is "supplied" by the virtual ground amplifier. At minimum gain, the input will handle 240 uA of peak input current without engaging the limiter.
The most important advantage has to do with the limiter circuit that we have in all our transmitters since we can make it work better in the SM. Our standard limiter is a shunt circuit that shunts excess audio signal to ground when input levels are too high. In the past we have had to buffer this low impedance limiter circuit from the relatively high impedance input circuit for the mic bias supply. The amplifier that we had to have between the mic input and the shunt limiter was subject to overload at high input levels. Generally, the lavaliere mic overloaded before the buffer amp but not in all cases. Some high current mics could overload the buffer. The buffer amp also had to have unity gain so its output didn't overload and this meant this low gain amp added at least 3 dB of noise. With the new input circuit, the shunt limiter can be right at the input. No buffer amplifier is needed. This is because the virtual ground input circuit is very low impedance and is just what the shunt limiter is looking for. The advantage is that the limiter range is at least 30 dB no matter what the transmitter gain setting or input level from the lavaliere mic. There is no other transmitter that has anywhere near this limiting range for high input levels.
Some careful design went into this circuit and it is compatible with almost all of our previous mic wiring recommendations including line level inputs. Some microphones can benefit from a slightly different wiring scheme and that is noted in the SM manual. Old wiring, new wiring and compatible wiring is listed. About the only thing that doesn't work is the 40 dB attenuator wiring for very high signal level line level inputs. This can still be accomplished by putting a single 25k resistor in series with pin 5 of the TA5F input connector.
The M152 uses the same element as the M150 microphone. The difference is in the wiring inside the 5 pin connector. The M150 is wired so that it uses the pin 4 source resistor built into Lectrosonics belt pack transmitters such as the UM100, UM200, UM400 and LM. The M150 is wired as a three wire microphone with the bias being fed to the drain of the internal FET and the audio coming from the source of that FET.
The M152 uses a 3.3k source resistor built into the 5 pin female connector. The audio is picked up from the drain of the internal FET. This arrangement gives much better results with the SM transmitter and is still fully compatible with other Lectrosonics transmitters. The reasons for this new input arrangement on the SM transmitter are given in the link at the end of this FAQ.
The M152 is exactly like a two wire biased microphone and the M150 is a three wire biased microphone. Like all mics that can be wired either 2 or 3 wire, the M152 has opposite polarity compared to the M150. If you are using the M150 with the M152 you should reverse the phase using the receiver menu on one of the systems.
The sensitivity of the M152 is about 7 dB lower on a UM100 or UM200 compared to a M150. The sensitivity is almost exactly the same on a UM400. The sensitivity of the M152 is about 6 dB lower on a SM again compared to a M150. The sensitivity of the M152 is about 3 dB lower on a LM transmitter. These variations are easily adjusted with the transmitter gain controls.
The blocks are a holdover from our early UM200 systems before we put in microprocessors. The frequency switches directly controlled a parallel input PLL (frequency chip). Since we had 256 possible switch settings using two 16 position switches and we were making 100kHz steps, we defined a block as 25.6 Mhz in size. Block 0 would have gone from 0Hz to 25.5 Mhz, Block 1 from 25.6 to 51.2 MHz and so forth. So the beginning frequency in any of these blocks would have been the block number times 25.6 MHz and the last frequency in that block would 25.5 MHz above that.
So, in a sense our blocks were pretty arbitrary and have little to do with the world outside Lectrosonics. As far as the outside world, you can call the sales department at Lectrosonics (800 821-1121), give them your location(s), what other wireless gear you have and they will make a good recommendation. If they mess up and you are using our recommended gear, we then have the responsibility to get you out of trouble. Our dealers are pretty good at frequency selection since they live in the neighborhood.
The other valuable resource is the RAMPS group since you can probably find someone that is operating in your area and can tell you what works. (See info on RAMPS in FAQ What is RAMPS?)
I assumed that you were speaking about our gear since you mentioned blocks. If you are interested in other brand equipment, call their offices and get advice for your area. The other manufacturers are generally quite helpful and if they aren't then try some one else.
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.
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.
Here's a long reply posted to the RAMPS news group. It has a variety of wirings depending on the output level you need.
To the Group:
To make the measurements on the B6 we used the same setup as in the previous post, "Lectrosonics MM400a and COS11 red dot wiring" (See COS-11 test setup in FAQ How should I wire a Sanken COS-11 or other three wire microphone to a MM400 transmitter?). We had available two B6 microphones and I assume (that word again) they are the standard 6 mV/Pa units since the overload point that we found was close to the specified values. One unit was one that Carl at Countryman kindly loaned us several years ago and the other was from a customer who was having difficulties matching to 400 series transmitters. We found several factors that could cause possible problems with the B6 and a UM400a. The bias resistor for the UM400 series is 4k. This is higher than what we have used on the UM200 by a factor of 4. We chose the higher value because of the improved noise performance of the 400 series. In an effort to increase input signal levels to get past self noise, we increased the bias resistor value between our pins 3 and 4 to 4k. We had also run into problems getting enough output out of some big name low current microphones that wanted to look into a 20k (!) load.
The B6 microphones that we measured here were pulling 750uA and 950uA at 3 Volts which is 2 to 3 times higher than the B6 spec sheet. At first I thought this might be the problem since this much current would pull the operating point well below 3 Volts.
I called Carl at Countryman and learned more about B6's than the average person should know.
What Carl said specific to the B6 is that the ideal voltage at the B6 mic terminals is 1.5 Volts at which point the mic will draw 500uA. This was a lower voltage than I expected and changes what we would recommend for biasing. The 500 uA does differ some from the Countryman web site values but products always change the most right after the moment you publish "firm" specs.
Countryman's original recommended UM400 wiring inside the TA5F, from their website was:
Carl now prefers another configuration, which is to ignore our internal resistors entirely and wire a 1.5 k resistor from pin 2 to pin 3 and and a 3.3 k resistor from pin 3 to the the hot lead of the B6. The reason for the new recommendation is to reduce the high B6's sensitivity and get it closer to other commonly used mics.With our bias impedance and the resistors this will drop the signal 6 dB total below our original Lectro wring and bias the mic at 2.1 Volts. So the wiring is:
Another wiring, for more attenuation, will change our 5 Volt bias to 1.7 Volts on the B6 and drop the level 14 dB below our original wiring is as follows:
So here are three wirings which will drop input levels 3, 6 and 14 dB below the Lectro recommended wiring. I agree with Carl that the 6 dB wiring is the best all around. However, there is a bit more to the dynamics of the situation than just limiting and clipping levels. The 3 dB wiring will let the B6 drive the UM400 into 10 db of limiting (compression not distortion), even with the gain at a minimum. The 6 dB wiring will be 7 dB into limiting. The 14 dB wiring will not drive the transmitter into limiting before the mic itself clips. The downside is that input noise levels will come up by the same amounts which might be a problem in very quiet environments.
The standard B6 is spec'd at a maximum input sound level of 118 dB. We measured gentle clipping at 114 dB on the high current mic and 117 dB on the lower current mic. These measurements are probably not as precise as Countryman's since they were made at higher voltage and current levels (more gain) but still are certainly comparable to Countryman's spec sheet. Considering that these are higher current mics than other electrets and at clipping, the mic is swinging the entire 500 uA bias supply, the std B6's are hot mics indeed. The gain reduction wiring above does nothing as far as increasing the sound level limit of the microphone itself to more than 118 dB spl. Therefore, the lower gain B6 (-10dB) version may be a good spare mic choice, certainly for loud situations since it would handle 128 dB Spl.
The long and short of it is that Carl wishes wireless mic manufacturers would standardize the input circuits and if not that, then at least not change the inputs willy-nilly. I agree with Carl and certainly we are guilty of changing the input values when we went from the 200 series to the 400 series. I would add that it would be great for the wireless manufacturers, if the all various mics had similar output levels and similar bias currents. What makes it tough, is that the bias currents between manufacturers vary by 15 to 1, the output levels by 25 dB or more and recommended loads from 1k to 20k.
Carl made a very interesting proposal which was to just provide a bias voltage (say 5 Volts or 3 Volts), a DC blocked audio input and a ground and let the mic manufacturers recommend the resistor values for the drain and/or source loads and build them into the mic connector. As Carl pointed out there is lots of room inside a Switchcraft TA5F connector.
We had available two B6 microphones and I assume (that word again) they are the standard 6 mV/Pa units since the overload point that we found was close to the specified values. We wired the two mics with a 10k resistor from tip to sleeve of the MM400 connector, which is the wiring recommended by Countryman. We found that the B6's overloaded at between 114 and 117 dB spl and that value is essentially the same as their spec sheet value of 118 dB spl. Our sound pressure arrangement was more practical than exact. Since the microphones swing their entire bias supply of 500 uA at the 114 to 118 dB spl input, they are fairly hot mics. This 10k wiring will develop 1 Volt pk at the normal input of a MM400a. The maximum level that the MM400a will handle before limiting is 0.42 Volts peak. I want to emphasize that this is limiting and not clipping. The signal will still be clean just compressed. So a B6 at maximum signal swing can drive the MM400a into 7 dB of compression. However the B6 itself will start to clip before the transmitter audio circuits begin to clip.
To reduce the gain of the standard B6 4 dB, put a 4k resistor in place of the 10k across the B6 leads in the connector, i.e., from the center pin of the MM400 connector to ground. Three dB is due to a reduced load and a dB or so due to reduced current. This will get the max output of the B6 close to the max non limiting input of the MM400a. If this is still too hot and you are uncomfortable with running the MM400a at minimum gain in loud situations, then I recommend the reduced gain B6 which is down 10 dB from the standard unit.
See also FAQ (What is the correct wiring for different versions of the Sanken COS-11 microphone and various Lectro transmitters?) for wiring a COS-11 for more normal sound levels than the 114 dB SPL assumed in this FAQ( How should I wire a Sanken COS-11 or other three wire microphone to a MM400 transmitter?.)
We recommend a 2k to 4k source resistor. Wire the white lead to the resistor and the other end of the resistor to ground. The black drain lead is wired to the center pin. We do not recommend just wiring the source lead (white) to ground. Below is a long post to the RAMPS group that explains why:
After my post about the COS-11 wiring, I was asked by a dealer if the lower output COS-11 red dot wasn't a simpler solution than building a resistor into the connector to simulate a three wire hookup on a MM400, which is a two wire system. So we went back and made more measurements. Lectro isn't set up to make precision sound level measurements but we faked it fairly well. I used some B&W 602's (7" Kevlar cone) to produce a surprisingly low distortion 400 Hz audio signal (less than 0.3%) and a Radio Shack sound level meter for level measurements. The Radio Shack was calibrated to a B&K sound level meter and is actually pretty decent. As further proof of the pudding, some lavaliere mics that were spec'ed as overloading at 118 dB, indeed did overload at those levels. We placed the mic about 3 inches from the loudspeaker cone so we could easily get levels of 125 dB at the microphone.
With 114 dB into the mic and using a two wire mic setup (signal taken from the black wire or drain), we set a mixer level of 0 dBm for a COS-11 grey dot(?) with a shorted-to-ground source, and measured -8.8 dBm for the same mic with a 2.2k source resistor. A COS-11 red dot measured -9.1 dBm shorted to ground and -18 dBm with a 2.2k resistor. Distortion levels in the same sequence were grey dot shorted 9%, grey dot with 2.2k 1%, red dot shorted 2%, and red dot with 2.2k 0.44%. These results are pretty much what would be expected; the source resistor reduces gain and provides negative feedback to the FET, both of which reduce the distortion. In addition, the red dot is a lower gain mic and therefore has less distortion at high sound pressure levels. What is most interesting here is that a grey dot with a source resistor has less distortion than the red dot shorted at the same sound pressure level and at the same output level (gain).
We now increased the sound pressure level to 124 dB and ran the same sequence. Distortion levels were grey dot shorted (unusable)%, grey dot with 2.2k is 1.75%, red dot shorted is 8%, and red dot with 2.2k, is 0.7%. Again, these results are pretty much what would be expected; again the source resistor reduces gain and provides negative feedback to the FET, both of which reduce the distortion. Again,the red dot is a lower gain mic and therefore has less distortion at high sound pressure levels. This is a repeat of the previous findings and once again a grey dot with a source resistor has less distortion than the red dot shorted at the same sound pressure level and at the same output level (gain).
These numbers jive fairly well with info from Sanken's site. So my recommended hookup for a MM400 is to use a 2k or 4k source resistor with either of the Sanken microphones. The MM400a transmitter will be driven about 6 dB into compression when set for minimum gain before the microphone starts to clip and this will be at sound pressure levels of 120 dB and 130 dB respectively. Without the source resistor, the levels will be more than 10 dB lower. For the UM200 or UM400 transmitters, always wire the microphones as three wire microphones which is shield to pin 1, black to pin 2, white to pin 3, and pin 4 grounded back to pin 1.
We make an adapter that converts the UH400 XLR input into a TA5M 5 pin equivalent for 2 or 3 wire lavaliere use. The model number is a MCA5X. You will need to set the UH400 for 5 Volt phantom power but it does have Zener protection if you forget and leave the UH400 on the 48 Volt setting. It's in this catalog:
You can also do the following, but you MUST have the UH400 set for 5 Volts, not 18 Volts or 48 Volts:
For a 2 wire lavaliere, pin 1 of the XLR is ground and pin 2 is bias (audio). Pin 3 not used for a lavaliere.
If you have a three wire mic like the Cos-11 it is a little more complicated but not bad. Hook shield to pin 1, a 2 k to 3 k resistor between pin 1 and the Cos-11 white wire (source load) and hook the remaining black wire to pin 2 of the XLR. This will give you a low distortion hookup comparable in gain to the UM series of transmitters. We strongly recommend putting a small 6.8 to 9 Volt Zener diode across pin 1 and pin 2 for voltage protection in case you misset the voltage. The cathode goes to pin 2 and the anode to pin 1.
The MCA5X is certainly an easier solution.