Desk Microphone for use with the IC-7300
This desk microphone has been designed specifically for the Icom IC-7300 transceiver. The supplied hand microphone performs very well indeed but because some people speak too closely into it the "plosive" effects can be excessive, many prefer to use a desk microphone with a foot switch arrangement as I do. Unfortunately the problem with many desk microphones made for this purpose is that they are incredibly expensive and many even suffer from RF pick-up and produce a rather poor HF frequency response (2 to 3Kc/s where we need it for amateur radio use) and generally too much LF gain, not good!
The complete "shell" of this microphone is the very popular and cheaply available BM800 from China, it is of all metal construction and has plenty of room inside for modification for this purpose. All of the internal components are removed including the printed boards and the original condenser insert which due to it's large diameter was found to be lacking in the range of frequencies we are interested in here.
The new microphone element itself can be a standard small electret insert which contains it's own FET which is used for impedance conversion but I found a rather better one made by Hosiden which measures 9x4mm and can be found on e bay etc.
The DC and AF path's have been split in order to provide the purest transfer of audio from the insert to the transceiver and of course allowing the frequency response to be modified to suit the rig.
The purpose of each of the components has been described in the parts list and those familiar with audio circuits will be able to follow the arrangement.
Most operators have found that when using the stock microphone a treble boost of +3 to +4 is needed together with a compression level of about 4 or 5 on the rig itself which equates to a boost of about +15 to +25dB at 2.6Kc/s. The treble boost in this microphone arrangement provides an analogue boost at 2.6Kc/s of about 20dB with a rapid roll off above 3.5Kc/s. The Boost/LPF circuit consists of L1/C6/C7, the additional capacitors C6a/C7a are selected on test to provide a peak centred on 2.6Kc/s. It should be noted that the boost is still about +10dB at 2 and 3Kc/s because of the smooth rise and fall of the response curve (see graph) meaning that it is effective in all three filter positions on the IC-7300. An audio signal generator and millivoltmeter or oscilloscope are needed to set this centre frequency up. With this arrangement a treble boost on the rig of +3 Maximum was found to be ideal.
The rig's bass level is set to "0" and the pre-set pot VR1 adjusted to the required level of LF response with regard to the distance you speak from the microphone which should be about 15cms (6"). Because this desk microphone arrangement has no additional pre-amplification it will be found that the microphone gain will need to be increased substantially from around 35% for the hand mic to about 80% for this one due to not "close talking" as you would tend to do with the supplied microphone, this is not a problem and is to be expected.
The pre-set pot VR2 would normally be set to minimum resistance to provide the full 20dB of boost but it has been included here so that the level can be reduced to about 10dB for use with other transceivers.
I personally have never suffered with RF pick up in the shack as my aerials are well away from my operating position but I know quite a few people do suffer from this even with some very expensive commercially made desk microphones due to their poor design. The capacitors C2/C3/C8/C9 will effectively remove any possibility of this throughout the whole frequency range of this transceiver and up to 144Mc/s if transverting.
The microphone should be used "Forward Facing" that is talking in to the end of the mic at a distance of about 6 inches as the insert is mounted "side on" in order to prevent breath blast, see pics.
The original BM800 microphone is supplied with a large foam shield which is intended to sit over the top of the microphone mesh, this foam is modified by cutting out about a quarter of it, see pic, and this is then fitted "inside" the microphone mesh cage and the new mic insert and it's mounting bracket are simply pushed in the middle of this making a much neater yet effective arrangement.
The "SHELL" of the original BM800 microphone which is available in various colours.
Original PCB and insert removed
The new board showing the pot cored inductor L1 and the two pre-set pots VR1 and VR2
The new electret insert front and rear glued into position with hot melt
Hosiden Electret Insert
Cut out and discard about a quarter of the windshield foam with scissors, leave the top part of the foam in tact as this is where the insert will be.
Fit the cut down foam windshield inside the mesh microphone head
The blue line on the graph below shows the analogue 20dB boost centred on 2.6Kc/s and the rapid roll off over 3.5Kc/s. The red line is the response of the electret insert.
Circuit Diagram
Parts List
R1 10K 1/4 watt (AF Isolation)
R2 10K 1/4 watt (Drain load for FET within the electret insert)
R3 68K 1/4 watt (can be left out if not needed)
VR1 20K Lin pre-set (Bass level adjustment)
VR2 5K Lin pre-set (2.6Kc/s Treble boost level adjustment, approx 10 to 20dB)
C1 10mfd Electrolytic, 25v wkg. (AF de-coupling)
C2 0.01mfd Disc ceramic (HF de-coupling)
C3 470pF Disc ceramic (VHF de-coupling)
C4 0.033mfd Mylar/Poly (Treble coupling)
C5 1mfd Non Polarised (Bass coupling) (can be two 2mfd tantalum capacitors back to back)
C6 0.047mfd Mylar/Poly
C6a Select on test for peak at 2.6Kc/s
C7 0.047mfd Mylar/Poly
C7a Select on test for peak at 2.6Kc/s
C8 100pF Disc ceramic (144Mc/s de-coupling)
C9 4700pF Disc ceramic (HF de-coupling)
D1 Red LED (can be left out if not needed)
M1 Hosiden 9x4mm Electret Insert
L1 100mH Pot cored or fixed inductor
SKT1 XLR connector (original at the base of the BM800 microphone)
BM800 Microphone shell, available in various colours.
VR1 is adjusted to provide the required level of "Bass" response (Rig Bass level set to "0")
VR2 is normally set to minimum resistance but can be adjusted to reduce the Treble boost by about 10dB with resistance at maximum when using the microphone with other rig's.
NB, L1 may prove troublesome to obtain although fixed inductors are available on the internet and other arrangements can be used here including iron cored inductors but the "Q" needs to be high in order to obtain the 20dB peak at 2.6Kc/s.
A pcb can be purpose made or just use an experimenters board as I have, the component layout is unimportant, see pictures.
This microphone arrangement can of course be used on any transceiver with a DC supply of 5 to 15v on the microphone audio line but for rig's that do not it is simply a case of feeding this DC separately by disconnecting R1 from the audio line at "X" and feeding the supply here.
Because of the low pass filter roll off above 3.5Kc/s this arrangement is also Ideal for AM operation providing crisp audio where it's needed but effectively removing the unwanted higher audio frequencies which of course the electret insert is capable of.
A simplified version of this arrangement can be made without the "HF boost/low pass filter" circuit but retaining the control over the bass response by fitting a link in place of L1 and omitting the components C6,C6a,C7,C7a,C9,L1 and VR2, the rest of the circuit remains the same.
