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.
Since 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.
- Speaker Level (<+42dB)
Two 6mm sockets in parallel are provided to allow the AM403 to be connected in series between a power amp and its speaker(s).
Important Note When Using the DI Speaker Input:
When connecting a DI (Direct Inject) device to a power amplifier's speaker output, make sure that the amplifier is also connected to a properly rated speaker, speaker cabinet or suitable resistive load.
Improper output loading, sometimes referred to as a Floating Output, may result in damage to your amplifier's output stages ... translation: very expensive!
( Amplifier Dummy Load for use with a Direct Inject (DI) Box )
- Line Level (<+20dB)
Single 6mm socket. For use with the line out from a guitar amp or other device.
- Instrument Level (<-20dB)
Two 6mm sockets in parallel are provided to allow the AM403 to be connected in series between the Guitar/Effects and the guitar amplifier.
- Ground Lift Switch
Physically disconnects the Input Ground from the Output Ground.
- Microphone Level (approximately)
For a balanced 600Ω load (or greater) suitable for connection to a mixing/recording console via standard XLR connector.
Possible Uses Include:
- Capturing the full output distortion of a tube amp, by taking the signal from the output of the Power Amp rather than from the Line Out from the Pre Amp. To avoid damage to your amp, make sure the amp's output is appropriately loaded when not using speakers. This is actually a great way to get a nice fat tube distortion sound without having to worry about outrageous sound pressure levels.
- Being able to dump a line or instrument level signal straight into a mixing/recording console.
- Reducing noise and signal degradation levels where extra long leads are required by replacing unbalanced leads with balanced ones.
This becomes progressively more significant with lead lengths beyond about 4 metres (13 feet). A combination of signal loss over distance, RF (Radio Frequency) noise, other induced noises (mains hum and lighting control glitches) and also increasing capacitance within the guitar lead itself - all conspire to reduce signal integrity over distance.
- Solid / Sturdy
- Seemingly Good Quality Parts
- Simple to Operate
- Compact (couldn't physically be any smaller ;-)
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.
- There is no online support, web site or even official PDF manual to download (a scanned copy is available here though). This is very much a case of you get what's in the box and not a skeric more. This product (and it's related products, as seen in the manual, may not be available all the time).
- The Ground Lift switch is the only thing that protrudes from the case, making it vulnerable to breaking off (it happens!). A flat sliding switch would have been preferable (but would also increase manufacturing / assembly costs).
- An RCA connector would have been useful as a Line In option.
- Speaker attenuation - The available attenuation ratio of 100 to 1 is rather severe. A 50 to 1 attenuation ratio option would have been good for those of us who don't run 100W+ amps, flat-out, all the time.
- An attenuation selector switch for more options. A simple 0dB / -3dB (or perhaps 0dB / -6dB) option would do.
- The extremely compact nature of this DI Box makes modifications and/or additions pretty much impossible. To mod this unit will almost certainly require a larger case ... which would be somewhat of a waste.
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.
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.
- Features - 9 / 10 (would benefit from a padding switch)
- Ease of Use - 9 / 10 (speaker loading required)
- Build Quality - 10 / 10
- Durability & Reliability - 10 / 10
- Value for Money - 10 /10
- Overall Average Score ... 9 / 10 (ridiculously close to a 10/10)
160000 - (-----) None so far.
Incept Date: Wizard - 160510
Last Update: Wizard - 181006