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What's the difference between active and passive direct boxes? #1534109 02/06/06 03:06 PM
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RockNRoll Offline OP
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Hello all!

Just was wondering what was the essential difference between active and passive direct boxes? I intend to use this to connect my guitar to a mic-input to help alleviate ground loops/noise...Actually how do direct boxes compare/differ to those 'hum eliminator' devices? Thanks for your feedback!

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Re: What's the difference between active and passive direct boxes? #1534110 02/06/06 04:14 PM
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Fingerstyle_Jim Offline
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Active requires batteries or an adapter, otherwise: power, passive doesn't. An example of a passive tone control is the tone knob on any electric guitar that doesn't require batteries. The tone control will let all of the signal through when the knob is turned completely one way, and then as you turn it the other way it cuts out frequencies thereby changing the tone. An active tone control will have the ability to add frequencies giving you more control, however, sometimes active systems add noise along with whatever they are doing.

Jim

Re: What's the difference between active and passive direct boxes? #1534111 02/06/06 04:54 PM
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doug osborne Offline
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An electric guitar has a high impedance/ high level output. A mic pre wants to see a low impedence/low level input. A direct box steps-down the signal so it doesn't overload the input of the mic pre, and changes the impedance so the mic pre doesn't load the guitar, which can result in a muddy sound. A DI is necessary to connect a guitar to a mic pre, although some mic pres have direct inputs built-in.

There are a few ways to achieve this. A passive direct box is simply a transformer - it lowers the output and the impedance simultaneously - the transformer has fewer windings on the output than the input, for exaple a 10:1 ratio. This can be a pure and great-sounding path, but a cheap transformer can sound bad. Also, it presents a load of its own to a passive magnetic pickup equipped guitar, which can load the guitar and make it sound dull.

An active DI uses an electronic circuit to lower the level and impedance. It does need power, which may be derived from a battery, phantom 48v power, AC, etc. It will not present a load to the source, so the sound will be sent to the mic pre with no different coloration whether it is passive pickups, active pickups, a piezo pickup, an unbalanced electronic instrument (synthesizer), etc. Like all electronic circuits, this circuit must be designed so it adds little noise, distortion, coloration, etc., so active DIs may sound different from one another.

As with any audio gear, your ears are the only judge as to which design sounds best with your source.

The hum-eliminator boxes are completely different animals, helping you to connect line-level unbalanced -10 devices with balanced +4 devices.

Re: What's the difference between active and passive direct boxes? #1534112 02/06/06 05:02 PM
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fantasticsound Offline
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First, a DI is an impedance matching device. It's designed to allow a high impedance input (aka, an electric guitar, most synth outputs, etc.) plug into a low impedance input.

They take into account the differences between line and low (mic or guitar p'up) level using a pad which attenuates the input, generally between 20db-40db.

To your question of the difference between passive and active, power is the simple, but incomplete answer.

Passive DI's convert high impedance to low impedance using a transformer. No additional power (beyond the input signal) necessary to operate.

Active DI's use electronic circuits to convert high to low impedance and require batteries or phantom power, provided by the PA mixing console or a separate, phantom power supply box. Some active circuits require more power because they are used as signal processing and have their own power supplies that must be plugged into a wall socket.

But most importantly, passive DI's will lose some treble in the conversion. Few passive DI's are available that compensate for this. Active DI's will have a more transparent frequency response (though each certainly has its' own character) and, therefore, is preferred for more articulation. They have more likelyhood of noise issues, however, due to the additional electronics and use of phantom power from the console. Good passive DI's are also nearly indestructable. Most active DI's, while built like tanks, are not quite so robust as their passive siblings.

Radial makes a line of active and passive DI's that are extremely high quality, sound great, and while expensive, are not boutique items, per se.

If you're on a budget, A/B some DI's into a PA at a store. If you hear a difference, by the one that sounds better. If you don't, buy a relatively inexpensive passive DI. It should last you a lifetime unless you desire more open, articulate sound available from high quality DI's. ;\)


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Re: What's the difference between active and passive direct boxes? #1534113 02/06/06 05:09 PM
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fantasticsound Offline
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Quote:
Originally posted by doug osborne:
...The hum-eliminator boxes are completely different animals, helping you to connect line-level unbalanced -10 devices with balanced +4 devices.
Actually, the Hum Eliminator boxes are not for connecting "consumer" line level (-10dBV) to "professional" line level (+4dBV). They are 1:1 transformers for electonically disconnecting a noise source while allowing balanced or unbalanced connection between input and output. Otherwise, for long transmission of your signal, you can output a differential balanced signal. (Provided the input is differential balanced, of course.)

For connecting low to high line level equipment, Ebtech makes a "bump box" with the hum eliminator circuit, but the two boxes are unique.


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Re: What's the difference between active and passive direct boxes? #1534114 02/06/06 07:05 PM
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doug osborne Offline
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Quote:
Originally posted by fantasticsound:
...

For connecting low to high line level equipment, Ebtech makes a "bump box" with the hum eliminator circuit, but the two boxes are unique.
That's the one I was thinking of. The message for RockNRoll is that neither of these are DIs.

Re: What's the difference between active and passive direct boxes? #1534115 02/06/06 07:14 PM
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Bill@Welcome Home Studios Offline
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Quote:
Originally posted by RockNRoll:
Hello all!

Just was wondering what was the essential difference between active and passive direct boxes? I intend to use this to connect my guitar to a mic-input to help alleviate ground loops/noise...Actually how do direct boxes compare/differ to those 'hum eliminator' devices? Thanks for your feedback!
Short answer:

DIs (direct injection) are used to convert signals from one output type to a different input.... usually the destination (input) being the mic input on a console.

A passive DI uses a transformer to achieve this.

An active DI uses circuitry (and requires power) to do this.

But without a ground lift switch, neither will prevent the hum you speak of. Not all DIs come with one.

As has been mentioned, humbuckers are not DIs. ("humbucker' being the generic term for "'hum eliminator' devices. "

Bill


"I believe that entertainment can aspire to be art, and can become art, but if you set out to make art you're an idiot."

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Show business: we're all here because we're not all there.


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