Slaying the Hummer

[William Rossiter]

I've installed a 1/4" phono jack that cuts out the 3" TV speaker when I plug
in the phones. But I get a whole lot of hum in the phones. I am guessing
that because the phones are more sensitive than the speaker, I have to turn
the TV volume control down quite a bit, making the hum more audible than the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

 

[Myron Samila]

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!

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19
"William Rossiter" <ctn41205@centurytel.net> wrote in message
news:xaednbFSFZbp91-i4p2dnA@centurytel.net...

I've installed a 1/4" phono jack that cuts out the 3" TV speaker when I plug
in the phones. But I get a whole lot of hum in the phones. I am guessing
that because the phones are more sensitive than the speaker, I have to turn
the TV volume control down quite a bit, making the hum more audible than the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

 

[Watson A.Name \"Watt Sun]

William Rossiter wrote:

I've installed a 1/4" phono jack that cuts out the 3" TV speaker when I plug
in the phones. But I get a whole lot of hum in the phones. I am guessing
that because the phones are more sensitive than the speaker, I have to turn
the TV volume control down quite a bit, making the hum more audible than the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

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.



--
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[mikem]

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 installed a 1/4" phono jack that cuts out the 3" TV speaker when I plug
in the phones. But I get a whole lot of hum in the phones. I am guessing
that because the phones are more sensitive than the speaker, I have to turn
the TV volume control down quite a bit, making the hum more audible than the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

 

[Myron Samila]

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.

--
Myron Samila
Toronto, ON Canada
Samila Racing
http://204.101.251.229/myronx19
"mikem" <mladejov@CharlieEchoDelta.utah.edu> wrote in message
news:bpu4uq$m56$1@coward.ks.cc.utah.edu...

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 installed a 1/4" phono jack that cuts out the 3" TV speaker when I plug
in the phones. But I get a whole lot of hum in the phones. I am guessing
that because the phones are more sensitive than the speaker, I have to turn
the TV volume control down quite a bit, making the hum more audible than the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

 

[William Rossiter]

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
====================================


"mikem" <mladejov@CharlieEchoDelta.utah.edu> wrote in message
news:bpu4uq$m56$1@coward.ks.cc.utah.edu...

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 installed a 1/4" phono jack that cuts out the 3" TV speaker when I
plug
in the phones. But I get a whole lot of hum in the phones. I am
guessing
that because the phones are more sensitive than the speaker, I have to
turn
the TV volume control down quite a bit, making the hum more audible than
the
program sound. I am further guessing that I could put some kind of
attenuater into the circuit that goes into the phones, but not into the
speaker, and thus could turn the volume up to outshout the hum without
busting my eardrums. But I don't know where to go from there: do I just
wire a resistor into the hot side of the signal? Or what?

 

[mikem]

Myron Samila 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.

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.

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?

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....

And Taiwan
probably is a better manufacturer than most, but again, I haven't seen too many TVs made
there, mostly China, and Malaysia.

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

MikeM

 

[Mark Zenier]

In article <Q4OdnT49H-vvPl-i4p2dnA@centurytel.net>,
William Rossiter <ctn41205@centurytel.net> wrote:

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.

You don't understand

live
--------------------+------->|-------+-- + supply to horizontal output/power
line input | |
--------------------|---+--->|-------+
neutral | |
| +---|<---+
| |
+-------|<---+-- TV circuit ground

With a full wave rectifier, the voltage on the circuit ground, for one
half of the power cycle, is one diode drop above the neutral (grounded
side of the power line) voltage. ON THE OTHER HALF CYCLE IT IS A HALF
A SINE WAVE THAT PEAKS AT -170 VOLTS.

The hum you hear is probably the leakage current of this pulsating current
through you.

Mark Zenier mzenier@eskimo.com Washington State resident