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AM 403 Dual Passive Direct Inject (DI) Box
Review and Description
AM 403 Dual Passive Direct Inject (DI) Box - Review and Description
AM 403 Dual DI Box
Find an AM 403 on Ebay

The new price of an AM 403 Dual Direct Inject Box can be around AUD $60 on eBay (includes postage). Realistically speaking, you probably couldn't buy the parts inside it for that money, let alone fabricate a case and pair of circuit boards.

I've never seen a used AM 403 for sale, but if one came along for under AUD$40, it would be worth buying, if only for the two balanced output transformers.

My suspicion though is that anyone who has one of these will be keeping it ;-))  

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The AM403 (aka SoundKing) Dual Direct Inject (DI) Box can be summed up as simple, sturdy and efficient. The AM403 is an all passive DI Box that is well constructed from quality parts. The case is all metal, providing good RF shielding. The layout allows for useful implementation options.

The AM 403 is an extremely densely packed unit. There is literally no spare space inside. In fact, it's so tight, that once assembled it would be a major task to effect repairs.

What is a Direct Inject or DI Box:
A Direct Inject or DI box allows unbalanced and generally high impedance signals (e.g. guitar level) to be converted to balanced and low impedance signals (e.g. microphone level). These signals can then be sent down longer cables with less noise and degradation at the other end, where they can be plugged directly into the balanced microphone input of a mixing desk or recording console.

AM 403 Dual DI BoxSince the AM403 is passive, there are no batteries or power connections needed, and no internal amplifiers to add noise. There will however be some harmonic distortion due to the transformers, this can be as high as 5%. Sadly, transformer distortion figures are rarely available in passive DI Box specifications. But really, they should be.

In my case, I wanted to DI the speaker output from a Boutique 20W (aka Lunch-Box) valve amp. So any transformer distortion is likely to go totally unnoticed ;-)

For some uses though, you might be looking for better quality (and almost certainly more expensive) transformers. Where the manufacturer is prepared to boast about how little distortion there is.

I say this because the whole AM 403 costs less than what you may pay for just a single balanced output transformer. In the photo to the right, the transformers are enclosed in the shiny stainless steel cans in the centre of the unit.

See also - Amplifier Dummy Load for use with a Direct Inject (DI) Box

Download a scanned copy of the AM401 / 402 / 403 / 404 User Manual in PDF format.

For the AM 403, the following items are duplicated for the second channel.




Possible Uses Include:


To address and/or accommodate the cons mentioned below the device would have to be housed in a larger box. So the points mentioned below are more of a 'wishlist', rather than a criticism. If the AM 403 is taken as-is, there really isn't anything to complain about. It does exactly what it sets out to do.

AM 403 Schematic:
As far as electrical devices go, the AM 403 is very simple. Essentially, the the inputs are voltage divided to bring them down to an acceptable level for the transformer to deal with. The transformer then converts a high impedance unbalanced signal into a low impedance balanced one, ready to be connected to an XLR mic input on a mixer or recording console. Simple, clean and effective.

The quality of the transformers (which I can't comment on) would be the only potentially limiting factor. However, given that the quoted frequency response is 30Hz to 30KHz (+0/-1 dB), it's not looking at all shoddy. Being a transformer coupled device however, there is likely to be some inherent distortion. Though there are no distortion figures provided (which seems to be quite common for DI boxes), there should be.

AM 403 Dual DI Box

See also - Amplifier Dummy Load for use with a Direct Inject (DI) Box

AM 403 Dual Direct Inject Box Summary:

So how does the AM 403 shape up?

As far as features go, I would like to see a padding option (-3dB or -6dB), so 9/10.
The AM 403 doesn't need batteries or power, so it's just a matter of connecting the relevant ins and outs and you're good to go (remember the warning above about proper speaker loading if using the speaker connections), so it's 9/10 for ease of use.
Build Quality, Durability and Reliability are a straight up 10/10.
Value for money is Excellent. To build a unit like this, you wouldn't be able to source the parts for the price of this unit. Added to that, the price compares very favorably to other 'Brand Name' units which may cost twice as much or more. So another 10/10.

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