Passive and Active DI boxes

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

DI, or direct boxes are either passive or active. Active meaning powered. In both cases transformers are used. Depending on the application there are different turns ratios of the coils. Some manufacturers even list the impedance ratios of the coils. Sound quality-Frequency Loss-- depends on the loading of the input, and output. The loading factor, in terms of impedance ratios, becomes more critical in passive DI boxes. What I mean by that is the location of a passive DI box in the signal chain. If the passive DI box is drawing signal from a guitar pickup, that is much different than drawing signal from a line level signal. Line level being defined as up to one volt. So is the passive DI box first out of the guitar pickup, or is it following some powered pedal, or effect box that produces a line level signal?
I have been investigating turns ratios and impedance ratios of passive DI boxes, and find that the turns ratios are all over the place as is the impedance ratios. For example for turns I have seen everything from 12:1 to 500:1 In addition, some of these passive DI boxes are substituted, or used, for ground loop isolation boxes.
Who knows the quality of the transformers--and the purpose of the DI boxes and hum eliminators coming from China. Nowadays they all seem to come from China and there are vast numbers of them for sale on the Internet. I suspect the vast number of people who buy a hum eliminator have no idea what is in it, or where to put it, or what it is really designed for. It comes from China, it's cheap, and it's worth a shot in the dark. I suppose that is how many people think. It doesn't stop the hum, but it was cheap.
I know that Peavey made both passive and active DI boxes. Does anyone know information about the transformers that were used? Meaning turns ratios, and primary and secondary impedances? I have an old Peavey passive DI box designed to take a amplifier speaker output and convert it to a line level signal. Peavey made several different types of passive DI boxes. If you know anything about the different boxes Peavey made, and what is inside them, please share. Thanks!

Erv Niehaus · From: Litchfield, MN, USA

Keith,
I'm sure this is the passive box you were talking about.
Erv

Dave Grafe · From: Hudson River Valley NY

I am unable to find technical Peavey drawings or detailed specs, no doubt the small audio transformers used were sourced from multiple supply companies such as CTS and Asian manufacturers.

All effective DI boxes share the basic impedances.

Input impedance: A minimum 100 kohm to 150 kohm, with some boutique active units going as high as 10 meg. The top quality Radian passive box lists a 130k input impedance.

Output impedance: A maximum of 600 ohms transformet balance is a legacy standard, but that is a bit high for best performance from most mic inputs, so 100-150 ohms is more common, the lower values being the premium offerings.

Here is a common passive DI with a few extras, and the circuit printed on the case.

Active DI boxes with few exotic exceptions, specifically do NOT use transformers, being designed to that end in the first place. Some have active circuitry intended to emulate the sound "color" of transformers, but most feature powered IC op-amps with associated proprietary circuitry replacing the transformer for I/O and impedance matching. Some boutique manufacturers encase this circuitry in epoxy to discourage copying of their circuits, which also impedes repair of their inevitably expensive products.

Hope this is more help than noise. I'm certain that others here can provide more detail.

Keith Hilton · From: 248 Laurel Road Ozark, Missouri 65721

The world of DI boxes, hum eliminators, and ground loop eliminators is filled with snakes and snake oil salesmen. Of course that is just my opinion. If you read the snake oil literature, you might assume in addition to hum, it would cure cancer and the common cold. A discussion of DI boxes would not be complete without discussing ground loop noise, because cables connect, and cables contain grounds. Below is an article I found on the most common (3) ways to eliminate ground loop noise.
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Isolation transformer, optical link, or differential input: A ground loop that is intrinsic to the circuit design and so not accidental can be removed by inserting an isolation transformer or an optical break in the offending cable. A differential input arrangement could also be tried if the two cables in a twisted pair system each pick up the ground loop signal Vloop, so that by subtracting the signal on one cable from the other, Vloop is removed. This requires a differential input with enough common mode rejection to do this subtraction. In general using isolation transformers, optical breaks or differential amplifiers requires financial outlay and introduces signal degradation. Signal degradation occurs because transformers operate over a finite bandwidth so that signals outside this bandwidth will be lost. Differential inputs can degrade signals because each of the two differential inputs functions over a limited voltage range and saturates if the common mode signal, i.e., Vloop, is outside of this range. For example, if Vloop =10 volts and the differential amplifiers saturate at 1 volt then the signal will be corrupted because the subtraction will fail. Optical isolation involves converting the signal voltage to an optical signal that is then converted back to an electrical signal. Both the up- and down-conversion involve nonlinearities which can lead to distortion. Also, up- and down- conversion typically involves a requirement for isolated electrical power being supplied on both sides.
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The people making transformers say transformers are the best way to go in eliminating ground loop noise. Problem is--- high quality audio transformers run around $150.00 or more. Just for the little transformer! You can use less expensive audio transformers if you know how to design around their limitations. Both optical, and differential op amps are not that expensive and come in ready made chips to install in a circuit. Most people seem to prefer a passive device, and that eliminates optical and differential op amps.
Getting back to snake oil and the snake oil salesman, 99.999999% of all of the DI boxes and hum eliminators come from China. Some people will do anything to get rid of hum, even spend money on snake oil, hoping it might cure their ingrown toe nails.
My motive: I find the subject fascinating. I am enjoying looking at the circuits, and building samples. The trick is to not color the sound.

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