The BM800 Condenser Microphone
Conversion for Amateur Radio Use
These very well made all metal cased microphones are commonly available on the Internet in the UK for under £15 including delivery and come complete with a shock mount, foam wind shield and a connecting lead. They are available in many colours and there is plenty of room available inside the cylindrical case to carry out the modifications. As they stand they are really not suitable for Amateur Radio use for a number of reasons but with the following modifications they make a very good desk or boom microphone for the shack with enough output to drive any transceiver.
The problems which need to be overcome are as follows.........
1) The specification states that a "phantom" 48vdc power supply is required.
2) The microphone provides a balanced output which is not required for our purpose.
3) The frequency response range is far to much for Amateur Radio use at both the HF and LF audio ranges.
4) The audio output is not enough for some rigs.
All of these problems have been addressed and very conveniently some of these microphones come with not only the populated pcb but also a completely un-populated one behind it which I have used to build in the new circuitry. If your's does not have this spare board then one can be built to fit or you can use "dead bug" construction, hum pick up is not a problem due to the microphone's all metal construction.
Summary of Modifications
1) Separate microphone dc supply from a 9v PP3 battery or 5 to 12vdc supply from the rigs microphone socket.
2) Microphone audio output is changed to "unbalanced"
3) Low and High pass filters fitted to address the wide frequency response issue.
4) The splitter stage (Q5) changed to a pre-amplifier stage enabling up to 500mV P to P, variable output.
5) Fitting the wind shield inside the microphone cavity which makes its profile much neater.
There is another microphone available called a BM700 which employs the same circuitry but the top of the unit is "wedge" shaped but all of the mods shown here are equally applicable.
The pictures below show the microphone with its accessories as received from the supplier and three of the microphones themselves, note that the white one on the right is a BM700 with the wedge shaped top.
The following pictures show the populated and un-populated boards with the microphone lower casing removed.
This is what the un-populated board looks like after mods. The pot at the top (VR1) adjusts the output level from 0 to 500mV p to p.
This is a close up of the condenser element. The wind shield can be cut down and fitted over this alleviating the need to fit one externally.
This frequency response graph is supplied with the microphone and shows how level it's response is over a very wide range. The RED line which I have added shows the response curve of the newly fitted low and high pass filters.
This is the circuit employed in the BM800 and BM700 microphones before any modifications, there may be small differences in circuit values on some units but they are of no significance. The FET Q3 is required as the condenser element is very high impedance, Q5 is a splitter transistor which feeds the balanced output transistors Q1 and Q2, Q6 is the 9v series regulator transistor which feeds Q3 and Q5.
Circuit courtesy of Henry Spragens.
Circuit courtesy of Henry Spragens.
Original Block diagram
New Block Diagram
Q3 is the high impedance input matching FET, Q5 is the pre-amplifier transistor, Q6 is the 9v series regulator transistor, F1 is the low pass filter and F2 is the high pass filter.
Q3 is the high impedance input matching FET, Q5 is the pre-amplifier transistor, Q6 is the 9v series regulator transistor, F1 is the low pass filter and F2 is the high pass filter.
New Modiied Circuit
The low pass filter comprises R22,C22 and C23 and the high pass filter C25 and VR1. NB, if additional low frequency response is required the value of C25 can be increased to 0.47mfd and if additional high frequency response is needed C22 and C23 can be decreased to 0.022mfd.
The low pass filter comprises R22,C22 and C23 and the high pass filter C25 and VR1. NB, if additional low frequency response is required the value of C25 can be increased to 0.47mfd and if additional high frequency response is needed C22 and C23 can be decreased to 0.022mfd.
Parts List (Additional)
These parts are in addition to those already present on the original main pcb
R22 1K ohm, 1/4w
R24 470 ohm, 1/4w
VR1 2K ohm, min pre-set pot
C20 2.2 mfd, Electrolytic/Tant
C22 0.1mfd, Poly or Mylar (see text)
C23 0.1mfd, Poly or Mylar (see text)
C24 470pF, Disc Cer
C25 0.2mfd, Poly or Mylar (see text)
C26 470pF, Disc Cer
C27 470pF, Disc Cer
Below is the parts locations on the main pcb
These parts are in addition to those already present on the original main pcb
R22 1K ohm, 1/4w
R24 470 ohm, 1/4w
VR1 2K ohm, min pre-set pot
C20 2.2 mfd, Electrolytic/Tant
C22 0.1mfd, Poly or Mylar (see text)
C23 0.1mfd, Poly or Mylar (see text)
C24 470pF, Disc Cer
C25 0.2mfd, Poly or Mylar (see text)
C26 470pF, Disc Cer
C27 470pF, Disc Cer
Below is the parts locations on the main pcb
The new circuit and parts list are shown above, the circuit only draws around 2mA at 9v so if you are using a PP3 battery it will have a very long life.
Follow the steps below to make the changes....................
1) Remove the connecting leads from the XLR plug to the pcb except the earth lead which is pin no 1.
2) Remove C3 and C4
3) Fit a 2.2 mfd Electrolytic or Tantalum capacitor across R4 ( Note polarity, NEG to Q5 emitter)
4) Fit the new LPF/HPF sub pcb consisting of R22 and 24, VR1, C22,23,24,25,26 and 27.
A suitable connecting lead is now required with an XLR socket on one end to fit to the microphone and whatever plug is required on the other to match your transceiver. The lead which is supplied with the microphone is no use at all as it is only a single cored screened lead and we need two cores plus screen however the XLR socket on this lead is re-useable for this purpose. I use a foot switch for TX/RX switching and for it's connection I use a standard 1/4" "free" mono jack socket which is connected to the TX mic plug via a short lead.
Setting up the gain of the microphone with VR1 is simple, remember that this mic will now provide about 500mV peak to peak with the pre-set at maximum and some rigs will not need this much audio. Using the transceivers supplied hand microphone set the mic gain to the correct position for normal operation then remove it leaving the mic gain where it has been set and connect the BM800 making sure that VR1 is set for minimum output, slowly advance the pre-set pot for NO MORE than the same output as the hand mic when talking in your usual voice about 4" to 6" from the BM800, this pot need never be touched again, the lower half of the shell can now be re-fitted.
Follow the steps below to make the changes....................
1) Remove the connecting leads from the XLR plug to the pcb except the earth lead which is pin no 1.
2) Remove C3 and C4
3) Fit a 2.2 mfd Electrolytic or Tantalum capacitor across R4 ( Note polarity, NEG to Q5 emitter)
4) Fit the new LPF/HPF sub pcb consisting of R22 and 24, VR1, C22,23,24,25,26 and 27.
A suitable connecting lead is now required with an XLR socket on one end to fit to the microphone and whatever plug is required on the other to match your transceiver. The lead which is supplied with the microphone is no use at all as it is only a single cored screened lead and we need two cores plus screen however the XLR socket on this lead is re-useable for this purpose. I use a foot switch for TX/RX switching and for it's connection I use a standard 1/4" "free" mono jack socket which is connected to the TX mic plug via a short lead.
Setting up the gain of the microphone with VR1 is simple, remember that this mic will now provide about 500mV peak to peak with the pre-set at maximum and some rigs will not need this much audio. Using the transceivers supplied hand microphone set the mic gain to the correct position for normal operation then remove it leaving the mic gain where it has been set and connect the BM800 making sure that VR1 is set for minimum output, slowly advance the pre-set pot for NO MORE than the same output as the hand mic when talking in your usual voice about 4" to 6" from the BM800, this pot need never be touched again, the lower half of the shell can now be re-fitted.
