Help with disabling loudness on this chip

[tschanrm]

I've been modifying my Mazda's stock head-unit because it contains a made in Japan indash 6 cd changer. I've added pre-outs by splicing into the volume chip, a Mitsubishi M62490FP. Datasheet can be found here:

http://www.alldatasheet.com/datashee.../M62490FP.html

My problem is that loudness is currently enabled. I want to disable the loudness feature. The datasheet shows what switches need to be on and off to disable loudness, but does not say how to flip those switches on and off. Are these switches controlled by the MCU? Any help on disabling loudness would be much appreciated.

[singa]

My best bet. pin30,7 input for setting frequency characteristics of loudness block.There is a capacitor 820pF ( see schematic of datasheet) ,it may be different in your system.Just desolder one leg and pull it out of the circuit.This could cut it out of the signal chain.

[kevinkr]

I take it the head unit does not have a loudness switch or a combination of key presses that defeat loudness. (My old Ford premium sound head unit had a loudness switch... The Honda had a key sequence.)

The components on pins 6 & 7 and 30 & 31 set the loudness contour which in this design incidentally includes boost both at the high end and low end of the audio spectrum.

Removing everything except the 2.2uF caps that go to pins 5 & 32 will defeat the loudness control. (You might just want to remove the resistors and leave the caps in place - see schematic, this will accomplish the same thing and is reversible.) You may or may not need to add a small set of resistors from pins 7 & 30 to pin 1 the half supply reference if you need more attenuation at the bottom of the volume control. I assume a few hundred ohms would take care of it. (Try it without first.)

Note that you can defeat the high frequency boost and not the low and vice versa by removing the appropriate components. Highs are handled by the networks on pins 30 & 31 and 6 & 7. Lows on pins 30 & 7 to ground. You can also change the amount of boost and the inflection frequency on the low end by changing the 1.6K resistor and the 0.22uF cap. A larger cap will limit the boost to lower frequencies, a smaller resistor will reduce the boost and raise the inflection point. (Recommend increasing cap to avoid this.) Maintain the same time constant or lengthen it, don't go the other way as this will boost the mid bass as well as the low bass.

Hope this helps.

[tschanrm]

Thanks for the help. The more I look at this schematic, could I bypass the tone section completely? I really have no use for it. I would sever the connection between 5 & 6, sever 12 & 13, sever 31 & 32, and sever 24 & 25. Then I would connect pin 5 to pin 13, and connect pin 32 to pin 24. Would this way work as well?

[kevinkr]

I'll have to take a look at the thing again, defeating the tone controls should be possible, but I can't guarantee this. Do this part first though and get it working - easier to troubleshoot one change at a time if it doesn't quite work as expected. If this is a baxandall configuration (IIRC it is) it has no effect in the centered position.

Good luck!

[mrshow4u]

it looks like the tone, loudness, and volume functionality are all controoled by serial data all being clocked into the chip. It looks like setting data bit 0 {1|0} toggles Loudness. I get the feeling there's no "Loudness" button to toggle the function??

[tschanrm]

mrshow, thats right, I hve not found a button, or combination of buttons that can disable loudness. I can access diagnostic modes, but those can't do anything. I can also access an "Ant-On" mode and also an "Amp-On", but no combination of buttons seems to toggle those settings on or off. I have the service manual for the car, and it doesn't even describe the "Ant-On" or "Amp-On."

[mrshow4u]

Hmmmm, it looks like clock and data come in on pins 18 and 19. It looks like an MCU is calling the shots. I don't see a data latch input. My guess is the data is clocked in when needed only, 16 bits at a time. have you scoped the clock line to see if it only is toggles when a volume or tone adjustment is made. My guess is (if the clock runs in a 16 cycle burst) that the voltage on the data line at the first clock edge will be High (Loudness ON). A digitizing scope would be really handy for this measurement

...it also looks like D15 D16 address which row of functions to select. For volume and loudness functions, D15 and D16 should be Low

[tschanrm]

Sorry, I don't have a scope where I'm at. I can tell you for a fact that loudness is on. I've measured the freq. response at 1/3 volume and at full volume. At 1/3 volume the freq. response is kinda 'U' shaped, and at full volume the freq. response is flat.

[rpapps]

Hi tschanrm
Can you ascertain what type is the microcontroller and which pins on it are connected to pins 18 & 19 of the M62490?
Thanks
Rob