Building a computer mic preamp & needing help

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andrewmaxin

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Hey all

I am asking around if there is any electronics geeks like myself out there that may know how I can achieve a 48V phantom power voltage using the 12V Power feed from the computer Pwr supply. I know that there is a way to boost DC using a switching regulator, or perhaps by using a transformer in reverse-fashion. either way, I would really like help in building this

Thanks

Andrew
 
You need a switching regulator like the one built into UPS systems.

Not sure where you can get a hold of one, but it'll put out about 10 volts into a transformer primary. You will need a custom-made transformer to push it up to about 55 volts, and then line-regulate the output using zener/transistors.

Not sure if there's another way to do it. Some car power amp ICs can self-regulate upto 30V from the 12V rail using two hefty capacitors, but I'm not sure how that's done.

My best shot would be to take the incoming mains line and use a transformer down to 55 and then regulate it. Seems the smart thing to do.
 
Sangram

Thanks for your reply. Yes I had considered that I may have to do it that way. The reason I was shying away from that method is because of two things.

One, I would have to tap the mains which doesn't excite me. Especially since it is an ATX pwr supply that is turned on&off by the MB, and not by a physical switch. So that would mean an extra pwr switch for the unit itself.

Two, I would be putting in a transformer which will probably introduce a schwack of EM interference to the circuit and surrounding components. As well, I will have to regulate the supply which will give me a heat dissipation problem.

That being said, The tradeoff is that the old-fashioned power supply method will give me a less-limited current source, as well as more flexibility as to operating voltage.

The big question is what is the lesser of two evils?

I guess I have some moral pondering to do :)

Would you know of any resources/schematics of these power supplies that you mentioned?

Thanks again

Andrew
 
The only other thing I can think of is a phantom unit mounted outboard, which plugs into the soundcard inputs at one end and the mic plugs into the unit.

Yes a transformer could introduce EM but I guess if it's a really low-current transformer the EM would be very low, since we're only talking a few milliamps. Ditto for regulation, you won't need a high-power regulation circuit since the mics draw very little phantom power, maybe a couple of milliamperes. I've used a 9V battery on a regular condensor mic mounted inside my guitar and the battery is going strong two years on. At 48 V, mics draw even less current, maybe just 1 ma. Likely you won't even need anything more than a standard plastic transistor + zener combination.

And no, I'm not sure where you can get PSU schematics but I'll see if I can look around for you. The old-fashioned method I'm sure needs no further explanation. If you need a circuit, I can organise to send you one. The switching circuits, well, I'll have to see. Basically it's a sine-wave generator (even square, in some cases) which is coupled to a very high power current amplifier, which drives the transformer.

Extrememly inefficient, the base curcuit for a UPS. You'll probably need an ampere or so, to get about 150-200 milliamps at the output tap (1/4, the approximate ratio for your transformer) which should be enough to power a few mics.

The other thing I was thinking was the fact that you'll be loading up your PSU unessecarily. You could try the dual PSU method (try googling it, I found an intersting way to add two ATX PSUs together) and mod the other PSU to provide 48 V which you can then use to power the phantom.
 
Thanks Sangram

Maybe I can just use the computer pwr supply to power a relay that can switch on my power supply.

I think I will use a traditional pwr supply after all. You're right about the interference, I can always shield the xfrmr within the unit.

Luckily, my cct is so small that I have lots of room


You say that mics only draw a few milliamps, but what about the average DI? Is there an established standard that they can draw?

Andrew
 
Well a decently made three -transistor DI should be no more than 5 mA max, if designed right, unless you're driving very low impedance from the output in emitter-follower mode.

In any case should be no more than 10mA per channel.
 
Are you doing all this just to get phantom power on the soundcard preamp? What soundcard are you using?

If it is a soundblaster than those preamps won't accept regular low impedance mics anyway.

If you are determined to build it yourself your best bet will be using a battery for power. That is how most discreet phantom units work.
 
I am building something similar to the e-pre (from ARX)

I just have the standard soundcard that is built into my Asus motherboard (for now) All I am building is a preamp to give me the +15 dB boost that my Aux input needs.

I am basing my input section off of a Soundcraft front end(which I know is clean & quiet), and I plan on using a buffer stage with a fine trim (so that I can adjust for different soundcard input impedances)

I was going to put a limiter on it, but I need to find something solid before I do that. I will probably do a dual pwr supply (one +48V feed for phantom, and one +/- 17V feed for the op-amps.

My goal is to be able to plug a condensor/mic/mixer into the front of my machine & go.

Andrew
 
I looked into that as well. I'm an electrical engineer but more importantly, I'm a build-it geek. Look at the way PAIA biases their tube circuits. They have schematics on their website,

www.paia.com

They use a simple oscillator driving a multiplier to get the appropriate voltage. It should be simple enough to use that circuit to get around 50v and then regulate it with a zenor diode. You don't have to be too accurate as I recall but it should be pretty clean if you pick your components carefully.

Let me know how you fair, I am interested in the same thing. Wanna share your preamp circuit?

Bye the way, PAIA is using a LM5532 type op-amp in their gear now and it's really quiet. It is an order of magnitude better than the TL082 family devices. I like it a lot.

Happy inventing,

Harv
 
Hey. That's great!

I didn't think of usng a hex inverter as a signal generator. The diode/cap array looks almost like a cascade voltage multiplier, but the design is a little bit different than I have seen before.

I will mock it up on my circuitmaker program and see how well it holds up.

And yes I would be happy to share the final schematic with y'all. There only seems to be one of these built in mic preamps around. With any luck, I will be happening in no time.

Regards

Andrew
 
Harv

I tried out the cct on the paia site on my cct Sim, and it didn't exactly give me what I would call phantom power. I put in a sine wave of +/- 15VDc and got an average 20VDc out the other end. I emailed paia to see if there is anything I could do or hadn't done.

Any ideas?

Andrew
 
Andrew,

I don't know how much voltage they actually get out of these things. I've used that cascade voltage multiplier a number of times to generate really large voltages like a kilovolt or so, but I admit I haven't built this up. I'm not in a building mode right now, but if you ge time, prototype it on a quick circuit board or whatever. I'll bet if you choose the components correctly, you'll get some pretty good numbers. I do know it will generate enough current to drive a mike.

Harv
 
Take a look at switching regulators from Linear Technology or National Semiconductor and the like. It does not need lots of components, gives you a stabilised voltages and enough current to feed mics that need it. This can be a problem with multiplier circuits.

Just as an idea: http://www.national.com/pf/LM/LM2577.html

It is a bit oversized, but this range of circuits is not very critical to get functioning. Maybe the higher frequency types are better from an interference viewpoint, but a couple of RC's after it will probably be enough.
 
I agree

Havoc,

I agree. I have used these many times for negative voltages and for their design rating they are great. I don't know about using them for 48v ranges, but National or Linear Tech probably have an application note that will handle it. I'd definately recommend them, thanks.
 
Well...

I think I am just about finished here. I ended up just scrapping the idea of running off the computer power supply. I wasn't able to get the voltage levels and stability that I wanted to make this a clean unit. What I ended up doing is mounting a small adapter connector on the back of my case, and using a relay, I used the 12Vdc line from the power supply to switch on/off the preamp. Also by doing it this way, I was able to use a simple cascade voltage multiplier to get my 48Vdc.

As Soon as the design is finalized and thouoroughly tested, Perhaps if anyone is interested, I might be into sharing the project details. Would it be fair to ask for 5-10$ for the schematics?

Anyway, thought I would give y'all an update on my progress for the interested.

Bye

Andrew
 
Well I will be able to tel you in a day or two when my gain controls come in. The circuit needs a fancy 10K reverse Audio Taper Vertical mount potentiometer. I just couldn't get them in the city, so I had to wait a week. I just used a fixed resistor to test the circuit for now. So far, it seems to work. I haven't put it in to my computer yet for testing.

Andrew
 
Hey All.

Finally Finished.... Yep it works...

I need to tweak the phase & gain controls a little, but yeah.. everything works..

As for the power issues...
I use an external power supply (16VAC) that is shielded and run through the case. The power is switched by a 12V relay that is powered off of the 12V line on the computer pwr supply...

It is pretty good with regards to power drain, only about 150mA average.

If I went through the trouble of putting together a how-to for this, do you think someone would anyone actually PAY for it?
 
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