Massdrop x THX AAA 789

Thanks for the input! Oddly the voltage across is only 12V on both even though you're right it takes 24V. Further down the PCB it seems be almost in series with 16V caps around 3300uF. I'm not interested in mapping out the whole circuit, so I might put something similar.

Yah, this thing has microdynamics now. I gave it a good listen for an hour and compared it to my Flux FA-12 and it's pretty undeniable. It also seems to struggle more with complex passages now. I don't know if this because I'm using a 50K pot when the original was 5K, so I'll order a new one before I comment any more on it. This behaviour was observed with BA IEM's too, ThieAudio Hype 2's. Listening fatigue seems to be less too.

 

So all that circuitry is a rail splitter then. Puts ground in the middle of the floating input voltage.

If there is indeed 3300 uF downstream from there, then more capacitance is unlikely to hurt. It's also unlikely to help. But hey, it's cheap, go nuts.

 

I've never bypassed a relay before, but here's my attempt. Some hand holding so I don't burn my house down would be much appreciated.

Below is the pinout of the relay:



Using the above I'm guessing I just need to short these 2 pins on either side



The thing is the 789 has relay's up the wazoo. I figure this is the reason I can't check if I'm connecting the right pins, because the signal from the RCA jacks to this relay isn't actually connected, from another relay I'm assuming, until this thing powers on.

Am I doing this right? I've desoldered the relay and am just waiting for the go ahead to short pin 2 to pin 4 and pin 9 to pin 7. I know for sure that pin 2 and pin 9 are shorted to the attenuator's input for their respective channel.

Thanks!

 

Yah, sorry scratch all of the above.With some probing with the amp on I found none of the pins connect to the RCA inputs. If some of the pins are shorted via a multimeter in ohm mode to ground the amp does briefly go into protection mode. I think given the proximity to the source selection button this relay just mutes the output when you switch between RCA and balanced inputs. The amp works fine with it completely removed. Scratch all my assumptions and idiocy above.

 

I confirmed this morning that the missing caps are indeed in parallel with the existing 3300uF 16V caps. Would it be safe to add additional 6800 uF 16V caps in parallel, or do I have to be concerned about too much additional capacitance causing oscillation? Would in rush current be an issue? TIA!

 

Behold, a 789 with triple the filtering capacitance, over 10,000 uF, and two metal film bypass caps, 1 uF and 0.1 uF for shits and giggles.

So two conclusions:

1) This doesn’t really do a whole lot for headphones. IMO still a mediocre sounding headphone amp.

2) It does something magical for IEM’s. It gives more controlled and more dynamic bass along with an airyness that’s more tubey/holographic. In some forums this amp has a pretty bad rap with r2r DACs, but after this mod I could clearly hear the extra reverb my r2r dac has to offer, but only for IEM’s.

I was really hoping to make this a better headphone amp, but I can live with making it a better IEM amp.

Hopefully this is the end of my retarded obsessiveness with fixing broken things for now.

 

I have heard some people like this amp best with BA IEMs so I think you are onto something...