Hey guys! Been searching the forum and reading for 3 days now, but can't figure out/comprehend what's happening in my situation.
It's a battery-powered boombox that uses an automotive head unit, 2x TDA7498s, and a Boost Converter to give the amps 30ish volts.
12v from battery goes to head unit and boost converter. All grounded to the battery.
If I use both wires for a channel, going to the amplifier, there is no music from the speakers. If I unhook one of them, it plays perfectly. I feel like it's some kind of grounding problem, but I clearly don't know enough about this to figure out my problem. I've been hooking up radios and amplifiers for over 30 years, now, and it blows my mind that I can get sound with just one wire!
What am I doing wrong? And what Is this phenomenon (in my eyes lol)?
It's a battery-powered boombox that uses an automotive head unit, 2x TDA7498s, and a Boost Converter to give the amps 30ish volts.
12v from battery goes to head unit and boost converter. All grounded to the battery.
If I use both wires for a channel, going to the amplifier, there is no music from the speakers. If I unhook one of them, it plays perfectly. I feel like it's some kind of grounding problem, but I clearly don't know enough about this to figure out my problem. I've been hooking up radios and amplifiers for over 30 years, now, and it blows my mind that I can get sound with just one wire!
What am I doing wrong? And what Is this phenomenon (in my eyes lol)?
You have an head unit Probably with with BTL amps and want to connect it to the TDA's ?
If the head unit has BTL amps it won't work.
Your head unit uses 4 amps, 2 in BTL configuration for each channel. OK 2 amps in one chip, and I am not meaning front & rear amps, but both 4 are used for the front speakers.
You are shorting one of the amplifiers when connecting one of the wires to ground and the chip goes into protection mode !!
Study Bridge Tied Load amplifiers and You will understand. BTL does not use a common ground.
Check with a multimeter if there is continuity in the -spk wire to the head unit chassis or the black negative supply wire. That tells You if it isn't BTL.
I would be looking into domestic sub schematics, since they usually accept both type of high level connections. Grab the signal before the head unit amp chips, or use some sort of transformer in-between (ground loop isolator for example)
or found this:
https://mosconi-system.it/wp-content/uploads/2019/07/BTL_SE-tec-info_en.pdf
Last picture I believe a speaker instead of the the resistor would do the trick. Replace then with a high wattage resistor if it works.
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The whole BTL thing confuses me! haha
I can connect mp3 player (phone) to TDA7498 and it works as expected. I can also hook the same mp3 player up to a car audio amplifier in a vehicle and it will also work as expected. I cant figure out why the same doesn't apply for the board amp. If the phone will play flawlessly through both amps, Why doesn't the head unit?
It can't be because of the grounds, because in a vehicle everything is also grounded to the battery.
I'm about to retire for the day, so I will address the rest of your post tomorrow. The easy answer is just playing it with one wire disconnected like i have been doing for the last year! (even though I have no idea how this is working)
If it's all powered from the same 12V supply, the power ground to each unit is already providing the *signal ground*, albeit with a small DC offset (the difference in I*R drop between the power source and each unit).
Maybe it would give a *flash of insight* if you tried connecting the other amp output wire (of the same pair) to the '7498 input.
If that doesn't make sense, re-read MAACO's post -- he writes better than I do.
Cheers
Maybe it would give a *flash of insight* if you tried connecting the other amp output wire (of the same pair) to the '7498 input.
If that doesn't make sense, re-read MAACO's post -- he writes better than I do.
Cheers
High level inputs to car power amps often just don’t connect the “cold” side of the speaker outs from the head unit. It might make connections, but not go anywhere (on purpose). Or perhaps go to a resistor network to ground. So that you don’t short out half of a BTL amp in the head unit. The power ground connection provides the speaker return. Better car amps use differential inputs to prevent ground loop problems and the noise pickup you can get from using the power ground on the head unit for the signal return. On a battery powered boombox that won’t be an issue because there is no alternator noise - just leave the “cold” side of the high level in’s disconnected and call it good (if it isn’t broke don't fix it).
I'm pretty sure none of Your speaker out wires (pink, grey, white and green and the corresponding returns with the same color from the ISO speaker connector) connect to ground. They go straight from the head-unit to the speakers. This is for a head-unit with front & rear outputs. For a front speakers only radio pink and green are missing. That's why I've asked You to make the multimeter test.
If The head-unit amps are "normal" single ended, The -FR or -FL spk wire is connected to the black ground wire or chassis inside the unit. Assuming an ordinary negative polarity chassis. (In ancient car days, chassis was battery positive)
Wat's the model of the head unit ? What Watts ? More than say 20W must be BTL because 12 or 14V (alternator charging Voltage) can't give You 50W of power.
It could work if Your car could provide +14,-14 & GND. You have only two wires (+12V & GND) & probably a yellow 12V for settings backup.
This is solved by BTL as one amplifier is in phase and the other works inverted.
So between +14V BTL amp 1 & -14V BTL amp 2 (because of the inversion) You now have created 28V at the speaker terminals, but with no common ground.
This is just Math. I'm not accounting for losses in the circuit design.
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I read the PDF and it just confused me, and make me think the opposite is true haha. I'll see if I can find a video on youtube and see if that makes it more clear for me.
I will get the radio out in the next day or so and do the multimeter check.
So, a DC ground is completing the AC signal, rather than a signal return line? I just can't wrap my head around this! haha
There is 8.2K ohms from negative speaker wire to chassis/ground wire, 8.2k ohms between positive wire and chassis/ground, and 160 ohms between each channel's + and -.
In BTL circuits, one chip provides the positive half and the other the negative half of the signal from the amp.
By removing a wire, you are breaking the circuit.
Look it up in in the net, or the data sheets of the old classic Class AB chips, 2030, 4440, 1875 can all be used as straight or BTL.
The 4440 is rated as 6W/channel in stereo, or 19W/channel in BTL mono.
Here the local amps sometimes use 2 of those chips (4440) as output, because it runs well in cars, and buses, on 12 volts, the others need higher voltages for that output.
And 19 watts is OK in a bus too.
By removing a wire, you are breaking the circuit.
Look it up in in the net, or the data sheets of the old classic Class AB chips, 2030, 4440, 1875 can all be used as straight or BTL.
The 4440 is rated as 6W/channel in stereo, or 19W/channel in BTL mono.
Here the local amps sometimes use 2 of those chips (4440) as output, because it runs well in cars, and buses, on 12 volts, the others need higher voltages for that output.
And 19 watts is OK in a bus too.
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I don't think you read my post correctly. I'm having the opposite problem that you mention, here. Mine ONLY plays with one wire disconnected.