Active Pickup Phantom Power

[ChuckNoblet]

I'm attempting to make an active pickup. I've searched forums and I hear all this great talk about supplying active pickups with phantom power. But no one seems to have actually done it. I would like a power supply to be 18 volts if possible. Or, better yet, a variable voltage power supply.

I saw a variable power supply kit on ebay: http://cgi.ebay.com/Adjustable-Volta...742.m153.l1262

Would this work for a guitar with actives? I don't want to blow anything up!

I also wonder about something like M-Audio's Audio Buddy:

http://www.americanmusical.com/Item--i-MII-ABO-LIST

Could this be used for phantom power? How much voltage does this thing supply? I don't know much about electronics but am fascinated and willing to learn.

Glen

[David King]

Phantom powered mics typically run on 48V filtered DC. That's what most desks (aka mixing consoles) supply. A variable voltage supply might be interesting to mess with just to see what voltage sounds best but you need really good filtering of the supply and I'm not sure if the ebay supply would give you as clean DC as a battery could.
All 13 pin midi guitar systems also use 9V phantom power on pin 7 or 8 (I forget which)
Both the Alembic Series I and Series II basses I think came with off-board power supplies to power the internal electronics. The basses used a 5 pin jack for a bipolar supply of + and - 12 Volts I think. They started doing this back in the late 70's or thereabouts.
If you want to use phantom power then I'd stick with a 3 pin XLR output jack and design your electronics to run on 48V. You might as well make your pickups fully balanced HBs with the floating ground so you get all the benefits of balanced cable and fall within the norms of typical low impedance mics i.e.150-200 Ohms output impedance so you can plug straight into the mixing desk

No good reason to do phantom as modern, low noise preamp designs have very low current needs and the typical battery life of 500-1500 hours is usually less trouble than a nonstandard connection and a guitar that's useless without it's supply.

[David King]

That M Audio mic preamp is phantom powered from the USB connection to the computer which would be pretty limiting (I think USB is only 4.75V).
Many better quality mic preamps do have a filtered, switchable, 48V phantom power for condenser microphones.

So you could take the M Audio thing apart and stick it's guts into the guitar and use a USB connector to plug the guitar into the sound card of your computer but I'm not sure what you'd gain from doing that.

[ChuckNoblet]

Interesting

You are right about batteries being easier. I noticed the Variax guitar uses a 9 Volt power adapter/footswitch for phantom power. You may wonder why I want more power than 9 volts? These are serious actives!

On that note, how much voltage can you go with and not hurt an amp or computer input? I've heard of people using 2 batteries (18 V) for basses and even 3 (27 V) but they say it makes little difference in headroom. Is there a voltage max input for amps? Or current?

Ideally I would like a separate power supply such as the one made by Variax but with more power. Could I somehow hot-rod one or should I look for some schematics for a separate one? Thanks for your input Dave. I'll also look into Alembic.

[David King]

Chuck, I guess it all depends on what the problem is that you are trying to solve here. Are you getting distortion? What part of the gain stage is it coming from? You can use 130V phantom power like the B & K mics do if you want but we still need to know what the specs are for the next part of your signal chain. Pro audio equipment is set up for +4dBu (whatever that means) Meanwhile home audio and your computer are probably looking for -10 dBu. If you make some very quiet pickups, you will need lots of gain and will need to buffer them carefully. If you make hot pickups then they hardly need to be active do they?

[ChuckNoblet]

I'm making very weak pickups. But I'm still in planning mode.

My signal chain? I have a computer that I would like to play and record through directly to the sound card and I want it to be able to be heard through my Fender Princeton amp.

130 Volts? Wouldn't that destroy everything? I thought 48 would even be too much. But I don't know these things for sure.

When you say I need to buffer them carefully, how do I even begin to do that? I'm an amateur here!

An interesting link about phantom power:

http://groups.google.com/group/alt.g...451b1a3e12c0a1

What sort of values do you need to know for the buffer? The resistance?

I do appreciate your help.

[David King]

This is a sort of chicken and egg problem, you either build the buffer first and wind the pickups to match the input impedance of your buffer or you wind your pickups and build the buffer to match.

That battery box is a solution in search of a problem that doesn't exist. If the the guy's peavey bass uses two 9V every 6-8 hours then there's an obvious electrical fault in the circuit. He doesn't need new batteries -he needs a bass that doesn't say Peavey on it. (It's kind of like drummers who break drumsticks or bass players who break strings to show how tough they are. Hey I break my cereal bowl every morning and bend my spoon too. What the hell does that have to do with making music? NOTHING! Music doesn't have to be a contact sport.)

Wind some 150 ohm humbuckers using 30AWG wire and connect the grounds to the floating ground pin of an XLR jack, connect the other two leads to the other two pins. Plug the instrument into any mic input on a mic preamp or mixing board or sound card with mic level inputs and voila you have a low impedance pickup with no noise. You don't need onboard preamp and off-board power supply to power it.

[David King]

A quick google of "mic low impedance buffer" yielded this very old discussion:
http://groups.google.com/group/rec.a...2296b0c540a008

I don't know shit about electronics so i can't tell you where to look for the right circuit but there are plenty of mic pre kits (like Paia's) that you can pilfer. Just know that a lot of wiser heads have solved this problem a million times already and cheap, off the shelf solution are piled up in junk yards everywhere.

High impedance guitar pickups are stupid in so many ways but they happen to sound great so once you've heard all the glorious extended range of your 150 ohm humbuckers through a great mixing board be sure get a reality check from a nice, passive, high impedance guitar...

Oh and 130V and 48V supply voltage is all a way of getting a very secure 2V peak to peak signal. It's ac, not dc you're plugging into your computer or amp so not likely to do much damage provided you get the proper output levels going. In theory that 4.75V USB power is plenty to get a 2V peak to peak signal so don't ask me why some dumb ass bass player thinks he need 27V to make his bass sound better. He either has a very small prick or he can't play because he spends all his time of forums blathering about external power supplies.

[David Schwab]

Chuck, I make active pickups. They are low output, and I use a common self biasing FET buffer circuit. I can get more gain than I need from a low Z pickup... up to line level actually, which is enough to clip most amp inputs!

You don't need a FET, since they are high impedance, but it works great and is low noise.

Keep in mind that low output pickups will sound different than regular hi-z pickups. EMG guitar pickups are pretty much regular hi-z pickups, but they are buffered which lets you plug them into things like sound cards.

Just google Alembic Stratoblaster for a great simple FET buffer.

[eddie]

Hey all!

How about this:
http://www.precisionmusictech.com/xbat.htm

It shouldn't be to hard to power this with an 9/18v external power supply. There are ones available that just connect to the battery terminal. Plus I can't imagine it would be too hard to make one, looks like it just uses a stereo cable as an extension cord per say. I imagine it uses the same stereo connector for on/off functions as most active systems use, but shifts the batt and on/off function down the line of the stereo cord into the box. Why didn't I think of that before?
Plus check out that programmable pick up controller they have. Might save on wiring but how many hours do you think you could spend programming that thing!?

[David King]

A metal box and two cords for $60?

[eddie]

Don't forget to add shipping.....

[Artie]

The M-Audio Audio Buddy doesn't get its power from USB. In fact, it has no USB connection at all. Its strictly an "audio" device, as opposed to a computer device. It gets its power from a 12-volt AC wall-wart. It then uses an internal voltage quadrupler to boost it up to 40 volts DC. Thats still not proper 48-volt phantom power though.

I've got a circuit all designed that provides 9 or 18 volts into a stereo guitar jack, that uses standard phantom 48-volt power. However, I don't want to publish it until I can prototype it up and try it out.

I'll let you know as soon as she "flies".

[David Schwab]

A metal box and two cords for $60?

Exactly! It would be real easy to get a 9V wall wart and make this yourself.

You should also add more filtering.. some cheap wall warts are a bit noisy.

That thing is not phantom power though... that implies that you are using the same wire for the signal and power, and separating them in the device. The standard phantom power circuits would work here (I mean the the end inside phantom powered mics).

And there's another consideration. Unless you are putting in a circuit that uses high current op amps, or runs on more than 18V, I'd stick with batteries.

They last a very long time, and don't induce AC line noise into your circuit.

If you are worried about the battery going dead while you are playing... I've never had that happen to me in the 30 years I've been putting battery powered preamps in my instruments.

[eddie]

Thats what I'm trying to say...it would be easy to make and power!

Plus you don't have to worry about stuffing one or more batteries into a control cavity or routing for a battery box. Since it doesn't work like phantom power it would be versatile enough to use with EMG's, SD Livewires/Blackouts or whatever preamp setup you want to use.


+1 on the battery vs. ac adapter. The adapter would be nice as you would never have to worry about losing power but we're not talking about powering some stadiums sound system. Batteries are much more quiet!

I would hate to forget my "box" at home on gig night! It would be easier to scrounge up a battery than make a new box on site.....