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

[Millstonemike]

... I'm a little hesitant to tackle the biggest blob of plastic on this thing...

TomZ

In the absence of a Dremel cut-off, I would try a fine tooth hacksaw blade to cut it off close to the board. That might allow the individual pins to be removed one at a time.

+1 on Neil's wire size post. I like reclaiming wire from Ethernet cables. The wire comes in twisted pairs, each pair has a signal and a ground in the Ethernet application. Seems perfect for audio line level signals to minimize noise.

[tomzarbo]

Thanks Mike, that's good advice.

I did forget to flux up the wire before I tinned it but I didn't really have any trouble getting it to stick. It's probably more of an issue of getting things lined up.

I magnified this with a loop to take the pic on my phone so it's looking way larger than normal, but it's really kind of tiny stuff to fiddle with as you know. I have a cool flux pen that works well, and I have a few rolls of 'speaker tinsel lead'... um, I mean desoldering braid copper line, along with some solder suckers too. It all comes in handy when you need it.

I had turned up the heat a little because even when tinned, the very tip of the iron doesn't get quite as hot as a few millimeters up the iron. What degree heat would you suggest for a fairly fine pointy tip?

The #14 and #15 pins did produce sound, so that checks out. Well, I forgot to check if my audio output was front or rear on the fader... next time.

So I got a few things off the board...


the antenna and RCA jacks. I'm a little hesitant to tackle the biggest blob of plastic on this thing... the dual connector for voltage input and speaker output. Any ideas on how to get this off without borking the board? I do have a heat gun that I could use.

Also, here is the faceplate with the rest of the metal frame removed....



It's about 3/4" thick minus the four mounting tabs that will serve as an attachment point for the rear board at some.... um, point.

TomZ

[neildavis]

22ga wire is way overkill. 30ga wire-wrap is much easier to work with, and it you have the expensive teflon-coated stuff, the insulation won't shrink back when you solder it. You can tack it down into place with a dot of hot melt glue or silicon glue, and the repairs will look nice and last forever. 30ga wire is the equivalent of a 1.5mm trace of 1oz copper, according to one online calculator. That's plenty for these signals.

[Millstonemike]

I'm on a satellite link today and posting pictures isn't working well. It's on page 4 of this link.

We may be thinking two different things.

Single supply bridged amps: in the general case when there isn't DC buck circuitry increasing the nominal 12 V car electrical system. Most all head units use the 12 V as-is and implement Bridged Tied Load (BTL) mode.

On the positive half of the output wave, the power out circuits use Bat- as output Gnd and Bat+ as output Vcc+. On the negative half of the wave, the power output polarity is reversed. Bat+ become output Gnd and Bat- becomes output Vcc-. Thus the output signal can swing between the two extremes of Vcc+ and Vcc-. This increases the maximum output power by a factor of 4 over non BTL mode.

The result imitates a dual supply amp with Gnd, Vcc+ and Vcc-. except Gnd is ever changing with each half cycle. That's why car stereos have two leads for each speaker. Neither of which can be ever be connected to the car frame (i.e., battery ground). The output would short on each negative portion of the cycle. When the circuity reverses polarity as Bat+ would be connected to chassis ground.

So the output swings from Bat+ to Bat-. without a constant ground and never connected to chassis/battery ground. A "virtual" ground in the output wave would be 2 x Vcc / 2 (e.g., Vcc.).

On another note, the line level signal in single supply amps use a DC blocking capacitor to bias the input signal to Vcc/2. That allows the signal to vary from it's + peak to - peak without ever going negative. It's viryual ground being Vcc/2.

I just don't see how to convert the BTL output to a line level signal given that the RCA output conductor is tied to system ground. That would be like trying to use chassis ground for one side of the speaker connection in my explanation, above, albeit via some resistor network.

[Millstonemike]

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



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 ...

[tomzarbo]

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



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

[neildavis]

I'm on a satellite link today and posting pictures isn't working well. It's on page 4 of this link.

[Millstonemike]

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

[neildavis]

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.

[Millstonemike]

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?

[Millstonemike]

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.


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 ...

[tomzarbo]

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.


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

[tomzarbo]

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

[tomzarbo]

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...



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

[Millstonemike]

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.