Questions about Active and Passive DI's


We realize that one of the advantages of a passive DI is that it would not require either a battery or phantom power. What other advantages does a passive DI have over an active DI?

The benefits of a passive direct box like the Radial JDI are numerous. The first is the sheer fact that passive DIs do not require power to make them work. Instead they use a transformer to perform the impedance matching and balancing. Transformers create a magnetic bridge that allows audio to pass while blocking DC. This makes passive direct boxes very helpful at eliminating noise due to audio ground contamination from varying DC offsets and reference voltages between the instrument and the mixer or preamp that can cause buzz or hum. This is manifested in the form of ground loops due to power distribution issues.

The second is the sound of a transformer. Unlike active circuits that go from say 0.5% distortion to 100% distortion once you overload the input and exceed the rail voltage (aka clip the input), transformers do not distort the same way - they saturate. This creates a natural compression that we humans find pleasing and is often referred to as vintage sounding. The down side is the quality of the transformer varies significantly from manufacturer to manufacturer. We spend hours both testing and listening to various transformers to hear how they perform when used with acoustic guitar, active and passive basses, keyboards and electronic drums under various loads. You would be surprised at how different one transformer sounds compared to the next, even though they may have the same electrical spec.

There are only a small handful transformer manufacturers that understand audio and are capable of producing a quality transformer consistently. This includes balancing the core lamination between nickel and iron for best performance and price, scattering the windings for optimal signal transfer, adding faraday shields where needed and encapsulating the transformer with a MuMETAL® can to eliminate pollution from outside magnetic fields. By extending the frequency response beyond the 20Hz to 20kHz boundary and testing performance in the difficult 20Hz bass region, we are able to achieve very satisfying audio results that are often preferred over active direct boxes.

Do passive DI's present a different load to a guitar or bass as opposed to active DI's?

Passive direct boxes have had a bad rap for years, mostly due to misunderstandings and holdovers from the early days or touring. In the beginning, the 'transformer of the day' came from broadcast. The 600 ohm standard line level transformer changed the tone and feel of passive instruments like the Fender P-Bass due to loading. What happens is the magnetic pickup in the bass is driving the signal to the bass amp and hundreds of feet of cable to feed the PA system, splitting the signal evenly between the two. When you do this with the wrong transformer, the bass player immediately notices a lack of fidelity and punch. Today, we specify our Eclipse transformers in our direct boxes at 130,000 ohms, a far cry from the 600 ohms that were once used. Therefore the loading is no longer as problematic as it used to be. In fact with the advent of high output active basses, passive direct boxes are often preferred as they are able to handle the output without clipping.

What other differences are there between passive and active DI's?

For low output instruments such as an old vintage Jazz Bass, even this bit of loading can cause a bass player to notice a minute drop in level. For these artists, an active direct box like the Radial J48 tends to be a better solution. This also applies to some vintage instruments like a Fender Rhodes piano. The active DI has a built-in buffer or unity gain amplifier that deliver a stronger signal than the passive counterpart. The benefit with a passive direct box is the simplicity of not having to worry about local power, of course isolation and extreme signal handling. In the past, high output instruments would overload the typical active DI, and only in recent years has new switching power supply technology enabled us to increase the signal handling to where this is no longer a problem.

We often associate active direct boxes to condenser mics and passive DIs to dynamics. You can use either in many cases, but if you want more reach - say on a violin - the condenser tends to be a better choice just as an active DI will produce more air from an acoustic guitar. Active DIs also benefit by enabling us to control various parameters that are simply impossible using a passive circuit. For example, we produce a direct box called the JDX that captures the sound coming out of a guitar amplifier head while also capturing the back electro-magnetic impulse from the loudspeaker. To make this sound natural, a series of filters are employed and this circuit requires power.

Are there applications that you recommend active or passive DI based upon instrument?

As a rule of thumb, if the source is active such as a keyboard, we tend to recommend passive. If the source is passive like a Fender bass, we tend to recommend active. This reduces the gain stages in the signal path which reduces noise. Where the lines blur is with acoustic guitars that have built-in pickups. Folks are comfortable with active direct boxes and since we can now handle higher signal levels, this seems to work well. When it comes to piezo transducers, active is the only choice. These finicky devices sound best when they see a very high input impedance. Our latest PZ-DI direct box has a 10 meg-Ohm (10,000,000) impedance for this very reason. This smoothes out the peaks, eliminates the squawk and extends the frequency response making it much more pleasing for orchestral instruments such as upright bass, violin or banjo.

The challenge of course is noise. When you increase the input impedance, you have very little signal left to work with. So it often takes months of trial and error to eliminate the noise problems. Another concern these days is computers. They are omnipresent and tend to always have noise problems. We produce a hybrid direct box that combines transformer isolation to block DC and active buffers to block AC coming back from the PA system. This 'best of both worlds' works well for sound cards, CD players and iPods that produce CD quality audio.

When using an active DI that offers a choice of battery or phantom power, is there a difference in performance when using the battery vs. phantom power?

Folks hate batteries. Not only are the bad for the environment, they always go down right in the middle of a show. To avert disaster, touring professionals spend thousands of dollars on batteries, putting in fresh ones in the wireless systems and guitar pedals before every show. When the power in the battery begins to drain, the circuit starves and distortion manifests itself. Today, all mixers and preamps are equipped with 48V phantom power. This, one would think, would eliminate the need for batteries in a DI box. The problem is ground loops. When they manifest, the solution is to lift the pin-1 ground. But with most active DIs, when you do so, you cut off phantom power which of course shuts the DI off. The battery is basically a band-aid solution. We figured out an alternate way of solving the problem inside the J48. The trick is in the power supply: This converts the 48V DC phantom power into AC using a switching power supply while it raises the internal working voltage for more headroom. The clever design enables us to lift the audio ground inside the power supply, not at the connector. This way, we eliminate the ground loop without cutting off the power. No batteries are required. This of course costs more to do, but the results speak for themselves.