mungo@cs.strath.ac.uk (Mungo Henning) (04/28/87)
A relative is slightly deaf and gets annoyed when he tries to listen to the TV because his hearing-aid picks up all sounds in the room. His TV does have a headphone socket, which is great when only he wants to listen to the TV but is useless when anyone else wants to listen too: he sets a `reasonable' volume for the headphones which makes the loudspeaker volume too high. I bought a small general-purpose amplifier from Tandy (Radio Shack) and connected it up to the headphone socket so that he could adjust the volume to his own requirements without adjusting the TV volume. The trouble is that at a reasonable TV volume the little 9v amplifier clips when its volume is turned up too much: giving an annoying buzz in the headphones. The tone is not too hot on the little (cheapo) amplifier. There seem plenty of articles regarding TV amplifiers in popular magazines, but they usually use a microphone close to the TV speaker. The directness of the current set-up is very desireable, but I'd like to cure the clipping and the lousy noise output from the current configuration. Any ideas out there would be welcomed. I thought that one possible solution is to attenuate the `headphone' signal which comes from the TV: I think it is too strong for the little amplifier. Thanks Mungo Henning P.S. I may not get back to any replies for a while, but I'll do it eventually. -- ARPA: mungo%cs.strath.ac.uk@ucl-cs.arpa, mungo@cs.strath.ac.uk UUCP: mungo@strath-cs.uucp, ...!seismo!mcvax!ukc!strath-cs!mungo JANET: mungo@uk.ac.strath.cs
bill@videovax.Tek.COM (William K. McFadden) (04/30/87)
In article <578@stracs.cs.strath.ac.uk> mungo@cs.strath.ac.uk (Mungo Henning) writes: >A relative is slightly deaf and gets annoyed when he tries to listen >to the TV because his hearing-aid picks up all sounds in the room. >I bought a small general-purpose amplifier from Tandy (Radio Shack) >and connected it up to the headphone socket so that he could adjust >the volume to his own requirements without adjusting the TV volume. >The trouble is that at a reasonable TV volume the little 9v amplifier >clips when its volume is turned up too much: giving an annoying buzz >in the headphones. The tone is not too hot on the little (cheapo) >amplifier. >Any ideas out there would be welcomed. I thought that one possible >solution is to attenuate the `headphone' signal which comes from the >TV: I think it is too strong for the little amplifier. You are correct in that attenuation is probably needed. I assume from your article that the TV does not disconnect its internal speaker when something is plugged into the headphone jack, otherwise no one else in the room would hear the TV very well. Anyway, if the amplifier is the kind I think it is, then it expects a very small input level, like you'd get from a microphone. If you put this circuit between the TV and the amplifier, it should work a lot better: >----- R1 -------+--------< Center Conductor | FROM R TO TV 2 AMPLIFIER | >------------------+--------< Shield For starters try 100,000 OHMS for resistor R1 and 1000 OHMS for R2. This will attenuate the signal 100 times. If this is too much attenuation (e.g., not enough output from amplifier), lower the value of R1. If the amplifier still clips, lower the value of R2. (I don't think this will happen, though. 100 times is quite a bit of attenuation.) If you want to get fancy, you could make one of the resistors variable. Hope this helps. -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 UUCP: ...{hplabs,uw-beaver,decvax}!tektronix!videovax!bill GTE: (503) 627-6920 "How can I prove I am not crazy to people who are?"
wtm@neoucom.UUCP (Bill Mayhew) (05/04/87)
You can keep the volume at the TV headphone jack high enough for person with hearing loss and avoiding 60 Hz buzz by cutting down on the volume in the set's speaker. One thing you can try is to switch the speaker in your set to a speaker in series with a resistor. You need to watch the impedance of the combination. If the set has a 16 ohm speaker, try replacing it with an 8 ohm speaker in series with an 8 ohm, 5 watt resistor. You could keep the original seaker by placing the speaker in parallel with a 16 ohm resistor and that in turn in series with an 8 ohm resistor: that would make sure that the impedance stayed the same. There are a zillion different types of audio outputs in TVs. Some sets use class A outputs driven from very high voltage (135 volts) and 40 ohm speakers. Other sets are more "normal". You should be very careful if you consider digging around inside your set. On a lot of color TVs, the audio circuit contains components that are **NOT** isolated from the AC input. There is a chance you might get seriously zapped if you don't know what you're doing. I'd recommend getting the schematic for your set first, so that you can assess the saftey of any modifications you might make. You might try using an operational amplifier chip to make a notch filter for the 60 Hz buzz for the ear phone listener. You should be able to find the design for a 60 hz filter in the applications notes in an integrated circuit catalog. I think the spec sheet for the National Semiconductor LF444 or Texas Instruments TL-072 should have and example notch filter. --Bill (wtm@neoucom.UUCP)
rep@genrad.UUCP (Pete Peterson) (05/04/87)
In article <566@neoucom.UUCP> wtm@neoucom.UUCP (Bill Mayhew) writes: > >You can keep the volume at the TV headphone jack high enough for >person with hearing loss and avoiding 60 Hz buzz by cutting down on >the volume in the set's speaker. > >You might try using an operational amplifier chip to make a notch >filter for the 60 Hz buzz for the ear phone listener. > The original request for this information didn't make it clear whether the problem occurred at high volume settings (since he mentioned clipping) or low settings which makes the residual hum ,at zero volume control setting, significant. In the first case, the attenuator on the output as mentioned in a previous posting would make sense; if the problem is residual "buzz", the attenuator on the TV speaker that Bill mentioned would probably help. Two additional points: Trying to get rid of "60 hz buzz" (as opposed to 60 hz hum) with a 60 hz notch filter will probably not work out too well since you would still be left with the harmonics, which are what makes it sound like "buzz" rather than "hum". You could perhaps use a high-pass filter to cut off frequencies below 300 hz (telephone-quality sound). Hearing loss tends to be frequency-dependent; using an amplifier with some control over the frequency response (at least treble-boost if not an inexpensive graphic equalizer) might allow for greatly improved intelligibility without blasting the person's ears. pete peterson {decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep
lacasse@randvax.UUCP (05/06/87)
One person asked for help where the sound level in the headphones plugged
into a TV for a hard-of-hearing person was insufficient when the volumn
control was set at a confortable level for other viewers to listen on
the still-connected speaker.
Someone replied and suggested attenuating the speaker inside the set.
He cautioned of electrocution and other risks of altering the set.
I re-iterate his caution: don't modify a TV set if you are not "qualified
service personnel".
But it is interesting to note that many TV sets have a resistor in
series with the headphone jack to attenuate it. I suspect they do this
to make the volumn control more usefull for the headphone listener.
Since in your set the speaker does not cut off when a headphone plug
is inserted (most do), the manufacture may also attenuate the headphone
plug in the hope of making the speaker and headphone level more
commenserate. In general, a headphone takes a LOT less power then
a speaker. If your set has such a resistor, you might be better off
it if wasn't there (shorted out). Then you could use a simple "L-PAD"
(100 ohm 5 watt potentiometer) in series with the headphone, near
the headphone, adjusted by the hard-of-hearing person. You might
want to take your TV to a good repair center and ask if this modification
could be made.
markalab@ur-tut.UUCP (Daniel F. Luna) (05/08/87)
Another problem that you may be experiencing is that there is a (probably) substantial impedance mismatch between the headphone jack and the amp you are using. Usually headphones are 8ohms, and input to line amps are on the order of 10's of thousands. Although in this direction (low to high) things work, they are not efficient, and much power is lost. Try adding an impedance matching device like an 8ohm to 10K transformer in the line. -- Daniel F. Luna PC-Person. uucp: ...rochester!ur-tut!alab arpa: ur-tut!alab@rochester
alab@ur-tut.UUCP (Daniel F. Luna) (05/08/87)
Someone suggested disconnecting the resistor in the headphone jack. This may be OK, and may be quite dangerous. Some amps NEED a termination in order to be "happy". The resistor is to assure the amp of this. When the headphone connects, it either disconnects the resistor, or parallels it. The resistor is usually of a higher impedance than the headphones (being 8 ohms) and therefore effectively disappears. -- Daniel F. Luna PC-Person. uucp: ...rochester!ur-tut!alab arpa: ur-tut!alab@rochester Standard Disclaimer: If it don't work, it ain't my fault.