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Re: Radio for Radio
To: Willie Barnett <wbradiolists@gmail.com>, CRTech <crtech@crtech.org>
Subject: Re: Radio for Radio
From: "vern@wa0rcr.com vern@wa0rcr.com" <vern@wa0rcr.com>
Date: Fri, 14 Sep 2018 21:53:06 -0500 (CDT)
In-reply-to: <CAOkhgWrLrozzDVPE1HhkOjuUs6ba7nZ=VgFO7VuKT8M-9MTC_w@mail.gmail.com>
References: <830893665.249995.1536607774559.JavaMail.zimbra@bevcomm.net> <165c5176197.b0be18a199763.1590082261514861472@rt66.media> <CAOkhgWrLrozzDVPE1HhkOjuUs6ba7nZ=VgFO7VuKT8M-9MTC_w@mail.gmail.com>
Reply-to: "vern@wa0rcr.com vern@wa0rcr.com" <vern@wa0rcr.com>

I set up  a GM digital tune FM car radio to feed one of our FM translators. I put 16 ohm resistors across each radio speaker output

to provide load to the car radio output amps..

I am using a  150 ohm to 600 ohm  audio transformer spec-ed at 20 Hz to 18 KHz audio transformer that I had picked up used many years ago. It is spec-ed  at 5 watts. The 600 ohm output feeds a Henry matchbox Hi Z unbalanced input. The balanced output feeds the XLR balanced input on the Nicom transmitter. Since a loss of power could potentially cause the receiver to revert to 98.1; it is powered by a 12V, 7A  regulated power supply floated across a 12AH  deep cycle battery.  We had a power loss to the site of two days earlier in the summer. The receiver stayed in proper operation the whole time. I do have a very small 16 ohm speaker connected across one car radio output channel, to monitor. The system works great on air; and sounds great. I estimate that we have about 35db of headroom on the received signal through the car receiver. The car radio is set to where I can just hear it on the speaker.

Vern Jackson

On September 14, 2018 at 11:41 AM Willie Barnett <wbradiolists@gmail.com> wrote:

A word of caution... accidentally setting the volume too high while feeding into the 8 ohm secondary of a 70v transformer, could put 70+ volts on the primary! (Remember: MURPHY!!!) Kiss the inputs of whatever it's connected to "goodbye".

A plain 600 ohm 1:1 transformer will work perfectly.

One thing to be aware of, is if using an 8 ohm step-up to 600 or whatever, you will also be stepping up any amplifier NOISE on the speaker outputs... and you'll need to run the volume control very low to compensate for that step-up, so the noise will be very much an issue. (Don't ask me how I know this.) ;)

If the only transformer you have is an 8 ohm to some higher impedance, put a resistor between the radio and the transformer, something like 100 ohms or more, as protection against MURPHY. It will also let you run the volume control a bit higher... then, use a simple resistor voltage divider on the output. That will attenuate the signal to a useful level, while at the same time attenuating the amplifier noise. :)

The UPS is a good idea... unless what you're using is a good old fashioned analog car radio. :)

Willie...


On Mon, Sep 10, 2018 at 4:06 PM, Jim Turvaville <jim@rt66.media> wrote:
My experience with car radios for EAS monitoring as well as translator relay use has been great.  A couple of thoughts:

1.  Many new radios have RCA line outputs designed to be plugged into external power amps, which allows us to not connect speakers at all to the unit. Then a simple matchbox can take the unbalanced signal to +4 studio grande balanced audio. 

2.  In absence of the RCA plugs I have used 70-volt transformers to create a better impedance match.  Take the radio outputs and connect to the 8-ohm side and put a 600-ohm load resistor across the 70-volt side and it will typically load just fine and not have notable audio distortion.  Volume out on the radio itself will determine level on the side of the transformer, so adjust carefully. 

3.  Use a good UPS to hold the radio power supply so it will not go off or in standby mode in case of a power blink.

References: Radio for Radio
(Mark Carol Bohnett <mbohnett43@bevcomm.net>, 10 Sep 2018 19:29:59 -0000)
Re: Radio for Radio
(Jim Turvaville <jim@rt66.media>, 10 Sep 2018 20:06:17 -0000)
Re: Radio for Radio
(Willie Barnett <wbradiolists@gmail.com>, 14 Sep 2018 16:41:25 -0000)
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