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Tear down of Massdrop THX AAA 789 Headphone Amp

tomchr

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#61
I'm not familiar with rajapruk's proposal. I would look at how the 74HC109 flip-flop and JRC signal detector are used and rewire them to do what I needed them to do. The '109 has both set and reset pins. It's possible that all you'd need to do is to lift one of those off the board and wire it to VDD or VSS.

First step would be to trace out the power control circuit and create a schematic for it.

Tom
 

amirm

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#62
I would love to be able to modify it to automatically turn on with power in though, or maybe with an external trigger.
Anybody got an idea on how this could be done, beside @rajapruk proposal? :p
Since there may be a power on pop, you would need a delay of a few seconds and a "one shot" trigger. Wire that to the appropriate pin on the power button and it should power it on automatically without messing with the existing circuit. I can build one when I get mine and test it if there is demand.
 

rajapruk

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#64
Seeing all these TI parts makes me wish TI could just start selling their reference implementations as basic unglamorously simple audio products without any gimmicks, but with SOTA performance. Like a dac, for example.
 

AndrewMason

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#65
@AndrewMason, is there a way to make the amp automatically turn on when main power is turned on?
Hi @pos
This modification to make the unit power on at startup is WARRANTY VOIDING and UNTESTED, but here it is if you want to be the first to experiment...
-Order some spares for U701 SN74HC109DR in case you need to undo this, or in case the rework messes it up.
-Gently remove U701 ideally with hot air tool, taking care not to rip any pads off the PCB. Ripping pads could be disastrous and irreversible. Don't proceed with this step unless you are confident in your rework skills.
-Cut or break off pin 6 from the U701 chip (i.e. clip the pin from the package. Leave the PCB alone)
-Re-solder U701 into place on all pins except the missing one U701.6.
-Solder a jumper wire or make a solder bridge from U701.7 to the pad where U701.6 used to connect.

This will make the amp use the 1Q* output of the chip instead of 1Q to control on/off state of everything. Once 24Vdc power is applied, it should start up.
If it doesn't work, remove all the mods and put a new chip in U701.
 

restorer-john

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#68
Don't proceed with this step unless you are confident in your rework skills.
Kapton tape. Plenty of it covering the SMD devices you don't want to fall off, OK? :)
 

obc

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#72
do i need to turn the thx789 off every time before i switch to another headphone?
and can i plug in 2 headphones at the same time?
 

Jimster480

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#73
do i need to turn the thx789 off every time before i switch to another headphone?
and can i plug in 2 headphones at the same time?
I don't switch it off and yes I have 2 headphones plugged in.
Its rare that I unplug anything though because I have both plugged in at the same time.
 

solderdude

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#75
I recommend to be a little careful when plugging in 2 totally different headphones.
When one of them is low impedance and rated 20mW or so and the other is 300 Ohm and 200mW and you listen to the 300 Ohm headphone at a decent volume chances are the also plugged in 20mW low impedance headphone, still lying on the desk, died without you knowing it.

When headphones are quite similar in dB/V ratings then it isn't as much as a problem, even when the impedance differs.

No need to switch it off. It is protected against shorts and everything.
Still I would highly recommend to turn the volume down before switching as well.
Reason is the same as described above.. different power ratings, impedances and efficiency.
 

graz_lag

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#77
The idea to add a Khadas DAC inside the case is brilliant. Is anybody modding this? Somebody should :)
Inside ? Why ?
Just stack it on the top and you're done ...
 

maxxevv

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#79
Was just wondering @amirm or anyone who has the amp, whether it has ventilation holes underneath the case ? I can't seem to spot anything on the few Youtube videos or online photos.

Much appreciate if anyone can snap a photo if there are any vent holes on the underside of it.

Thanks in advance! :)
 

Jimmy

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#80
I think that if this is just a licensed THX's design it would be beneficial to distribute it under another brand, maybe adding an internal connector for a DAC board like f.e. the Moon 430, Jotunheim, ..., and maybe include a better potentiometer too (Alps blue velvet, Vishay pro, ...), selling it as they are doing now is seriously limiting its potential user base.

Because one can :cool:. I personally would like to have just one device, not several.
 
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