Bo Hansen
Well-known member
First of all, an active DI-box design is no big rocket science, and as you all here know, it usually consists of an impedance converter that also acts as a buffer to drive a transformer for the balanced line to the console microphone input.
(unless it is an electronic balanced DI-box with two OP-amps without transformer on the output)
So I write this with a twinkle in my eye.
A year ago at the beginning of this thread, you talked about both Rupert Neve's "RNDI" and my "1975 active DI box" and I can add in this context that both of these two designs have a very great similarity how we solved this impedance converter/buffer compared to many other active DI-boxes on the market.
In 1975 when I made this very simple design, (published here on the groupdiy forum in 2004) I chose two cascaded emitter followers, the first a NPN driven with low current for high input impedance and low noise, and the second a PNP driven with higher current for good driving of the transformer.
I chose one transistor polarity of each i.e. NPN and PNP, because already at this time I thought this gave a nicer clipping behavior and more stable bias point.
Around 2014 when Rupert's RNDI came on the market, it was the same idea, except for the first transistor where Rupert chose an N-channel JFET instead of a BJT NPN as in my case, but otherwise very similar in all other respects, except for true output, speaker mode pad and ground lift, where instead I only have low impedance monitor/amp output and fixed ground loop suppressor.
Both of us also chose about 10 times lower input impedance than active DI boxes usually have, to imitate the standard input impedance that most guitars and bass amps have, that is around 1 Mohm.
We also both chose almost the same current consumption from the phantom feed, Rupert 4.5 ma and I 3.5 ma, this is a good compromise between output swing and drive capacity.
Ps! I know how RNDI design is made because I helped RND in Texas to service their products here in Sweden for a couple of years before they got a dealer with their own service workshop.
It is with some pride to know that I had the same thoughts back in 1975 as Rupert when he made his DI-box 40 years later.
--Bo
(unless it is an electronic balanced DI-box with two OP-amps without transformer on the output)
So I write this with a twinkle in my eye.
A year ago at the beginning of this thread, you talked about both Rupert Neve's "RNDI" and my "1975 active DI box" and I can add in this context that both of these two designs have a very great similarity how we solved this impedance converter/buffer compared to many other active DI-boxes on the market.
In 1975 when I made this very simple design, (published here on the groupdiy forum in 2004) I chose two cascaded emitter followers, the first a NPN driven with low current for high input impedance and low noise, and the second a PNP driven with higher current for good driving of the transformer.
I chose one transistor polarity of each i.e. NPN and PNP, because already at this time I thought this gave a nicer clipping behavior and more stable bias point.
Around 2014 when Rupert's RNDI came on the market, it was the same idea, except for the first transistor where Rupert chose an N-channel JFET instead of a BJT NPN as in my case, but otherwise very similar in all other respects, except for true output, speaker mode pad and ground lift, where instead I only have low impedance monitor/amp output and fixed ground loop suppressor.
Both of us also chose about 10 times lower input impedance than active DI boxes usually have, to imitate the standard input impedance that most guitars and bass amps have, that is around 1 Mohm.
We also both chose almost the same current consumption from the phantom feed, Rupert 4.5 ma and I 3.5 ma, this is a good compromise between output swing and drive capacity.
Ps! I know how RNDI design is made because I helped RND in Texas to service their products here in Sweden for a couple of years before they got a dealer with their own service workshop.
It is with some pride to know that I had the same thoughts back in 1975 as Rupert when he made his DI-box 40 years later.
--Bo
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