TV speaker amplifier

[micky]

Well, last night, I finished the first draft of the amplifier for my
bathroom tv speaker.

Thanks for the advice earlier

It works well** except for two problems.

A) Allong with the TV sound, it blends in the strongest AM station in
town. The amplifer module I bought just has 5 unshielded wires
coming out of it, and for the input, I used 20 inches of lamp cord
connnected to a 1/8" phone plug, which plugged into the earphone jack
of the tv. I guess the fix most likely to work is to replace the
lamp cord with co-axial shielded cable. Right?

Or I could just wrap some kind of shielding around the wire that is
there. Like heavy duty aluminum foil taped in place. Good idea.
Easier becaue I wouldn't have to hunt for some shielded cable, or
solder a phone little plug to the end. .

B) It's probalby much too loud. I have to turn the tv volume down
from a maximum of 50 to about 15. Then let it get amplifed again by
the amp. I'm figuring the TV output has less distorition, and that
using it at low volume which then gets amped by this thing introduces
distortion. (Though I caouldn't tell because the AM radio was too much
of a distraction.)

The remedy for this seems to be to change wall warts. Right now I'm
using a 12VDC, 500mA adapter, which I chose mostly because it was the
first I saw in my box that had the end, the tip, cut off already.
The spec says it can use 4.5 to 12VDC


**I'm using a Kemo M031N monaural amplifier module that sells for
about 8 dollars at MCM Electrronics. Rated at 3.5 watts. I'm glad I
didnt' get the 8? wattt version that was about 13 dollars.

It's "box" is open on one side but filled with some sort of hard
filling. It has 5 unshielded wires coming out of one side. Two for
the input. And two for the speaker plus two for the power, which
share one negative wire. Which makes 5 total.

I paid no attention to polarity, since it's a monaural output, but
when I put in a shielded input, I could make sure that the shield is
conected to what the instructions say is the ground, if it matters???

Unlike their drawing, I don't have the pot wired into the input,
because the TV is 9 feet away from the bathtub. Rather it's in the
wires that go to the speakers, from a 1930's record player, mounted
above the tub (for the last 30 years.) .

Thanks.

 

[William Sommerwerck]

A) Allong with the TV sound, it blends in the strongest AM station in
town. The amplifer module I bought just has 5 unshielded wires
coming out of it, and for the input, I used 20 inches of lamp cord
connnected to a 1/8" phone plug, which plugged into the earphone jack
of the TV. I guess the fix most likely to work is to replace the
lamp cord with co-axial shielded cable. Right?

RIGHT!!!

Story... Back in 1978 I was working at a hi-fi store in Pennsylvania. Around
this time, fancy speaker cables and interconnects were becoming popular. One
customer bought some Monster speaker cable, then called us to say that he had
a hum problem. When I saw his system, I broke into uncontrollable laughter. He
had attached RCA plugs to the Monster cable, and was using it as an
interconnect!


B) It's probalby much too loud. I have to turn the tv volume down
from a maximum of 50 to about 15. Then let it get amplifed again by
the amp. I'm figuring the TV output has less distorition, and that
using it at low volume which then gets amped by this thing introduces
distortion. (Though I caouldn't tell because the AM radio was too much
of a distraction.)

If anything, the TV introduces more distortion than the amp. Keep the TV
output low and let the amp provide the needed gain.

Stick with the supply you have.

 

[Jamie]

Oh dear ...

Arfa

That's gotta be one of the funniest post I've seen in some time now..

Jamie

 

[micky]

Excellent first step... and the Kemo data sheet for this amp
specifically says that the input cable must be screened.

Yes it did, and I read it, but I sort of blew that off in my mind,
thinking it was referring to very small signals rather than line level
input. And I did that despite the blurb in the MCM online catalog
saying it accepted line-level input.

If you've got AM incursion, you should also try to "choke" the wires
coming into, and out of the amplifier. You can get small "clamp-on"
ferrite RFI blockers - they're similar to the tubular ferrite cores
that you find on many USB and video cables, but they come in two parts
in a plastic shell so you can clamp them onto an existing cable. Use
one on the input cable, one on the power cable, and one on each
speaker cable... this should keep the AM signal out of the amp.

Aha. Maybe I have a couple of these that I cut off discarded
equipment, or I'll buy them if the AM is still getting in.

Aha, I found them for sale at a mail order place where I needed 7 more
dollars to avoid paying a 5 dollar handling fee (for orders under 10
dollars) . I still need to spend 4 more dollars!

Reducing the voltage will probably reduce the *maximum* volume (before
it distorts) but probably not reduce the volume at any specific TV
volume setting (that is, it won't reduce the amplifier's gain).

Yeah... I just looked at Kemo's web page and it says that this amp has
an input sensitivity of "< 80 mV". That's not much voltage at all.
Your TV set is almost certainly "overdriving" the input with too much
voltage, and could be forcing it into distortion quite easily.

Just to be clear, I didn't mean that I had experienced audible
distortion already, but that was because I kept the tv volume at 20
out of 50. And I kept the pot low, but after I posted I turned it
almost to the max and of course the volume didn't seem to go up so
much in the last third or half of my turning it.

But I figured there was distortion I wasn't hearing, that would show
up if I listened to music, or would make my ears hurt after a while
(even if at one level it sounded okay) .

I also kept the tv volume down for fear of damaging my 80 year old
speakers. (BTW, I never take showers or hot baths (only warm baths
with no steam, not even on the mirror when I'm done) , so the
environment for them above the bathtub is not so bad. But my brother
has visited and he takes showers (I don't know how steamy they are)
and the grill cloth (designer burlap, that is, light tan-colored
burlap) is water stained, I guess from the steam. I'm lucky the
speakers didn't get hurt and next time he visits I will have him
shower in the bathroom off my bedroom.

Kemo's data sheet shows the use of a 10k potentiometer (presumably an
"audio taper" type) as a volume control. I'd suggest that you get one
(Radio Shack or similar) and wire one up as the data sheet shows...
this will let you adjust the gain. See

http://www.kemo-electronic.de/datasheets/m031n.pdf

Ah, let me ask about these lines from the instructions:

With too high ohmic voltage sources it may occur self-oscillation of the amplifier
(constant sound). In that case it is convenient to place directly at the module and parallel to the cables of the current supply (+
and -)

Parallel to the cables? Not across the cables? I know this is
translated from German, but still.

an elca
Elca?

of approx. 1000 ìF 12 V=. The amplifier achieves peak power at an operating voltage of 12 V= at a 4-ohm
loudspeaker. With lower voltages and highly ohmic loudspeakers, power will increase accordingly.

Power will *increase* with lower voltages??

OTOH, with "highly ohmic" speakers, 8,, 12, 16, more power will be
needed, right? Or not? But either way, the power won't increase by
using higher ohm speakers?? So for both situations, they must mean
"decrease".

Watts=Volts*Amps=Ohms*Amps*Amps-R*A

Yes. Do as they say.
Okay.

You should have a pot (or a simple two-resistor fixed attenuator) in
the input circuit, so that you don't overdrive the amplifier and force
it into distortion. You don't need to remove the pot you have placed
in the speaker wiring... just add a 10k pot as the data sheet says you
should do, use this to "turn down" the amp's sensitivity to the point
where it's acceptable, and leave this pot at that setting.

I had planned on using the tv's volume control, but your idea would
keep it safe when I forgot or when someone else visited. (Though I
don't remember any of my guests using the tv or the radio in the
bathroom. They probably think I'm weird.)

 

[micky]

"Parallel to" is more standard terminology in electronics.. "across"
is colloquial English.

Thanks. I would have thought the word was perpendicular, not
parallel.

They really mean the same thing in this
case... connect the capacitor's + and - terminals to the + and - power
terminals on the amplifier right where you connect the wires from the
wall wart.

OK. Still working on this. It may be done in 2 years, in time for
my Masters Degree in tinkinering. (MST)

 

[Leif Neland]

micky formulerede spørgsmålet:

Thanks. I would have thought the word was perpendicular, not
parallel.

If you think of "Parallel to the end of the cable" it makes sense.

Leif

 

[micky]

micky formulerede spørgsmålet:
If you think of "Parallel to the end of the cable" it makes sense.

Leif

Hmmmm. Thanks.

 

[Leif Neland]

Den 05/03/2013, skrev Pat:

Parallel as opposed to "in series with". In other words, across.

Which makes the interesting question: Is a bulb/speaker/motor/whatever
connected in parallel or in series with its connecting cord?

Leif

 

[William Sommerwerck]

Which makes the interesting question: Is a bulb/speaker/motor/whatever

connected in parallel or in series with its connecting cord?

In parallel. Think of stripping the power cord of small sections of insulation
and attaching additional loads.