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The 6-pin header, looking at the schematic, is an easy way to plug in a 3rd eeprom for development. They must have a separate board for that.

The regulator is in the correct orientation in the picture you posted above in post #12

You're welcome. I was in Turkey with the family last year. We stayed at a resort on the beach in Antalya.

I've shared the two versions of DI at PCBWay... https://www.pcbway.com/project/shareproject/THAT1646_Balanced_XLR_using_singled_ended_power_supply_20_to_30VDC_0480a454.html https://www.pcbway.com/project/shareproject/THAT1646_Balanced_XLR_using_dual_ended_power_supply_15VDC_12cb3570.html

PCBs for the small DI boards arrived. I built one of each to test and they are working fine. Two versions: Single ended power supply as an add-on to the power amps (24V); Dual supply (+/-15V). Shoot me a PM if you want one.

Exactly. Three questions during engineering: How easy is it to see an overvoltage? How risky is an overvoltage? How much does it cost to eliminate the risk? It's trivially easy to make the mistake. Catastrophic to some designs (those with voltage doublers and only zener protection). Trivially...

It's not an extra mile. It's an extra inch.

I just saw a post over on the gear page about folks blowing up their KTR by applying 18V accidentally. IMO, pedal makers should expect such a thing to happen and handle it gracefully. The common voltage doubler chips have a max operating voltage of 10V. Pedals that use them usually have a zener...

Would be cheaper for you to order direct from PCBway than for me to ship to Australia. Once I've tested them, I can share the project for others to order. I'm making 2 types: single ended power supply for this add-in board and one that takes +/-15V.

I don't understand? There is one power jack. Do you mean this? That's a push button gnd lift switch for pin 1 of the XLR.

I think it will all fit in a 125B. This add-in board can be glued to the bottom of the foot switch or the bottom plate of the case. Here's what it will look like.

Have ordered some...

Designed a PCB for an add-on DI using this schematic... 1"x1.5" Have not ordered any as yet. The annotations don't align with the above schematic. I started from scratch to do the PCB.

This would be the schematic to make an add-on board for DI installed in an existing Volume-Only power amp build using the old board. The signal is grabbed after the first gain stage on the TL072 and then divided down to get the right level into the 1646. But, it's probably safe to go directly...

By the way, the reason I can combine the power amp and XLR send is that I have a cab sim ahead of this built into my all-in-one preamp, which is over here...

Nope. The new board has a THAT1646 chip to do the XLR driving. It's a significant circuit addition. But, there might be room to add a stand-alone balanced send using the same circuit. I've since added a ground lift switch as well. The above picture shows no ground connection to pin 1 of the XLR...

Same supply to both but I have an RC filter on the preamp boards. Here's a schematic snippet of the power supply. J1 is the power coming in from the power jack and that is sent directly over to J6 to feed the power amp. R16 and C1 are the RC filter to provide clean power to the preamp circuit...

By the way, In using LA2 pedals now for years live and recording, something learned is that it's fine to have the cab sim right after the dirt and prior to all the wet effects. Cab sim does not need to be at the end of the chain. I've been trying to explain this to some guitar players and they...

If I were doing this again from scratch, I would have done the frequency analysis ahead of time and probably made the cabsim a separate PCB. Although I did compare drive and clean channels behavior and tone, I didn't study the cabsim enough before doing the full builds. Then had to figure out...

Another tweak. RB16 controls the mid scoop. The original value of 51k has too much scoop. Changed to 100k. This is the full frequency comparison of real versus mine with the tweaks. Ignore the noise. Takes way too long to get the full spectrum. The real curves would be smooth. Red is real, Blue...