Here's an update to the battery powered SC-1 Mic Preamp (I know, I know... finish the cases, and the DI box... I'm not neglecting those. This is just a little sidetrip.)
I've been incubating in my brain how I can power my preamp with batteries. This is for use by the "tapers" crowd, you know... the a'la Grateful Dead fans that tape concerts.
Preliminary testing by powering it with (4) 9-volt batteries was successful.
But FOUR 9V batteries? That's a lot... and expensive. And bulky.
So I investigated DC-to-DC conversion... I stumble upon a solution using a single 9Volt battery, I can produce 18Volts. So now, I just need (2) 9-volt batteries to produce 18-0-18. Great! A 50% savings in battery.
Testing the preamp with the DC-DC converters work... sort of. It was NOISY...very NOISY!!! .... The preamp works, but there's a background noise that sounded like a mosquito that wouldn't go away. Literally... it sounds like a mosquito buzzing around your ear.
Took about an hour trying all sorts of things trying to fix the problem but no-can-do. And as always, just when I was about to give up... I'll try one more "last-ditch" solution.... I mean, who cares... it doesn't work, it wouldn't hurt to try this last thing.
And what do you know! .... that fixed the problem! It was quiet again! Yay! Running on DC-to-DC converters... I can't believe it.
Now, the big problem of NOISE is gone. I'm happy.
But I'm the type of person that's always looking for some hidden problem, instead of being content and calling it a day.
I found (2) things to "moan" about....
1) My phantom power is only 18volts... not 48Volts... It's not even half of 48Volts! While most condenser mics will work with +18, a few mics just sounds bad running on +18V phantom power... or not even work at all.
2) The +18V supply fluctuates between 17.8 to 18.1 then back to 17.8 again... 17.8, 17.9, 18.0, 18.1, 18.0, 17.9, 17.8.......
I don't know why it's doing that...I know it's caused by the DC-DC converter, but why... I don't know.
Already, I have a possible solution that will solve both problems.
My DC converters can go up to +28Volts on a single 9V battery. I think I'll crank it up to it's maximum output! Then maybe 28Volts will be enough to operate power-hungry mics like FET condenser mics. It's going to be better than 18V at least. That *might* fix problem #1.
Also, I can build a regulator to regulate down that +28V max output down to +18V... this will give me better regulation, and a buffer of about 10V. So I think even if the battery runs low, I'll still be able to get 18V. That fixes problem #2.
Should I design a new PSU, or just mod the existing PSU-1848 PCB.... I think 80% of the PSU-1848 schematic can be reused on the new battery powered PSU. Besides, I need space on the new board for the DC-DC converters.
So... there we are on this new project. Coming soon... SC-1 preamp powered by (2) 9-volt batteries!... with phantom power!