routing 2 channel audio to 5.1 speaker system

[Driber]

I have a Genius 5.1 speaker system that looks something like this:

https://www.newegg.com/genius-sw-5-1-1500/p/N82E16836179002

I have it connected to my TV using only a stereo connection. On the back I have 6 RCA inputs for all the separate channels, including a dedicated LFE channel. The problem is that when the source is 2 channel stereo there is no bass as the LFE channel is not being fed. What I've done up until now is split the right channel source and route it to both the right channel input and the LFE input.

The downside of this is that sometimes there is a sound on the left channel only which then results in a bass-less output. Obviously I can't split both left and right to the LFE input as that would combine both channels and the stereo audio would become mono.

My idea is to make a splitter using diodes. I came up with the following schematic and would love to hear some feedback on whether or not this is going to work.




This should theoretically combine both L and R channels and feed it to the LFE channel while preventing the FL and FR inputs from being directly connected together.

If this would work, is there any specific type of diode I should get, or could I just use any?

Looking around eBay I came across the following listings:

https://www.ebay.com/itm/260956278011?hash=item3cc234ccfb:g:hb0AAOxyY3ZRzZRA

https://www.ebay.com/itm/133672189448?hash=item1f1f7bbe08:g:FSgAAOSwnCdcJuDL

Is there any difference?

[Mod Note: resized huge image]

 

[Audioguru]

We NEVER use a diode to feed audio. It produces EXTREME distortion. Diodes can mix DC voltages but audio is AC voltages.
You need an audio mixer product that has a frequencies crossover circuit that feeds high frequencies to the little satellite speaker amplifiers and feeds only the deep low frequencies to the subwoofer amplifier. It might already mix the left and right low frequency audio signals properly to feed the subwoofer amplifier since usually an audio system has only one subwoofer..

 

[VenomBallistics]

You probably want an active solution rather than a passive one.
Unity gain op amps can give you the isolation you need to maintain right and left channel signal integrity, the mix them post op amps to feed to the sub

 

[dave9]

^ Or a single opamp, with the L & R inputs pulled down by resistors for faux-isolation, then opamp gain as needed. I'd not do it but it's only bass.

I wonder though if it's worth the bother. That Genius setup is only a 20W sub and 5W satellites and that probably at high THD. We can go along with the intention to Make This Work, but the result may not be any better than just throwing together some basic class A/B amp chip circuit, no more difficult to implement than the opamp (few dollars for a PSU and bigger caps), and only feeding two channels but with better speakers.

I'm just suggesting that the outcome should be worth the effort. If on a tight budget, sell the 5.1 setup, buy used speakers, or even a whole amp if you don't want to DIY... basic datasheet example chipamp circuits are easy to do but by the time you add up all the little misc parts, unless you have parts bins waiting for you to use them already), rolling your own amp can cost more than something used on craigslist or ebay, or even one of those ~$15, ready made class D amp boards on ebay is likely to affect sound quality less than the speaker change because that Genius setup is sort of a token gesture at 5.1... it does it but a lot of concessions to get there, none of which are desirable if feeding it only 2 channels.

Then again with enough searching, someone in China probably already makes (and sells on ebay or alibaba, etc) a unity gain dual opamp widget that is ready or can be easily modded to do as VenomBallistics suggested, so you aren't buying a part at a time at premium prices + shipping ,would be the least expensive and fastest (least work) path to the solution, depending on search skill and time invested to find that product. I'd imagine something like that could cost under $4 delivered if you can wait a month to get it.

 

[Driber]

Thanks for the responses, everyone.

The op amp thing sounds interesting. Would something like this work for my use case?

https://www.ebay.com/itm/184348160543

Looking up the part number of this eBay listing gives me the following datasheet:

https://www.analog.com/media/en/technical-documentation/data-sheets/AD827.pdf

My aim is to fix up something inexpensive using the speaker system that I already have. I'm not an audiophile and apart from this little LFE channel shortcoming I'm quite happy with the system.

I'm more than willing to spend time and effort into a solution as I love to DIY and tinker with things. I also don't mind waiting weeks for eBay items to arrive, I'm quite used to it

 

[Audioguru]

The appearance and extremely low price of the ebay very high frequency AD827 dual opamp smells like a fake.
Ebay sells one at $1.02US.
Digikey sells one at $27.80US.
Mouser sells one at $26.48US.
Why buy a fake very high frequency opamp made for video (and what will you get at that extremely low price?) from ebay instead of a much more stable and less expensive audio opamp sold locally by a reputable real electronics parts distributor?

Why doesn't your cheap 5.1 audio system already have a frequencies crossover circuit and a mixer that already feeds mono low frequencies to the subwoofer amplifier? Did you ever use it as a 5.1 or as a 2.1 system? What audio source has a stereo output plus a subwoofer output?

 

[Driber]

Update, I managed to fix my issue. I looked elsewhere on the web for other solutions and someone suggested using resistors instead of using diodes. So I gave that a try and it works perfectly. I used 10k ohm resistors where the diodes are in the schematic mockup image from my first post.

And I didn't even need to purchase any overpriced 25 dollar chips that surely only cost a tiny fraction of that to actually manufacture, more closer to the "fake" eBay listing, I bet.

Why doesn't your cheap 5.1 audio system already have a frequencies crossover circuit and a mixer that already feeds mono low frequencies to the subwoofer amplifier?

I think you already answered your own question - because it's cheap

Did you ever use it as a 5.1 or as a 2.1 system? What audio source has a stereo output plus a subwoofer output?

I did, many years ago. I initially got this system for hooking up to a PC with dedicated surround (5.1) audio outputs. But now I have it hooked up to my TV audio out, which is just 2 channel stereo.

 

[Audioguru]

Ebay and you mentioned a very expensive AD827 opamp that works as high as 50MHz. Audio goes only to 20kHz so why pay a lot for video and radio frequencies when a much less expensive audio opamp is available. A fake ebay opamp can be VERY cheap.

 

[crutschow]

I looked elsewhere on the web for other solutions and someone suggested using resistors instead of using diodes. So I gave that a try and it works perfectly.

Yes.
I was going to suggest that as it's a perfectly good solution.
I'm surprised no one else here did.

 

[Audioguru]

He has a stereo system. A mixer as simple as two resistors reduces the channels separation.
There are tweeters and a woofer. Where is the crossover network?

 

[crutschow]

A mixer as simple as two resistors reduces the channels separation.

Only if the source impedance is not low compared to the resistors in the network.
I would expect the TV audio output impedance to be low.

 

[Audioguru]

Some TVs have an electrically "alive" common because they are missing a power transformer, then they do not have outputs. Connecting anything to the internal low impedance speaker wires can cause electrocution.

If a TV has stereo aux outputs then since many Noobs short them together for mono then the outputs are not a low impedance and the two resistors combining the outputs for the bass amplifier will reduce levels and reduce stereo separation.

 

[crutschow]

If a TV has stereo aux outputs then since many Noobs short them together for mono then the outputs are not a low impedance and the two resistors combining the outputs for the bass amplifier will reduce levels and reduce stereo separation.

I'm confused.
Obviously if you tie the two outputs directly together you will have no stereo separation.

I'm referring to connecting the two (line level) stereo outputs together through two equal value resistors (e.g. 10kΩ) for the subwoofer input.
That should have no significant effect on the stereo separation of the two outputs.

 

[Audioguru]

An audio mixer circuit uses an inverting opamp so that the inputs are separated from each other. Two resistors also are a mixer but they reduce separation if the input signals have any series resistance.

The audio amplifier/speakers in this thread might be a little portable with the speakers so close together that it does not produce any stereo separation anyway.

 

[Driber]

Just to clarify, with this solution involving resistors I'm not hearing any noticeable reduction in stereo separation. How it was explained to me on the other forum is that a 10k ohm resistor will let through (most) low frequency audio while blocking (most) high frequency audio.

I ran this stereo test clip several times before the change, intermediate testing during soldering the wires together, and afterwards when everything was all finished and set up. It sounds exactly the same - left channel audio comes from the left speaker and right channel audio comes from the right speaker, plus now with the added bass coming from the subwoofer.

Ebay and you mentioned a very expensive AD827 opamp that works as high as 50MHz. Audio goes only to 20kHz so why pay a lot for video and radio frequencies when a much less expensive audio opamp is available. A fake ebay opamp can be VERY cheap.

This thread is literally the first time I've ever heard of opamps so I know next to nothing about them, that's why I posted a link of an eBay item I found when simply doing a search for "op amp" on eBay, and asking for advice.

I don't exactly know what makes a chip "fake" when it's just something generic and not branded or anything, but it looks like a very simple chip containing something like a couple of transistors or something, so my guess is that 25 dollars is way overpriced for something like that. But maybe I'm wrong.

I just know that often eBay listings for generic chips only cost a couple of bucks when it's in fact coming from the exact same (Chinese) manufacturer that also makes them for big brands like Apple, etc. So it's often the big brands that place a huge mark-up on components that are otherwise dirt cheap to produce.

The audio amplifier/speakers in this thread might be a little portable with the speakers so close together that it does not produce any stereo separation anyway.

As I stated in the first post, reduction of stereo effects was my main concern about connecting left and right together

The front left and front right speakers are about 360 cm (12 feet) apart, with my TV sitting right in between, which is pretty standard for a home cinema setup I'm guessing?

 

[bertus]

Hello,

Have a look at this article:
https://audioxpress.com/article/Build-a-Passive-Surround-Sound-Decoder

As delay, you could use some PT2399 board.

Bertus

 

[Audioguru]

Just to clarify, with this solution involving resistors I'm not hearing any noticeable reduction in stereo separation.

Good, then the TV audio source has a very low output resistance (opamps).

How it was explained to me on the other forum is that a 10k ohm resistor will let through (most) low frequency audio while blocking (most) high frequency audio.

Absolutely not! An inductor does that, for audio it will be huge. A resistor passes all audio frequencies, ultrasonic frequencies and many very high radio frequencies.

I posted a link of an eBay item I found when simply doing a search for "op amp" on eBay.
My guess is that 25 dollars is way overpriced.

The 25 dollars IC is a special very high frequency one. Such high frequency performance is rare and expensive. The ebay seller picksup and sells garbage, fakes and defective junk which is why ebay sells the $25 opamp for only $1.

I don't exactly know what makes a chip "fake" when it's just something generic and not branded or anything.

There are huge cities in China where they make fake parts or unsolder and sell defective used parts. Then they are sold cheaply on ebay, on the other Chinese websites and Amazon.

I just know that often eBay listings for generic chips only cost a couple of bucks when it's in fact coming from the exact same (Chinese) manufacturer that also makes them for big brands like Apple, etc. So it's often the big brands that place a huge mark-up on components that are otherwise dirt cheap to produce.

Name brands have excellent quality control and test and sell only top quality parts. But ebay sells fakes that look alright but work poorly and sell the failed parts thrown out from the name brands.

As I stated in the first post, reduction of stereo effects was my main concern about connecting left and right together.

If you directly (no resistors) short together two audio channels then you might damage them.

The front left and front right speakers are about 360 cm (12 feet) apart, with my TV sitting right in between, which is pretty standard for a home cinema setup I'm guessing?

Then you will have excellent stereo separation.