Passive to active pickup, how?

[Ruel]

Someone I know is asking how to convert a passive pickup to an active one. He came accross an ad for an active pickup conversion kit for about US22$ and is wondering if it can be done "DIY". I promised to post the question here.

As I've read here in some previous threads, an active pickup is low impedance, wound with less turns than a passive pickup. I also remember it being said that thicker wires are used. So how does one go about doing the conversion?

[Satamax]

What is active??? It's just adding a preamp to a pickup. I used to do that with 741s

[David Schwab]

Someone I know is asking how to convert a passive pickup to an active one. He came accross an ad for an active pickup conversion kit for about US22$ and is wondering if it can be done "DIY". I promised to post the question here.

You can use something like this circuit. This is an Alembic Stratoblaster, which was originally made to buffer the output of a Strat and also give a boost.

You can use this after an individual pickup, or at the output of the instrument.

As I've read here in some previous threads, an active pickup is low impedance, wound with less turns than a passive pickup. I also remember it being said that thicker wires are used. So how does one go about doing the conversion?

That's a low impedance pickup, which is not necessarily the same as an active pickup. If you rewind a pickup with less turns, you will get a very hi-fi type of tone. That works great for bass, but not all guitar players like that kind of tone. Low impedance pickups need to either be preamped or run through a transformer.

EMG guitar pickups are more or less regular high impedance pickup with an active stage after the coils. Using a buffer circuit like this will give you a similar tone, with an extended frequency range without rewinding the pickup.

[Ruel]

Thank you very much for the info guys.

[Mark Hammer]

Worth noting that "active pickups" technically do not exist. What we call active is simply a conventional pickup with a buffer/gain stage after it.

Conventional "passive" pickups tend to have more windings on them, with thinner gauge wire, which produces a decent output level, but can result in greater susceptibility to hum.

So-called "active" pickups tend to be produced with fewer windings, using thicker wire. This results in a pleasingly low impedance pickup that is much less suceptible to hum. Unfortunately, it also produces considerably less output, so a gain stage is added afterwards to compensate for that low output level.

It's like the difference between a tall guy with muscles, and a short thin guy standing on platform heels. They're both men but one needs a little help to be as tall as the other one.

Sometimes, as well, it is easy to think that pickups themselves are "active" if the electronics are built into a circuit-board attached to the underside of the pickup. The pickup coil itself is still a normal passive coil around a magnet, but the physical attachment of the electronics to the pickup base leads us to think it is different. Again, this is like our short guy with thick heels glued to his feet so that you think its part of his body, when it isn't.

And finally, as David correctly notes, sometimes the electronics are not there specifically to bring the level up, but to provide better frequency response by creating better impedance matching between the guitar and the amp. While it is not done frequently, active electronics can also be used to wire a pickup just like a "balanced" microphone where the noise picked up along each of the two connecting wires from the coil is cancelled out at the preamp.

[David Schwab]

And finally, as David correctly notes, sometimes the electronics are not there specifically to bring the level up, but to provide better frequency response by creating better impedance matching between the guitar and the amp. While it is not done frequently, active electronics can also be used to wire a pickup just like a "balanced" microphone where the noise picked up along each of the two connecting wires from the coil is cancelled out at the preamp.

Yes, and just the fact that you have less turns of wire provide better frequency response. But even buffering a standard high impedance pickup will improve the response by isolating it from the cable and controls.

From my understanding EMG has a balanced input to the opamp.

[Joe Gwinn]

Conventional "passive" pickups tend to have more windings on them, with thinner gauge wire, which produces a decent output level, but can result in greater susceptibility to hum.

So-called "active" pickups tend to be produced with fewer windings, using thicker wire. This results in a pleasingly low impedance pickup that is much less suceptible to hum. Unfortunately, it also produces considerably less output, so a gain stage is added afterwards to compensate for that low output level.

I would think that this is self-scaling: Fewer turns lead to lower hum for sure, but also to lower music signal as well, so the music-to-hum ratio is unchanged. Buffering and amplification will not change this ratio. So, there has to be real humbucking in there somehow. Not to mention electrostatic shielding.

[Mark Hammer]

Well, whatever the case, the active electronics are there to provide needed assistance to the pickup.

[mkat]

You can use something like this circuit. This is an Alembic Stratoblaster, which was originally made to buffer the output of a Strat and also give a boost.

I can't seem to find a J201 or 2N5457. Is there another compatible JFET I can use with the Stratoblaster?

[mkat]

It's ok, I found some.

[David Schwab]

RadioShack sells the MPF 102.

[DrStrangelove]

JFETs

I can't seem to find a J201 or 2N5457. Is there another compatible JFET I can use with the Stratoblaster?

http://www.mouser.com/search/refine.aspx?Ntt=2N5457

Over in the Tech Tips at Geofex, RGKeene has a nice little jig to test the FET's
gate cutoff voltage if you want to sort a few dozen for consistency.

http://www.geofex.com/Article_Folder...h/fetmatch.gif

-drh

[mkat]

Thanks for the info David and DrStrangelove. I did see the mouser listings for the Fairchild J201 and the various 2N5457 FETs. RG Keene's jig looks good.

[David Schwab]

Thanks for the info David and DrStrangelove. I did see the mouser listings for the Fairchild J201 and the various 2N5457 FETs. RG Keene's jig looks good.

I get my parts from Mouser. You can't beat $.09 for a FET compared to buying a few from RadioShack for $1.29 (or something like that)!

[Joe Gwinn]

You can use something like this circuit. This is an Alembic Stratoblaster, which was originally made to buffer the output of a Strat and also give a boost.

There are two problems with that circuit: http://music-electronics-forum.com/a...8&d=1174315690.

First, it needs a 1Mohm resistor to ground from the JFET gate, or operation with a capacitor-out anything will fail. Only if you are guaranteed that a pickup or transformer coil will be used without series capacitor is it safe to omit the 1 Mohm resistor. The gate must have a DC path to ground.

Second, it needs a 100Kohm resistor in series with the gate, to prevent punchthrough on peaks. The punchthrough will not hurt a JFET, but the distortion is not at all pleasant (unlike some kinds of distortion).