Re: [stationpro] Hum problem on StationPro II
- Paul,I will open it up and get back with my findings.The XLR going into the PR781 is a 3 pin so there is bound to be common ties. This is a standard Heil cable.73NJ0F
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On Mar 22, 2013, at 8:58 PM, "Paul Christensen" <w9ac@...> wrote:Andre,I am interested in the wiring configuration between your PR781 XLR and the 8-pin plug. If you can, please report back with your PR781 wiring configuration that describes the number of mic cable conductors and the wiring connections between the XLR and 8-pin connector -- as well as any common ties between the cable shield and any other conductor(s). A quick hand-drawn diagram would be even better!Paul, W9AC----- Original Message -----From: Andre VanWykSent: Friday, March 22, 2013 9:11 PMSubject: Re: [stationpro] Hum problem on StationPro IIPaul,Thanks for the info.I sat messing with the unit this afternoon and found the bad apple. As soon as I unscrew my Heil PR-781 from the station pro, the hum is gone. If I push the metal ring a little away form the unit, while still having the mic plugged in, the hum stays away.As soon as the metal part of the 8 pin mic plug touch the unit, the hum is back. I will open the plug tomorrow to investigate.73'sAndre NJ0F> 5.Double-check the pod wiring (especially the mic wiring) for your transceivers. Icoms are a bit tricky, as I recall, because they usually supply +V (roughly 5V) on the MIC+ line. Be careful where you ground the MIc- line, if you don't keep it floating.Here's one potential solution. It's often used in cases of last resort when interconnecting unbalanced equipment. At the destination end of the cabIe, a coupling cap (typically 0.1 uf) is used from the audio shield to the ground connection or Mic (-) line. You can try it both ways with the (-) floating or connected to the equipment ground point. The idea is to break-up the ground loop caused by the simultaneous connection between the two shield ends of the audio cable and any other secondary ground pathway between equipment.Again, this is a solution of last resort. Well, maybe second to last (see below). Probably the best solution is to seek out the root cause and try to manage it. Don't be too concerned at first about the size of the coupling cap. If you find that it reduces or eliminates hum/buzz, then you can optimize its value. I can provide you with a simple formula that allows for a -3dB roll-off at say...100 Hz. Increasing the cap size beyond the desired -3dB point only serves to increase the potential for low frequency hum. For a mic circuit, there's no need to go below a 100Hz turnover point.I think someone mentioned a commercial audio isolation transformer. That's the easy but expensive way to the solution. However, adding an audio isolation transformer at the transceiver mic input does have the advantage that any RFI is almost sure to never become a problem. The better audio transformers have extremely high common mode rejection ratios (CMRR). But, if you have multiple rigs, adding a transformer for each can become very costly. So, one can experiment by moving the transformer back to the SPII. It's not ideal as the effect of transformer's CMRR is best realized near the input to the mic amp.You may be tempted to use one of the blister-packed transformers sold by Radio Shack. It may be worth trying one just to see if any improvement is possible. But don't make it permanent. These transformers are inexpensive but even when the secondary is terminated into its optimum load impedance, the transient response and resulting distortion products are horrible. I once evaluated the two types of RS transformers with a square-wave generator set to 1 kHz. The result is anything but a square wave on the output. Yes, we don't speak in square waves but the square wave test is an excellent way to test the transient performance of an audio transformer. It can be argued that in a bandwidth-limited system (e.g., 100 Hz - 3kHz), the transient response isn't too important. I generally avoid those arguments. By contrast, a Jensen audio transformer, even when improperly terminated will produce a near-perfect square wave. But that performance comes at a very high cost -- about USD $80 per transformer. If you get to the point of wanting to try a transformer, I recommend either Jensen and CineMag. These can be purchased with or without enclosures.Paul, W9AC
- I am probably way off base here but I wonder if it has something to do with the Heil mics DC blocking cap to remove the IOCM phantom mic power provided on all modern ICOM radios? I also would be interested knowing how you are wiring the mic to the SPII.