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 way to measure input Z is to use a low capacity scope and a signal generator. Set the sig generator to the desire input level and monitor the input signal on the scope. Then place a series resistor with the signal generator output and increase its value until the scope reads 1/2 voltage of what it was before. The resistance of the series resistor is then the same as the load resistance represented by the input circuit. Gary is correct on the high freq attenuation but I don't see where it would drop the audio level much at hollering frequencies. You said a loud noise but is this a whistle or a shout. Perhaps a little more scientific evidence is required here to arrive at a verdict as to whether the cable's 300-400pf cap is guilty or not. HIHI. If the output of the preamp is indeed 600 ohms balanced or unbalanced I would terminate in the amplifier with a resistor at least as low as a few thousand ohms but not below 600 ohms John, WA5BXO -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brett Gazdzinski Sent: Tuesday, January 13, 2004 1:03 PM To: [email protected] Subject: RE: [AMRadio] RE: r390a, sx17, t4xb for sale The mike preamp is a berringer ultra voice, 600 ohm output I thought, its balanced, I unbalance it at the output (berringer says this is ok), and it goes to the input of a 120 watt per channel audio amp, it actually feeds both channels so I have 4 outputs right and left, a and b. I have no documentation on this amp (Nikko), it was a pawn shop special... I have not had a chance to run checks yet, but it sounded like the coax cable was really cutting out the highs. Before replacing it, making loud s sounds a foot from the mike gave 20% modulation on the mod monitor, which I thought was normal because its so high in frequency. After installing a cheap radio shack cable, the s sounds give over 100% modulation!!!! I cant guess why if a cable works to vhf, why it would cut high audio frequencies so much. I suppose it has to be capacitance and inductance, but then how does it work so well at 30 mc? Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of John Coleman > Sent: Tuesday, January 13, 2004 1:03 PM > To: [email protected] > Subject: RE: [AMRadio] RE: r390a, sx17, t4xb for sale > > > Coax cable will have a certain amount of capacitance per ft. which if > terminated to a high Z load in the speech equipment will > result in a loss of > high freq audio. If how ever the line is terminated with the > characteristic > Z of the line then it can be very long. This works ok for very low Z > microphones but not for high Z mics. I build a transistor > preamp with a low > Z output such as an emitter follower into the microphone case > and then run > low Z cable to the speech equipment where it is terminated > with a 50 ohm > resistor. It is then XFMR coupled to the speech equipment circuit to > prevent common mode hum induction. The XFMR coupling may not > be necessary > in all cases. A 600 ohm microphone (that is a microphone > that is designed > to work into a 600 ohm load as many studio mics are) will > generally work > well with 50 ohm coax and a 600 ohm termination provided the > cable isn't a > mile long. > > John, > WA5BXO > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Brett Gazdzinski Sent: Tuesday, January 13, 2004 7:14 AM To: [email protected] Subject: RE: [AMRadio] RE: r390a, sx17, t4xb for sale I made an interesting discovery Sunday. I have had a few reports of my audio not having a lot of highs, even though I boost the mid and upper voice frequencies. There is nothing to limit the highs in any of the rigs, other than the mod iron, and even crappy mod iron will typically pass high enough frequencies to sound good, and found the cord between the mike preamp and the power amp was the problem. It was a 15 foot piece of rg8m, the little coax, with phono ends on it. You would think something that works for RF service up into VHF would be able to pass high audio frequencies, but no. I wonder why it was so poor for audio, cutting stuff above about 3000 cycles a lot???? Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Brett Gazdzinski > Sent: Monday, January 12, 2004 11:13 AM > To: [email protected] > Subject: RE: [AMRadio] RE: r390a, sx17, t4xb for sale > > _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio _______________________________________________ AMRadio mailing list [email protected] http://mailman.qth.net/mailman/listinfo/amradio
