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

[tomzarbo]

Man, you are amazing with this stuff Mike. I really appreciate you and Neil giving us so much info with this.

So I actually understood your explanation, instead of 5v going right to pin 4, it goes through a 10k resistor, then to a cap through to pin #4. The cap's other terminal goes to ground. Upon turn-on, 5v goes through the 10k resistor, then to the cap/ground, and it begins to charge, once it charges sufficiently to let at least 3.5v through, which is enough to turn on the amp via pin #4, the amp will power up. That brief delay while the cap is charging is enough to allow things to normalize so there isn't a bunch of clicking/popping upon turn-on. (I can't be sure I've understood something until I can explain it to someone)

Well, I guess I might need some clarification on "the large SMD cap to the left of the TDA7388." I don't really see it... unless you're being funny? C34?



Thanks guys,

I'll do more again as soon as I can. Thanks for the ribbon info too.

TomZ

[Millstonemike]

I wonder if the ribbon cable that connects the front and rear boards could be replaced with a longer cable to facilitate the rear board being placed independently in the cabinet. As-is, the unit is maybe three inches deep, and that cable is maybe 4" long at the most; the rear panel can't be moved very much currently -- I can just barely get it perpendicular to the front plate.
TomZ

It seems the cable is readily available in a wide variety of pins and length (I counted 20 pins from your pic). And I believe the PCB connectors provide the termination for the cable and the existing cable can be removed and a new cable inserted. You have to determine some parameters like pitch (measure the total width of the cable and divide by 20) . And you need to determine if you want the end connections on the same side or on opposite sides of the cable (e.g., pay attention at the existing orientation in the connector).

AND you need to verify this as I can't clearly seem the terminations from the Pic.'s.

Ali has them much cheaper. But you need to search the term "FPC FCC". Price low to high. Then click on one and scroll down to the sellers other items for you specific cable. An 8 " cable can be had for ~$2.

[Millstonemike]

You should be able to turn off the power amp by grounding pin 4 of the chip (Standby). In standby mode, the chip draws very little current. But check to make sure pin 4 isn't tied to 12V. The data sheet shows that this pin is usually tied high with a 10K or so resistor and a 1uF to ground, which gives a "virtually pop-free" turn-on (or so they say...).

The Bluetooth is provided by a different chip than that other decoder board. The chip is made by Buildwin, part of the AppoTech Group, and it is probably a CW6690G with a custom part number for that code. It is probably decent enough quality, even though it is intended for the low/moderate priced market. That chip shows up in several makes of Bluetooth speakers and the only "reviews" I found are favorable.

This decoder/preamp would be better if that TM2313 chip was on the main board rather than the amp board. It's a preamp chip with analog switches, tone and volume controls with decent specs, but putting it on the same board as the power amp means you can't get rid of the amp. Not a huge deal, but it it means you can't just unplug that amp board to make the assembly smaller and lighter..

Yes, while the TDA7388 consumes ~200 ma while idling, in standby mode it consumes 20 ua. Standby is Pin 4 (Pin 1 is left, count front and back pins as if they were all in one row).. Standby is used to control on/off pop via an RC ciruit that allows the standby pin to come up once the power chip has settled down after power up. I would bet that the large SMD cap to the left of the TDA7388 is in that RC circuit. If so, one side should be connected to ground, the other side to pin 4. If that's true, shunting that cap with not allow the chip to come out of standby mode.

Do a continuity test on the cap to ground and pin 4. If I'm right, hook up a speaker and play something. Shunting the cap should force the amp into standby and stop all output to the speaker.

Here's a pic of the standby circuit. The standby input wants to see a minimum of 3.5 V to turn the amp on. On power up, +5 V is fed to via a 10 K resistor to a capacitor connected to ground and on to the standby input. The voltage at the cap and pin 4 will begin to rise as the cap charges eventually reaching 3.5 V thus turning the amp on. About 12 ms delay to turn-on. By shubting the cap, the 5 V source will be connected to ground via the resistor and Pin 4 will never rise.

[tomzarbo]

You should be able to turn off the power amp by grounding pin 4 of the chip (Standby). In standby mode, the chip draws very little current. But check to make sure pin 4 isn't tied to 12V. The data sheet shows that this pin is usually tied high with a 10K or so resistor and a 1uF to ground, which gives a "virtually pop-free" turn-on (or so they say...).

The Bluetooth is provided by a different chip than that other decoder board. The chip is made by Buildwin, part of the AppoTech Group, and it is probably a CW6690G with a custom part number for that code. It is probably decent enough quality, even though it is intended for the low/moderate priced market. That chip shows up in several makes of Bluetooth speakers and the only "reviews" I found are favorable.

This decoder/preamp would be better if that TM2313 chip was on the main board rather than the amp board. It's a preamp chip with analog switches, tone and volume controls with decent specs, but putting it on the same board as the power amp means you can't get rid of the amp. Not a huge deal, but it it means you can't just unplug that amp board to make the assembly smaller and lighter..

Great, you answered the main question we had about this piece, Neil. Thank you for digging into this for all of us.

So it looks like the rear board needs to stay connected to operate.

Since most would probably be thinking of using this as a table radio/boombox head unit like myself -- where the entire radio may be 6 to 10" deep, I wonder if the ribbon cable that connects the front and rear boards could be replaced with a longer cable to facilitate the rear board being placed independently in the cabinet. As-is, the unit is maybe three inches deep, and that cable is maybe 4" long at the most; the rear panel can't be moved very much currently -- I can just barely get it perpendicular to the front plate. The actual metal 'case' could be removed leaving a 3/4" thick front panel to mount, and that would allow for more leeway when mounting the rear board. The rear circuit board can be removed from the total rear panel/heatsink, which makes it a good 2" shorter. If one were to not use the built-in amp and had disconnected it, loosing the heatsink wouldn't hurt anything.

I suppose any lengthening of the cable would make it more susceptible to noise though, and I haven't tested this out with an amp board in close proximity, only a Lepai 2020 amp 2 feet away. I should hook up the 320-606 TDA7492 amp that I frequently used next to it to see what the noise issues are.

Anyway, thank you again Neal for clearing up some questions for us. If the click on/off issue can be eliminated, The more I tinker with this, the more I think this may be a real nice unit to mess with for small radios and such. As much as I like the feel of knobs and buttons, the touch screen on this seems like it would work well for a close-proximity radio unit in the kitchen or what-have-you; and all of the other small faceplate preamps don't have this knob/touchscreen combination.

TomZ

[neildavis]

You should be able to turn off the power amp by grounding pin 4 of the chip (Standby). In standby mode, the chip draws very little current. But check to make sure pin 4 isn't tied to 12V. The data sheet shows that this pin is usually tied high with a 10K or so resistor and a 1uF to ground, which gives a "virtually pop-free" turn-on (or so they say...).

The Bluetooth is provided by a different chip than that other decoder board. The chip is made by Buildwin, part of the AppoTech Group, and it is probably a CW6690G with a custom part number for that code. It is probably decent enough quality, even though it is intended for the low/moderate priced market. That chip shows up in several makes of Bluetooth speakers and the only "reviews" I found are favorable.

This decoder/preamp would be better if that TM2313 chip was on the main board rather than the amp board. It's a preamp chip with analog switches, tone and volume controls with decent specs, but putting it on the same board as the power amp means you can't get rid of the amp. Not a huge deal, but it it means you can't just unplug that amp board to make the assembly smaller and lighter..

[tomzarbo]

Sorry some of these pics are still a little fuzzy, I took a boatload of pics and these were the best believe it or not.

The ribbon that connects the front and rear boards have very close wires which I doubt I would be able to test out without shorting. It may be possible though because where it attaches to the front board they alternate top and bottom with their attachments, so there is essentially double spacing there. That may be more than my fat, feeble fingers can handle, though.

From the first pic in post #27 it looks like there is a voltage regulator(s) ? on the rear board that I'm guessing brings the 12v down to 5v to send to the front board? I really shouldn't say because I'm just guessing, but that is what they do so... There are two of them, only one has markings. There are some other chips on that rear board as well, maybe it's all basically amp-related? I think there is probably something on that rear board that triggers the 12v for the power antenna lead though, but there again, I'm guessing.

Seems like just cutting the pertinent line to the amp chip would be the easiest and cleanest way to reduce milliamp draw like Mike said.

Actually, without some kind of way to attach the rear terminus of the ribbon cable to something that could easily be soldered to for antenna, RCA output, power in, etc. I think I'd have a hard time messing with this to get to the individual cables on that ribbon; and again, that would eliminate the 12v antenna power lead which I think is mandatory... see below.

*** One more thing, I messed with this a bit more today, there is definitely a click/thump coming out of the RCA jacks when the unit is turned on/off. I think it would bother me a bit to hear that each time it's cycled. I didn't notice it being so loud previously, probably because I had the external amps volume lower, but it's actually loud enough to be kind of annoying.

Upon power-up, there is a fairly big 'click' almost immediately.
There is an approximately 2 second delay on power-down then you hear the click; that one is milder.

Possibly a time-delay relay device like Mike suggested previously would be able to take care of that being an issue though.

More to think about...

TomZ

[tomzarbo]

These are from the front board:


That should be the important ones I hope.

[tomzarbo]

These are from the rear board as well:


A few more...

[tomzarbo]

I took some higher resolution pics this afternoon...

These are from the rear board.


More...

[Millstonemike]

Dukk,
I think it's just Bass and Tremble, and various combinations of those for the Rock, Jazz, Pop, etc. presets on the EQ. They're not all bad sounding though.
There is also a 'Bass Boost' or I think they call it 'Loudness' button as well.

Mike,
So cutting the power line to the amp chip will be enough to kill the extra draw?... I'll take some higher resolution pictures tonight of the innards so we can see what types of chips are used. It looked to me that there was more circuitry than just the normal 'amplifier' stuff on the back, but if we could figure out which lines are the DC V + and -, the antenna, RCA output lines, I guess it could be bypassed. We'd have to see if the front board works on 12V or 5V, I'm guessing it is reduced to 5V for that one.

Man, that would make a really thin piece of kit there without that stuff on there. I'm thinking a continuity check on the ribbon cable unplugged from the front board would give me what I need to do that.

I'll hopefully check this out in a few days. It's interesting! It has promise I think.

Ahh, back to work!

TomZ

Like I mentioned. The white linear IC backed up against the back plate is likely the power amp. You could cut the V+ on the leg holding it to the PCB. That would insure the remaining circuitry was still powered.

Lots of power amp chips using this format are in consumer electronics. It's likely a class A/B amplifier. Not as efficient as Class D. But maybe not much different when idling. Then again it's using older solid state design. A similar, currently available, quad amp chip for car radios from ST has a typical idle current of 190 ma (range from 120 ms to 350 ma) If we can't get the chip's datasheet, you could always measure the pins.

Oh. Good point, the DC-DC converter is 5 V. You likely need 12 V. Not to worry, There are isolated converters that will do 24 V DC to 12 V DC @ 500 ma. They just cost more (~$15).

But it "Kills two birds with one stone". You can power the main amp with 24 V DC and convert that 24 V down to an isolated 12 V DC for the face plate.

You could do a 4 woofer, two tweeter rendition and use the the units existing amp. If you use 4 ohm woofers, you'll get a decent spl out of the stock units quad output at 12 V.

Decisions decisions ...

[tomzarbo]

Dukk,
I think it's just Bass and Tremble, and various combinations of those for the Rock, Jazz, Pop, etc. presets on the EQ. They're not all bad sounding though.
There is also a 'Bass Boost' or I think they call it 'Loudness' button as well.

Mike,
So cutting the power line to the amp chip will be enough to kill the extra draw?... I'll take some higher resolution pictures tonight of the innards so we can see what types of chips are used. It looked to me that there was more circuitry than just the normal 'amplifier' stuff on the back, but if we could figure out which lines are the DC V + and -, the antenna, RCA output lines, I guess it could be bypassed. We'd have to see if the front board works on 12V or 5V, I'm guessing it is reduced to 5V for that one.

Man, that would make a really thin piece of kit there without that stuff on there. I'm thinking a continuity check on the ribbon cable unplugged from the front board would give me what I need to do that.

I'll hopefully check this out in a few days. It's interesting! It has promise I think.

Ahh, back to work!

TomZ

[Millstonemike]

I can say a couple of things now...

* It's almost weightless, very light.

* It doesn't look like the 'Wattage' speaker output circuit can be removed from the equation; or I should say it's not on it's own 'Amp-Only' board... The board in the back looks to have more than just 'output generating doo-dads' on it, but I'm not 100% sure about that.

* I'm not sure how much amperage would be saved by removing the output stage from the equation. The amp chip is screwed to the back aluminum 'heat sink'... could the leads to that chip just be clipped, saving those milliamps? After about 20 minutes of messing with it, it didn't get even warm, so leaving it as-is wouldn't be a problem I'd think.

....
TomZ

All good info. I agree, removing the "amp" from the circuit might not save much. But, perhaps, just enough. IIRC, you mentioned 280 ma draw sans speakers. The amp chip might draw 50 to 150 ma idling. Removing that draw gets you in the vicinity of 200 ma (just cut the power pin). That's why I posted the 200 ma DC-DC isolated converter. I think isolation may be needed if you add your own amp to the mix. And you can't isolate the main amp. It requires too much power.

Can you post a high-res pic of the chips , lettering. I'm pretty sure the white Linear IC is the power amp.


On another note, look at the mechanical construction. When embedded in a custom build, I think you can lose the back and side panels (I love how the back panel is formed like a giant heat sink - maaaketing). Then you can mount the "amp" board close to the main board and save a ton of space.

[Dukk]

^ Thank you for the redirection. I will have to check it out - frankly I have been dissatisfied with any 'Ali..' website but maybe I have always viewed only Alibaba

Any EQ function on that piece Tom?

[Millstonemike]

I have yet to ever buy from Ali as I find it frustrating to navigate and when I do find something I need a minimum order of eleven million of them

Some units have display settings that can turn it off when powered down.

Not Alibaba, Aliexpress ...

[tomzarbo]

Here is another much shorter video for a super-quick look at the unit up close: https://www.youtube.com/watch?v=gKt8l2BlDsg

TomZ