hey now!
time to revive this old dog of a thread as im about to order my lectrosonics SPDR which can power the CMRs directly (same front end as all the transmitters they make, which these are designed for)
so basically CMR specs look like this
lectrosonics front end looks like this
Dynamic mic: 300 Ohm impedance, 0.5 mV to 50 mV input level
Electret lavaliere: Input is virtual ground with servo adjusted constant current bias (very low impedance), Nominal 2 mV to
300 mV input level
Line level: 2.7 k Ohm impedance, 17 mV to 1.7 V input level
lectrosonics recommended wiring to mic-in as follows:
Schoeps shield to pin 1
Pin 4 jumped to pin 1 (shuts off servo bias)
Schoeps blue wire to pin 2 (+5 Volts)
Schoeps white wire to a 1.5k resitor in series with pin 3 (audio in) You can substitute a 500ohm resistor for the 1.5kohm resistor if you are looking for more gain.
i asked bernhard about this he said
"That resistor controls the gain. The smaller you choose it the higher the sensitivity of the input - but this may lead to overload for higher SPLs. We do not recommend to choose the resistor such small, we would use 3k3."
so basically if the resistor is too small it might (brickwall?) the input? in another email Bernhard said
any resistor in line with the mic in will increase the ground noise floor, so we recommend to keep it as low as possible, this means adapted to the maximum sound pressure level under your special recording conditions. We recommend not to choose too much headroom for this reason.
so we dont want to choose too high of a resistor and raise noise floor.
The lectrosonics claims a wide dynamic range (117 dB), but we have no idea how clean the pres are. so we dont want to put a big resisitor in line (noise) and then add a bunch of preamp gain (more noise).
Knowing that our application is the typical concert environment, max 130 dbA with 50-70 dB of dynamic range at best in a room, what would be a good resistor value? Again on mic in:
Dynamic mic: 300 Ohm impedance, 0.5 mV to 50 mV input level
cmr output level into lowZ input: something less than 0.56V (though CMR manual recommends minimum 5K impedance... i am confused
I asked Bernhard:
Lectrosonics provided me more information on the input impedance for the various inputs. i believe the CMR would be considered a dynamic mic. Do the numbers below help with resistor selection? ideally I want the dynamic range of the mic output to be a match for the recorder, in loud situations (80-130 dBA). I notice on the CMR spec it says max voltage of 0.56V at 2kohm, and 0.9V at 20 kohm. it also says minimum recommended input impedance of 5kohm (much higher than any of the transmitter's inputs). so in this case is the resistance additive? in other words if i add a 4.7K resistor to the mic in, is the effective impedance then 5K ohm?
If i went directly into line-in with impedance of 2.7K, it seems the max voltage of CMR would be slightly above 0.56V ( based on the 0.56V@2kohm, but possibly reduced from this as the mic is only getting 5V input).
I am not so concerned if the levels are very low going into the Lectrosonics unit, as the dynamic range of the Lectrosonics should exceed that of the mic by 30-40 dB
the reason i mentioned reduced output due to 5V input is because apparently the CMR can take 4V-10V, and has lower output at lower voltage, though bernhard couldn't quantify the difference between 4V and 10V when asked (i guess based on the widely varying input impedance of different devices). This is as much as he would say:
With a 10V supply it will have a higher output capability but will not reach the CMC's value. 4V are the absolute minimum requirements, the maximum output level is significantly lower already under these conditions.
.. no I did not measure it but following the schematics it is obvious. The precise value also depends on the load impedance. So to reach maximum level you should switch the following input to its highest impedance, if possible.
my battery boxes have always run directly off a 9V in the 7-8.5V range. i have no idea if the 5V provided by the lectrosonics circuit above will compromise performance