Hi John, Crystal and ceramic mikes (piezoelectric mikes) generally act like a combination of a relatively low impedance source (I don't know exactly, a few thousand ohms or so), coming through a series capacitance of roughly 500pF. This tiny internal effective output coupling capacitance is the reason that we need a very high load resistance to maintain the low frequency response.
A 15 foot cable with 30 pF per foot would present a 450 pF capacitance to ground, and this would work with the piezo microphone's effective output capacitance to act as a capacitive voltage divider, which would cut the mike level about in half (-6dB), or about 100% to about 50% modulation, which is about what was observed. The effect of this capacitive voltage division should be uniform across the audio frequency range, but the load resistance causes another effect at the low frequency end.. The total effective series capacitive reactance would be equal to the effective microphone output capacitance and the cable capacitance in parallel, or about 950 pF, so in this example the presence of the 15 foot cable would reduce the mike level by about 6 dB, and the relative low frequency response with any given load resistance would be extended down by about an octave. That can be a dramatic response difference if the low frequency cutoff is in the 300Hz range. Bacon, WA3WDR ----- Original Message ----- From: "John Coleman" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Thursday, January 15, 2004 11:08 AM Subject: RE: [AMRadio] Mike Cable Capacitance One of us, and it very well could have been me, must have missed something. I though Bret, N2DTS, said that the coupling problem (loss of audio level and high frequency response) was the cable from the preamp to a power amp using line level coupling. I was only mentioning that a high Z input, such as an old style XTAL mic input on the power amp would be a typical problem as you mentioned. But I don't think he is using that scenario. I think Bret said that he had a preamp for the mic and that it was the coupling cable from the output of the preamp to a power amp line input that was in question and that replacement with a very short cable increased the audio level and high frequency response. This doesn't seem very normal to me, that changing from a 15 ft to a 1 ft cable at normal line level medium impedances would make that much difference in frequency response and especially a noticeable gain difference form 20% modulation to 100% or more. A friend of mine is noted for placing resistors and capacitors inside a cable connector for the purpose of EQ or attenuation, rather than modifying the equipment. You have to be careful about using just any old cable in his place; it may have a 1 meg resistor in series with the cable built into the connector end. HIHI. John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jim Bromley Sent: Thursday, January 15, 2004 4:06 AM To: AM Radio List Subject: [AMRadio] Mike Cable Capacitance John Coleman, WA5BXO, posted: (Regarding 50-ohm coax attenuating the high audio frequencies when used as a microphone cable). > I feel there must be some other thing that is overlooked > about the connector or cable. 15 ft of cable would not > normally cause any noticeable effect on audio unless the > terminating Z was greater that a few mega ohms as it would > be for some of the older equipment made for Xtal mike input > but I can't imagine a modern day line input being greater > than 100K ohms.... The impedance to be concerned with in this case is not the terminating impedance, but rather the source impedance of the microphone itself. In the case of the D-104, although the impedance of the microphone is specified at a nominal 10K-ohms, its actual source impedance is much higher. It is a well-known fact that the element must be terminated in a resistive impedance of at least 10-Megohms to obtain adequate low-frequency response - indicating a source impedance of at least that amount. Additionally, since there is no DC path through a crystal acoustic transducer, one would be suspicious that whatever the actual source impedance is, it would contain a capacitive reactance as a series component. The best solution to the problem is that advocated by several other posters here - incorporate a preamplifier very close to the microphone element (inches, not feet, away) having an imput impedance in the tens of megohms and capable of driving a 500-ohm termination at unity gain. Jim Bromley, K7JEB Glendale, AZ James E. Bromley Tel: 623-848-8711 5128 N. 69th Ave. E-Mail: [EMAIL PROTECTED] Glendale, Arizona 85303 _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio
