it was only some home built passive Di-boxes with small Beyer or Ke-Mo
high ratio transformers around the studios here in Sweden.
When I recorded bass guitar I always missed the claer steady bottom,
so I must do something, and the result was the "DI-box 1975"
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some important construction hints
The metal case/housing for the DI electronics, shall only be grounded on the XLR output
connector pin 1, and also linked to the extra ground tag on the XLR connector,
and connected to a solder tag fixed in one of the XLR fasten screws.
(that shall be the case/housing ground)
Of course the connections from XLR pin 1 to the electronic ground rail (the bottom line in the schematic)
via the 0,047 uF, 47 ohm and the two diodes shall still be connected.
The two tele jacks must be isolated from the metal box, use plastic jacks, (Cliff brand, with plastic nut or
crome nut with isolated washer) the jacks ground/sleve shall only be connected in the electronic ground rail.
The body on the transformer shall be grounded in the electronic ground rail,
and not have connection with he metal case.
It is also usal that there are a extra ground cable or connection on the transformer, this shall also be
connected to the electronic ground rail. (pin 8 on the Lundahl LL1538)
Lundahl LL1538 configuration for 5:1 ratio
Transformer connection pin 6 to amp output (10 uF and 10 kohm)
pin 5 to ground rail, pin 1 and 4 to XLR pin 2
pin 2 and 3 to XLR pin 3, pin 8 to ground rail
a small electronic correction to improve the bias stability
not corrected in the schematic above
Change the two 470 kohm to 100 kohm
Change the 1 uF to 10 uF/35 volt
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