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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

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AM 403 Dual DI Box
AM 403

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, higher energy signals (e.g. line Level, guitar level) to be converted to balanced and low impedance signals (e.g. microphone level). These balanced, low impedance signals can then be sent down longer cables with less noise and signal degradation. At the receiving end 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 may 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 most of 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 really should be THD figures for such devices.

AM 403 Dual DI Box

Some AM 403 Applications:

Please note that in all the examples below, it is assumed that the balanced output will be connected to a balanced input (e.g. mixer), though this is not a requirement.
AM 403 Dual DI BoxAM 403 Dual DI BoxAM 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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Your Comments:

191003 - (Excellent) Thanks for all the detailed information!
Not sure I got the whole part about the amplifier inputs but I'll read your "Amplifier Dummy Load for use with a Direct Inject (DI) Box".

Question though - Don't you find it irritating that the input levels printed on the case are not specified as dbV or dbU?

One more question - say I record an electric guitar and I want to split it - get a crappy clean DI sound in addition to the Amp sound (recorded using a microphone) how do I do so with this DI? Obviously I hook up the pickup to the instrument input (to be outputed to a recording console via XLR) but which of the DI's outputs should I use to connect to the guitar input of the amp?

Editor's Note:
Re: "amplifier inputs": I guess a diagram would be useful there. When I get a minute or five ... [Done!]

Re: "specs in dBv or dBu".
Admittedly, this is a very messy and potentially confusing space. In part this is because the terminology has been adapted across several distinct fields of knowledge, e.g.: physics, electronics, acoustics. Then also subdivided into professional and domestic use categories, just to make sure that no-one can easily get their head around it

However, for those who don't know:
dBV and dBu are reference voltages that are stated as specific values. For example:
stands for deciBel Volts, where 1.0000 volt ≡ 0 dBV, and
dBu stands for deciBel unloaded where 0.7746 volt ≡ 0 dBu
(see here External Link for further details and conversions)(≡ means 'exactly equals').

The dB (deciBel) is used to describe a relative value and is therefore used to compare two levels - rather than state an absolute level or value. To say that speaker A has a sound pressure level that is 6dB more than speaker B, is the same as saying that speaker A is twice as loud as speaker B (acoustically). This is a comparison without referring to wattage or actual input and/or output levels. Note however that a speaker's SPL (Sound Pressure Level) is usually specified as a ...1 KHz signal at an output level of 1W (Watt), measured at a distance of 1 metre.

For the DI Box above, there is only a simple resistor voltage divider network in use to reduce the level of the various inputs. Essentially, one could connect any level of signal to any of the inputs (as long as the resistors or transformers survived), and that signal would always be reduced by the same relative amount.

So the DI Box inputs are marked to "suggest" what might be appropriate for a given task based on the signal level one is working with.

For example: The output of a guitar multi-effects unit will often lie between instrument level and line level. So, if you are using the device (cranked up) closer to or at line level, use input 3. If using the device with the level trimmed down closer to instrument level, use input 4 or 5. Then tweak or trim at the mixing console.

It may take a little experimentation to get the best outcome for a given situation. Ideally you send the maximum signal level to the mixing desk (or whatever next stage) without clipping, so that the input gain at the desk can be trimmed right down for minimum noise and distortion. Knowing this shit is essentially what audio engineers get paid for ;-)

And to the last question:
A straight guitar signal would go into one of the "instrument" inputs (input 4 or 5). These are tied so it doesn't matter which.
Line Level goes to input 3 and speaker output (via a dummy load) to inputs 1 or 2.

Incept Date: Wizard - 160510
Last Update: Wizard - 240401

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