problems using KD55-AG9 as centre speaker in an audio system

[merseyswine]

I am having audio quality problems when I use the TV as the centre speaker on my 5.1 audio system. When I crank up the system volume above half way, the audio goes intermittent on a rapid staccato cycle. It's ok at low volume. I wonder if it is an impedance mismatch problem but I don't know what the TV's centre-speaker input impedance is. My system is designed to work with 8 Ohm speakers but my previous centre speaker was a 6 Ohm, 120W Wharfedale and it works fine with that speaker. Or perhaps my audio system (50W per channel) is over-driving the TV's speakers at higher volumes.

[royabrown2]

@merseyswine 

Sounds like motor-boating, the sound of an unstable amplifier in considerable distress, and liable to blow up an expensive TV, especially as this set provides its audio by vibrating the screen.

So we had better hope it is an amp, with good overload protection (though it does not sound like it), and not the two 20W actuators hitting the bump stops.

How are you even accessing the TV’s speaker at anything other than line levels and impedances? I.e. what output from your amp are you feeding into what input on the TV?

Whatever it is, I suggest you stop using the TV like this, even at lower levels, until we can sort out what is going on.

[merseyswine]

 I’ve just been feeding the “centre speaker” output from the amp to the appropriate input on the TV via a length of speaker cable and selecting the “audio system” option on the TV. After seeing your message, I’ve now stopped doing that and I’ve gone back to using my Wharfedale centre speaker. It’s a bit of a bummer, though, as I really wanted to use the TV as the centre speaker. But I don’t want to have to replace the amp (a nice Cambridge Audio unit) The TV sound seems to be working just fine, thankfully. Thanks for your advice.

[royabrown2]

@merseyswine 

I’m taking it that the centre speaker output from the amp is a pair of terminals intended to feed a speaker - high output, low impedance, varies with the volume control?

And you will have had to terminate the speaker cable with two RCA plugs (or just maybe a 3.5mm minijack, though I think that unlikely) which might have been a clue that doing this would be a little, er, unusual?

The RCA connections on these TVS are expecting a low power, constant level input into a high impedance. So not a good electrical match.

If it’s any consolation, though, matching the centre speaker to the front L R speaker pair is possibly the major consideration in 5.1, and the TV is rather unlikely to have been a good sonic match.

I don’t suppose your amp has line level (RCA phono sockets) outs for centre speaker, intended to be used with a powered speaker? But if it does, you could safely try that with the TV, though you would probably need to adjust the volume of that speaker, separately with the TV remote, every time you turned the amp up or down.

Might be worth trying, to convince you that the TV would not make a good matching centre, though; or alternatively, if it does make a good match, that it’s worth the hassle of two volume controls.

[merseyswine]

ok -there are quire a number of issues in your latest reply. Thanks for taking the trouble, by the way.

1. Physical connections. The output terminals on the amp are the binding-post type that accept bare wires that you poke into a hole and then tighten the sleeve. The terminals on the TV center-speaker terminals are the same sort of thing. Neither accept jacks or RCA connectors.

2. Impedance, etc. The amp is designed for 8 Ohm passive speakers only. The output varies with the volume control and there is no line output.

3. Use of TV as a centre speaker. According to the tech info, the TV acts as a passive speaker when used in centre-speaker mode. There is no possibility of using it as an active speaker.

4. TV vs. Wharfedale as centre speaker. The sound is better with the Wharfedale, when watching Blurays. I'm disappointed, as I wanted to eliminate the latter. But at least I get the benefit of the TV's acoustic surface + technology when watching ordinary TV without using the sound system. It's a cut above the soundbar that I had.

The next issue for me will be that my system is only 5.1, so I will need a Dolby Atmos system. But the Missus would have a fit, since I already spent £2400 on the TV, a new Bluray player and new HDMI cables. Roll on Christmas.

[royabrown2]

@merseyswine 

Centre speaker mode - who would have thought it? I completely missed that - apologies 😢

in which case, the instability is in your amp - probably because it wants to drive into 8 ohms, but the centre speaker is only 4 ohms, or possibly because of it being a lower wattage the centre speaker can handle compared to what the amp can provide.

Both things could potentially be fixed with a 4 ohm resistor, rated for whatever wattage the amp can provide, in series in the run of cable.

Do you know what the resistance of the Sony TV centre speaker is, in ohms, and the power handling capability in watts? I can’t find these details in the literature, and the ‘bot won’t even offer me the AG9.....

[merseyswine]

I don’t know the impedance of the TV speaker but I’ll see if I can find out from Sony. The amp is specified for 8 Ohm speakers but all my current 5.1 speakers are 6 Ohm and it seems happy with those.

The amp’s output is rated at 50W RMS per channel. I don’t know how much power the TV can handle but its output is 20W (RMS or peak? They don’t say) for each of the main speaker actuators. So I guess it’s likely that the amp is just trying to blow up the TV when I crank up the volume.

[merseyswine]

Further to my earlier reply, I just found out from the Sony website that, with Android TVs that can be used as the centre speaker of an audio system,  the input impedance of the TV speaker is 8 Ohms. They say the TV can be used with an amp that has up to 200 Watts (they don't say if it's peak or RMS) when connected to an audio system.

Does this men that the problem is with my amp? It works fine when connected to the Wharfedale 6 Ohm centre speaker. The problem with the using the TV with the amp only occurs if I turn up the volume to a level that I like to have on action movies. It's not excessive, by any means.

[merseyswine]

I seem to have solved the problem. I found that one of the front speakers had become disconnected. Now I've reconnected it, I can use the TV in centre-speaker mode even at high volume. I guess the amp was going unstable with the one speaker disconnected. I know little or nothing about amps!