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Add-on sub PCB for RSA SR-71/71A/71B overdischarge protection and prevention for DC leakage on output

 For english-based people's convenience, this is independent but very important article for introduction of add-on sub PCB for the earphone/headphone burners.


 As I have concerned and warned about this issue again and again during last decade, the Blackbirds from RSA has fatal and dangerous problems. These tragic reconnaissance aircrafts can burn voicecoils in headphones or BA drivers and tantalum capacitors on the network in IEMs instead of JP-7 fuel.

The worst point in this issue is the amps can damage and burn your earphones/headphones but the amps themselves won't be damaged and never get broken. In recent portable rigs, prices of earphones/headphones can overwhelmingly be higher than amps, there will be huge financial loss when this issue occurs.


The symptoms will appear when one battery voltage gets low. If both batteries could be running symmetrically, voltage will get continuously stabilized......but never goes like that actually.

Like a chariot in Ben Hur, one or the other battery will run out earlier finally.

Then the fastest manned aircrafts turn to be heatwire controller as below image shows.



I told and warned Ray about this 10 years ago but he's staying to ignore it and still leaving in their products.

And owners around me are still suffering this and being afraid of this symptom.

Hence I need to have and develop countermeasures for.



This above is a prototype for add-on sub PCB for shutting down when one battery get terminal discharge voltage.

You can set shutdown voltage by the switches on center and already programmed into MCU on right upper to make optimum discharge curve for alkaline, NiMH, and LiPo 6F22(wall-wart) batteries.


 BTW, weird thing in RSA's portable lineups is that they are using 100ohms feedback resistors (finally he stopped that in 71B).

Lower value resistors can make lower thermal noises, but there's needless to amplify above 100kHz in audio amplifier by using too lower value feedback resistors, and in use of JFET input type OPAMPs, we need to be conscious about 1/f noises below 1kHz.


(From datasheet of 2SK30ATM)


This is reason why 71/71A's bass gets dull and masked.


Regarding to 71B, they quitted to use 100ohms as for feedback, but buffers moved to outside from the feedback loop as in following image.


(Press to zoom)


This makes piling up 634's bias voltage into outputs and we can observe 40mV DC leakage in each channel constantly, too bad. Also OPA1632s on balanced input are not connected to GND via Vcom pins, this also helps to pile DC voltage up to outputs.


 There is so much defects on their products (also P-51 and Tomahawk have another safety problems around their LiPo batteries too) and honestly this is somehow troublesome for me but this is also my mission as proponent of these issues.


The protection PCB $149

The protection PCB w/ entire mods for 71/71A $449

The protection PCB w/ improvements 71B $199

The protection PCB w/ entire mods (including LiPo balancer for safety) for 71B $499

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