TV's Optical Digital Video Out directly to Speakers?

crutschow

Joined Mar 14, 2008
34,281
Okay. Can you tell me how to connect them and what the real life vs. theoretical advantage is compared to our original plan?
Here's the connection using three 3.9Ω resistors in parallel, connected in series with the speaker.
upload_2018-3-23_8-29-30.png
Theoretical Advantage:
Gives an exact 4 ohms to the amplifier, and has a higher power rating of 6W total so the resistors have less stress.

Real Life:
Likely will be little noticeable difference in the sound, the amplifier operation, or the resistor life, between using the three 3.9Ω resistors versus one 1Ω resistor.
 

Thread Starter

c627627

Joined May 18, 2011
63
Thank you.
So I just got the resistors. Yes it took a full month for them to get here from a Chinese eBay seller.
The package containing the amp however came with nothing inside + a USPS letter that it was lost, someone must have swiped it en route from a shipping container. This is just a fun project for me so I'm not too upset, the seller has refunded the inexpensive amp purchase anyway, but I am looking to put the transistors and speakers to good use.

To that end, I recently reacquired my original Plasma TV which many years ago was released without a tuner and with no speakers. I have the tuner. Now to connect the speakers.

This is what the TV manual says:
SPEAKERS (6 Ω) 16W [8 W + 8 W] (10 % THD)

TVManualSpeakers.jpg


PlasmaTVBack.png


As posted earlier I have 2.7Ω 7.5W speakers.
I now also have Twenty 1Ω, 2W resistors.

What happens when connecting the speakers without any resistors? Would you need a really high volume to blow them? How can I connect and use the resistors to now make this new project work?
Thank you for your advice.
 

crutschow

Joined Mar 14, 2008
34,281
What happens when connecting the speakers without any resistors? Would you need a really high volume to blow them? How can I connect and use the resistors to now make this new project work?
You could connect them directly as long as you keep the volume at a normal level.
A high volume level would more likely burn out the amp, not the speakers.

To match the 6Ω amp to the 2.7Ω speakers you need 6Ω - 2.7Ω = 3.3Ω in series with each speaker
I would just add three 1Ω resistors in series, which is close enough.
That will then give a power in the 2.7Ω speakers of ≈3.6W when the amp is outputting its maximum 8W.
 

Thread Starter

c627627

Joined May 18, 2011
63
The "amp" in this case would be the TV itself, that would not be good. So like this:

SpeakerToTVSeries.jpg

What are the blue lines on this picture, just regular wire, can I use old PC power supply wire?
Thank you.
 
Last edited:

wayneh

Joined Sep 9, 2010
17,496
Any wire will work as long as it's not very thin.

But you've shown the resistors in parallel and that will accomplish next to nothing - they need to be in series.
 

wayneh

Joined Sep 9, 2010
17,496
I've corrected the image, if you refresh the page, that should be right then?
Yes, that will add 3Ω of impedance to the speaker and thereby reduce stress on the amplifier.

Maybe. This will also reduce the volume produced at a given setting. I think of you turn up the knob to drive the speaker to the same volume, there will be equal stress on the amplifier that there was before. I think. This arrangement reduces the chance of inadvertent damage if the volume is set too high, but it comes at the cost of speaker volume at a given setting.

A more elaborate and efficient way to accomplish this is an impedance matching transformer. I'm not suggesting this for you, just pointing out that there is such a thing.
 

Thread Starter

c627627

Joined May 18, 2011
63
Thank you for this information. I look forward to testing this in real life, the volume differences you described.
Thanks for pointing that out.
 

crutschow

Joined Mar 14, 2008
34,281
I think of you turn up the knob to drive the speaker to the same volume, there will be equal stress on the amplifier that there was before. I think.
Not quite.
With no series resistors, the output voltage is lower for a given output current, which means the amp would be dissipating more power (unless it's a Class D amp).
 

Thread Starter

c627627

Joined May 18, 2011
63
So you are saying the volume difference would go in the other direction? How significant do you think the difference would be?

Hey what if I tested it, the TV is connected to my computer, maybe if I had a consistent tone sound file and I could use something to measure the output.

Any ideas of a consistent tone file and what program can I use to measure this?
 

crutschow

Joined Mar 14, 2008
34,281
With the 3Ω of series resistors and your 2.7Ω speaker, you will get about 3.6W of speaker power versus 8W with a 6Ω speaker.
That's about a -3.5dB reduction in the perceived sound level, noticeable but not a large amount.
Generally you can just perceive a 1dB change in sound level.
Unless you operate your TV at loud sound levels, I think you will be fine.
 

Deleted member 115935

Joined Dec 31, 1969
0
Just a point of interest.
If the speakers are used in open air, they are going to sound 'horrible'
You need to stop the rear sound of the speaker reaching the front sound
Like a box,
 

Thread Starter

c627627

Joined May 18, 2011
63
Okay thank you.
Finally, what if things were in the other direction:
If I have the TV asking for SPEAKERS (6 Ω) 16W [8 W + 8 W] (10 % THD)
and if I instead I had two speakers that are 8 Ω speakers, which do not appear to have any watt value assigned to them, it just says that their rated impedance is 8 Ω?
 

crutschow

Joined Mar 14, 2008
34,281
if I instead I had two speakers that are 8 Ω speakers, which do not appear to have any watt value assigned to them, it just says that their rated impedance is 8 Ω?
No problem.
The speakers sound will likely seriously distort long before you apply enough audio power to damage them.
 

Thread Starter

c627627

Joined May 18, 2011
63
You mentioned the amp being damaged if the speaker Ω value was lower.
So if the speaker Ω value was instead higher, as in this case using 8 Ω speakers instead of 6 Ω speakers, the 6 Ω amp inside the TV will not be damaged, it's the 8 Ω speakers that would be damaged instead, and then only if you keep the volume high even after you hear serious speaker distortion, does that summarize it correctly?
 

crutschow

Joined Mar 14, 2008
34,281
So if the speaker Ω value was instead higher, as in this case using 8 Ω speakers instead of 6 Ω speakers, the 6 Ω amp inside the TV will not be damaged, it's the 8 Ω speakers that would be damaged instead, and then only if you keep the volume high even after you hear serious speaker distortion, does that summarize it correctly?
More or less.
A higher impedance speaker puts a smaller load on the amp so it won't be damaged.
A amp is perfectly fine with a infinite impedance (no)load.

But why are you referring to the 6Ω speakers inside the set?
Are you planning on having those connected also?
That's a different kettle of fish.
 
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