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OK, who out there has a sheet metal break and wants to make me an RF shield?

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@Ricardus What are you trying to shield, and what shape are you looking for?

@toroidalcore I'm only kidding with this. Doing this over the internet is probably not the best way to do this. I'll need to find someone local, se we can kinda fit it as we go.

But it's a shield between the power supply and preamp boards in my latest project.

It would be about 10 1/4 inches long by 1 5/8 high, and go like this red line.

@Ricardus This just looks like a linear supply then, you're just stepping line voltage down, rectifying and filtering it, then running it through some linear regulators? And you're trying to keep line noise out of the audio?

I was just going to say, make sure you have a good electrical connection to the rest of the chassis, and enclose the supply part as best you can. I was going to suggest looking for some copper tape as well to help seal cracks too. Some common mode filtering to chassis on the input might not be a bad idea if you don't have any already.

@toroidalcore It's already dead quiet, but a little extra protection can't hurt.

My friend Kevin has a sheet metal break but I haven't been able to get with him to do this.

@Ricardus Should be pretty nice. I'm used to working with switch mode stuff, which is a lot noisier.

@Ricardus I think they're normally the way to go, but you have to manage EMI. For a lower power application it's not too bad.

@toroidalcore They're getting better all the time.

This tube amp was designed to be powered by a 48 volt SMPS, and his B+ and filament voltage was derived from the 48 using SMPS on the board. I put a Meanwell 48 volt unit in the case and it's pretty damn quiet.

He put filtering on the board since he designed it for a SMPS, but I put it on a spectrum analyzer at 192K and there was some switching noise at about 80K, but it was about 105 dB below full scale so I can live with that.

@Ricardus Was he using a like a small forward converter to get B+ then?

We were using something like that on a board at work to power an isolated current sensor. It was failing EMI, but I ended up fixing this with a small cap next to the transformer to cross the two ground planes. You can get these pretty clean if you keep the common mode loop small.

@toroidalcore He was using one device for B+ (220 ish volts) and something else for the filament voltage (6 ish). All derived from 48 volts.

Here's the schematic.

pmillett.com/micpre.htm

@Ricardus My first thought for going around 4x in voltage would be to use something with a transformer instead of a straight boost converter, but if it's stable that works.

This looks like a neat preamp, and I think I've run across his site before. Looks like good inspiration, I haven't really done much in the way of audio circuits, and I'd like to try building an amp sometime.

@toroidalcore He's mostly home hifi stuff. He used to own a company. He's a pretty particular designer, and I stumbled into his site 4 years ago or so, and no one I knew built his mic preamp so I figured I would. But I did a 2 ch. version since I hated his one channel enclosure.

I also added a balancing transformer to the output since he designed it to be unbalanced for cost and simplicity. I used Sowters in the first build, but I just got two OEP's for free so I'll use those in version 2.

@Ricardus How has that worked out? Also, what are you using for an audio interface?

@toroidalcore The tube mic pre? Very happy with it. Can't wait to start the 2nd one.

I have two interfaces. Nothing fancy. Focusrite 2 channel and 8 channel.

@Ricardus Cool. I've got a Saffire Pro, it's nice but I've been thinking of getting a newer one with USB.

You just run the output of the mic pre into one of the line level inputs then?

@toroidalcore Yup. Line inputs.

I wish Focusrite would just give me a USB3 box with 16 line level inputs and good converters in it. I don't need the preamps.

I know I'm an exception to the rule because I have a collection of cool preamps, but why doesn't someone make this?

@Ricardus I feel like I've seen interfaces that are just a bunch of line ins and outs. I want to say it was RME who made it? I'm guessing that people in big studios where they have a high-end recording console would want something like that (or several). There's a sweet spot in the market for an interface with 8 mic inputs, I suppose. It works well enough for a lot of people.

I'm actually running direct outs from a mixer into mine, not for sound quality reasons but mostly for signal routing convenience while noodling around on my bass or whatever. I've got an older desktop I built that I think is having weirdness with firewire devices and Ubuntu's low latency kernel, so I've been thinking about trying a USB interface. That said, I may just try upgrading this machine and sticking a firewire card in first.

@toroidalcore I could run a commercial studio with 16 channels of conversion just fine. In studios I've only needed 18 channels once for basic tracking. Once you're done with basics you only need 1 or 2 channels at a time for overdubs. And if I needed to I could have done the session with 16 for basics.

RME makes a lot of nice stuff, and it's totally worth it, but I'd rather pay the Focusrite price. 😉

@Ricardus Agreed, although I haven't looked at used RME stuff. I did get the Focusrite used.

It seems like you should be fine with 16 channels, but I could also see that being something you end up using more and more of if you have them available...

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