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Author Topic: What is he purpose of the transformer In a DI box?  (Read 6328 times)

Dave Potter

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What is he purpose of the transformer In a DI box?
« on: March 11, 2013, 02:29:03 pm »

Just to frame the question better.  I understand that DI boxes are primarily for impedance matching, but you can do that with op amps.  That leaves isolation.  What can happen if you don't isolate with a transformer?  What are the benefits?
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Tim McCulloch

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Re: What is he purpose of the transformer In a DI box?
« Reply #1 on: March 11, 2013, 02:49:23 pm »

Just to frame the question better.  I understand that DI boxes are primarily for impedance matching, but you can do that with op amps.  That leaves isolation.  What can happen if you don't isolate with a transformer?  What are the benefits?

The transformer is there for impedance matching AND balancing the output.  Ground isolation is a side benefit.
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Mark McFarlane

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Re: What is he purpose of the transformer In a DI box?
« Reply #2 on: March 11, 2013, 03:27:21 pm »

...  What are the benefits?

A transformer can add some non-linear behavior, that you may or may not like.  Also, passive DIs with a transformer don't require power, which in some cases may be considered a benefit especially if you have a bad memory like me and forget to turn on the power :)
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Don Boomer

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Re: What is he purpose of the transformer In a DI box?
« Reply #3 on: March 11, 2013, 03:53:00 pm »

How about safety as you no longer have a direct electrical path between pieces.
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John Roberts {JR}

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Re: What is he purpose of the transformer In a DI box?
« Reply #4 on: March 11, 2013, 04:29:16 pm »

The classic old school transformer DI is purely to "transform" the high instrument source impedance, down to something lower and suitable to send output signal over longer wires without negatively affecting the sound quality. Coincidentally this also drops the signal level down low enough to not overload mic preamp inputs, but that is secondary effect. Likewise galvanic isolation of grounds is a secondary benefit.

Active DI do not match impedance per se but use active electronics to buffer it. Similar in effect but not the same. Active DI are generally not transformer isolated but present the instrument with even higher impedance for even less loading.

The benefit of not using a transformer, is that transformers (especially cheap one) are not perfect, so active can be cleaner even when using relatively cheap electronics. (Cheap electronics is a lot better than cheap transformers.)

JR
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Dave Potter

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Re: What is he purpose of the transformer In a DI box?
« Reply #5 on: March 11, 2013, 07:03:28 pm »

Okey dokie.  I just fitted the optional pro Transformer mod to my Bass amplifier.  Since it has no audio effect whatsoever ("warmth" "saturation" etc) other than a 180 degree phase shift, I was wondering why I did it.  No reason apparently.  ::)
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Greg_Cameron

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Re: What is he purpose of the transformer In a DI box?
« Reply #6 on: March 11, 2013, 07:26:11 pm »

Ground isolation is a side benefit.

Or when you have an audible ground loop, it's the primary benefit :)
« Last Edit: March 12, 2013, 10:03:08 am by Greg Cameron »
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John Roberts {JR}

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Re: What is he purpose of the transformer In a DI box?
« Reply #7 on: March 11, 2013, 07:31:14 pm »

Okey dokie.  I just fitted the optional pro Transformer mod to my Bass amplifier.  Since it has no audio effect whatsoever ("warmth" "saturation" etc) other than a 180 degree phase shift, I was wondering why I did it.  No reason apparently.  ::)
A passive transformer DI used for an instrument is a completely different application than a transformer output for an amp.

Is this a manufacturer approved mod?

It is generally a good thing to have transformer isolation between a guitar amp and a console.

JR
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Dave Potter

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Re: What is he purpose of the transformer In a DI box?
« Reply #8 on: March 12, 2013, 06:47:46 am »

Quote
Is this a manufacturer approved mod?

It is generally a good thing to have transformer isolation between a guitar amp and a console.

JR
Yes it is the approved Markbass "super DI transformer" module.
"Good thing"?  Other than ground loops, what specifically?
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Frank DeWitt

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Re: What is he purpose of the transformer In a DI box?
« Reply #9 on: March 12, 2013, 09:14:56 am »

Yes it is the approved Markbass "super DI transformer" module.
"Good thing"?  Other than ground loops, what specifically?

Balanced output is the big one.  It allows long cables with little or no pickup of noise along the way. Example from your amp to a FOH mixer
It protects your gear from there gear.  The only thing that can get through a transformer is audio.  Phantom power, messed up grounds, and unknown problems in there gear are stopped by the transformer.  It also protects there gear from your gear.

It is worth noting that a transformer will pass audio in either direction.  A passive DI box can be used as intended to take a unbalanced line level signal and convert it to a balanced mic level signal or it can be used the other way.  I have used two of them to move line level from a guitar amp a loud distance to the line input of another amp.  BTW a good transformer doesn't change your sound.
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Re: What is he purpose of the transformer In a DI box?
« Reply #9 on: March 12, 2013, 09:14:56 am »


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