I found my way here via google search for connecting headphones to speaker terminals. I was not able to get much out of those posts since this is a slightly different problem, and my electronic skills are limited.

I'm trying to connect the transmitter for a WHR120RS IR wireless headphone to a television set for someone with hearing loss. There is not a dedicated headphone jack on the television. I put a stereo jack on the back of the television connected directly to the speaker terminals. It is not a type of jack that disconnects the speakers with the insertion of the plug. Others in the room wish to adjust the television volume to comfortable levels while the person using the headset can adjust the volume independantly on the headset itself. (I realized that it won't be totally independant. If the tv volume is adjusted, the headsets will need to be adjusted correspondingly. But that's not a problem)

The television (as I kind of expected) slightly overdrives the headsets so that they begin to break up and distort when turned up to listening level. With the television volume turned way down, the range of volume control on the headset itself is fine. With the television set volume turned to moderate, the headsets cannot be turned up very much before distorting.

There are no impedance specs available for the wireless headset. I can't find specs for the television either. It is a Zenith SY2551S that is probably about ten years old.

High fidelity is absolutely not a priority here. Would someone kindly offer a suggestion as to what approximate value of resistor, or other components to put in series, parallel, on the connection between the jack I installed and the speaker terminals in the set? I just need a little attenuation to get the headset transmitter in a useful range. I'm handy with a multimeter and have a few good ones around. If I need to I can take whatever measurements are recommended and post them here.

Thank you very much for any help.

I can't imagine why a 1/2 voltage division with two 1k's wouldn't work, if you're just on the verge of not clipping. Otherwise use a 1/4, I guess. Most audio inputs have >2kohm resistance anyway - they're impedance bridged, not impedance matched.

Thank you. But I have to reveal my igorance further here.

Where would those 1k resistors be applied? Together in series on either of the plus or minus wires leading to the jack I just installed in the tv?

Or by voltage division to you mean that I'd put them in a different arragement?

One (the larger of the two resistance values, if they're different) would go from the positive terminal on the speaker to the positive terminal on the wireless input. The other would go from the same positive terminal on the wireless input to the negative terminal on the speaker.

If it's too quiet, either reduce the value of both resistors (say to 250 ohms) or make the bottom leg (between the +/- terminals) more resistive than the top leg.


NOTE: I am not a licensed professional engineer, I do not take any responsibility if you lose an arm over this, do this modification at your own risk, etc etc.

Decreasing the value of both resistors would actually make it quieter (assuming you don't also change the ratio).

Wrong. Decreasing the value of both resistors would actually make no difference (assuming you don't also change the ratio and that the impedance of the source driving the divider is significantly lower than the impedance of the divider).

Cheers, Slipstreem.

That's right, and Axon is right as well in that as you increase the resistance of the each resistor in the divider, the input impedance of the device being driven will have a greater effect on the ratio, causing a decrease in level.

My days as an engineer have long passed.

Was there concensus in there somewhere?

I'll just begin then with...

"One (the larger of the two resistance values, if they're different) would go from the positive terminal on the speaker to the positive terminal on the wireless input. The other would go from the same positive terminal on the wireless input to the negative terminal on the speaker."

...using 1K resistors all around. I can't imagine this tv using more than a couple of watts (if even that at moderate levels) amplification. Will 1/2watt resistors be ok? I suppose I could hunt further down in the junk drawer here and find a some 1 watt versions. Do I need beefier ones yet?

Thanks for all your help guys.

A pair of 1/4W 1k resistors should be fine.

If the level is too hot, increase the value of the resistor connected between the two positive terminals.

After my gaff, I'd understand if you wanted to get a second opinion.

This type of circuit is called voltage divider.

If you use the Wikipedia schematic, Vin is the signal coming out of the TV (and into the voltage divider). Vout (out of the voltage divider) goes into the headphone amplifier/transmitter.

With stereo, you will need two pairs of resistors.

The output voltage is determined by the ratio Z2/(Z1+Z2). If the resistors are equal, you will get half the voltage (a 6dB drop.) If Z1 is 9k Ohms and Z2 is 1k Ohms, you will get 1/10th of the voltage (a 20dB drop).

Another option would be to use a potentiometer (volume control), which is really a variable voltage divider. (catalog/picture - schematic - more info)

A potentiometer has 3 connections. In this application, they are connected as In, Out, and Ground (just like the voltage divider schematic). The output is the center connection, and if you mix-up the other two connections, it will simply work "backwards" (counterclockwise = louder).

They come in linear and audio taper. Audio taper would be preferred, but it's not necessary for a set-and-forget application like this. They also make ganged "stereo" pots, but they are a bit harder to find. (The catalog-link I gave you shows a a potentiometer with an on-off switch attached, which you do not need.)

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About impedance / resistance (Ohms)

Don't worry about "matching" the impedance. Resistors around 1k should be fine...

The TV's amplifier is designed to drive 4 or 8 ohms. You don't want to go that low, because that will double the load on the amplifier, and you will burn-up a standard 1/4 Watt or 1/8 Watt resistor.

On the other end, the Headphone amp's input probably has an input impedance of 10k - 100K Ohms. If you use resistors in this range, the impedance of the headphone amp will come into play and you will get higher-than-calculated loss.

However, decreasing the values of the resistors could potentially improve immunity to noise. As long as the sum of the two resistances is several times greater that 8 ohms, so as not to load the amplifier output, and you do not exceed the power rating of the resistors, which in this case is unlikely, I would recommend fairly low values of resistance.

True (although the resistor noise involved with 1k resistors may not be particularly audible).

I was mainly concerned that in the unlikely event that the load resistance of the wireless input was particularly low - say, 500 ohms - you'd need to drop the voltage dividing resistors particularly low to have any hope of a high enough signal level. And at the same time you don't really know what impedance the speakers are, so it's probably a good idea to keep the resistors above 100 ohms, to avoid reducing the speaker volume.

None of this is probably going to matter, and if it does, you can wire additional resistors in parallel on the + to + leg to increase the volume.

I wasn't actually considering the resistors as a noise source, which is totally negligible in this case. I was more concerned with noise that could be picked up in the cable, and which would be attenuated by a lower resistance value.

I would go along with something on the order of 100 ohms total for the two resistors.

Ok, the fix is in, so to speak, with 1k resistors all around.

Works as hoped, but with a tad too much attenuation.

So I don't want to lower the value of both resistors since I'm getting
the idea here that this may not make a difference. What do I do now?....

...decrease value of the series resistor on the pos side (say with 200 to 300 ohm resistors)
and leave the 1k in place bridged across the plus and neg at the phone jack?

This could indicate that the input impedance of the wireless headphone is lower than expected, most probably because it was expected to be plugged in in place of regular headphones, whose impedance is intermediate between speakers and line inputs.

Try a smaller value between the two positive terminals.

Ok, I tried a 150ohm between the positives on both sides and am very happy with it. It seems that the bass response is thinner but that is probably just my overactive imagination. That would be for the better in any case since the remedy here is entirely to enhance verbal comprehension (dialog).

Thank you all very much for the assistance here. The hearing impaired individual (me mum) couldn't be happier. She admitted that for a while now she had been stoically missing out on 3/4 of the dialog in movies when company is over for fear of raising the volume too loud on the set for others.

Now she just relinquishes the remote and when everyone is comfortable, she adjusts the headset to her needs.
Seriously... she was ecstatic about the improvement.

Incidentally, if anyone else is considering similar things..

I tried a device at radio shack which was essentially a little walkman sized/styled personal amplifier. A battery powered microphone in a case with volume control and headset. The sound reproduction was HORRIFIC. Man... I got nothing against cheap Chinese stuff if it satisfies a category that exists nowhere else, but for twenty dollars, it shouldn't be the sort of quality you'd expect out of a gumball machine. Brought it back.

So I got this RCA wireless headset for the same price on sale at another store and hoped i could hook it up to the television (since at this point it is really the only place where she has problems understanding what is being said to her). I can recommend this one.

Hell.. once the apex of this baby-boom curve hits the econony in full they'll probably be offering these things as an option with every television set.