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 Post subject: Two Sound Cards or "Sources" Y'ed into One Amplifier Input
PostPosted: Jan Fri 10, 2020 6:53 am 
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hello folks,

this may sound elementary, ridiculous, or just plain outlandish, but I have a question.

can I use a Y cable and connect the audio out of two computer sound cards into an amplifier with ONE input ? also, could I connect two CD players with Y cables to and amplifier with one set of L + R inputs ?

I use two big beast dell boxes for my audio, music, and video editing and want better sound than what the "dell sound bar" provides when I am working on the projects.

I don't want to clutter up my wrap around corner computer workstation with a stereo receiver or even a small one that has multiple inputs on it. I also don't want to take the time to push buttons when I am moving back and forth between each computer and playing audio from them, sometimes at the same time. I don't want to install two little lepai amplifiers or have two sets of speakers.

I would like to have two computer towers hooked up to one amplifier with one set of speakers without an A/B switch to switch between each tower.

(boo hoo, I know... :P )

I don't have a "switch box" that would allow me to switch between computer tower 1 and computer tower 2 to send the audio to the little tri path amplifier's single input.

I want to mount one of those little Lepai 2020 amplifiers under one of the computer desk shelves, put good speakers on each side of the workstation on the floor and just hear my music better from the two towers.

I am not worried if I lose some gain by combining the outputs of the sound cards to the amplifier or if the audio suffers a tad bit on the high end or low end to the impedance mismatch that I have now created.

I just want to know if there is any damage I will do to the two sound cards (left and right channel high impedance headphone output) if they are paralleled into an amplifier with one input.

it would be like this:

left channel computer 1 + left channel computer 2 into a Y cable that parallels both computers' L channels into the single left channel cord that will go into the little amplifier's single L input

right channel computer 1 + right channel computer 2 into a Y cable that parallels both computers' R channels into the single right channel cord that will go into the little amplifier's single R input

yes, I know this might be a very stupid question and even an elementary one at that, but I have never needed to combine two stereo audio sources into an amplifier with only one input.

will I hurt anything except for some slight sound degradation or is doing something like this NOT recommended due to hurting the sound cards ?

if it is not recommended to use a Y cable that will "connect" the outputs of each sound card, is there a way I could do it to isolate the sound cards from each other before going into the single input amplifier ?

thanks for any help

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Fri 10, 2020 2:12 pm 
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Firstly, it's not a stupid question at all. The only stupid question is the one not asked....

Yes, you can parallel your computers outputs, PROVIDED that they are line level out, and not speaker or headphone level out. If the latter, I'd be concerned about damaging one or the other. You could still do it but you'd have to pad the level way down both to avoid damage to either computer, and to not overdrive the input to your receiver.

A few things to keep in mind ... there may be some loss of fidelity (frequency response) when you do this. There could also be, and probably will be, a drop in output level. Unpredictable, and probably not a big issue, since you are going to be able to make that up with the input level to your receiver.

I would start out by lowering the output level of your computer output to just over minimum, and see if it's enough level to work with your receiver.

To the final question, it would be best to "isolate" the two computers from each other. A Y cord is a brute force method of combining them. Resistor matching networks would be best. A switch box or isolation transformers would be even better. You don't need a huge switch box to do this ... a single miniature 4PDT toggle switch should do the job unless your computers provide balanced outputs.

If the computers are balanced out, forget all the above. It won't work.

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Last edited by Barry H Bennett on Jan Fri 10, 2020 3:41 pm, edited 1 time in total.

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Fri 10, 2020 3:12 pm 
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Resistor in series with each will help equalize levels (otherwise the one with lower output impedance will be louder). There may still be some level shift if one is not powered.

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Fri 10, 2020 5:50 pm 
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I agree that resistors in series would be best, otherwise the two outputs will be "fighting" each other trying to output 2 different voltages. Another thing to keep in mind, as more circuitry is connected to the input, you may add more capacitance and reduce fidelity, as has already been mentioned.


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sat 11, 2020 2:24 am 
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To answer the question if it will do any damage to the cards -- it depends. There's no specification for brute force combining active outputs and it's poor engineering practice to count on "it depends" for things to work.

A line level output is (according to Wikipedia) "typically" (grain of salt) 100 Ohm to 600 Ohm but there's no reason why it couldn't be .1 Ohm. "It depends." What is spec'd is that it drive a minimum 10K load.

One way would be to place a 10k resistor in series with both outputs and then tie the other end of the resistors together feeding the one input. That should meet the minimum impedance requirements and protect both cards. Assuming all devices meet spec, if only one is putting out music it'll be attenuated by around 10 dB since you would have, worst case, signal attenuated by 10k into a 5k load (the inactive 10K || 10k input). If the amplifier input impedance is higher than 10K (quite likely) there'll be less attenuation but 'best case' is still down 6 dB. The volume control on the amp can likely compensate for the attenuation in either case.

Of course, an active mixer solves all the 'compromises' and, if designed properly, would allow volume matching (independent gain on each channel). A simple dual opamp (12V supply) would be sufficient for a basic 2 Chanel stereo mixer. Basically, the opamp simply adds some gain to make up for the attenuation.


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sat 11, 2020 3:47 am 
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great answers

i did not think it would be so simple, especially when it comes to NOT damaging the sound cards.

they are very nice soundblaster cards that cost me some bucks.

what if i put each computer's output into a ground loop isolator and then go into the Y cable ?

https://www.walmart.com/ip/PAC-SNI-1-3- ... 0302008005

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sat 11, 2020 11:43 am 
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I don't think a ground loop isolator is going to have any effect here, one way or the other. Ground loops are not the issue here.

I concur that a simple two (or in this case dual stereo = 4 channels) mixer is simple enough, and solves ALL the pending issues.

That said, I also think simple Y cords will work just fine IF you are dealing with line output, not speaker level output, from the sound cards.

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sat 11, 2020 5:11 pm 
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yes, this is line level.

when i took apart one of those ground loop isolators once, there was a 1:1 transformer on each channel, isolating the input and output.

i too questioned the ability of a GLI in this application b/c it still is "sending a signal to the other sound card" thru the 1:1 transformer.

i just might break down and buy an A/B switch for RCA inputs.

i just wanted to get this all hooked up today as we are putting the living room all back together after a complete remodel.... :evil:

consolation: the whole house is done now :D .

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sat 11, 2020 8:56 pm 
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DR,

Look on the bay for Optimus SSM 50 mixer. This little mixer sounds unbelievable and will give great flexibility


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sun 12, 2020 3:55 am 
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4 10k Resistors seems to me to be about as simple as one can get. What's to lose? If the volume is 'too low' you simply start over, lesson learned, no harm no foul.

Put the resistors at the amplifier end. That keeps the cable runs low impedance.


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Sun 12, 2020 4:01 pm 
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i gave the resistor method a shot. it works well with minimal loss. the little amplifier has a lot of power and can easily compensate.

thanks.

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Mon 13, 2020 4:50 am 
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Dutch Rabbit wrote:
i gave the resistor method a shot. it works well with minimal loss. the little amplifier has a lot of power and can easily compensate.

thanks.

steve
Super duper.

I'm curious, did you have the 'other' channel off or powered up and just not outputting anything?

The reason I ask is it occurred to me that the 'volume' calculation may have been a tad pessimistic in the case of one or the other channel powered off since it's possible the fallow channel, if 'off', may present a high impedance and, hence, not load the other with it's 10k (it would be 10k into a high impedance rather than 10k into essentially '0' when powered).


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Mon 13, 2020 1:40 pm 
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not outputting anything

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Tue 14, 2020 10:36 am 
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Greetings to Steve and the Forum:

It seems as though you have already solved this, but here's a little article about a lattice type of resistive splitter and combiner. The neat thing about it is, while it has 6 dB loss through it, it has 40dB of isolation between the outputs (or inputs, in your case).

Attachment:
Lattice Splitter Passive.pdf [489.66 KiB]
Downloaded 7 times


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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Tue 14, 2020 9:55 pm 
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thanks jim,

if i were to use this setup, in the first picture, is it equal both ways ?

i would have to switch things up and have one output and two inputs.

steve

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Wed 15, 2020 2:05 am 
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Greetings to Steve and the Forum:

It is not the first picture that you want to use; it is the second... the one with the angled crossing wires. The first picture is the standard resistive splitter; the second picture is the lattice type splitter that provides good isolation. The text states that the device works both ways, as a splitter or combiner.

The catch is that the lattice splitter requires balanced audio lines. If I recall correctly, most computer audio cards are single-ended hi-Z.... and the lattice splitter will not work with such. I don't know what you have, but perhaps your high-end audio cards have a balanced output.

If you have only unbalanced lines to play with, you will have to use either the isolation resistors in the setup that you already have, or I think you could use the standard resistive splitter as in the first diagram. It looks to me as though it wouldn't care if all the lines were unbalanced, but best have someone else look at it to check me on this. The catch is that this "conventional" circuit has only 6 dB of isolation between the inputs (or outputs; both circuits are Newtonian, i.e. they will split or combine).

Regards,

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Wed 15, 2020 5:40 pm 
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The mixer solution is best.

On the wierd (hair pulling) side , perhaps those two things you want t0 to
Y together have supersonic audio components that would just love to
cause a hetrodyne.

Off the wall ?

Just recall what stereo multiplex demodulators did when their pilot signal
got into some tape recorders and formed a beat with the bias oscillator.

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Wed 15, 2020 6:19 pm 
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methinks we've wandered into the overthink phase of this discussion ....... ;-)

Try it with a plain Y cord. If that doesn't work, use a series resistor. If that doesn't work, try a lattice network. If that doesn't work, move up to a small active mixer, dirt cheap these days.

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Thu 16, 2020 1:17 am 
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Greetings to the Forum:

My broadcast engineering background is bleeding through. Notice the magazine where the lattice splitter article first appeared. The rule in broadcasting is never use an active device where a passive one will do. Active devices are much more prone to failure and failure in broadcasting translates to lost $$. If you have an active mixer take a dump during the super bowl, that can cost the network a million dollars. Resistors are more reliable, and therefore safer.

Just my distorted viewpoint. :D

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 Post subject: Re: Two Sound Cards or "Sources" Y'ed into One Amplifier Inp
PostPosted: Jan Thu 16, 2020 1:34 am 
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Barry H Bennett wrote:
methinks we've wandered into the overthink phase of this discussion ....... ;-)

Try it with a plain Y cord. If that doesn't work, use a series resistor. If that doesn't work, try a lattice network. If that doesn't work, move up to a small active mixer, dirt cheap these days.
The only problem with that theory is that the 'plain Y' cord might take out his expensive sound cards.

My understanding is the simple resistor method has been tried and is working just fine so I don't exactly know why this horse is still being beaten.


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