Alternative to Faceplate/Preamp unit for Table Radios...

[Millstonemike]

So I ended up just sawing pin #4 on the chip with a tiny Exacto blade till I got through, and bending the board side of the pin away a bit, then soldering a tiny piece of 'twisted pair' phone cable line from the chip side of Pin #4 to the board attachment point of Pin #2. I felt like it would be easier for me to reattach these points to get it working again over trying to bridge the trace if this isn't successful...

...and it wasn't: all output stopped: the built-in amp was quiet, but the RCA outputs were silent too as well.

...

TomZ

1 out of 100 chance, just for you Tom

The regulator in your link is not a buck inverter. It's the common LM7812 linear regulator (despite the offers description). Other than it doesn't provide isolation, it would work. You would be reducing 24 V to 12 V, a 12 V drop through the regulator. At 0.5 A that will produce 6 W of heat in the regulator. With the heat sink, the LM series are good for up to 15 W.. But it will get hot. Imagine a couple of nightlights throwing off heat inside your enclosure.

This Link is to a true buck regulator, pictured below. This is not an isolated regulator. At 85% to 90% conversion efficiency, you could expect less than 1 W of dissipated heat. And the chip, LM2596, will be hot to the touch as it doesn't use a heat sink.

[tomzarbo]

So I ended up just sawing pin #4 on the chip with a tiny Exacto blade till I got through, and bending the board side of the pin away a bit, then soldering a tiny piece of 'twisted pair' phone cable line from the chip side of Pin #4 to the board attachment point of Pin #2. I felt like it would be easier for me to reattach these points to get it working again over trying to bridge the trace if this isn't successful...

...and it wasn't: all output stopped: the built-in amp was quiet, but the RCA outputs were silent too as well.

It looks sloppy in the pics, and it kind of is, but it's attached and not shorting despite how the pics may look. (These pics were taken on my phone with a loop in front of the lens)



So, I'm thinking next I try to reattach Pin #4 to the board lead and get it back to where it was and make sure I didn't screw it up before proceeding further?

This unit draws half an amp roughly using the RCA outputs with the built in amp "on" but not powering speakers, so assuming noise isn't an issue, something like this buck convertor could be used to bring the 24 volts most would likely need for a beefier amp down to the 12 volts that this head unit wants to see: https://www.parts-express.com/lm317-...lator--320-617

It says it has a 1.5 amp current limit... I believe bringing 24v down to 12v at half an amp may tax this little thing a bit, but not enough to be dangerous I wouldn't think. I have a few of these already on premises, so I can test that out easy enough in the future. My fear is that if I have to do too much complicated surgery to this thing to get the amp out of the equation, I fear that others may not want to delve into it to that degree, and I think that's part of the point in fiddling with this.

Assuming it's reasonably quiet with something like this TDA7492 amp that I seem to keep using... https://www.parts-express.com/tda749...2x50w--320-606
it would be probably enough output for nearly anything radio or boom/box wise that most would ever want.

Just thinking out loud of course...

Thanks again guys for the guidance.

TomZ

[Millstonemike]

Think it would be worth trying to cut the #4 pin on the amp and connecting a jumper line to #2 (ground) and see if the line out is still functioning? I could undo if there was no RCA output.

TomZ

That would be the next step. Putting just the amp in standby to see if the line outs still function. If they do, that means the RC delay circuit is affecting other functions that control line out. And that would be good. The unit would settle down before providing output to an secondary amp. If the line out's remain inactive, the amp chip may be providing the line out despite the earlier test, or, (and I don't see how), an output on the amp is controlling other functions.

Tom, I can see the trace between C34 and pin 4 in one of your hi-res pics. You could slice that and test with a jumper from pin 4 to ground. Which ever way you feel most comfortable ....

[neildavis]

That makes it 99% the line level is not coming from the amp.

This is not correct. Think of a bridged amp as two amps 180 degrees out of phase with each other, each referenced to VCC/2. If you wanted to add a line output, you just need a blocking capacitor and a resistor network to limit the voltage output. Connect the "hot" to either amp through the capacitor and resistor network and use the ground connection as a reference. This is how the headphone jack on an older single-supply transistor amplifier was implemented--it works fine.

So there is still the possibility that the RCA line outputs are derived from the power amp output. If that is the case, putting the amp in standby mode will disable the line outputs I would buzz out the line outputs and see where they connect. It shouldn't be too hard to determine whether they come from the volume control chip (TM2313) or from the amp output via a capacitor and some resistors.

I would still recommend a look at one of those other amp boards with the PT2313 and the TPA3116. That cable will have 6 audio lines, power, ground, and SDA/SCL to control the PT2313. That's not that many connections to deal with. If you could find a breakout board for the flex cable, that would help...

[tomzarbo]

That makes it 99% the line level is not coming from the amp. The amp's high level output cannot be tied to ground as it uses BTL mode to maximize power out given the nominal 12 V auto system. But if it was, the resistors used to lower the high level speaker output to a reasonable signal level would provide some isolation from ground. That would be a bad design. Though I have seen some questionable design decisions in the past ...

Think it would be worth trying to cut the #4 pin on the amp and connecting a jumper line to #2 (ground) and see if the line out is still functioning? I could undo if there was no RCA output.

I'm pretty sure I can pull that off. I can't get to the traces, but I think I would be able to put a thin X-acto blade and cut the leg off near the board and just lift it off a bit. Soldering a small line to #2 shouldn't be too hard, I have a pretty pointy soldering iron temp controlled. I could bend it down and re-attach if it didn't work.

Neil, I think if I can get the onboard amp to power down and still have the option to use the RCA jacks to run to an amp, that would be all I would personally need. That way it could be used two ways, amp-to speakers, or line-out to amp. I could use one of the isolated DC to DC converters of a higher amperage to run this thing while having the increased voltage to directly power a bigger amp. I'm afraid my success rate on changing boards would be pretty slim. I can tend to bite off more than I can chew with this stuff, although you guys have walked me through more than I could have figured out on my own.

TomZ

[Millstonemike]

The RCA ground does have continuity with the board ground mounting holes, as well as the antenna ground connection.

TomZ

That makes it 99% the line level is not coming from the amp. The amp's high level output cannot be tied to ground as it uses BTL mode to maximize power out given the nominal 12 V auto system. But if it was, the resistors used to lower the high level speaker output to a reasonable signal level would provide some isolation from ground. That would be a bad design. Though I have seen some questionable design decisions in the past ...

[tomzarbo]

Yes, a possibility. There could be an easy test for that. Pick a line out RCA connector's ground and see if it is connected to PCB ground. If it is, it's likely connected to the TM2313 as that chip uses ground as one side of it's signal output.

The RCA ground does have continuity with the board ground mounting holes, as well as the antenna ground connection.

TomZ

[Millstonemike]

It's possible that the RCA outputs are connected to one of the amplifier outputs through a resistor and capacitor. [/ATTACH]

Yes, a possibility. There could be an easy test for that. Pick a line out RCA connector's ground and see if it is connected to PCB ground. If it is, it's likely connected to the TM2313 as that chip uses ground as one side of it's signal output.

[neildavis]

Also, You might want to consider dumping that board and using a different amp/TM2313 board. Search on Ali for TM2313 or PT2313 or TDA7313 and you will find boards that have the TPA3116 with a PT2313 chip. The microprocessor on your main board could be connected to the PT2313 using 3 wires: SDA, SCL and ground, and it wouldn't know that anything had changed, assuming the PT2313 and TM2313 are truly software compatible, which they are supposed to be(?) . You would need to make sure the audio is connected correctly, but once you did that, you would have a stereo 50W class D amp instead of a 15W class AB amp with its bulky heatsink.

[neildavis]

It's possible that the RCA outputs are connected to one of the amplifier outputs through a resistor and capacitor. The amplifier outputs are bridged, which means it is using two amps, one out of phase with the other. The speaker is connected between the two out-of-phase amps, which gives you effectively twice the voltage. But you can use a dropping resistor and DC blocking capacitor on either of those amps to create a line-level signal. If that is how the RCA outputs are connected, whenever the amp is put in standby or has the power cut, the RCA outputs will go dead.

However, it is also possible, and more likely, that the RCA outputs come directly from the TM2313 chip. The TM2313 pinout is shown below--it would be a connection from pins 22, 23, 24 or 25 to the RCA jack through a capacitor (need to block the DC voltage). The TM2313 doesn't have a dedicated mute pin--you must mute the device by sending serial data via the I2C bus (SDA and SCL) with a microprocessor. So if that is how the RCA outputs are connected, you could "fake out" the micro by having that standby signal go to the micro but isolate the standby signal from the amp (see next paragraph).

If you find out that the RCA outputs are connected to the TM2313 rather than the amplifier outputs (through capacitors and maybe some resistors, of course), then you can shut down the amp by isolating the standby pin from the board and grounding it at the chip. Just cut the trace going to pin 4 and add a small jumper from pin4 to ground (pin2).

If the RCA outputs are derived from the amp output rather than the TM2313, I would rewire them to use the output of the TM2313 instead (cuts and jumpers). Then you can disable the amp and still have audio

[tomzarbo]

I tried connecting to Pin 4 (standby) on the chip amp and testing for continuity on all the pins on TM2413 and didn't get any hits. Same with all the ribbon connections. Then I did it again and still no hits. Occasionally, I'd touch the right side of C34 to check the lead hadn't come off and every time I had good continuity to I think the results are valid.

Once I'm sure how this is going to be used, I can determine if the on-board amp is enough, or does it need additional watts, and a little surgery on the amp. So basically, I'd cut the trace going to Pin #4 on the amp, so it's not making a connection, then find a ground and solder a piece of wire to the ground and pin #4 on the amp. Either that or if I can't find the trace, I'd have to exacto blade the base of the pin to cut it, then lift it off a bit and solder a ground line to that. That would leave the chip circuitry doing what it's supposed to, but still keep the amp in standby mode, reducing current draw.

Also, the other smaller faceplate/preamp unit I was messing with had about a 200 milliamp draw, there was no amp on that one. This does seem a bit high I guess comparatively.

TomZ

[Millstonemike]

So I tried to short out the cap C34 and it seemed to work.

Interesting... when I shorted the cap out, it did stop the output to the speakers, and amperage draw dropped from a bit over 500 milliamps to just under 300 milliamps.... but, doing so also muted the RCA line-level outputs, which I didn't expect. I had one channel of the onboard amp connected to a subwoofer speaker, and I also used the RCA output to connect to my Lepai and neo nano speakers.

I thought the onboard amp would stop playing and go into standby while the RCA outputs would continue to have output, but that didn't happen; all output was muted.

Just to be clear, I basically took an old set of multimeter probes with the banana ends connected, and shorted out the C34 cap...



So now I'm confused. I'm assuming that I connected to the cap incorrectly, and basically did this wrong. Everything still works though so at least I didn't blow it up.

If I did do this right, why did the RCA jacks stop putting out music?

Sorry if this is obvious to everyone else, I seem to have a harder than normal time understanding this it seems.

TomZ

Your probe test was correct.

That makes some sense. Muting the unit's line out until all the electronics have a chance to settle down lest an external amp produces start up pop/noise. The audio processor , TM2313, doesn't seem to have a specific mute or standby input pin. All it's mute functions are controlled by its data feed.

I'd try a couple of things. Test continuity between the C34's connection to Amp pin 4 and all the TM2313 pins. One probe on pin4 and thent run the other probe along the pins to see if you get a hit,. Try the same test along the ribbon cable connector on the odd chance the RC delay circuit is being sent back to the main board.

If putting the AMP in standby becomes a priority. You may have to cut the trace from C4 to pin 4 on the amp. Then ground pin 4. That would leave the RC delay circuit intact for the other functions using it.

Also, based on your earlier measurements, I expected the ma draw to be much lower when the amp's standby is forced low by shorting C34.

[tomzarbo]

Neil, I had the power on and music playing first, then I put the test leads on the cap to short it out.

Well, I'm thinking that this unit can be used in two configurations...

(1) using the built-in amp(s) with a hefty 12v power supply (they have decent output on my pair of Bantams - a low sensitivity 3" woofer two-way using the ND91-4 woofer),
or
(2) just using the RCA outputs to your own amp; clipping the power leads to the built-in chip amp and hooking this up with an isolated DC-DC convertor...

It looks like from the previous diagram that Mike posted in post #32 that just clipping the sixth lead in from both ends (Vcc) would just cut the power to the chip altogether and there would be no draw?





TomZ

[neildavis]

Did you short out C34 before applying power, or after?

If you applied the short before turning on power, it might make some sense. It looks like the RCA outputs would be coming from the TM2313 chip, which is controlled by the microprocessor (the F1C100s chip). Maybe the standby signal is routed back to the microprocessor, and it keeps the TM2313 chip muted until the power amp is out of standby mode. That's not a good way to design the circuit, IMO, but whatever....

But if you applied the short after power was applied, what you are seeing is really weird, because it doesn't make any sense for the micro to monitor that signal.

[tomzarbo]

So I tried to short out the cap C34 and it seemed to work.

Interesting... when I shorted the cap out, it did stop the output to the speakers, and amperage draw dropped from a bit over 500 milliamps to just under 300 milliamps.... but, doing so also muted the RCA line-level outputs, which I didn't expect. I had one channel of the onboard amp connected to a subwoofer speaker, and I also used the RCA output to connect to my Lepai and neo nano speakers.

I thought the onboard amp would stop playing and go into standby while the RCA outputs would continue to have output, but that didn't happen; all output was muted.

Just to be clear, I basically took an old set of multimeter probes with the banana ends connected, and shorted out the C34 cap...



So now I'm confused. I'm assuming that I connected to the cap incorrectly, and basically did this wrong. Everything still works though so at least I didn't blow it up.

If I did do this right, why did the RCA jacks stop putting out music?

Sorry if this is obvious to everyone else, I seem to have a harder than normal time understanding this it seems.

TomZ