TDA1552Q in computer speakers and headphone out
I have a very annoying problem. I have pretty old Jazz speakers (each has 3 mid-freq speaker and one tweeter). The sound from the speakers is fine, but when i connect the headphones i get this terrible 100hz hum. The hum is not dependent on the volume settings. It comes only from headphones, but not the speakers (see below).
The speakers are bases on tda1552q. I traced the problem with a o-scope and here is what i discovered.
The hum originally comes from a the power supply (it is a transformer + 4 diodes + 3300 uF cap), but it is okay, because tda1552q can compensate it. The power supply noise rejection mechanism as i understood is very simple and effective. They pass the noise to both wires going to a speaker, so, the noise become synch and does not affect the speaker at all (for example, we have 100mV spike on one wire and the same 100mV on other, so, the speaker gets 0V, no noise).
This is all fine, but then comes the headphone connector. The headphone connector has only three outputs, L, R and COMMON. When the headphones are not connected the signal goes to the speakers, whn the phones are connected the signal is doconnected from the speakers and goes to the headphones. It is done mechanically in the jack. However, as i see it, they get COMMON from the popwer supply negative (common) wire, and pass ONE of the output wires from left and right channel to the phones. But there is nothing to cancel the noise on the left and right channel this way! Thus 100hz hum in the headphones. I cannot figure out
how to fix the problem.
Does anybody has idea how to connect usual 3 wire headphones to things like tda1552?
All i can think of is an op amp 1:1 inverter for for each channel to subtract common noise and only then pass it to the headphones output. That's crazy.
Or.. totally change the PSU to a regulated type (say 12V would be just fine). But i'd rather prefer simpler solution.
No simple way round it for a BTL amplifier designed for in car use.
Possibly wire in two small audio transformers to get the
voltage drop and the 100Hz hum balanced operation.
1) Wire audio transformers where? I am sorry again, i did not get your idea, because i never done audio before.
2) Do you think switching to regulated PSU will help? (Switched PSU does not help, it gives its own noise, so, i mean switch to a linear regulated PSU).
3) I really might consider throwing out all the guts and getting some other chipamp which has a separate headphone output and BTL out for the speakers and rebuilding the pcb. Can you suggest such chipamp?
Damn, those stupid engineers. They for sure knew what they were doing and released such idiotic speakers!
For BTL amplifiers you must never common the "negative" of L&R,
but you can common transformer outputs, its 4 wire to 3 wire.
If your driving the speakers from a PC headphone out, you could
rewire the socket to the input, using the PC for volume control.
Adding another small chip that does not use BTL is also possible,
this of course should have good PSSR, and/or add a regulator.
Another alternative is use a headphone amplifier, starting point :
Fiio E3 Portable Headphone Amplifier ipod mp3 | eBay UK
Another alternative is to gut a pair of USB powered speakers
for the small board and controls they contain and use that.
(quite a few have headphone sockets)
I will most likely go for bypassing input from pc to the headphone output and use pc as volume control. Seems to be the simplest way to fix it. Also, i will disassemble another set of speakers (which work find with speakers) and see how it done there. I am intrigued!
Presumably your other set of speakers are not BTL, so have a common earth,
i.e. 3 wire connection of the speakers, L- = R- = common earth. Most likely
be a dual rail supply rather than AC coupled output single rail supply.
Unless they use a switching amplifier .... BTL again, but no ripple.
Uses screened connection wire for your mod, else the speakers will hum.
Stereo wire with overall screen should be fine. On the main board, if
its socketed short wires near the plug, otherwise use short jumpers.
It is incredible how important in life to meet right people at right time. Your responses enlightened me in audio amps more than all my previous life. I finally see how things works, i just needed those dozen words you said.
Now, about class D amps.
I looked at them. The cheaper ones (like TDA8920) require a lot of external components
and the costly ones (like MP7722 which require less external components) are.. well.. costly :)
I might redo these speakers on one of the Class D chip-amps in the future. It does not seem to be very difficult, just need all the components.
I even now looked at so called "class T"(tm) chips, but they seems to be total overkill for the dull speaker i have.
FWIW for class D your far better off buying boards or modules, that
is for an experienced constructor, nevermind the less experienced.
I did check (after posting, wrong way round) what I said, and its
probably not true, BTL class D will most likely have the same issues.
The speakers I'm intrigued by, 3 bass mid units + tweeter ? or 3" ?
Anyway if it ain't broke don't fix it, particularly applicable to amplifiers.
Umm.. Then i really have to dissamble the set of speaker which works well with headphones and see how it is done there. I'll do in on weekend.
As for the speaker i have the issue with: I wrote it wrong, each box has 3 speakers total, 2 equal mid-bass speakers and one tweeter, nothing special.
However, i looked at the D class description and how it works. I am pretty sure, that if you power it from switchinh power supply with switchingh freq about 1mhz then no noise will be on the output on the grounded speaker because the LP filter will just filter it out. So, the headphones with common wire (we assume the common wire is always at zero potential) maybe connected to such output. Just a thought.
Another thought about fixing my speakers. Well, maybe i should get the output to the headpones from the output of sound processor. It is NEC uPC1892 and as i understood the volume contrrol is done at the level of sound processor and as i saw with my o-scope the output of from the sound processor does not have this 100hz noise and goes directly to the tda1552 amp where the noise is introduced. Again, just a theory.
the datasheet for the sound processor
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