Isolation Transformer for Homemade e-call

R

rangerdavey

New member
I am building a homemade e-caller and am hooking up the Nady 351 and Radio Shack Amp up to the same power source. I am getting feedback when I sure the same power source. I read on this forum that if you install an Isolation Transformer it will fix the problem. I cannot find the wiring diagram or explanation on how to install it. I sure would appreciate help with this!

Thank You!
 
Just wire the transformer inline with the power source.

red to battery positive
yellow to Positive leads on the amp
black to battery negative
white to negative leads on the amp.

Hook the resiever to the battery as normal

there are some internal pots to adjust the gain you can adjust. there is some good info here.

More Info
 
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may I suggest a slightly different approach?

If you are going to use the Radio Shack Audio Isolation X-former ,
then try experimenting and wiring it as follows:

Red to Battery + , Yellow to Either Amp or RX (but not both)
Blk to Battery +, White to Either Amp or RX (but not Both)

using this schematic from radio shack:
http://support.radioshack.com/support_supplies/doc9/9885.htm

the reason, is that with the connection mentioned previosly, you are introducing a voltage drop in the return ("ground") side, and sometimes that will cause a ground loop, whcih can cause feedback problems. OTOH, it may be you are getting good common mode rejection /ubbthreads/images/graemlins/smile.gif I Dunnno.

the connections and use of the "isolation transformer" are actually being connected, and electrically operated as a series choke, in both the + and - battery supply lines.

you might also check the ohmic wire resistance of that xformer. It is listed as 60 Ohms on the secondary. if the primary is the same ohmic resitance ( possibly, since it is 1:1 for AC) then the total series resistance could be as much as 120 ohms.
that would be introducing a voltage drop, reducing voltage to your setup.

other possible replacments for the x-fromer would be a low resistance choke and filter cap on each power line to the E-Caller units (AMP, RX)
and heck, even a small value of series resistance, say 3.3 to 10 Ohms.
the choke (inductor) value would be approx: .15H or 150mh, to achieve the same 600 ohm impedance at 300hz. with the iso-xformer's fairly high ohmic resistance though, it may be doing more of the "isolation work!! /ubbthreads/images/graemlins/smile.gif

the most rejection can be had by using a 3 terminal regulator to one of the units (the RX) it can provide up to 70dB of rejection to noise/ripple, blah, blah, but your RX will have to bale to work at the lower, regulated voltage.

best regards,
bryan
 
Dang Timbertoes! You make that seem a bit too easy /ubbthreads/images/graemlins/laugh.gif


Happy to report I've never gotten feedback with my set-up which also uses one 8 AA NiMH cell pack driving both my Nady 151 Rx and the Hobbytron/Velleman 7W mono amp (as described in Wireless Remote E-Caller tutorial linked off the PM tech pages).

I've since noted that most of the posts about this problem have used the Nady 351 and another amp than the 7W Velleman driven off the same battery pack. Thinking I might soon spring for another Nady (the 351VR-LT)but thought to ask a couple of you guys who've experienced feedback a couple of questions before I do: Have you narrowed the problem down to whether it's a problem common to certain combos of amps and receivers and/or RF channels? Were the receivers brand-new units or older versions? Is there still a net weight and size reduction after you have to go with an isolation transformer? (With the 151 vs 351 the transmitter is the same WLT-15 unit, it's only the Rx is different and smaller, right?)

Maybe one of you fellers can tell me, while you're at it, does an isolation transformer (or what else to put in-line?) eliminate the "pop" I've always gotten I turn my MP3 player on and off, with my transmitter set to anything but mute? Somebody addressed this in a thread awhile ago but I've lost it. Only lately has this "pop" been bugging me.)

LionHo
 
LOL, it is never as easy as it seems. I bet Elkhunter will testify to that. We have Electrical Engineers here, who dont experiment (and learn) as much as you guys do!!! LOL.

Pops in audio are usually a problem thats "designed in"
hehe.

not on purpose, of course but it does happen.

Lots? of Audio gear uses some sort of timed cut out of speakers connections at turn on to eliminate those annoyances.

it is usally caused by the series coupling capacitors in the audio amp paths charging and the transistors biasing up, when the unit is first turned on. one thing I can think of is adding a miniature series connected relay, using a RC time constant to delay the closing of the relay. when the relay closes, the audio path to the Power Amp or Speaker is completed. more parts, more hassle. there is 100% chance of a better way...... but no, no,no, I am not talking about a Hand call.....
/ubbthreads/images/graemlins/grinning-smiley-006.gif /ubbthreads/images/graemlins/grinning-smiley-006.gif /ubbthreads/images/graemlins/grinning-smiley-006.gif
 
Yeah, handcalls. Well, been there done that, too; for about 15 years I didn't use much of anything besides a handcall. Took me awhile but I finally figured out that I don't want to

a). be busted by
b). get eaten by
c). become the unwilling sexual partner of

the mountain lion that I'm trying to photograph using and fawn bleats, juvie ML chirps and female-ML-in-heat vocalizations. Even though I can do all these by handcalls or by voice, I've chosen to record them.

LionHo
 
You are correct, thanks for correcting my mistake. A little cranial gas I guess. I hook it to the amp only. It's not the what the isolation transformer is for but it's better than anything else I could come up with to eliminate the feedback. Filter caps did not help and I tried a lm7809 regulator with no sucess as well.

Adjusting the internal gain on the reciever and transmitter will do as much as anything to clean up the sound too.
 
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Lion-ho,

the isolation transformer is 1/2" x 1/2' x 3/8" so there is not a weight issue there. The 351, or at least the two that I have used do not "pop" like the 151. It seems to be built in like he said. Like I posted above, there are 2 pots inside each of the reciever and tramsmitter and by playing with those two, you can get the sound almost crystal clear. Mine will actually out perform my comercial unit in the sound quality department.

I tried the wellman but never was happy with the sound I got out. I tune the volume on my reciever and amp so I can run my rio up full blast and still get good sound quality. That way I can't over-amp it. I get the best results with the reciever on about 3/4 volume and the amp at about 1/4 volume. I tried adding an audio pot to the wellman but still did not get the results I wanted. I did not ever use it with the 151 though, only the 351

There is a good bit of differences between devices of the same design. They never seem to act the same way twice. I guess that "accurate within 5%" catches up with you eventually. I built one for a buddy with a 151 and nothing I did would ever get rid of the static/feedback. I ended up not connecting the negative on the output from the reciever to the amp. funny but with only the positive hooked up it was really clear. adding the negative messed it up.
 
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