Miscellaneous Audio Questions

[Menticol]

Hello everyone!

Some months ago I made two (very simple) audio related projects, but both of them failed: The first one isn't serious, but the second was catastrophic. I have a couple of questions since then, and I hope you can (laugh) and help me.

Case 1:
I got a very nice stereo "UV meter" (scrapped from a 1970's receiver) and wired it to my home theater. Actually is not a VU meter, it's labeled POWER W and it has an odd scale (0.3-1-3-5-10-20-30-60), but I don't mind, is just for aesthetic purposes.

The indicator is connected in parallel to the home theater speaker lines (I used a small diode and a resistor in series to make it work). The problem is, every time I crank the volume too high, the needle jumps out of the scale. As expected.

Question: Using a larger resistor (to keep the needle on scale) will damage the amplifier? What's the correct way to limit the needle movement without messing the impedance?

________________________

Case 2:

My DIY project required to add a "Line In" to a cheap 1980's sony boombox, to be used with an external mp3 player. Tried to use the cassette head line - didn't worked, sound was awful. I attribute it to impedance mismatch.

Then I tested another approach, one that I successfully used before on another boombox: Connecting the external mp3 player directly to the amplifier input pins. It worked great, but volume and tone controls were unusable (I totally bypassed them, so that was expected). To fix the problem I made a simple volume control (see the attached file) and connected it between the player and the IC.

It worked, and guess what: Boombox's original volume control magically started to work too. I happily glued my potentiometer on a hidden place, and assembled the unit. Proceeded to use my new creation... and the mp3 player burned after 5 minutes

Question: Where is the fault, on the volume control or the gross amplifier connection? I didn't detected any voltage coming out of my Line In, and the same approach works great on other equipment

Thank you very much for any input guys. By the way the song's title on the picture says it all, it's a shame.

 

[crutschow]

Case 1:
A higher resistance will have no effect on your amplifier. It's small resistance (thus higher current) that can damage an amplifier)

Case 2:

Don't offhand see that anything you did would have damaged the player.

What's an "IC" input?

 

[Menticol]

What's an "IC" input?

I'm sorry Crutschow, I mean the Input pin of the amplifier IC.

Maybe the potentiometer value was inadequate? It was 50K

 

[crutschow]

A 50K should not have caused a problem with the player. It's a puzzle to me. Did the volume control seem to work normally over its adjustment range when it was working?

 

[Menticol]

Well now that you mention it, the volume operated normally except when one end of the potentiometer was reached -the maximum volume. At this moment the left channel was VERY loud, much more than the right channel. I glued the potentiometer on a medium setting (both channels equally loud) and used the boombox original volume control instead (which as I said, inexplicably started to work only after I attached my 50K pot)

I've made a small diagram to explain what I did.

 

[ronv]

Maybe it needed to be coupled through a capacitor?

 

[crutschow]

The problem could be the two volume control pots in parallel. The boombox control may have put too large a load on the player's output. Basically the boombox control was likely operating by shunting the player signal to ground. You should probably have added a switch so that either one or the other was selected to isolate them from each other.

 

[Menticol]

Thanks to your advice I was finally able to finish my blog post.
Thank you very much Carl!