TV speaker amplifier

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.

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

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.

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.

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

Yes. Do as they say.

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.

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.

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!

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.

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

amplifier

and parallel to the cables of the current supply (+

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

Elca?

voltage of 12 V= at a 4-ohm

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

Okay.

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.)

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

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

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

micky formulerede spørgsmålet:

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

Leif

Hmmmm. Thanks.

(+

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

Den 05/03/2013, skrev Pat:

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

Leif

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