12V phantom power mod for KB/A 50

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dak
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12V phantom power mod for KB/A 50

Post by dak » Fri Sep 23, 2022 7:13 pm

I am not a fan of batteries, so I prefer powering my electret microphones via phantom power (by now, converters from XLR phantom power to mini-XLR 3-pin are ubiquitous). 12V is enough for the typical adapters available here and is what all the opamps in the KB/A 50 work with, so it's readily available on the PCB. The standard calls for 680 ohms resistors for 12V of phantom power, and one does not want to have this kind of drag on dynamic microphones or on the TRS plug of the XLR combo jack by default. So I decided to make this completely detachable, and using a double switch here for both resistors would suffer from a lack of simultaneousness, making for a much bigger bang when switching phantom power on/off than desirable.

So I decided to use MOSFETs as switches instead. Due to the body diodes, there are a few drawbacks: voltage on the XLR pins must not be above 12V (shouldn't happen unless something else injects phantom power in a reckless manner), the capacitors blocking phantom power from the actual input need to discharge when phantom power is switched off (so that the body diodes stay definitely non-conducting). The KB/A 50 has 100k resistors doing that. I put in a slide switch for engaging phantom power, and I use the ground switch in the XLR combo socket for switching off phantom power when plugging in a TRS plug (the requisite GN pin of the socket is unconnected on the underside of the PCB). It would be cleaner not to route phantom power to the TRS socket at all but I didn't want to cut too many traces on the PCB, and the KB/A 50 keeps XLR and TRS connected.
phantompower_schem.png
phantompower_schem.png (70.77 KiB) Viewed 3680 times
R1 and C1 don't do a whole lot of noise/transient suppression I'll admit (corner frequency something like 800Hz so should at least cover conspicuous hissing), but the results are quiet enough. Values are just from memory. I'd have to recheck.

Oh, and R3 and R4 need to be closely matched, like by using 0.1% tolerance resistors.

One thing that still needs changing next time I have this amp at home again (only going to happen once I have the schematics and can do a few other improvements) is to use a bigger resistor than 2.2k for the blue LED: I thought that was already conservative but it's still stupid bright and distracting in concert. Probably 10k would do the trick.

When I get the amp here again, I'll add a few photographs of the mod. Somehow I find it a nuisance that keyboard amps (not just Peavey) rarely bother providing any kind of phantom power: instrument microphones often are electrets that could be powered that way.

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dak
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Re: 12V phantom power mod for KB/A 50

Post by dak » Wed Feb 08, 2023 9:10 am

Ok, so I finally got hold of KB/A 50 schematics and have taken the opportunity to get the amp back home for some tweaking (minimizing hum by fixing some ground routing issues possibly introduced during revisions of pot and socket placements). Good opportunity to make some shots of the phantom power mod.
Piggyback PCB behind XLR socket
Piggyback PCB behind XLR socket
P1150405.jpg (5.35 MiB) Viewed 2884 times
Side view
Side view
P1150420.jpg (4.65 MiB) Viewed 2883 times
Front switch and LED
Front switch and LED
P1150424.jpg (2.84 MiB) Viewed 2884 times
Not particularly pretty but functional.

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studiodtk5
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Re: 12V phantom power mod for KB/A 50

Post by studiodtk5 » Thu Feb 09, 2023 5:17 am

Excellent mod. Thanks for sharing.
Darren
ITOC: 08-00190

Peavey stuff I have: Masterpiece 50, Custom Shop 212, Stereo Chorus 212, T-60, T-40, Signature Select, Odyssey II Prototype, Generation Custom EXP, Firenza P90-ACM, VB-2, Stomp Boxes, Radial Pro 1000, lots of mics, etc...

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