Boss - DI-1 Direct Box  [schematic]

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Re: Boss - DI-1 Direct Box

Postby Pollo » 03 Feb 2017, 17:24

On a second note I'm trying to read up on differential amplifiers and how they work but it sure is hard to find something in plain English that I can understand. Does anyone know some good sources?
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Re: Boss - DI-1 Direct Box

Postby Pollo » 03 Feb 2017, 22:51

After reading some information on differential amplifiers I understood a little bit better what needs to be done. I did some experiments and it seems to work as intended. Apparently the amplification factor of the op-amp is determined by the ratio between Rf and Rin, in this case R26 (Rf) and R25 (Rin). In this circuit R25 equals R26 therefore the amplification is 1. I added a few resistors in series to R26, getting a total of 87 KOhm and I got 4.1 dB of extra gain. I calculated 5.35 but it is close enough.

So I could replace R26 with a 500 KOhm resistor and would get approximately the 20 dB I want. The disadvantage of this approach is that I lose the benefit of a balanced connection, the noise reducing properties. So it would be preferred to get 10 dB gain on each side of the balanced line. And that's where I am not sure how to proceed, because on the other side it looks a bit different. Would I get the 10 dB if I inserted a resistor between terminals 6 and 7 of IC1b? Apart from that it's not easy to do since they are linked on the PCB, I doubt if it's going to do what I want. Anybody has any ideas?
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Re: Boss - DI-1 Direct Box

Postby Pollo » 05 Feb 2017, 16:58

After much head scratching I came up with the following modification:

di-1 Mod.jpg


It works. It gives me about 14 dB of extra gain. Unfortunately it also increases noise with about the same amount. Since this was intended for use with recording my DI-ed guitar it is not that useful; I can increase the gain digitally with the same result. So I ended up undoing the change. But I kept the phantom power modification. I kept the capacitors C12 and C13 because they not only block the phantom power but also some DC voltage from the op-amp.

I also found out that there is an error in the original design. There is switch to swap the inverting and non-inverting line on the XLR connection but when the switch is in 'normal' position it is actually inverted (pin 2 normally carries the 'hot' non-inverted signal). But maybe that is different in Japan?

All in all I learned a lot from this.
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Re: Boss - DI-1 Direct Box

Postby clintrubber » 28 Mar 2018, 11:46

Pollo wrote:I also found out that there is an error in the original design. There is switch to swap the inverting and non-inverting line on the XLR connection but when the switch is in 'normal' position it is actually inverted (pin 2 normally carries the 'hot' non-inverted signal). But maybe that is different in Japan?


That's correct, various Japanese gear (at least used) the opposite 'standard' of pin #2 negative (inverted) polarity.


Bye
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