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Post by Guitar on Sept 8, 2020 12:54:20 GMT -6
This is really cool! Love to see a little bit of the technique that goes into it. The only problem I would have with those mics are the capsules themselves. I did some mods on some of those mics and in the end, solid state, I felt that the capsule coloration was holding me back ultimately. If you plot the frequency response there will be weird peaks and dips in the top end, it's just something inherent to those capsules. Microphone Parts claims to have fixed this and they sell capsules from their web store, with a supposedly flatter response, but I never tested them. I am wondering if there are other capsules out there that would fit those bodies, since there are so many of those kinds of mics floating around. Honestly I don't mind these mics with the tubes. They sound just fine. The problem now is that out of the power supplies I bought only a couple even powered up and those promptly stopped working with any kind of load on the output. Guess that's what I get for ordering chinese boards. I still need to make an order for some brand new ICs and see if those bring these back to life. I really should have just done my own boost supply design but I was interested in cutting development time off and I had such an easy time with my first chinese boost board I figured it would be a slam-dunk. Oh well. That's really interesting. I ordered some $2.50 DC to DC converters (yes, real price) and I haven't tried them yet. I was also thinking about trying to do my own layout. I was trying to get from battery power up to about 15V DC for some THAT mic pres, that I want to use for mobile recording. That project is on the back burner right now. I guess I should test them out and see if they even work and for how long with a load. |
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Post by svart on Feb 9, 2021 9:23:09 GMT -6
Unlike our government spokespeople, I'm circling back around to this.
So I was determined to figure out why these chinese HV supplies seem to be really iffy. I traced back through the circuit and drew it out and then worked through the values of parts they chose. Not quite the values I'd use but it should do something while these are doing nothing.
I replaced the generic 3845 IC with a new one from TI and it did not power up either.
I then decided to modify the circuit to something more like the datasheet and I got a very high voltage (~300V) output. So I then backed off the mods until it stopped working again. It seems whatever transistor they used for the "Shutdown" option is wrong. I can't read the markings but trying to use a diode tester to determine the pinout seems to suggest that it's the wrong footprint or wrong part for this footprint.
Anyway, without the transistor the supply turns on just fine. Strange. I think I'll swap the transistor from the original working unit or swap in something from the lab at work and see if it works with a known good part.
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Post by svart on Feb 9, 2021 9:38:25 GMT -6
I guess I'll also verify what happens if I use the 470uH inductor on a working board. Sims show that it might be a much reduced available current output, but also a much reduced inductor peak current too.
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Post by svart on Feb 10, 2021 9:46:43 GMT -6
So I grabbed an unworking HV supply and a new carrier board and hooked them up. No workie as expected. I then pulled the unmarked shutdown transistor off the board and it immediately came up and worked. I guess those are just junk.
Anyway, so some sims show that the 470uh inductor would supply 10x less current to the output with a given load. I haven't tried this with a physical test yet though. Sims also shows that a 10k load would be OK for 47uH but would load the supply down so much with the 470uH that 100V would become 30V. The 470uH needs more like 1M as a load to meet voltage. I'm also simulating around 40% duty cycle, so there's some room to increase/decrease voltage.
I honestly don't know what the load for the two SDCs would be, so I'll err on the side of using the 47uH inductors I think, mainly because the other supply worked OK with 47uH and I expect these to be no different.
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Post by svart on Feb 22, 2021 8:17:04 GMT -6
So I tested the board loading with my stereo Royermod SDC setup. Both the 47uH and 470uH worked OK. Neither got hot, although I did not test current sourcing ability. I have not tested the audio yet, but I will. It seems that the HV boards from EBAY/Alibaba will work with the 470uH, at least with these mics. I'd like to try with a higher current mic or tube preamp though.  |
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Post by rocinante on Mar 29, 2021 23:22:15 GMT -6
Svart
This is really intriguing. Can you break down what you have done and are there any pcb's available?
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Post by svart on Mar 30, 2021 8:09:13 GMT -6
Svart This is really intriguing. Can you break down what you have done and are there any pcb's available? I mean, the thread kinda explains it, no? It's a carrier board with necessary filtering for a couple chinese Ebay switching power supplies to use as the power supplies for low voltage (<200V) projects. One supply produces 85-200V and the other supply produces 3-12V from a single 12-15V input. I've used the first one for my royermod SDC mics because the original linear supply got very hot with two mics but the voltages weren't stabilized if only one mic was used. Since this one is switching, it runs cool and is always stabilized. These are prototype boards but they aren't worthy of selling as bare boards. If you want one I can sell you a completed one. |
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