Mono to Stereo Line Level Audio Adapter?

[Stevereno]

I'm trying to tie a mono line level audio output into both L&R stereo inputs of an amplifier, but encountering problems. When I use a "Y" adapter for the mono output and connect it to both L&R inputs on the amp, it kills the audio. If I connect just the single mono output to either L or R input by itself, it'll work, but then I don't get audio out of both channels on the amp.

Do I need some sort of adapter, transformer, or resistors to isolate the outputs? The amp doesn't seem to like having both L&R inputs tied together.

Thanks in advance for your help.

[Chris Smith]

Id start with a Y type of feed and add in a diode to each line on the Y. [Try both directions]

Also the impedance could be out?

You can also try a small ceramic cap to each of the Y lines to isolate the signal from DC, while still sending a AC coupled signal.

[MrAl]

Hello,

I would start by making sure the Y adapter is really working ok.

All the amps i have seen have identical channels and it never
hurts to connect the two inputs together. I guess there is the chance
that you have a very odd amp however, in which case i have to
second Chris's idea to use two small ceramic disc caps...one for
each channel. A good value would be 0.1uf. If this emphasizes
the highs too much go to 1uf.
It could be that the two connected together bothers the bias, but
this is a rare thing.

[haklesup]

What you tried should typically work. You're dividing a voltage signal between two high impedance inputs (presumably) which should be OK. If you were mixing two outputs to form a mono output, that would be different and you would need resistors at least.

If the Y adapter for some reason swapped ground and the signal conductor on one side, it would appear to work one input at a time but together, they would cross ground both inputs. Easy enough to check its continuity with a meter though.

Same would be true if one of the inputs on the amplifier were reverse wired. Normally you might just see a phase reversal but in this case, it would ground the inputs. Not very likely but I thought I would mention it. Check continuity of the ground shields on the cabling and amplifier.

If it's RCA connectors, try inserting only enough for the center pin to contact leaving ground open on just one side. Ground should be common inside the amp so you need only one to make contact to the cable shield for proper continuity. BTW, many stereo amplifiers just require input to the left channel and it automatically outputs that as mono to the speakers, does yours not do that?

If for some reason you start to think that the L and R signals are cancelling each other out inside the Amp, you can convince yourself otherwise by connecting two different resistors in series with the mono output and sending each one to the left or right inputs. if you use say 5K and 10K then the volume will be unequal at the two inputs and the output should come on. I don't really think that is it though.

[Chris Smith]

There are plenty of reasons it wont work, polarity inputs may be the opposite per leg, [one uses a neg, the other pos, together they are AC input] impedance mis match, voltage input thresholds, draw down current, etc.

Isolate the signal using caps, a diode, a pot, etc and find out.

[Stevereno]

Thanks for all the input folks. You've provided me plenty of troubleshooting ideas. Much appreciated.

Great Forum!

[jollyrgr]

Why would you want to put a diode in series with an audio source? This is called an RF detector used for AM radio. The problem with ordinary diodes is they are non linear devices and most conduct only above 0.6V. Most line level devices might put out 1V PtoP if you are lucky. The caps will eliminate a DC loop and are not a bad idea if you use the right value.

In stereo amps both channels are the same. I think something must be shorted in the Y adapter like MrAl and hacklesup have said. I've connected many inputs together to get mono signals on both channels. Some devices use the left input as the MONO input. Devices like this will automatically connect the right and left inputs together. (The short out switch is in the RIGHT input and is shorted to the left input when nothing is in the RIGHT input.) Usually when devices have this type of arrangement you will see the left input labeled as LEFT/MONO. See hacklesup post.

I'd check the Y adapter for a short or reversed connection at one of the connectors.

There IS another thing that can happen. If you have an open ground between the source and the two inputs you can get a neat effect called differential canceling. In mono signals you will get nothing or next to nothing based on the slight errors in design. If you have a stereo signal and do not connect the ground to the source you will get the DIFFERENCE between right and left signals. This will cancel out anything mono. By inserting only the tips of the RCA jacks into inputs you can get the signal to cancel. Depending on the source you will cancel vocals and get a cheape and dirty "surround sound" processor. I used this method for surround sound back in the 80's.

[Chris Smith]

914 signal diodes will tell you the story.

If a Polarity exists on the two separate signal inputs it wont be known unless you experiment with them.

Often the input can be a push pull situation where the left is waiting for a negative part of the sine wave, while the right is waiting for a positive part of the sine wave, and only a diode setup will separate the canceled signal.

Tying the two together will simply short out the signals and neither will work.

My choices are not answers, only suggestions on how to find out what is actually going on.