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.

Thanks in advance.

 

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.

 

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.

 

@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.

 

Another datasheet is more comprehensive specs has PCB and helpful hints. Nice amplifier project.
https://www.st.com/resource/en/datasheet/tda7266.pdf

 

@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.

 

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.

 

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.

 

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.

 

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.

 

Quote: " Do not rely on "short circuit and thermal overload" protections frequently. They are designed for occasional errors not for continuous use every day."

TOO LATE! I'm using it several hours per night and it sounds so good, I'm going to let it run until it burns out the TDA. I'll come back and report about it. It'll be fun installing a new TDA, I like that sort of thing. At that time I'll switch to the 250uF cap and to grounding the headphones directly. You've peaked my interest in if what you are saying is accurate. Not saying it's not correct, just wondering if the conclusions are accurate.

It'll be a fun test!

 

It's now Jan. 13, 2022, and my circuit modifications as shown above in the first post are still working fine. Many many hours of operation and no issues. Nothing in the circuit let any smoke out. TDA device still working fine. I used it at max volume to compensate for my hearing for the entire test.

But, being the curious sort, tonight I opened it up and replaced my setup with the setup shown above in AudioGuru's attachment 'With Headphones'. Used 220uF, 25 WV caps and moved the headphone jack common to circuit ground. Kinda hard to tell if there was an audio improvement but I will say that perhaps the bass is a somewhat stronger.

Fidelity is fine for my poor hearing situation so I'll leave it this way. But I think maybe a pre-amp would help things as I could use just a bit more range on the volume. Maybe a 3 db boost would do it. Right now, 5 Vp-p is typical output to the headphones and the PSU is 12 volts so there's room to boost output a little. Might clip on occasion but doubt if I'd notice.

So I didn't learn anything, except that in this situation, using this commercial device, connecting a bunch of caps to the outputs routed to the headphones works fine...probably. HAH! Fun with electronics.

 

A boost of +3dB is only a little louder, +10dB sounds "twice as loud".

I have "normal for my age of 76" high frequency hearing loss. My low frequency hearing sensitivity is a little less than when I was younger but it works OK. My hearing aids boost 8kHz by 52dB(!) which is about 3 times more than 17dB max treble tone controls. All the important consonant sounds in speech are at high frequencies.

 

6 db would double the 'average' voltage to 10 Vp-p (with a 12 Volt supply) so that would be a good target test. Don't need to worry to much about heat as the Kinter case is all aluminum with a large surface area and I use an open mounting (not inside anything). Need to check the specs for the TDA first of course. Might be able to boost the supply voltage to 15-18 too.

Watched a movie last night and the bass is MUCH improved. Noticed that right off. Can't tell if I lost anything at the top end frequencies but I'll set up my arbitrary waveform gen to 'sweep' the audio range today and check it out.

 

Watched a movie last night and the bass is MUCH improved. Noticed that right off.

The 10uF output capacitor in series with the two 47uF ground capacitors cut frequencies below 710Hz (all the bass) into 32 ohms.
The 250uF that I recommended cuts inaudible rumbles below 20Hz and passes all bass sounds.

 

Decided, after some generator and scope work, to install an Av=5 pre-amp to my existing audio amplifier. Working pretty well. There is some peak clipping but that's not common.

I've attached the schematic. If anyone sees any issues, let me know. [Edit: I see I forgot a decoupling cap, 0.1uF, across the LM2904 power supply. Oh, well.] I had planned on using 2.2uF throughout but found that many in my stock were mis-marked from the factory as 2.2uF but were actually 1.8uF. Huh. Well, turned out that 2 of them measured 2.2uF and used those on the inputs. They are special devices that are nice and small so they fit in the space allotted, which is why I used them, plus 2.2uF was spec'ed in a pre-amp circuit collection.





So far working well, not overheating the TDA device. Low distortion and crisp clean audio out, either with headphones or through the surround sound system this is fed to is what I'm hearing. Got chided by the neighbor about it being too loud last night. The 5X Av gain is just the right amount for my poor hearing and the excess volume last night was just a test of the TDA, listening for distortion or for overheating.

This project is not complex as some, but was fun to do. A little brain work now and again when you're retired doesn't hurt. I'm liking using commercial devices that are so readily available these days to base a circuit on. So inexpensive too. Years ago, building from scratch was the way to go, now days, there are so many pre-built modules that it's becoming less of a thing. The lack of hobbyist electronic magazines points to that too.

Anyway, all done with this one. Thanks to everyone, especially AudioGuru, for the tips.

 

The LM2904 that you are using is never used for audio because it has crossover distortion, noise and a poor slew rate that cuts frequencies above a few kHz since it is the same as an LM358 and LM324. You should use an audio opamp.

 

However, in practice, NO crossover distortion was noted from tests with the signal gen or from signals from an audio source...a Vizio smart TV.

After decades of practice, I'm well aware of CoD and watch for it on the scope. In this case, NONE detected over many hours of observation during circuit breadboarding or after build. Chips were newer and I'm pretty sure much headway has been made in reducing it. Also, isn't CoD ONLY present in dual supply designs?

And then the biggie...it's what I had in stock.

 

Biasing the output stage of an audio amplifier in class-AB eliminates crossover distortion. But the class-AB biasing produces an idle current that is low but is not zero. The lousy old LM358, LM324 and LM1904 are made with a very low idle current because the output stage is biased with no current. The low current also produces the noise (hiss) and poor high frequency slew rate.
Audio opamps operate in class-AB and produce very low distortion, low noise and a bandwidth to 100kHz.

Any opamp can be biased at half the total supply voltage and work the same when it has either a single or has a dual supply voltage.

Crossover distortion adds a buzz sound to low level sounds. Some electric geetar players like it since they rarely play at low levels.