Active Pickup Phantom Power

[David King]

I still don't understand what the benefit of an active pickup is over a passive, low impedance, balanced output?

Sooner or later you need to plug something into the wall so you can hear it.

[David Schwab]

I still don't understand what the benefit of an active pickup is over a passive, low impedance, balanced output?

Sooner or later you need to plug something into the wall so you can hear it.

How many instruments have passive low-Z balanced outs? I can't think of any. You can do that, but you need something like a mic pre, or a transformer to use with hi-z amps. It is the way they used to do it though. I think it's all because of the hi-z amps we use. Active was an easy way to buffer the pickup.

If you wind a passive pickup to get extended highs, you have a weak low end. If you wind it for a full low end, you get reduced highs. It's real damn tricky to get both.

The easiest way to get a very wide frequency response is with less windings, or with low resistance windings, but then you need to boost the level up.

Also lower impedance pickups have a smoother response free from peaks and humps. Some people like that.

Other reasons would be to use a buffered dummy coil, mixing two coils without them loading one another, or other tweaking of the pickups EQ.

Putting a preamp after a passive pickup is fairly close to having an active pickup. Like Rick Tuner has said, you first gain stage can be anywhere. Closer to the pickup is better for less noise. In the pickup would have to be for noise reduction, and just to make things easier to hook up.

[Corvus]

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 (or anyone) have you actually tried this before?

I have a multi-channel application in which I'm having each string plug directly into a mixer and your passive balanced output idea sounds perfect for that.

[David Schwab]

David (or anyone) have you actually tried this before?

I have a multi-channel application in which I'm having each string plug directly into a mixer and your passive balanced output idea sounds perfect for that.

I used to plug my low impedance pickups into the mic pres on my mixer. I didn't wire them up balanced like David said (which is the way to go), but it worked fine. A preamp is a preamp, regardless to where it is in the signal chain.

[Corvus]

It Works!

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.

David King I just wanted to thank you for posting this idea.

I've made my own balanced humbuckers and now have great sounding multi-channel output for my 7-string guitar.

I'm running output from each string into my firewire audio interface and running separate amp simulation and effects for each string on my Mac using MainStage software.

Having never build a pickup before it took me about four attempts to get a reliable design working but now I'm extremely happy with it.

Thanks a million

[peskywinnets]

Old thread I know, but....

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!

Actually the Variax phantom supply is about 7V & even that gets internally regulated down to 5V.

I'm actually making balanced active pickups (DCR = 80 Ohms) right now & 5V is way more than sufficient....but it's the rail supply I've settled on

I think people get way over fixated with the word 'headroom' (witness all the EMG 18V mods that abound - I mean WTF?!)

[Mark Hammer]

Ideally, you want enough clean headroom. You also want anyrequirement to replace batteries to be as infrequent as possible. So the current drain of the on-board circuit should be as low as possible, the current capacity of the batteries shold be as high as possible, and the means for replacing any on-board supply should be as non-invasive as possible.

One of the compromises between full-blown mains-powered phantom supplies and on-board battery supply is phantom battery power. The user can park a suitable battery supply in a box on the floor or wherever, and the battery can provide pure sweet DC to the on-board circuit via a 3-conductor "stereo" cable. This lets you run a standard XLR or TRS-type phone plug to the instrument and adaptor box, and then run either balanced or unbalanced from that point outward.

The batteries you stick in there can be your choice. For instance, you could use rechargeable 18v power packs, or you could use an octet of AAs in a clip for 12VDC, or whatever. An octet of AA batteries, even cheap ones, lasts a LONG time in a preamp circuit that draws a couple of ma at most.

[peskywinnets]

Ideally, you want enough clean headroom. .

I see that phrase a lot & it puzzles me - just wondering what dirty headroom is?!

Headroom is headroom!

To my mind, it equates to the 'wall' where the signal starts to hitting the limitations/extremeties of the circuit's capabilities (the main component of the 'wall' being the positive & negative supply) - so if if an opamp isn't rail/rail & say the (single) supply is 5V, then the most you can shoot for through that circuit is just under 4V pk/pk. No need to mention clean or dirty 4V is your limitation in this instance.

the average guitar amp is only expecting to receive a signal in the order of about 80mV RMS, even after as ridiculously strong pluck it's only peaking at about 1V pk/pk *max* (passive pickup) ...so all this talk of 18V supply to get enough 'clean headroom' is just taking the p1ss!

[Mark Hammer]

Okay, then "realistic headroom"!

I guess I mention "clean" because some folks seem to think that if you feed an op-amp an average 50mv signal, apply a gain of 100x to it, and power an op-amp that can swing as wide as within 2v of the rails, with a 9v battery, that somehow there is all this fabulous "headroom". Clearly, at a certain point, with certain amounts of gain applied to certain signal inputs, the circuit is going to go "Nuh-unh. Not gonna do it. You can't make do it.". There doesn't have to be clipping diodes anywhere in the picture for that circuit to clip intermittently.

Now, were the expectations of the user reasonable, and less gain was applied, then the circuit in question would provide sufficient headroom to accept transients without running out of swing. So yes, you're right. When it comes to headroom, it is either there or it isn't.

What you aim for with on-board electronics, of course, depends on what you intend on doing with the signal next. Some players (and here I suspect I am thinking primarily of those who will apply no effects; bassists and acoustic guitar players) will want sufficient gain that they can simply plug into a power amp or the line input of something like a rack limiter. Others will simply want to optimize their S/N ratio and get a little more "push" at the amp input stage. If the former, then one likely needs a higher supply voltage to provide the requisite headroom. If the latter, then 9v is more than adequate.

[Rick Turner]

You all are bandying about the term "phantom power" as though all it means is using an external power source. NOT!

The Alembic system, which goes back to 1970, not late '70s, uses a five conductor cable with plus and minus 15 Volts coming up on two conductors, another two being used for bridge and neck pickups (NOT stereo, actually dual channel), and the 5th being ground. That is not phantom power.

With phantom power, you send a single ended DC voltage up on the same conductor(s) used for the signal going down stream. You have to de-couple the DC from the AC audio signal with a blocking capacitor. The other issue is that there are several phantom power standards in the industry...one is 48 volts, commonly used for condenser mics. There are also 100 volt mics. Then there is the (often) 9 to 15 volts used by electret mics. A lot of the time you will not have sufficient current capacity with the lower voltage systems to properly run preamps and buffers for mag pickups.

For my uke pickups, I use an off-board battery box with two 9 volt batteries in series for 18 volts. I send that up the cable to the uke on the "ring" connection of a TRS stereo cable and jacks, and bring the audio down on the usual tip connection. And yes, a box and a cable and a couple of jacks should cost a lot more than $60.00 if you're actually in the business of making this stuff and trying to make a living at it. Just go price Hammond or Eagle aluminum boxes. Now go buy a decent TRS 10' stereo cable. And a couple of Switchcraft jacks. And a couple of battery snaps. Oh, and a couple of 9 Volt batteries. If you're going to sell that rig for $60.00, I guarantee you won't be in business much longer than Mubarak will be president of Egypt.

Please get real about costs, markups, and being in biz. It's disingenuous to be contemptuous of real world business. Not everyone here can afford to be an amateur at this game...

You want real world? Take your raw materials costs and multiply them by ten. That's going to be close to retail. If you can't do that and live with the numbers, you shouldn't try to make pickups...or anything else...for a living.

[Rick Turner]

Dirty headroom is what you get when you hit the brown note really hard.