Slaying the Hummer

Most devices that cut out the internal speaker use a resistor to attenuate the volume so that it would be suitable for headphones. Hmmm, the hum may go below the noise floor when you wire in a resistor, but what value? hmmm?!? I don't know!

You can put a 10 ohm resistor (or whatever the speaker impedance is) across the output to load it to its normal load. This 10 ohm resistor should be capable of dissipating a few watts if your output power is a few watts. That may help a lot but if the hum still persists, then you should put a 10 to 1 resistive divider between the output and the headphones. I would try a 220 and a 22 ohm resistor, and if that isn't enough, lower the 22 ohms to 10 ohms, making it a 22:1 divider. You will then have to turn the volume up to get the signal to override the hum.

Be carefull!!!

Since you say that the speaker is only 3", I'm guessing that you have an el cheapo Taiwanese TV set. Likely, there is not even a power transformer (or an audio output transformer) in this beast, and one side of the ac power line is probably connected to the audio common, meaning that depending on the polarization of the TV's line cord, your headphones could wind up with 120Vac on them. Imagine your kid or your dog chewing on the headphone cord...

Most of the inexpensive sets use a relatively high impedance speaker (like 32 Ohms). If you connect a 4 or 8 Ohm headphone, the level will sound very loud, because the Power in the headphones is inversely proportional to the load impedance.

I would use an audio transformer to provide isolation, and to match the voltage levels. If you could find an audio transformer like the ones used in paging systems to match a "70V" line to 4/8/16 Ohms, some combination of the taps on the primary and/or secondary will reduce the level, and provide safety isolation thereby keeping the line voltage off the headphones. Test any transformer you scrounge with an Ohmmeter to determine that it is not an "autotransformer".

MikeM

William Rossiter wrote:

I've rarely if ever have seen headphones are are 4/8 ohms. They are usually MUCH higher than that.

The only headphones that are of low impedance are club dj headphones to maximize the most out of a low power op amp.

Most TVs that I've ever come across do not have 32 ohm speakers, they'd have to use larger amps to drive the speaker, wouldn't it make more sense to use an 8 ohm driver? And Taiwan probably is a better manufacturer than most, but again, I haven't seen too many TVs made there, mostly China, and Malaysia.

Thanks for the caution:

I've mounted the jack on the plastic housing, so I'm not too worried about a ground on that score. I'll check both sides of the speaker leads to see if there's a possibility of getting the 120 v. I have fooled around with an L-pad, and found that if I adjust it properly I can bring down the signal to a manageable level and overcome the hum to some extent. I thought I would construct a fixed l-pad to put in the phone circuit but not in the speaker circuit, if I could figure out the values for the resistors in the l-pad. The one I used for the experiment operates in the kilohm range, so it is pretty hard to get it adjusted closely enough to measure the resistances with a VOM.

I'll keep on fooling aorund with it. Have made out my will, just in case.

Thanks for your thoughtful comments.

BR ====================================

plug

guessing

turn

the

MUCH higher

maximize the most

I have several stereo hi-fi headphones which are 4 or 8 Ohms. The headphones which are supplied with Walkman/Diskman type devices are typically 32 Ohms. If you take an 8 Ohm stereo headphone and short R to L to drive it from a mono source, you will effectively have a 4 Ohm load.

to use larger

No, just the opposite. It is much more expensive to build an amplifier capable of driving 4/8 Ohm speakers than it is to drive a higher impedance, assuming that you have a relatively high supply voltage (>25V) to start with. Only 12V powered car stereos like the low impedance speakers, because they can get 10W into them starting with only 12Vdc. Since the elcheapo TV sets dont have a power transformer, they naturally have high voltages available (rectified 120V line), so the audio stages work at higher impedances....

many TVs made

I was using "taiwan" in the generic sense...

MikeM

You don't understand

live

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