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Today I received this nice 90's Panasonic RX-CT900 boombox in the mail.


Panasonic RX-CT900


The specs claims it is 2x60W PMPO (PMPO b**llsh*t throw-of- a-cliff specification) but in fact it is just 2x5.5W RMS (Panasonic AN7133N IC). The amplifier is described in the service manual (right click with mouse and open in another tab to view/zoom it full size):


Panasonic_RXCT900_sch_amp


Sound is pretty good however when you crank up the volume is starts clipping (this is an amp problem - not a speaker problem), especially when using the XBS function.

Edit 11/13/2020: Discovered it is actually a speaker problem: foam rot!

What I want to try is to replace the amplifier IC with a modern, more efficient, more power and better specs one. I have many of these small OEP50Wx2 boards, they are pretty good (used many - excellent sound quality) however Class D (Texas Instruments TPA3116 IC):


OEP50Wx2 module


The question is, to you specialists, is this a stupid idea, does somebody tried such thing already (for example replacing obsolete IC) or can do this without problems.

What I want to do is to create an adapter socket (on the main-board) that allows me to plug-in (with compatible pin arrangement) the new amplifier module (and to easily switch back when it is no success or for other reasons - don't want to stress the solder connections too much).

In the service manual they specify a 15V input, which is perfect for the module because it outputs only 25W max (in theory). Because a class-D amplifier is more efficient (and consumes less current and more power at volumes) I think this a could be a great improvement in sound quality or is this just a crazy idea? I know the speakers maybe don't match the output power however I have some heavier replacements and test speakers (max 100W) that can be used to test the new setup (speakers are detachable) first.

Before I try, what do you think, possible or a source of many problems? Here is the full schematic of the mainboard (right click with mouse and open in another tab to view/zoom it full size):


enter image description here


Notice:

  • If this is possible it could be a great method to replace obsolete IC's with modern tech to keep devices away from the landfill;
  • The pictures of schematics are assembled of pdfs with separated pages (can contain some allignment errors).
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  • \$\begingroup\$ "Edit 11/13/2020: Discovered it is actually a speaker problem: foam rot!"- makes sense, because 5.5W rms should be plenty for speakers that size. \$\endgroup\$ – Bruce Abbott Nov 13 at 22:56
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If you want to socket the amplifier, then no. It's bridge output, not compatible with the existing speaker and headphone socket wiring, unless you leave the SPx- outputs unused, which reduces your output power by a factor of 4. Even with that restriction, it should still deliver more power than the existing. You should test the module for stability and distortion with one output unloaded however, some bridge output types may require a balanced load, others might manage without, check what yours does before committing much work to the project.

If you modify the speaker wiring to bridge, then you'll have more confidence that it will just work, and get the full power. The headphone socket will still need to be single ended though.

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  • \$\begingroup\$ Good point about the existing wiring, thanks for the answer. Okay, socket is not possible for all connections but when I use separated connections to the speakers, it could be possible, right? What do you think, is it possible to remove the amplifier and only connect the inputs to the new amplifier with separated output connections? \$\endgroup\$ – Codebeat Apr 24 at 14:14
  • \$\begingroup\$ Is it possible? Yes. Could you do it? How's your dismantling foo? I usually find something unexpected and break something when trying to modify stuff, but that's me. \$\endgroup\$ – Neil_UK Apr 24 at 14:18
  • \$\begingroup\$ And what if I keep the amplifier intact (as headphone amp) and drain the inputs only, that doesn't have to be destructive, right? \$\endgroup\$ – Codebeat Apr 24 at 14:25

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