I am hoping that a EE guy out there can direct me in the proper direction to make this happen. Basically I have a reasonably good set of computer speakers that take 5.1 channel inputs in the form of 3 headphone jacks (FR,FL),(SR,SL),(Cntr,Sub). I am trying to use headphone splitters to make these speakers work with 2 different sources (computer and old DVD player - for the wife )

When I hook up the system using simple 'Y' style headphone splitters the gain on the signal from the DVD player is almost completely lost to the computer... I figure the computer grounds out the signal for some reason... So I want to install Diodes as a one-way electronic gate for the signals coming into the speaker system. I just don't know what kind of diode...size/quality, where to source said diodes, etc...

If there is a better way of doing this without buying a receiver please let me know. My computer has some available inputs (i think) so maybe I could use it as a receiver but this just seemed like the simplest soln. (if i have to buy a receiver i'll probably just foot the bill for a logitec harmony remote and take the time to teach the wife to use the HTPC ).

Thanks for your help

If I understand your post fully, the output level from the DVD player is a very low one. I assume your referring to the the stereo RCA jack outputs on a typical standalone set top DVD player, if so those outputs are typically around a 200mV to a 500mV output, and thats why there seems to be no gain, because there isnt any.

as for why your not getting the proper signal into the PC, check the mini-phono plug type on the splitter. How many inputs into the PC are there. Is that input a stereo-mini or a mono-mini. If your trying to put a stero-mini into a mono-mini or vice versa you sometimes get bad results.

Adding a diode will not solve this problem safely or probably not even at all. I diode will only block the current in the direction you place, and stopping that current from going to a ground, isnt going to revert where you need it to go.

Heres what I would research

The signal level input into the PC card (what voltage level does it want)
The type of input (do you have a 6 channel in card, that in mono each channel), or do you have just 1 set of stereo inputs
Check out the Y cable, and see what it does. see if it actually takes two mono signals and combines them into a stereo signal, of see if it just combines them into a mono signal.

Scubasteve,

Thanks for the reply... the way that I have it set up now - that I thought the diode would resolve - is the 'Y' connectors (x3) plug the RCA-to-miniheadphone plug cables from the DVD player as well as the headphone outputs from the computer into one headphone input in the speakers. When all of the connections are made, the DVD player can barely be heard. If I disconnect the computer, it returns to its original state and operates flawlessly. So I figured that the computer might ground the jacks to prevent buzz or something when they are not in use... or just the added connection sucks juice out of an already weak signal... all the same I thought diodes on the computer lines would clear up the problem. I'll try to post a simple diagram to better explain what I'm thinking.

here is a pic... hope it helps

Click to view attachment

Do you hear sound from the speakers when using the DVD player and the computer is powered on but not
sending out any audio?

Ok, so your splitting the signal from one source into two devices.

A diode isnt going to help you.

if you put the diode in forward bias towards the PC, then its just going to operate as a short, and there will be no difference.

if you put the diode in reverse bias towards the PC, then its going to be an open towards the PC, and the signal will not reach the PC, leaving you in the scenario of essentially not have the cables connected to the PC.

I think we need to look into another alternative.

you could hack the cable up, and install switches so that you could enable each mode.

But I think the most proper approach would be to teach your wife how to use the PC. Ive personally found this to be the most effective method, you can always do a windows restore to last week in the event of a complication

do not use a diode on an audio signal line!

Audio signals are AC, so a diode will cut off half the audio signal, and seriously distort the rest. The ONLY audio circuit that I've ever seen that uses diodes in the signal path is a distortion circuit for a guitar.

If you want to switch sources, use a switch.

If the signal is too weak, use a pre-amp. I could suggest Elliot Sound Prodictions Project 88. This will put out a decent signal. (NOTE: you'll need a total of 6 channels to get 5.1)

I told him not to use the diode anyway,

but ........ and I dont have alot of experience with things at the PRE-amp level, but arent alot of audio signals riding on a DC bias? It may have been only in the signal/frequency generator work Ive done, but the signals that we pumped into some amplifiers were an AC signal (1kHz to 15kHz) and rode on a DC bias that kept it from going into the negative region.

The amplfier (permitting the class A, A/B, D), then uses a center taped transformer to create the negative half cycle of the output to the amplfier.

A DC bias is sometimes used for single-supply circuits, that is where there is only a positive voltage and a ground supplying the power, however, this is very rare in modern equipment, unless the entire pre-amp stage is battery powred, and even then, there are a plethora of cheap and reliable circuits that will allow a split-rail supply, which works better overall.

Nothing intended for output should be using a DC bias, as this is simply wasteful. Most devices capacitively couple the inputs and outputs, to ensure that the signal line centers on 0V. In any case, a diode will introduce unacceptable distortion into the audio.

Tovarishrob, you need a pre-amplifier to bring the level of your DVD player up to a similar level as the computer. Easiest way is to buy yourself a cheap preamp, or you can build one with not much effort.
Preamp design is very simple, and there are hundreds of designs out there, just do a search. Many designs only require one IC, a potentiometer to set gain, and a couple of resistors and some power, as simple as that.

no... two sources... one set of speakers



This is what I am hoping will happen... I think that the signal is significantly weakened by the Diversion to the PC when I want to use just the DVD player. Sorry my sceme was confusing... I use PPT 2-way arrows everywhere and It didn't explain that I want to use both the computer and the DVD player as sources.



Supra,

ok... does this still apply? Do typical headphone line-outs run AC? is there a better way to passively block the computer from sucking power out of the DVD player signal (for not too much money?)

an audio signal is AC, and supra is right. A Diode is a very bad idea. He hit the head on the nail, with the particular experience I had at the pre-amp level, anyway even if the signal was riding on a DC bias, a diode would NOT work well.

Most diodes have a .7V drop, in which voltage is lost across them, in their reverse bias. This would reak havok on a pre-amp signal.

I understood the two sources - one set of speakers on your diagram, but for somereason my brain realized what you were doing, but my hands typed something differently. Oh well.....

In short, the only way your going to pull this off, is either figure out a way to put switches in line on the mini's to isolate the DVD player from the PC, or teach the wife!

A logic circuit could be designed to autosense the incoming signalt, then tri-state buffers could be used as logic relays to auto-select the appropriate source, but I dont think this application would be worth the effort of the design, the time to build, nor the cost of parts

Get a switch for the 2 sources, so that only one is on at a time.

Many items "short" the outputs through a low value resistor when they're off, so that they don't have a "turn on tump." This may be what's happening with your devices.

Using a multi-pole switch to switch between inputs is a perfectly valid approach, and works well. These are avaialble cheaply (Or at least relatively so.)

A small pre-amp circuit will also make sure that you can use the signal well. This can be as simple as a single op-amp chip (dual op-amp like a TL072 for example) which will handle impedance matching for you. TL072 is a CHEAP component, but will handle low or unity gain with a very clean signal. More than fast enough for audio signals. Such a filter would be easy to do yourself. If you're going with active components, then it wouldn't be too hard to make the system work with a single switch to handle all 6 channels, either. This is probably fancier than you need.

I'd offer to build you one for the price of components, but I'm kinda pressed for time, and I don't know how long it would take before I got the time to build the circuit. Besides, it's an active circuit, so it would need power. A small AC adapter should do the trick, the problem will be getting one that puts out the appropriate voltage, 12VAC woud be perfect, and 9VAC would be acceptable, 100mA would be sufficient current, I'd think.

Thanks for the help guys... but barring another MS in EE i think I'll tackle the hurdle of teaching the wife.

Steve,

You never replied to my last crazy idea in the outland... transmitting wireless VGA using the laser pointers...



http://www.devtoys.com/Default.aspx




Supra, What do you think of the prospects? (the cool factor here would get me to learn enough EE to make it work...)

Well, for one, you don't need 15 signals to transmit VGA. Not all of the pins are used.

The biggest problem though is that VGA is an analogue medium, and your lasers wouldn't be linear enough to make it go.

Second, there's 2 way commiunication with the monitor. That's why you can't just put a splitter in to control multiple monitors. (Well, you can TRY, but it doesn't work well.)

Third, there would be signal lag. It takes time for a solar cell to build up a voltage. When you need milisecond response time for synch signals and the like, the best that you could transmit would be noisy and glitchy AT BEST.

I had the same problem as the OP. I ended up buying a PC data switch with 9 pin connectors. I bought a bunch of 6ft male to male audio extension cables and cut them in half. I then hooked the cables up to 9 pin d-sub connectors and I was set. 3 cables go to pc#1, 3 cables go to pc#2, 3 cables go to my speaker inputs. I was a bit afraid of noise, but haven't noticed any thus far. I wasn't able to find any commercial switches for 5.1 audio... if anyone knows of one please post it.

I agree... Diodes will be a problem on many levels. If you have a little money and lots of time you can roll your own mixer. Here is a simple audio mixer: 2 ins and one mixed(both) out. If there is interest I will post a complete design.

Click to view attachment

www.bridgeelectronics.com

OK, forget about diodes for audio. They are useless on many levels. Normal silicon diodes have a 0.7V forward voltage drop which will kill most of the audio signal and leave only the peaks. Schottky diodes bring that down to ~0.3V, but it will still distort the signal heavily. Besides the fact that diodes only let current go in one direction (DC) and audio is AC! Unless you bias the audio on DC level, diodes are useless.

To mix audio from several sources simply using resistors is one way. Connect two resistors to the sources and the other ends together to the output. Grounds connected all together of course. Experiment with resistors close to the impedance of the audio signals (10Kohm is typical I believe).

Headphone splitters simply connect both inputs together. If one is driving low and the other one is driving audio, this will short the audio out if the impedance is 0 (which it may very well be with some audio sources). In this case the computer probably has a lower impedance than the DVD player, and grounds its signal. Resistors will not allow either source to blatantly affect the other, although there will be some differences. If one source mutes the other while not playing anything or turned off, it is applying a DC voltage, so use capacitors to stop it. Experiment with the values, but I believe it should be on the uF scale, try 1, 10, and 100uF and see what you like best. Lower values will tend to kill bass / lower frequencies, while higher values will take more time to settle to DC changes (and are larger physically).

If one source is significantly louder than the other and you can't compensate using volume controls, and/or you want to mix them, use variable resistors as volume controls. A set of two resistors, one from the input, another one from ground, joined together at a common point that you can connect to your mixer, will serve as a voltage divider. Varying the relation between the values will vary the output voltage, where the output will be R1/(R1+R2) times as loud as the input. So, if R1 and R2 are the same, output volume will be 50% (linear scale, so likely you will not perceive it as 50% since the human ear is logarithmic. Experiment.). A variable resistor (potentiometer) would be connected with left to source, right to ground, and center to mixer. Be sure to use values close to the impedance to minimize issues.

The audio mixer circuit shown above is using exactly that: resistors to mix the audio, plus capacitors to kill the DC level (you might want to try using them too), and two op-amps as line amplifiers. If the sound level is too low, try using the op-amps too, but then you'll need a power source ( a simple wall-ward adaptor will work)

So, to summarize: diodes are a no-no. Resistors mix audio. Capacitors make sure that what gets through is only audio and not killer DC signals (which can kill the other audio source). So, capacitors and resistors are likely what you want: caps to prevent the PC from killing the DVD when idle, and resistors to mix both signals into one. You could also try only capacitors with no resistors - not sure how that would turn out, but if you try it it would be interesting to hear the results.

I'm not an audio master BTW, so if anything that I've said doesn't work I take no responsibility (I'm more used to the digital domain, but those tips about audio signals should about work for the most part).

Oh, I'm new here You'll probably see me quite a bit on the electronics forum

easiest thing to do would be to run your dvd player into your line in of your soundcard. if you go that route you will only get 2 channels. if you want 5.1 get a soundcard with optical or coax in and run it to the soundcard. that way your soundcard will take the sound from the dvd player and pass it to your speakers.