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

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  • #61
    Originally posted by Millstonemike View Post

    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...

    Click image for larger version

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    Free Passive Speaker Designer Lite (PSD-Lite) -- http://www.audiodevelopers.com/Softw...Lite/setup.exe

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    • #62
      Originally posted by tomzarbo View Post

      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 ....



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      • #63
        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)

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        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
        Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
        *Part 2 *Gluing multiple curved laminations of HDF

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        • #64
          Originally posted by tomzarbo View Post
          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.

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          • #65
            You could keep the amp in standby and try Neil's suggestion of using the line output of the PT2313 processor.. Pick up the line level signal at the TDA7388's input pins. That way the signal will have gone through the DC isolation caps on the PCB. Start with a low volume on the external system using that signal. It may not have any output delay and pop on power-up.

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            • #66
              Thanks Mike for the link, that's a much better solution to reducing the voltage.

              So using your suggestion Neil... if I can tap into the TDA7388 chip's leads to grab the input signal cleanly, I'll have my RCA line-level outputs there more or less, minus the 'pop and click upon turn-on' protections that likely exist on the board. Got it.

              I drew up a pin diagram on the links you guys provided...

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              I'm thinking that I probably need to connect to the shorter black-arrow pins 11, 12 and 13, where 13 is the ground for all four channels of input. The longer gray pins 14 and 15 are probably the rear R and L which I won't use... at least that seems the likeliest to me.

              I think I have a enough of those little 'pin grabber' test leads for the multimeter that I should be able to get the signal from there if I'm careful.
              I'm going to try and connect directly (no amp) to an efficient speaker driver first to see if I'm on the right track -- I should be able to hear something -- then I'll connect to my Lepai amp with the volume way down to see what it does. Good!

              Mike and Neil, I appreciate the guidance... it's been fun. Probably pretty interesting also for guys near my skill level to see what can be done to 'tap into' a mass-produced product to see how it can be modded to be even more useful as a pre-out faceplate unit.

              TomZ
              Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
              *Part 2 *Gluing multiple curved laminations of HDF

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              • #67
                Man, the really didn't want anyone to mistake the input and output sides of this thing with that big old arrow... did they?

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                Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                *Part 2 *Gluing multiple curved laminations of HDF

                Comment


                • #68
                  So Pins 11, 12 and 13 did supply signal (music) when connected directly to a small speaker. I forgot to test 14 and 15 as the alternate inputs from the faceplate for the rear R and L, I'll try to do that next time I mess with it to verify the rear and front connections. I couldn't really use the 'pin grabber' multimeter leads... too big to use on the chip pins between the chip and the board, and not enough ''meat" to hold onto on the rear... I just soldered the ends of some small wire and connected to a speaker and carefully tested with that.

                  It drew anywhere from 0.27 to 0.33 of an amp in the different modes and volume levels, so that's more manageable I think.

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                  I'm going to solder leads to the back of the rear board onto the little pin 'nubs' as it is the easiest place to hook up to. I'll run those lines to one of the little PE amps and see how noisy it is. If it's fairly quiet then I guess this may end up being a decent mod to create a table radio with. The hardest thing will be for someone to 'snip' the #4 pin and then solder to the #2 pin without borking the chip up or 'glumping' up the solder and bridging the neighboring pin. I actually did that a tiny bit, but was able to correct it.

                  I guess the next thing after that would be to figure out which of the lines coming from the two multi-wire 'clip connectors' on the back supply 12v + and -, and 'power antenna' 12v power when switched on so that bulk can be removed. Then get rid of the RCA outputs as they occupy some real estate as well like Mike said. Then you'll have a pretty svelte board which can be moved where it's most convenient to position it.

                  I'm thinking that I'd like to actually try and remove the plastic front panel from what's left of the metal 'frame,' use the stock ribbon cable to connect the two, and design a simple plastic means of connecting the two boards physically to each other, considering noise issues as I tinker with it of course. Zip ties and some hot glue may do the trick nice and easy.

                  But first, I have to make sure the output is fairly noise-free from this signal attachment point.

                  More to come...

                  TomZ
                  Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                  *Part 2 *Gluing multiple curved laminations of HDF

                  Comment


                  • #69
                    Originally posted by tomzarbo View Post
                    So Pins 11, 12 and 13 did supply signal (music) when connected directly to a small speaker. I forgot to test 14 and 15 as the alternate inputs from the faceplate for the rear R and L, I'll try to do that next time I mess with it to verify the rear and front connections. I couldn't really use the 'pin grabber' multimeter leads... too big to use on the chip pins between the chip and the board, and not enough ''meat" to hold onto on the rear... I just soldered the ends of some small wire and connected to a speaker and carefully tested with that.

                    It drew anywhere from 0.27 to 0.33 of an amp in the different modes and volume levels, so that's more manageable I think.

                    Click image for larger version

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                    I'm going to solder leads to the back of the rear board onto the little pin 'nubs' as it is the easiest place to hook up to. I'll run those lines to one of the little PE amps and see how noisy it is. If it's fairly quiet then I guess this may end up being a decent mod to create a table radio with. The hardest thing will be for someone to 'snip' the #4 pin and then solder to the #2 pin without borking the chip up or 'glumping' up the solder and bridging the neighboring pin. I actually did that a tiny bit, but was able to correct it.

                    I guess the next thing after that would be to figure out which of the lines coming from the two multi-wire 'clip connectors' on the back supply 12v + and -, and 'power antenna' 12v power when switched on so that bulk can be removed. Then get rid of the RCA outputs as they occupy some real estate as well like Mike said. Then you'll have a pretty svelte board which can be moved where it's most convenient to position it.

                    I'm thinking that I'd like to actually try and remove the plastic front panel from what's left of the metal 'frame,' use the stock ribbon cable to connect the two, and design a simple plastic means of connecting the two boards physically to each other, considering noise issues as I tinker with it of course. Zip ties and some hot glue may do the trick nice and easy.

                    But first, I have to make sure the output is fairly noise-free from this signal attachment point.

                    More to come...

                    TomZ
                    If you take the signal from the input to the amp,I would think you can just shunt C34 to turn the amp off. Need to test that to insure that RC circuit is affecting anything else. I don;t think so, but there have been some surprises.

                    If clamping the RC circuit to ground does not mute the signal at at amp, then that's a good sign they took the line level signal from the amp's output. But, maybe that was the only way to avoid power up pop/noise ...

                    Comment


                    • #70
                      Originally posted by neildavis View Post

                      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.
                      In bridge mode, each out-of-phase amp is referenced to ground with the each peaking VCC+ and VCC-.. How does the blocking capacitor reference VCC/2? It must as the RCA connector's outer conductor was tied to ground. Just not sure how that is done as the DC blocking cap sees + voltage swings referenced to ground. That would imply the cap stays near ground. Have a sample circuit?





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                      • #71
                        Originally posted by Millstonemike View Post
                        Have a sample circuit?
                        I'm not sure what you are asking, because these single supply amps don't use VCC-. For single supply amps like the one Tom has in that car radio unit, the output of each amp with no signal is sitting at VCC/2. The circuit shown in the figure shows both single-ended and bridged configurations. For the single-ended case, you just need a blocking cap to reference the output to ground. You could eliminate the cap if you could return the speaker to VCC/2, and that is what the reference circuit for the STA326 shows in Figure 22 of this link.

                        Free Passive Speaker Designer Lite (PSD-Lite) -- http://www.audiodevelopers.com/Softw...Lite/setup.exe

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                        • #72


                          PIc/gif not showing. Would like to have a look.

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                          • #73
                            I'm on a satellite link today and posting pictures isn't working well. It's on page 4 of this link.
                            Free Passive Speaker Designer Lite (PSD-Lite) -- http://www.audiodevelopers.com/Softw...Lite/setup.exe

                            Comment


                            • #74
                              It seems that hacking into pins 11, 12, and 13 works just fine for getting the signal out to an outboard amp.

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                              Why does all my soldering look like a 3-year old did it?

                              I used some 22 ga. multi-strand hook-up wire to attach to the protruding rear pins of the amp chip. Not too difficult. If I didn't have lighted magnification?... Fuggedaboudit! The stagger of the leads helps tremendously for stuff like this.

                              This mod does get the signal out sufficiently. I thought I heard a slight noise once when the LED's changed color, but I can't be sure that it wasn't me touching something; and I didn't hear it again.

                              Again, there was a slight click on turn-on (I think) but none on shutdown. It wasn't all that bad to tell the truth. The delay relay that Mike suggested on the amp would be perfect for something like this, and someday -- if they let the ones I've purchased out of China -- I'll add that as well.

                              One weird thing... the 'Mute' button doesn't do anything anymore. There are two ways to mute: either press the mute button on the remote, or 'short' press the power button on the unit. Neither do anything anymore. It used to mute the built in amp as well as the line out. I don't think that will bother me to loose that though.

                              So, next step is just trimming the extra stuff off of the rear board and finding the locations of the 12V power in and out for the power antenna to turn on the amp.

                              I forgot to test out the two rear connections on the chip for the 'rear' channels. Oops, meant to do that. Next time. I should also try to find out which lines are L and R just to make sure.

                              More hackery to come...

                              TomZ
                              Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                              *Part 2 *Gluing multiple curved laminations of HDF

                              Comment


                              • #75
                                Originally posted by tomzarbo View Post
                                It seems that hacking into pins 11, 12, and 13 works just fine for getting the signal out to an outboard amp.

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                                Why does all my soldering look like a 3-year old did it? ...
                                TomZ
                                Tin the wire first. Then place it on the connection and heat again. The existing solder on both sides will flow together nicely. Trying to tin the bare wire while making a connection to a pre-soldered contact is problematic. Also very useful to avoid heating up the PCB or component. The pre-tinned wire and contact will flow together very quickly.

                                And buy solder flux. Just a tiny bit on the wire and heat it until it boils. Then tin the wire as usual. Works like magic. And for removing a connection, solder braid. A tiny bit on the end of the braid, heat it. Then place on top of the PCB thru-hole lead, etc. and heat. Then just snip off the used end of the braid that is full of solder. You'll be amazed ...

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