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DIY DI Box

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scrawny git

New Member
I'm trying to build a very basic DI box. Passive and taking speaker and line levels down to mic level. I'm just not sure what transformer to use and this is apparently the key to a good DI.

The ratio, on the schematics I've seen is 1:1, so I was wondering about this?
High Performance PCB Mounting AF Transformers > Maplin

Here's one schematic I was thinking of trying:
p35-f2.gif

All suggestions welcome. Thanks.
 

Sceadwian

Banned
Looks good to me.
 

audioguru

Well-Known Member
Most Helpful Member
The transformer needs to have its spec'd load resistance or it might have a peak in its frequency response.
 

wasp22

New Member
Hey Scrawny

If you want to use this DI box circuit with an electric guitar you will find a 1:1 transformer will give you poor quality audio. Also the resistor and potentiometer limit the input impedance of the box to about 10K ohms.

A guitar pickup is designed to drive a high impedance input of 100K ohms or greater. With a standard microphone channel input on a mixing desk, the input impedance is around 1.5K ohms. If you use a 1:1 transformer your guitar will 'see' a reflected impedance of only 1.5K ohms. For best results a turns ratio of 10:1 or greater is required.

If you want to do this cheaply you could use a Maplin LT44 driver transformer (20K - 1K ct), connect the input jack to the 20K primary and use only half of the secondary (use the centre tap and either outside pin). This should give you give an input impedance of around 100K ohms. Give that a try first before plonking down money for an expensive transformer.

You second alternative would be to build an active DI, but that's a bit more complicated...

WASP
 

gabeNC

Member
A guitar pickup is designed to drive a high impedance input of 100K ohms or greater. With a standard microphone channel input on a mixing desk, the input impedance is around 1.5K ohms. If you use a 1:1 transformer your guitar will 'see' a reflected impedance of only 1.5K ohms. For best results a turns ratio of 10:1 or greater is required.

WASP

How about a piezo saddle type pickup on an acoustic? Usually they have a preamp on board does it bring it up to the same impedance level? Like Scrawny, I have an interest in this and would like to build something similar.
 

wasp22

New Member
How about a piezo saddle type pickup on an acoustic? Usually they have a preamp on board does it bring it up to the same impedance level? Like Scrawny, I have an interest in this and would like to build something similar.
Guitars with an internal preamp are much less critical regarding load impedance. A peizo pickup in most cases always uses a preamp because it requires an even higher load impedance than an inductive pickup.

Jensen Transformers (USA) make an excellent but costly DI transformer.
http://www.electro-tech-online.com/custompdfs/2009/04/as066.pdf

If you are running Bass guitar through your DI it is important to select a transformer that will handle high level low frequency signals without being driven into saturation and causing distortion. Some small audio transformers
perform badly with Bass for this reason.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Guitars with an internal preamp are much less critical regarding load impedance. A peizo pickup in most cases always uses a preamp because it requires an even higher load impedance than an inductive pickup.

Jensen Transformers (USA) make an excellent but costly DI transformer.
http://www.electro-tech-online.com/custompdfs/2009/04/as066-2.pdf

If you are running Bass guitar through your DI it is important to select a transformer that will handle high level low frequency signals without being driven into saturation and causing distortion. Some small audio transformers
perform badly with Bass for this reason.
Why not just use an active DI box?, suits pretty well any source (far more than a transformer), and they work extremely well - I wouldn't be without mine, and I have two double ones (use it as two separate DI's, or switch it to feed two balanced outputs from a single input).
 
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