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JoeDJ 1st November 2011 08:50 PM

Using a computer sub with a TV.... questions.
I have my TV sound set up to be fed into my stereo sys via the TV's 3.5mm anilog stereo jack when wanted.
However most of the time and late at night, I simply use the TV's speakers but they are sorely lacking in lower freqs.

So, I tried connecting an old IBM computer sub that I had packed away. It fits perfectly on the TV stand behind the TV and the sub's on/off switch is readily accessable. It's a perfect "fit" for that app.
The sub is designed to be fed a low level signal via its 3.5mm stereo input jack, It has an on/off switch and a seperate volume control.

Now, the problem is, when I turn that sub up (via its volume control) it has a nasty peak above about 150hz or so and makes things sound a bit "boomy'' of course.

What I would like to do is install a couple of lo-pass filters (one each for both L&R channels) between the TV's output and the sub's input...... at about 80hz.... just to give some missing "bottom" freqs to the TV
Taking the sub apart is not an option without destroying the case as it not designed to be taken apart.
The nearest I can determine is the sub is about 4 ohms.

So, my question is, can a given lo-pass filter that is designed for high level use be used in a low level apllication as described?
If not, any suggestions?

The main reason I'm even bothering with that sub is that it fits so peferctly behind the TV. If i can't use that particular sub I wiil not use any different one. Like I said, I already have the TV capable of being fed through my main 2.1 music sys that already contains a great sub..

The IBM sub is just an intetesting project for me. :)

firechief 1st November 2011 09:10 PM

"has a nasty peak above about 150k or so " you must mean 150 hz I'll bet. Yes you could try using a speaker passive crossover between the tv stereo channels and run that into the sub. But first I would be tempted to use a pair of resistors to combine the channels as this will help prevent the tvs output amplifiers from fighting each other. The values of the resistors will be on the order of a few hundred ohms. Im also not convinced that you need that crossover to solve the problem. The sub probally has both electrical and mechanical tendancy to filter out highs. Try just reducing the level going into the sub with resistors you may find it helps a lot.

sreten 1st November 2011 09:19 PM


What sort of sub is it ? if its bandpass there is a mechanical solution.

Electrically on the input board or input cable there is another solution.

How have you connected it without muting the TV speakers ?

Passive low pass filters on its feed are likely the simplest option,
High level filters won't work with active subs, the passive option
is far simpler and far cheaper.

rgds, sreten.

JoeDJ 1st November 2011 09:30 PM

Yes, 150hz.. I'm a terrible typist and had to make a few corrections.:)
BTW, keep in mind that the sub does have a volume comtrol but, it it is not useful in solving my problem.

The sub and the the 2.1 sys are both connected to the TV's anilog 3.5mm jack via a "Y" splitter.
The sub just has a single 3.5mm stereo female jack for its input.
The TV sound must be on when using the IBM sub of course. the TV's speakers would be used in addition to the IBM sub.
Of course, the TV sound is off when using my main 2.1 sys.

epa 1st November 2011 09:43 PM

maybe use something like this ,with a 12 volt adapter.

JoeDJ 2nd November 2011 04:03 PM


Originally Posted by epa (
maybe use something like this ,with a 12 volt adapter.

Thanks but, I do not want to complicate things. I just want to KIS.:)

I actually have an old 5 band stereo EQ I could use if I wanted to gp that route.

sreten 2nd November 2011 09:50 PM


So the 3.5mm socket does not mute the TV speakers ? It usually does ...

Something like a 3.3K resistor in series and a capacitor to ground, 0.47uF,
and then fed to the sub will roll off the sub feed at 100Hz, first order.

rgds, sreten.

JoeDJ 3rd November 2011 12:52 AM

OK thanks.
That is exactly what I wanted. :)
I will need two as the sub is fed a 2 channel signal

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