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Re: Radio for Radio
To: CRTech <crtech@crtech.org>
Subject: Re: Radio for Radio
From: Willie Barnett <wbradiolists@gmail.com>
Date: Fri, 14 Sep 2018 12:41:18 -0400
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In-reply-to: <165c5176197.b0be18a199763.1590082261514861472@rt66.media>
References: <830893665.249995.1536607774559.JavaMail.zimbra@bevcomm.net> <165c5176197.b0be18a199763.1590082261514861472@rt66.media>
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.

Follow-Ups: Re: Radio for Radio
("vern@wa0rcr.com vern@wa0rcr.com" <vern@wa0rcr.com>, 15 Sep 2018 02:53:16 -0000)
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)
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