I noticed in the Schlockwood SW200 pdf it emphasises that you have to use the supplied 18v power supply with the unit, so I emailed him saying that I was looking forward to the SW205 coming out so I could pair it with my Rangemaster and wanted to confirm that powering it directly with DC would be ok.
Well I got a very interesting response (in many ways) but I don't quite grasp what he's telling me, and am hoping someone here can clarify it for me.
Here was Jim's reply, the last two paragraphs is what I'm confused about, but I'm posting the entirety of it because it kind of explains why the problem (if there's a problem, which is kind of the question), and also because it was such an damn interesting response!:
Thanks for the note and the question. Also, I applaud your use of solar/battery "off the grid" operation of your station. Here's the sticky situation with DC power requirement.
Our designs always try to favor simplistic approaches to circuitry and the use of component parts that are universally available. We stick with through-hole circuit board layouts as many of the parts we use are 'legends in time,' with not only ready availability, but large backup stock with distributors as well.
Our SW300 AM mod-monitor utilizes three surface-mount chips. A numerically-controlled oscillator for the two local oscillators in the circuit, and two inexpensive switching regulators to generate the positive and negative supply rails from a DC input that can vary between about 8 and 24 VDC. This was at the insistence of a fellow who did some market research for us up-front, swearing that battery operation of the field strength meter function would be a 'must.'
Our second production run of SW300s occurred just as the COVID pandemic hit. And guess what parts we absolutely could not get from any source. No, we could find the oscillator chip in stock in the quantities we required, but the two switching regulators were totally unobtainable. Oh, sure, there were Asian knockoffs (probably out the back door of the Texas Inst. factory over there) that could be had for $20 each, but the price before (and after) the pandemic was less than a buck for the pair. Every other part on the board was immediately available from multiple sources.
The SW200 and its replacements use a 'phantom ground' power supply arrangement. That is, the 18VDC regulated switching supply that we provide with these units forms the positive and negative 9-volt supply rails, and 'ground' is simulated with an op-amp bridging that bridges the rails with its output grounded. This works like a champ, is easy and inexpensive to do, but it means that the negative connection of the wall-wart is 9 volts below ground, and the positive is plus 9V. Connect the negative side of the wall-wart to the chassis and you effectively short out the negative supply and pump the positive supply up to +18V. The incoming regulated 18VDC has to be isolated and floating.
The good news is that the SW205 draws less than 100mA from the 18V supply, or less than 2 watts. There are commercial DC-to-DC thingamajigs on the market that would power the processor, or the smallest inverter that will put out 115VAC would certainly do job just fine also. Sorry for the sad news, but in the interest of keeping the product cost low and using multi-sourced parts, it's the way we've chosen to go. I fervently hope that you might have a workaround, and would love to hear how you might solve such an issue.
I can answer this but to be continued tomorrow as it's now 2AM and late now just checked in for a quick look. I have the schlockwood 200 although right now I am using the studio interface processor that comes with the Procaster.
@mark Not to throw things off, but I've been curious why you quit using the Culbert processor you use to talk about having... Or maybe you had already answered that, I'm not sure. I had been considering buying one but when you had alerted us that Schlockwood was coming out with a new model, and at a reduced price that's not that much more than Cuthberts, I felt the Schlockwood is the best bet. But I'm still curious why you moved on from it
But returning to the point, I don't understand what all this positive and negative supply rails and floating ground by an op-amp bridge or whatever he's talking about concerning supplying the 18v DC that the processor wants. It sounds so complicated and I thought it was just a cut and dry simple thing.
Like I said, if it's all that big of a deal, I just plug it into an inverter.
@richpowers For the info on an isolated adaptor power supply vs an un-isolated one see this as it explains it. Basically it's the separation of the inputs and outputs via transformer for quieter operation and having two separate power supply voltages available as per the the answer you got from Jim Wood. The positive and negative supplies is explained here. The Decade FM transmitter has a similar thing.
You can use any 18 volt power supply as long as it's the isolated type. Triad brand from Mouser electronics has 18 volt adaptors that are isolated. I tried one that wasn't, a cheapy, and it didn't work properly!
But this doesn't matter if you want to use it from a power station as the DC outputs on power stations are 12 volts not 18 volts. So then you need the adaptor and power it from the AC via inverter with the power station. 12 volts is the most common voltages that work most things. But even if your power station had an 18 volt DC outlet it wouldn't work the Schlockwood. It couldn't run on a battery either. A floating ground is when a ground in a circuit isn't fixed like negative all the time. You can have a plus or negative voltage.
As for the Sean Cuthbert, I didn't like that there was a little glitch that when a song started there was a, maybe 1/4 second, delay till the processor kicked in with a slight distortion on the first note or drum beat of a song. Not all the time though. Maybe if I fiddled with it more I could fix it, who knows. Maybe it would be good for you as it works on a common 12 volt adaptor and you could power it from the DC outlet on the power station. I should try it again. Radio Jay Allen gave it a great review. I didn't contact Sean Cuthbert to ask about that, I should. In his demos on you tube it is great.
@mark It just shows my ignorance I guess, there have been numerous times I've bypassed power-bricks over the last 40 years with a direct connection to a 12v battery (in an RV I used to have, boats, camping and various other situations) and I never knew it could potentially cause problems or not work at all. It always worked and had an no perceivable issue doing it.
I guess I just got lucky
Well.. is there such thing as an isolated step-up to 18v direct from a battery?? -- is doing such thing unadvisable? Should I just forget the notion and just use the provided 110AC power adapter with an inverter ?
Probably what I'll end up doing
Oh, now I recall you talking about the Culbert flaw.. I had kind of suspected it was maybe just a bad unit you got
@richpowers You asked if you can step up 12 volts to 18 volts? It wouldn't be isolated even if you could. That has to originate in the power supply itself. If you get a Schlockwood you have to power it with the right AC adaptor 18 volts from the AC outlets.