DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

A lot of you have asked if it's possible to power the SC-1 preamp using batteries.

I have done some testing and am happy to report that YES, the SC-1 can be powered with batteries for field use.

I used four (4) 9-Volt batteries in series, in order to derive +18, Gnd, -18V and connected them to the preamp. For the 48V phantom power, I just used/shorted the +18V pad to the +48V pad. So my "phantom power" is only +18V in this configuration instead of the usual +48V.

Using dynamic or ribbon mics, the preamp works just fine, as normal.

For condenser mic use, I've tested it with some small and large diaphragm mics and it worked just fine. I tested it also with FET condenser mics and unfortunately, I have mixed results with FET mics. They require higher voltages than +18 and can't deliver a hot enough output.

If you have a battery powered condenser mic (like a Rode NT3), you don't need to turn on phantom power on the preamp and you can just use the onboard battery on the mic.

I will continue to research and do more testing hoping to reduce the number of batteries required, and provide a way to give a true +48V phantom power from a small battery. The goal is to use maybe (2) 9V batteries, or a 12V battery to power the preamp.

Current Consumption with (4) 9V batteries = 12.5 mA
with phantom power LED ON = 18.5 mA

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Wow thats an interesting request! It would never have occurred to me to run it on battery power. Maybe you should come up with a usb interface for the IO module. Then you could run it off of a USB port or at least derive power from there.

Where do you get a ground from if you're using batteries?

Also, how many hours would 4 9Vs last?

GND?... Between the 2nd and 3rd battery's (+/-) terminal.

Duracell alkaline I think are 580mAH... divided by 12.5 = approx 46 hours of use... maybe... probably less.

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I imagine the length of use would be whether you are a) needing to use the phantom power, and b) how high your gain is...

Gain has no effect on power consumption... which is surprising.

but yes, turning on phantom power, (The LED draws about 5mA) and the type of mic you used and how much that particular mic draws will increase current consumption (and lessen battery life).

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The conversion to voltage from current balances out for gain of course so this makes sense if you do the math (which I obviously hadn't done until you pointed out that gain has no effect!). I should have realized that from Ohms law. Duh. This stuff keeps me thinking. I love it.

Does the SC-1 operate in Class A? If so, it's drawing max current all the time, which would account for no variation according to gain. Class A is good though

Peace,
Sanaka

Hey Sanaka,

Basically, it's operating as an instrumentation amp.

I don't know if Class designations are applicable for individual opamps.

But what I can tell you is the current consumption is constant regardless of whether there is input signal or not, or whether set to low gain or high gain.

Hope this helps!

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So, since things like switching power supplies and low noise DC voltage multipliers are way out of my electronics league, I was thinking about some super simple sort of 'blunt force' battery schemes.

The center-tapped four 9v's in series, like owel tested, is simple and small and seems hard to beat. Presumably the SC-1 can operate over a small range, say down to +/-15V, so that even though the batteries' voltage drops over time would stay in a good range for a pretty long run time before they got too low. And hence no need for regulation or other circuitry.

48v for phantom is harder. The simplest is probably just five 9v’s in series which gives 45V – actually within the +- 4V spec for phantom. For true, unwavering 48V I was thinking of using six 9v's in series for 54V regulated to 48V with a high voltage linear regulator (a regular LM317 only handles up to 37V). But then I was looking at batteries in the store and realized that 9v’ are really expensive. Also they have higher impedance than 1.5V cells like AA, AAA etc. So I thought why not 32 AAA’s or AA’s in series for 48V? This would of course make a pretty hefty battery pack, but would at least do the job I think. And all relatively unencumbered by the thought process

None of this would be as good as what I hope owel comes up with: a small board that will supply clean, low noise, regulated +-18V, plus 48x10mA phantom all from two or three regular batteries. Go owel GO!

Peace,
Sanaka

48V is a carryover from the old systems of yesteryears. In reality, condenser mics will work with voltage less than 48V. As I said, most condenser mics will work with voltages as low as +18. Only a few will have some trouble working at those low voltages.



Not true. If you look closely at the specs datasheet, the difference between Vout-Vin cannot exceed 37Volts max, but you can have output voltages greater than 37V as long as the above condition is met.

See my +48V, and +44V psu. They're all using LM317.

So if you want a +48V output voltage using an LM317, you cannot inject more than 85V at the Vin terminal.

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Oh, right! Thanks! My bad. That's cool too because I have like 25 LM317 laying around.



That's true, and so for hacking phantom off the same batteries that power the amp is cool. But... For better or worse, the spec is still 48 (+-4). So any mic that uses phantom will run at 48. Many if not most perform better at higher voltages, and a few, like Schoeps, won't run unless they have the full 48. As in many cases in electronics, more voltage just helps things out. DPA even makes mics (and special preamps for them) that run at 130V instead of 48!

There was much gnashing of teeth and ill will towards the M-Audio Microtrack recorder (and actually a number of M-Audio products) when it first came out about it's anemic 30Vx3mA phantom supply. My overall impression is that serious recordists want real 48V phantom.

Peace,
Sanaka

I hear you....



I think the phantom power specs call for 10mA. That is very low.

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Yes, that's one of the beefs with the Microtrack. I have one, and the mic ins are only usable with a 27dB digital boost enabled, which unlike real gain boosts the noise floor by 27dB as well. Hence my search for a good portable mic preamp

Peace,
Sanaka

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.

Photo here:


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.


Possible Solutions:
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!

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