Hi All,
I am a new member and have found awesome discussions on dummy head microphones and HRTFs here on this forum. I have built a dummy head which I want to use for live video broadcasts of a local orchestra here in the Reno/Tahoe area. I had built one almost 50 years ago, when I was still in high school in Germany. I posted it to member Andysu:
I bought two matched EM272 microphones and tested the dummy head against two dynamic mics, and as I have read here on the forum, there is a emphasis in the 3-4 kHz range.
Above: Frequency response of the dummy head with EM272 relative to a pair of XML840s. I used a white noise recording played back through my 'calibrated' Denon and some awesome 'antique' Celestion SL6s with copper dome tweeters that I bought over the Internet from England. Later I got another pair with aluminum dome tweeters for surround sound. Back in Germany I had a pair of Celestion UL6s with a Monacor subwoofer and the sound was awesome. When I moved to the US in 1985, I left them with my high school buddy. The peak in the above diagram at 4 kHz is about 20 dB (!) above the XML840s. One has to consider that the level of the XLM840 (black) is about 8 dB higher than the dummy head (green).
So I looked at the surface mount circuit that is inside the XLR connectors of the EM272s and although I have good experience in electronics, I can't quite make sense of the circuit. One would assume that it would come with an FET circuit to better match the impedance for the XLR inputs of an external recording device. There is an IC with a stamp '5Ft' on it which I tried to find looking through various databases but with no luck. The only thing that would make sense is that it is a sort of voltage regulator, since the electret mics need to biased (correct?).
I measured across the diode and the reverse voltage starts at around 0.5 V and slowly, very slowly (over several minutes) goes up to 3.7 V or so, then at some point it is back to 0.5 V! But the sound is not affected by this. Very odd. (I also measured the forward voltage drop across the diode with my Fluke meter when the circuit was not powered, it was around 0.6V).
I am not sure if anybody can help here. My plan is to make a new surface mount PCB of the same size with an opamp that has a notch filter to eliminate the emphasis around the 3-4 kHz range. I would unsolder the original components from the Clippy PCB and reuse them in the new one. Alternatively, maybe as a first step, I would need to build a box with an external power supply that would take the EM272 signal and add the opamp with the notch filter, equipped with XLR jacks.
Regards, Stephan
I am a new member and have found awesome discussions on dummy head microphones and HRTFs here on this forum. I have built a dummy head which I want to use for live video broadcasts of a local orchestra here in the Reno/Tahoe area. I had built one almost 50 years ago, when I was still in high school in Germany. I posted it to member Andysu:
I bought two matched EM272 microphones and tested the dummy head against two dynamic mics, and as I have read here on the forum, there is a emphasis in the 3-4 kHz range.
Above: Frequency response of the dummy head with EM272 relative to a pair of XML840s. I used a white noise recording played back through my 'calibrated' Denon and some awesome 'antique' Celestion SL6s with copper dome tweeters that I bought over the Internet from England. Later I got another pair with aluminum dome tweeters for surround sound. Back in Germany I had a pair of Celestion UL6s with a Monacor subwoofer and the sound was awesome. When I moved to the US in 1985, I left them with my high school buddy. The peak in the above diagram at 4 kHz is about 20 dB (!) above the XML840s. One has to consider that the level of the XLM840 (black) is about 8 dB higher than the dummy head (green).
So I looked at the surface mount circuit that is inside the XLR connectors of the EM272s and although I have good experience in electronics, I can't quite make sense of the circuit. One would assume that it would come with an FET circuit to better match the impedance for the XLR inputs of an external recording device. There is an IC with a stamp '5Ft' on it which I tried to find looking through various databases but with no luck. The only thing that would make sense is that it is a sort of voltage regulator, since the electret mics need to biased (correct?).
I measured across the diode and the reverse voltage starts at around 0.5 V and slowly, very slowly (over several minutes) goes up to 3.7 V or so, then at some point it is back to 0.5 V! But the sound is not affected by this. Very odd. (I also measured the forward voltage drop across the diode with my Fluke meter when the circuit was not powered, it was around 0.6V).
I am not sure if anybody can help here. My plan is to make a new surface mount PCB of the same size with an opamp that has a notch filter to eliminate the emphasis around the 3-4 kHz range. I would unsolder the original components from the Clippy PCB and reuse them in the new one. Alternatively, maybe as a first step, I would need to build a box with an external power supply that would take the EM272 signal and add the opamp with the notch filter, equipped with XLR jacks.
Regards, Stephan