Mobile pre-amps


#19

re : soundsniffer

yes that looks good, hits the right spot for me. The maker should have left longer legs on the led to line it up with the small holes above the onn/off switch. Id prefer to do my own box and choose my own socket sizes but Ido like it. just wish the kit option was boxless and a bit less.


#20

Contact mic amps are pretty straightforward to build nice low-power ones, eg, the Alex Rice/Zach Poff circuit. That’ll also amplify a lot of things reasonably.

Phantom power looks a bit harder, though not as horrendous as I thought: and you don’t even necessarily need 48V; 12V might be enough, meaning you could run your preamp off a 12V DC supply, and utilising something like a TI all-in-one amplifier chip to do the amplifying.

But I realllly don’t need another project now…


#21

talking of which this might pair nicely with the Koma Field Kit


#22

renamed the thread as it’s not really norns-specific, even if norns prompted the thread. good to see all the suggestions!


#23

Since it would definitely want for some kind of battery, could be nice to make that battery tappable as a backup for Norns as well.


#24

As far as straight preamps, I have a schematic (and stripboard layout) for a Barcus Berry piezo preamp with gain/lows/highs which fits in an altoid tin, along with 9V battery, that sounds pretty nice. But no phantom…

So with the 12V vs 48V, what difference does that make? Is it down to the mic itself in terms of tolerance, or is it a headroom thing?

It’d be nice to find something that was quiet/nice, but wasn’t as big/expensive as the MP-1.

I’ve not used one, but am I correct in thinking that the Koma Field Kit is pretty hissy?

I’m wondering if holding out for this preamp is the way to go?


#25

depends on the mic, some will work perfectly with 12V (my oktava mk12 does iirc), some will not at all (MKH60 for exemple, is rated for 48V +/- 4V) and the output level drops to nothing below these values.


#26

It’s not difficult to boost even 5v USB to the 48V required for phantom power. The current draw is so minimal (microamps, usually, although some mics pull up a few tens of milliamps) that the circuit draws almost no power. So even a battery powered phantom circuit should be 48V as there’s effectively no reason for it not to be. Then all mics will work with it without worry.

Edit: here’s a nice reddit thread to design the circuit: https://www.google.ca/amp/s/amp.reddit.com/r/AskElectronics/comments/176r6v/generating_48v_phantom_power_from_a_5v_supply/ and a quick google search for “USB phantom power circuit” returns a bunch of premade micro designs too.


#27

Yeah, it looks like something straightforward like an LT8330 would get you 10ma of 48V out of USB. Then a phantom circuit is straightforward to knock out. So there’s a chance I might draw something up if people are interested in a powered-from-a-USB-battery-pack phantom power amp…


#28

quick update: appear to have a sketch of something. One circuit to generate 48V from 5V, attached to the datasheet circuit for the TI INA217. Mini-USB for power, XLR in, quarter-inch out, maybe about the size of a pack of cigarettes.

I might have to see this one through to completion. (It also has a suitable name already)


#29

Your main trick for really good sound/performance is going to be power isolation. What are you doing to keep powerline noise out of the audio circuit and the phantom power circuit?


#30

It is a mobile pre-amp but its friends call it … Tim?


#31

well, given I’m sat on a terrace on holiday and this is still at the back-of-a-cigarette-packet stage, I’m just going to see how it turns out. Possibly do some prototyping. Possibly read the paragraphs about layout in section 10 of the datasheet.

I mean, it’s probably not going to be cheaper than anything available or better, so maybe it’s just fun, and when I fuck it up you can tell me you told me so.


#32

Heh, I wouldn’t be the one to do that, but mostly just curious as to what your strategy is for it. Good on you for trying, for sure!


#33

Actually, I have an idea to use a common camera battery like an LP-E6N to power a device like this. You could easily charge it with a canon charger or we could build a little USB charging circuit into the device, also useful for side power. That way the batteries would be cheap and removable. You could do it with rechargeable AAs too, but those don’t hold as much power for their size. I’d be interested in helping you sketch this up, if you’re up for some help. With a battery you have less power supply issues to be concerned with - mostly just eliminating the switching frequencies and their harmonics from the booster circuit and the chip supply, and ensuring good board layout. A full ground plane on one layer will help with that, too.


#34

oh, arse. I misread things. So the main issue is that the nice available mic preamp chips want meaty bipolar supplies. So now I’m investigating bipolar DC-DC converters along with the bloody 48V supply…


#35


#36

I use my zoom h5 as a portable pre amp and also for getting external sources into my modular.


#37

Since the new Mixpre recorders are out there is less demand for the original Mixpre (the one before the Mixpre-D). They go on ebay for about £250 these days. Highly recommended, all analog, battery powered, metal chassis etc…


#38

I have a Mixpre-D I bought of eBay and use it all the time for lots of different routing, pre-amp bypass my Organelle dac and use it as a soundcard. Be aware though, it eats AA batteries in no time.

I’ve looked at these often too https://www.eventideaudio.com/products/stompboxes/mic-pre-fx-loop/mixinglink just no phantom power of 9V