Electronics help with ipod listen on vic-1 intercom

[ida34]

Question for any electronics guys out there. I read Bjorn's article in MVM about his adding CB audio to his vic-1 input to listen to the CB on through the intercom. He used an audio impedance transformer to connect the 8 ohm speaker output from the CB to the 600 ohm input of the vic-1. My problem is I am trying to connect my ipod to the vic-1. Ipod headphones have an impedance of 32 ohms. I need part number for a transformer that will convert my 32 ohm output from the ipod to 600 ohm input on the vic-1. If I can't find such an animal I might just connect an FM radio to the vic-1 and use an ipod radio interface. I have a transformer on the way that should give an 8 ohm input and a 600 ohm output and could use a radio with an 8 ohm output but would rather be able to connect the ipod headphone jack directly though the transformer without having an FM radio in the middle.

 

[ida34]

I found out that someone measured the impedance at the headphone jack on the ipod as being 5 ohm. I also found the attached plans for a FET impedance transformer. It looks like the input and output is variable by the values of a couple of the resistors. Anyone see any problem with this circuit? I also did some calculations and if I use a 75:1 ratio tranformer like the 600 ohm to 8 ohm then I would be showing about 375 ohms to the Vic-1. I think this would improve the audio loudness but would it be enough?


This is the webpage that the photo was taken from.
http://part15.us/node/1279

 

[OPCOM]

You can try the circuit. It looks like it provides voltage gain. It appears as though it would. R1 should be changed to 32 ohms and R2 to 76 ohms. Use the standard values of 33 ohms and 75 ohms. For an ipod, R1 need be only a common 1/2 watt or 1/4 watt size.

An issue might arise because the intercom has two terminals, niether grounded. Therefore one would be connected to signal ground (of this circuit) and one would be connected to the output.

If you intend to run from a 9V battery which is isolated from the truck electrical system, fine.

If you intend to take power from the truck and encounter excessive electrical noises, see about finding a 600 ohm to 600 ohm telecom isolation transformer to interpose between the circuit and the intercom to avoid crating a ground loop which will invite noise. You can try it without first, and see if you have a noise problem or not. If you need one, they are $2.00 at the surplus store. Or you can take one from an old dialup modem.

Some might complain that a telecom transformer will degrade the frequency response, but that's not much of an argument when listening through a VIC-1.

Please keep us informed of the progress and results.

 

[cranetruck]

Chuck, try using the "Audio Input" terminals on the VIC-1. It has a 6,000 Ohm input impedance and the signal is amplified within the 1780. I have used it for a CD player.

 

[ida34]

Thanks for the input guys. It is too cold to work on it right now but I am trying to get my ducks in a row before spring. I will report back with the results. I also need to modify a 2298 to power my bose ANR headset.

 

[OPCOM]

I forgot all about the audio input..

 

[Cdub]

Hey Guys,

Has anyone made any progress with using an Ipod with the VIC-1........??

Is it as easy as hooking the ear bud ends to the audio input terminals..........??

Thanks,

C'dub

 

[ida34]

I have made progress, sort of. I got the transformer but have yet to build up the system or make my ANR 2298 box. Too much else to work on. the problem is the impedance is not matched so it is not as easy as connecting ear bud cord to the audio inputs.

 

[gimpyrobb]

Cdub, you might want to try Tbanks here on the site. He sells and repairs the VIC systems, he might know.

 

[cranetruck]

Hey Guys,

Has anyone made any progress with using an Ipod with the VIC-1........??

Is it as easy as hooking the ear bud ends to the audio input terminals..........??

Thanks,

C'dub

Yes, that's what I did that one time, sacrificed an earphone and connected the wires to the "Audio" input terminals.
About the terminals, push down on them to open and insert the wire.

 

[Cdub]

Is the volume too low to hear cuz of the impedance is not matched ...........??

Thanks,

C'dub

 

[ida34]

Yes, but some have said the impedance input on the audio input side is more matched for the Ipod out put.

 

[Cdub]

Well I used a set of ear buds and hooked them up to the Audio input sides of the AM-1780. I can hear the music but the volume is much too low. Is there some easy way to get the Volume higher..........??

Thanks,

C'dub

 

[Boatcarpenter]

Has anyone figured this out so the volume from an IPOD is acceptable through the VIC-1?
Or how about directly from a laptop with a full music library?
BC

 

[ida34]

I have the transformer but do not know exactly where I laid it. When I find it I will put it together in a box to that I can plug the ipod into then plug it into the AM-1780.

 

[Greggor K]

This is what I did...
I went down to radioshack and got a cheap stereo jack extention that fit the Ipod. Cut it in half. Stripped the wires and twisted the two insulated left and right chanel wires together, the ground wire is the un-insulated wire. Hooked these into the TEL/REMOTE terminals of the VIC-1.
Works great, no stereo though LOL
Not super loud but you can hear it just fine. Ipod volume at about 75%.
Only bad part the kids like to sing along with the music and leave the intercom on so they can hear each other.

 

[Boatcarpenter]

Greggor K,
Thanks. Was worried that it might not even be loud enough to really hear well the way people were talking. Don't really care about the stereo part, just want to hear the tunes.
Thanks,
BC

 

[OPCOM]

I have the transformer but do not know exactly where I laid it. When I find it I will put it together in a box to that I can plug the ipod into then plug it into the AM-1780.

In a pinch or to experiment, you can use any of these transformers. low Z goes to the player, hi Z goes to the intercom.

beware do not connect a high power player to a step-up transformer (low volts or Ohms on one side, hi volts or Ohms on the other), there is such a thing as feeding too much voltage to the intercom input. If you have ghetto blaster, consider a 1:1 transformer as it just isolates the signal, of which there ought to be plenty. In fact anything connected to the intercon really should be isolated. Experimenting is necessary most of the time unless someone gives exact details of what they did.

type low Z high Z
6.3V filament transformer (any common type) 6.3V winding 120V winding
voltage ratio 1:19 impedance ratio 1:363

radio shack Catalog #: 273-1385 12.6V transformer 12.6v winding 120v winding
voltage ratio 1:9.5 impedance ratio 1:91

radio shack Catalog #: 273-1365 12V transformer 12v winding 120v winding
voltage ratio 1:10 impedance ratio 1:100

radio shack Catalog #: 273-1366 25.2V transformer 25.2v winding 120v winding
voltage ratio 1:4.8 impedance ratio 1:22.7

radio shackCatalog #: 273-1380 audio transformer 8 ohm winding 1000 ohm winding
voltage ratio 1:18 impedance ratio 1:125

radio shack Catalog #: 273-1374 audio transformer 600 Ohm winding 600 Ohm winding
voltage ratio 1:1 impedance ratio 1:1

from the ratios shown one can see that the ratio of the impedance (ohms) is always the square of the voltage ratio.

Thus a 6.3V-to-120V transformer connected to a 32 Ohm drive, would reflect an impedance of 32*363 or 11,616 Ohms to the intercom.

A 12V transformer in the same slot would reflect 32*100 ot 320 Ohms to the intercom.

Within broad limits and at these low powers, it is not always so critical what the transformer was designed for, but the voltage and impedance ratio that is important.

I hope this gives hints about how to figure out to use any transformers that might be lying around.

transformers; more than meets the eye

 

[Darwin T]

find out what freq the transmiters that you can use in your car transmit on and see if it overlap with a rt-524 if you have one and go wireless.

 

[Varyag]

Chuck, try using the "Audio Input" terminals on the VIC-1. It has a 6,000 Ohm input impedance and the signal is amplified within the 1780. I have used it for a CD player.

This is how we did it on our tanks.