Hi Guys
I have a new LG TV that has pretty reasonable internal speakers. These are driven by bridged class-D PAs, so the only analog signal in the entire circuit is what goes to the speakers. This is fine for what I want to do, and lets me use the TV volume control.
I added jacks across the bridged outputs with a switch to disconnect the internal speakers. I have a box with a pair of 10k:10k line transformers to tap the TV signals and isolate the ground between the TV and signal - besides, the TV is floating. I may have to attenuate the secondary side of the TXs going into the stereo, but I'm not that afar along yet.
I know that class-D amps are load sensitive, which leads to my question:
Do the amps need a load resistor of the same value as the disconnected speakers to exhibit low-distortion? or to have proper frequency response? or even just to function properly?
Or can I use a higher value load resistor inside the box to reduce wasted heat?
Thanks
I have a new LG TV that has pretty reasonable internal speakers. These are driven by bridged class-D PAs, so the only analog signal in the entire circuit is what goes to the speakers. This is fine for what I want to do, and lets me use the TV volume control.
I added jacks across the bridged outputs with a switch to disconnect the internal speakers. I have a box with a pair of 10k:10k line transformers to tap the TV signals and isolate the ground between the TV and signal - besides, the TV is floating. I may have to attenuate the secondary side of the TXs going into the stereo, but I'm not that afar along yet.
I know that class-D amps are load sensitive, which leads to my question:
Do the amps need a load resistor of the same value as the disconnected speakers to exhibit low-distortion? or to have proper frequency response? or even just to function properly?
Or can I use a higher value load resistor inside the box to reduce wasted heat?
Thanks
You have a new tv. It has an optical output, buy an optical to analog converter. They only cost a couple of $$. That's what I did for my wife'snew tv and old receiver. Most class D amplifiers are analog input. Class D amplifiers for the most part are not digital amplifiers.
Hi Mutlisync
My "new" LG is actually last year's model and I chose that for specific reasons.
Class-D amplifiers are not digital - the D has nothing to do with that and was simply an arbitrary assignment back in the 1960s when Sinclair and other needed a new amplifier class designation.
I tried using a gizmo that was supposed to take audio from the HDMI output, but the device did not work and I returned it
I just want to know if I have to give the class-D PAs the load they expect (8R), or if it is okay to run them with an essentially open output (10k), like you can do with linear PAs.
My "new" LG is actually last year's model and I chose that for specific reasons.
Class-D amplifiers are not digital - the D has nothing to do with that and was simply an arbitrary assignment back in the 1960s when Sinclair and other needed a new amplifier class designation.
I tried using a gizmo that was supposed to take audio from the HDMI output, but the device did not work and I returned it
I just want to know if I have to give the class-D PAs the load they expect (8R), or if it is okay to run them with an essentially open output (10k), like you can do with linear PAs.
Hi. Amplifiers inside tv even many years ago were fully digital, even in times of old samsung 1024x768 . For proper function they needs a load ,because of output filter , which may cause problems ,when unloaded. But you may increase load resistance slightly. As i saw , tv uses speakers 16-32 ohms mostly ,probably to reduce current. So look what's impedance of your original speakers are ,if they are 16 ohms , you may put a resistor like 22ohms as load. Also load required for damping your's isolatin transformers , to prevent windings inductance to resonate with filters output capacitor. But i'm worried more about your mentioned switch, which disconnects speakers. Amplifier of d-class must always have some load, so your switch must be switched only when tv is not operating for amplifiers safety.
The output filter will peak a lot without load - assuming there is an output filter, that the designers didn't rely on the loudspeaker itself to do the filtering to save an inductor and a capacitor.
Theoretical you are right, but - practically any speaker coil provides enough inductance to give enough rise in impedance over frequency - so there is no dampening in the resonance region about 50kHz. In other words - output resonance works generally undampended like open circuit. And as analog signal reproduced by DACs is strictly band-limited, there is no danger to excite this resonant tank. So I would not care about any dummy load here. Care should be taken not to overload the line transformer with speaker levels.
Hi nauta
You will play on the safe side if you keep loaded the internal class Ds with a dummy of approx. the nominal resistance of the internal speakers. Some 10W will be OK
George
Hi guys
Thanks for the replies.
I was figuring I would need a same-value load resistor. Thanks, George.
The audio power is listed simply as "20W per channel" without "RMS", or any other qualifier, so I am assuming RMS and therefore about 12Vrms maximum signal. My transformers are Hammond 140UEX, which can easily handle this voltage. Data sheet below. They are in mu-metal cans even though the sheet only says "metal".
On the secondary side I have a 9k09 + 1k divider on a switch to pad the signal out to my stereo, or not, as needed if I want to use this box elsewhere. The input RCAs are floating and the output RCAs are bolted right to the metal box.
The switch I added to the TV will be set one way or the other, likely always for external sound. It gives me the capability of using external speakers, or plugging through my little box to the stereo. Since this was the previous-year model, the store was selling them at a discount. We were amazed at the picture quality and even the innate sound, and I bought a second one to have in my shop. The latter may initially use external speakers. I got an even better discount on the second one as it was the display model. For both of them, we only use it with a bluray player - no streaming or internet connections.
I mentioned previously that there were some specific reasons for getting the old model versus the current one. One reason was that most of the new features were for all the streaming we do not do, so a wasted expense. The next is that the old version used an infrared remote instead of blue-tooth. We have no wireless devices (meaning RF wireless) in our house. Our ethernet is wired, and so are our land-line phones. The 2.4GHz is a dangerous frequency; higher or lower is safer ( until you get into high-microwaves / low-x-rays/gamma rays).
I remember seeing mid-1960s copies of Wireless World (now Electronics World) where Sinclair had a five-transistor class-D power amp. Minimal filtering at the output, relying on the speaker to do the bulk of signal integration. BJTs were very expensive then 🙂
Thanks for the replies.
I was figuring I would need a same-value load resistor. Thanks, George.
The audio power is listed simply as "20W per channel" without "RMS", or any other qualifier, so I am assuming RMS and therefore about 12Vrms maximum signal. My transformers are Hammond 140UEX, which can easily handle this voltage. Data sheet below. They are in mu-metal cans even though the sheet only says "metal".
On the secondary side I have a 9k09 + 1k divider on a switch to pad the signal out to my stereo, or not, as needed if I want to use this box elsewhere. The input RCAs are floating and the output RCAs are bolted right to the metal box.
The switch I added to the TV will be set one way or the other, likely always for external sound. It gives me the capability of using external speakers, or plugging through my little box to the stereo. Since this was the previous-year model, the store was selling them at a discount. We were amazed at the picture quality and even the innate sound, and I bought a second one to have in my shop. The latter may initially use external speakers. I got an even better discount on the second one as it was the display model. For both of them, we only use it with a bluray player - no streaming or internet connections.
I mentioned previously that there were some specific reasons for getting the old model versus the current one. One reason was that most of the new features were for all the streaming we do not do, so a wasted expense. The next is that the old version used an infrared remote instead of blue-tooth. We have no wireless devices (meaning RF wireless) in our house. Our ethernet is wired, and so are our land-line phones. The 2.4GHz is a dangerous frequency; higher or lower is safer ( until you get into high-microwaves / low-x-rays/gamma rays).
I remember seeing mid-1960s copies of Wireless World (now Electronics World) where Sinclair had a five-transistor class-D power amp. Minimal filtering at the output, relying on the speaker to do the bulk of signal integration. BJTs were very expensive then 🙂
Actually datasheet don't lists max voltage. I think it's better to put divider before transformer, but resistors need to be changed. Also data about max allowable dc is missing, so at best you should connect bipolar capacitor in series with primary winding.
As mentioned in post #2, the best way to extract audio is to use a simple and inexpensive Optical-to-RCA adapter to feed an external amp/speakers. - or even a pair of powered/wireless speakers.
Because putzing around and guessing with the internal amp is risky business.
With my LED tv I use the Optical audio output, which varies with the TV volume setting even if the internal speakers are turned off via the menu.
Because putzing around and guessing with the internal amp is risky business.
With my LED tv I use the Optical audio output, which varies with the TV volume setting even if the internal speakers are turned off via the menu.
Just add more integrators. No need to put dummy load add a 2 order passive low pass filter at output Of the class d amplifier. Resistor capacitor resistor capacitor.. Fc at 40khz. Class d are not load sensitive in general. The frequency response is load sensitive and that only include output filter. You can change output filter as you like to get appropriate response in the speaker . Basically speaker being part of the filter itself. If you remove speaker from equation you left with pwm signal which can be integrated as u like . A typical amplifier is 10k input impedance. So you can model the speaker as 10k .
All the babble over such a simple issue.
I guess some people just adore being overly complex and discussing things into the next millenium.
For a lousy $30 and a five minute job, the problem can be done, and call it a day and move on.
https://www.bestbuy.com/site/insign...nalog-converter-black/4226000.p?skuId=4226000
I guess some people just adore being overly complex and discussing things into the next millenium.
For a lousy $30 and a five minute job, the problem can be done, and call it a day and move on.
https://www.bestbuy.com/site/insign...nalog-converter-black/4226000.p?skuId=4226000
Here is another one.
https://www.ebay.ca/itm/Optical-Coa...1-0910-416b-b1db-7ccbf1b4cfb8&redirect=mobile
https://www.ebay.ca/itm/Optical-Coa...1-0910-416b-b1db-7ccbf1b4cfb8&redirect=mobile
It makes things MUCH easier when suggesting a source for parts IF we know the person's LOCATION.
Ok , would be probably modern solution . But as op mentioned , he has transformers already, also output socket and switch are connected too, so such device would mean that earlier done work was kinda useless. Also , this optical to rca converter requires power to operate , how you will control it's psu , while turning off tv with remote in example ? It will stay always on probably , thats acceptable not for everyone.
An unloaded class d output will have the output filter ringing and have high volts on it.
If output filter capacitor voltage rating is not high enough it can blow.
Or the high output volts will damage next item in the line.
If output filter capacitor voltage rating is not high enough it can blow.
Or the high output volts will damage next item in the line.