#### Jim@HiTek

Joined Jul 30, 2017
20
I retired from the electronics industry almost 20 years ago but do like to dabble sometimes. Since my hearing has gotten worse over many decades, I need some assistance in that area now and wanted to install a wired amp system so the flat screen TV that I run through a surround sound system wouldn't have to be so loud. So I came up with a commercial circuit from Amazon that I thought would work as an aid. BUT...turns out the +/- speaker outputs are not referenced to ground while the headphones I'm using (and all headphones that I know of) the Lft/Rgt channels are.

So I came up with the following circuit and it works great, but I wonder if any of you audio engineers would like to comment with any mistakes I made with the values or have a better idea on how to do the same thing simply? The values I chose just happened to be in my parts supply, and they measured fine with low leakage. I originally used resistor networks connected to the + ouputs and chassis ground but found I didn't need to drop the signal amplitude so eventually dropped the resistor dropping network idea. I found they weren't needed. That setup, hooking directly to the headphone and the ground to case ground was okay, but distorted a bit, plus I needed to crank the volume up too far. So I used another set of NP caps to create a virtual ground. That really helped a lot and now I have plenty of volume and excellent tonal quality, and stereo seperation is fine too. That surprised me, as I thought using the virtual ground would somehow make separation less noticeable.

The 10uF NP caps are 16 volt, the 2X 47uF back to back caps are 25 volts each.

I'm happy with the results, really sounds much better using the headphones than using the surround sound, but wonder if someone could run a Spice audio frequency scan on this setup and maybe suggest a fine tuned set of capacitors? As I said, I only used the ones I have because they were on hand. I need voice brightened a bit, and of course hearing upper freqs is difficult so having the high end there would help too. So maybe different values would be better?

The internal TDA7266SA amplifier is rated at and capable of 2X 7 Watt at 11 volts and they provided a 12 volt, 3 amp PS so though the ad copy lists it as a 2X 18 Watt, not strictly true in application.

I would appreciate any insight about the circuit or improvements to it. All the caps shown in the above drawing are inside the amp's case and are wired to a headphone connector though it's not shown that way in the drawing. I'm not connecting any speakers and I've isolated the headphone connector from the metal case. Using line levels, the volume is pleasantly loud enough for my hearing at 25% rotation. Coarse volume is accomplished with the remote. TV audio output is provided via RCA jacks with the output set by the menu at 'Line out' levels and I've turned off the internal speakers.

#### Audioguru again

Joined Oct 21, 2019
3,207
The bridged stereo amplifier has 4 amplifiers. You are shorting together 2 of the amplifiers with capacitors.
Your 10uF capacitors cut all bass frequencies at and below 500Hz for absolutely no bass. Instead you should use a tone control circuit to boost the high frequencies that older people (like me too) need to be boosted. I have excellent hearing aids that do it.

The amplifier produces 8W per channel at very high distortion into 8 ohm speakers when the supply is 12V. With the circuit I show the maximum undistorted power into 32 ohms per channel is about 300mW which is enough to explode them and destroy your hearing. Add an attenuator or reduce the supply voltage to 7V or 8V.

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#### KeithWalker

Joined Jul 10, 2017
1,789
The circuit should work without the capacitors. The OUT1- is connected to the OUT2- inside the amplifier chip and should be connected to the sleeve of the audio jack, with the tip and ring connected to OUT1+ and OUT2+ respectively.

#### Jim@HiTek

Joined Jul 30, 2017
20
@Audioguru: I understand what you're saying but the device has been working happily several hours every evening. The sound is excellent and the stereo separation top notch. I spend last evening doing my best to uncover any distortion or spiking or voice artifacts that would lead me to believe tying those outputs together with a 47uF cap would damage anything, including holding my finger on the TDA's potential hot spot to check for overheating on the case and it stayed cool. So I think that because of the random nature of any stereo signal, the AC 'shorting' of the two outputs is minimal. Even when I detected mono, I didn't hear any artifacts in the audio, or sense an overall decrease in volume. So I'm not too worried about that since it seems to work fine, plus, this is an experiment so if it blows up, no big deal. As far as over volume blasting my ears out? Not going to happen based on my tests so far. Like I said, I only put the volume control at 25% for comfortable volume. It behaves like a normal volume control on any amp. Then use the TV remote for coarse adjustment.

I just remembered something from decades ago, applying a test signal, a 1KHz tone to the two inputs, and shorting the OUT1- and OUT2- with caps like I'm doing can cause problems. I don't recall what happened for sure though but I believe it smoked the IC. So I won't do that. And I'm always in bed before the TV stations transmit a test tone during sign off (do they still do that?). And of course I may have just overdriven the inputs.

When I finish the current project, I'll try those 250uF caps and see if that improves (or maintains) the audio quality as it would prevent difficulties with the outputs. Right now the setup sounds excellent (at least for the last few days it's sounded excellent, my hearing changes slowly over time, I have tinnitus that comes and goes), but a better design is always better. And yes I have hearing aids but it gets tiresome to wear them all the time while watching TV. Headphones are my solution for the occasional respite from them.

@Keith: Check out my link to the TDA and you'll see that OUT1- and OUT2- are certainly NOT connected together inside the chip. AudioGuru's attachment also shows that. But thanks for the input. If I followed your advice, then I would definitely be shorting out the two outputs. With the caps, there's some protection against a DC short.

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#### Jim@HiTek

Joined Jul 30, 2017
20
@Sparky: Yeah, I saw that data sheet. It mentions "Short-circuit protection • Thermal overload protection", which I count on to keep me from getting into trouble when I do something odd with their IC. I used the TDA line of ICs years ago when it first came out and it was an improvement over the audio chips of the day...needed far fewer support components and really sounded good...back when I could hear better.

#### Audioguru again

Joined Oct 21, 2019
3,207
The circuit should work without the capacitors. The OUT1- is connected to the OUT2- inside the amplifier chip and should be connected to the sleeve of the audio jack, with the tip and ring connected to OUT1+ and OUT2+ respectively.
Absolutely not! The TDA7266 stereo amplifier IC has bridged outputs. Each wire of the speakers is driven and one side is inverted which effectively almost doubles the signal voltage which almost makes 4 times the output power of one output and a ground.

You should never connect any of the outputs together which would happen if a headphones is connected like you say.

#### Audioguru again

Joined Oct 21, 2019
3,207
Do not rely on "short circuit and thermal overload" protections frequently. They are designed for occasional errors not for continuous use every day.
What happens to an IC that becomes very hot then shuts off frequently? It heats then cools over and over then breaks with thermal fatigue.
Why connect it wrongly with capacitors shorting two amplifier outputs together? Do it properly like I showed.

Your idea of shorting the - outputs together with a capacitor is a problem only for high frequencies and for stereo. One - output wants to go + tshh and the other - output has zero signal and causes a dead short at the high frequencies between the - outputs.
It has nothing to do with DC because the amplifiers are matched on the same IC. Look at the speakers directly connected, they have no DC across them.

A volume control is supposed to be logarithmic, not linear so it matches the logarithmic sensitivity of out hearing. Then if you turn up the volume from 50% to 100% the output level increases 10 times and the power increases 100 times. Turn it up by mistake and kiss your hearing goodbye.

#### KeithWalker

Joined Jul 10, 2017
1,789
Absolutely not! The TDA7266 stereo amplifier IC has bridged outputs. Each wire of the speakers is driven and one side is inverted which effectively almost doubles the signal voltage which almost makes 4 times the output power of one output and a ground.

You should never connect any of the outputs together which would happen if a headphones is connected like you say.
You are correct. I thought I had deleted my answer after double checking the datasheet.

#### Audioguru again

Joined Oct 21, 2019
3,207
Why does the old ST Micro TDA7266 7W x 2 amplifier produce so much distortion at any power more than only 2W at any frequency above low audio frequencies? It is far from hifi. Digikey has them for $5.52US each. The Texas Instruments TAM3116 is much better, 50W x 2 and$4.12US but none are in stock. Millions of them are sold on Chinese modules.