DI input impedance

[seanm]

I have been looking at DI boxes. Some very well respected DIs, such as the Radial JDI, have very low input impedances. Many of the passive DIs are in the 10k range and the active in the 100k - 220k range.

I would expect that the impendance mismatch would affect guitar tone, but these DIs are very well respected in the studio where differences would be more noticable. Are people always putting a buffered device in front of them?

So that this is not off-topic, anybody know of a really good active DI schematic? I have already seent he one at ESP, but I would like to see some others for comparison.

[niftydog]

bear in mind that the JDI is completely passive, so maybe that's the mojo factor that helps engineers overlook the relatively low input impedance.

[scratch]

Hello Sean,

I don't have my schematics at hand but you could look into the SSM2142

http://www.analog.com/UploadedFiles/Data_Sheets/8871443ssm2142.pdf

You may also want to look into the NE5532 for it's excellant low impedance output drive capability (for driving long balanced line cables).

I've seen transformers that are 600ohm/600ohm, that I think are intended for changing unbalanced 600ohm microphone output, to balanced line. I wouldn't use one of these for guitar outputs ...

[seanm]

Hello Sean,

I don't have my schematics at hand but you could look into the SSM2142

http://www.analog.com/UploadedFiles/Data_Sheets/8871443ssm2142.pdf

You may also want to look into the NE5532 for it's excellant low impedance output drive capability (for driving long balanced line cables).

I've seen transformers that are 600ohm/600ohm, that I think are intended for changing unbalanced 600ohm microphone output, to balanced line. I wouldn't use one of these for guitar outputs ...

That SSM2142 looks cool.

[scratch]

I forgot to add, I think they run around 4-5$ per. Active (on Merivale) has them on the shelf last time I looked ...

[niftydog]

FYI;

600/600ohm transformers are simply signal isolation transformers designed to interface two 600ohm characteristic impedance lines.

One use for them is between two audio systems running on different power sources. The transformer prevents any unwanted signals from coupling into the inputs.

They are also used extensively in telecommunications.

There are also transformers that are intended for balanced to unbalanced (or vice versa) conversion, but these would be labeled as such. Each line of the balanced signal needs to see 600ohms, and one side of the transformer would be centre tapped.

[scratch]

Niftydog,thanks for the elaboration, I should have been more explicit in my transformer explanation ... I've got some of those for that purpose, a 600 ohm unbalanced microphone i need to run a long distance ...

I could go with an active DI type box and phantom power it, but a passive transformer is an easier build ;-)