DI Box question

[ricobasso]

Have just got an Inote Millenium DI box. It seems to work OK, but I'm puzzled by the instr/spkr switch. In spkr position it gives an extra 40dB attenuation plus some hf roll-off (100k/47nF). I understand the attenuation, although it seems a bit excessive, but why the hf cut?

[Speedskater]

This question might get more responses in the "Live Sound" sections.

[tomi]

Can't see why you'd want the HF rolloff...
My quick calculation suggests that makes it 6dB down at 33Hz - which can't be right! Would someone like to correct my sums?

[AndrewT]

I don't know what a DI box is.
The 40dB cut when in speaker position seems like it is designed to accept a line level signal without attenuation (or very little) and when the input is from a speaker, then the extra 40dB (divide by 100) brings the speaker level signal back down to near line level.

A series 100k followed by a 47nF to ground is a Low Pass Filter with F-3dB=34Hz.
It attenuates the bulk of the audio signal at a rate of 6dB per octave.

A series 47nF followed by a 100k to ground is a High Pass Filter with F-3dB=34Hz.
This passes most of the audio signal and blocks both sub bass and DC signals.

You need to tell us what you have, not making us guess.

[Speedskater]

It's a passive input device for musical systems. The following is a discription of one unit:

Passive direct box for connection of unbalanced musical instruments, line level sources and guitar amplifier outputs to Low-Z balanced microphone input for PA or recording. 1/4" (6.35mm) input (with parallel 1/4" (6.35mm) output); switches for speaker/line-level attenuation; high cut filter and ground lift; male XLR type balanced output. Uses DBT-1 transformer (20K/150 Ohms).

[AndrewT]

Originally Posted by AndrewT

The 40dB cut when in speaker position seems like it is designed to accept a line level signal without attenuation (or very little) and when the input is from a speaker, then the extra 40dB (divide by 100) brings the speaker level signal back down to near line level.

Originally Posted by Speedskater

........... switches for speaker/line-level attenuation;

confirms the -40dB switch.

[ricobasso]

Sorry I was a bit sketchy about the attenuator. It's a series 100k with a shunt of 1k resistor and a 47nF in parallel forming a low pass filter (all before the transformer). The series resistor is switched to 1k in the "Instr" switch position.

I think the performance is complicated by the transformer load as the cut off seems to start around 100~200Hz.