[non robotic] Audio level adapter

[chelmi]

Hi guys,

I have a question unrelated to robotic, but before asking it on a electronic forum and being abused, I'll ask it here
I'm sure some of you will be knowledgeable and kind enough to answer me.

Here is the issue: I recently bought a new TV and would like to plug it to my audio system. The issue is that my stereo system is crappy and does not have an auxiliary input. The only input I have on it is a "music port" (3.5mm mini jack) which is supposed to be connected to a potable music player. I bought the necessary cables/adapters to connect the audio output of my TV to it, but I have a very annoying "saturation" effect. Whenever the sound is too loud, I hear glitches

I'm not familiar with audio electronics, but my understanding is that the amplitude of the signal from the TV is too big (lowering the volume on the stereo system doesn't change anything)

My idea is to build device that would go between the TV and the stereo system to reduce the amplitude of the signal, i.e. an amplifier with a gain < 0dB.

Does it makes sense? if it does, what would you suggest as a circuit for a simple amplifier with negative adjustable gain?

Thanks in advance,

Chelmi.

[Asellith]

You can't just turn the TV down? what output are you taking from the TV? Composite RCA jacks? There are line to mic level converters or a cheap solution is a resistor voltage divider to attenuate the signal. Also is the stereo input designed for something like an Ipod? Might be why it seems to over amplify the signal.

[chelmi]

You can't just turn the TV down? what output are you taking from the TV? Composite RCA jacks? There are line to mic level converters or a cheap solution is a resistor voltage divider to attenuate the signal. Also is the stereo input designed for something like an Ipod? Might be why it seems to over amplify the signal.

Thanks for the quick reply !

The TV output is a double RCA jacks. Apparently, the volume control on the TV only works for the internal speakers.
Yes, the stereo input is designed for iPod and similar devices.

[chelmi]

I found the manual of the two systems:

the TV:
AUDIO OUT 500 mVrms (typical)

the stereo system:
Input Impedance Music port 250 mV 12 kΩ

That's twice the voltage !

What about impedance?

I see in the manual that there is a headphone output on the TV, maybe I will have more luck with this one. There is no data about it in the manual though. But since the media port is meant to be connected to the headphone output of the iPod, it should be compatible. I will check tonight when I get home.

[Soeren]

Hi,

Negative amplification = attenuation - what you need is a simple (passive) attenuator.
Your TV output appears to be twice what's needed, but that's max. parameters, better go lower and adjust at the stereo.
I'd say use around 1/6 max. both to have some headroom and to accommodate using a passive attenuator.
Too bad you don't have the output impedance of the TV, but try this:

(One channel only shown, double for stereo)

Code: [Select]

TV out o--[4k7]--+--o Stereo in
                _|_
                |  |
                |  | 1k
                |__|
                  |
                  /
Your output impedance should be around 1/10 of the input impedance it connects to (rule of thumb) to avoid to severe loading. I'm assuming an output impedance of 600 Ohm from the boob tube and if that hold true, this is the best (and cheapest/easiest) solution.
Tell me if there's issues with it, we can allways move things to make it the best fit to the impedance of both.

[chelmi]

Thanks a lot Soeren !

I built the circuit you suggested and it works perfectly so far. I will watch the Riget miniseries again and drink a Tuborg in your honor

What kind of symptoms should I expect if there is an impedance mismatch ?

Chelmi.

[Soeren]

Hi,

I built the circuit you suggested and it works perfectly so far. I will watch the Riget miniseries again and drink a Tuborg in your honor

Mkay... I never bothered with Riget myself (too weird for my taste - I'm not too keen on contemporary  Danish and Swedish movies, with a few exceptions) and I had my last beer in 1976 (Tuborg was my preferred brand back then though).
I'm more looking forward to seeing The New Daughter, a movie adaption of a novel by John Connolly, one of my favorite authors (and we just got his two latest books *Yayy*) Add to his grim environment, a large Scotch 'n' Coke and the world around me stop existing 

What kind of symptoms should I expect if there is an impedance mismatch ?

Bad sound - if it sounds OK, there's nothing to worry about.