Bo HansÚn
Gothenburg, Sweden

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tech page and
audio circuit advice


These circuit design and modifications on this website
belong to Bo HansÚn, and is free to use for private DIY purposes.
It is not permittet to use this material for commercial business without permission.




active DI-box, my work horse from 1975

API 312 mic amp DI input modification

API 325 line amp with DI input modification

power supply for API 312 or 325 amp card

simple FET impedance converter

phantom power supply

contact me

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active DI-box, my work horse from 1975

click on images
to enlarge


I designed this DI-box in the middle of 1970 when I worked as a recording engineer.
During this period, there were mostly just home-built passive DI-boxes with small transformers like Beyer, KeMo or Sennheiser, around the studios here in Sweden.
When I recorded electric bass guitar, I always missed the clear steady bottom, so I have to do something about it, and the result was the "DI-box 1975", whish I now present here as a DIY project.

For information, advice and praise from many who have built this DI-box around the world
can be read here on  Group DIY forum  over 300.000 visitors have been there.

circuit discription

This design is not "rocket science" it is a simple basic circuit that is easy to build and easy to get to work, but it is still of top quality that is hard to beat.

The front-end transistor is not of a JFET/field effect type that is normally in a DI-box, this design uses instead a bipolar silicon transistor.
The advantages are that they are easier to find on the market, and they are also more reliable.
The disadvantage is the difficulty in obtaining high input impedance as 10 Mohm, but 1 Mohm is ok, and good enough for this application.
An interesting note is that around 1 Mohm is probably one of the most common impedances on guitar and bass amps of all time, and is probably the reference impedance that the pickup manufactures use.

The circuit is a buffer amp consists of two emitter followers, the first one with low current to ensure high input impedance and low noise, and the second with higher current to deliver good drive capacity for the output transformer.
The internal voltage supply is selected to 24 volt to get a compromise between good headroom and low current consumption.

There is no groundlift switch that is normal, but instead uses a ground loop supressor built around two diodes, a resistor and capacitor, resulting in a safer operation for the DI-box electronics.

There is also a protection circuit for the input, consisting of two zener diodes and a resistor.
This offers a good protection against high signals such accidentally connected speaker signal.

ome important construction hints 

metal box/case ground
The metal case/housing for the DI-box electronics, shall only be grounded on the XLR output 
connector pin 1,
and also connected to a solder tag fixed in one of the XLR fasten screws.
(this is the only ground connection to the metal case)
Then a additional cable from XLR pin 1 to the electronic ground system rail (the bottom line in the schematic) in series with the 0,1 uF, 100 ohm and the two diodes.

isolated plastic jacks
The two tele jacks must be isolated from the metal box, use plastic jacks, with plastic nut or crome nut with isolated washer, the jacks ground/sleve shall only be connected in the electronic ground system rail.
This is very important, to get the ground loop suppressor to function properly.

shortet input jack
The input jack should be shorted to ground when it is un-plugged.
There is an extra connection/solder tag for the rear tip contacts that is the brake switch, this connection/solder tag shall be connected to the ground on the input jack.
(the link can be seen in the picture of the input jack mounted in the lid)
This removes noise, hum and interference with un-plugged input jack.

transformer case ground
The metal casing of the transformer shall only be connected to the electronic ground system, and not in contact with metal box.
it is not unususal that there is an extra cable or connector pin for the transformer core, this must also be connected to the electronic ground system.

LED type and connection (pc-card version)
Use a 3 or 5 m.m. red LED, because the low voltage drop. (other colours have higher voltage drop)
If the LED is mounted visible from the outside from the box, it must of course be connected with loose cables from pcb.
The long leg shall be connected next to the two 6,8K resistors.
If the LED is not used, use a link instead of LED.

some small electronic corrections

not corrected in the schematic

improved the bias stability
change the two 470 kohm to 100 kohm
change the 1 uF to 10 uF/35 volt

to reduce the component supply
change 47 ohm to 100 ohm
change  0,047 uF to 0,1 uF

transistor choice

This design accepts almost all transistors that are of a general purpose small signal amplifier type.
It is of course preferable to choose a low-noise type with the best selections of hfe, to ensure
that the transistor is doing a good job in this DI-box circuit.

Use only good brands and types such as Philips, SGS/Ates, Motorola, National, Fairchild.

For pc-card version, it is good if the transistors have same pin-out/package outlines as orginal NPN BC550C and PNP BC560C, and having legs arranged from left to right C-B-E. (seen from the top with the text facing you)
This package outline is called TO92 / TO226AA, case 29-04, style 17, according Motorola standard.

Here are some recommending European types that fits well:
BC550C = BC550B, BC549B/C, BC548B/C, BC547B/C, BC414B/C, BC337-25/40, BC237B/C, BC184B/C
BC560C = BC560B, BC559B/C, BC558B/C, BC557B/C, BC416B/C, BC327-25/40, BC307B/C, BC214B/C 

There are also many other useful transistor types in the U.S. package outline style, with legs arranged from left to right E-B-C, and this package outline is called TO92 / TO226AA, case 29-04, style 1.
This version will fit on the pc-card if the transistor is rotated a half turn.

Here are some recommending U.S. types:
BC550C = 2N3391A, 2N3904, 2N4401, 2N5088, 2N5210, MPSA18
BC560C = 2N3906, 2N4403, 2N5087, 

To make it even more complicated, so there is an additional U.S. package outline wish is also used in Japanese transistor types.
This version have the legs arranged from left to right E-C-B, and this package outline is called TO92 / TO226AA, case 29-04, style 14.

If this version will fit on the pc-card you have to cross the C and B leg.
Put on a piese of insulation sleeving on one crossed leg, in order to avoid short circuit between them.

Here is some recommending types:
BC550C = 2N3707, 2SC1775-AE, 2SC1815-GR/BL, 2SC2240-GR/BL
BC560C = 2N3703, 2N4058, 2SA872-AE, 2SA1015-GR/BL, 2SA970-GR/BL

other componet choices

The resistors can be standard 5% 0,25w carbon film, except for the two 6,8k that need to be 1% 0,25w metal film. (or use 1% 0,25w metal film for all resistors)

All capacitors of radial lead types, capacitance tolerance 5 to 20 %
0,1 uF capacitors of polyester or polypropylene type, minimum rated 50 volt, pitch between legs 5 mm.
10 uF and 100 uF aluminium electrolytic capacitors minimum rated 35 volt, pitch between legs 2-3,5 mm.

Zener diods 12 volt 500mw/0,5 watt. (for example BZX55-C12, BZX79-C12, 1N759, 1N5242)
Diodes 1N4004, 1 amp 400 volt. (1N4004 to 1N4007 is ok to use)

Good choice of plastic jacks are:
(you can use the same type but with PCB pins if normal solder lugs is sold out)
Cliff CL1160A / S2BNB, (CL1169 / S2BNBPC)
Neutrik NMJ4HFS, (NMJ4HFD2)
Rean NYS2122, (NYS2162)

three types that fit directly into the PCB layout

Lundahl LL1538 configuration for 1:5 ratio
or LL1538XL for more low end headroom
data sheet

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.

buy Lundahl transformers

Haufe ST8456
a good alternative to Lundahl.
ordering can be done here

price cheaper than above, but still good quality.
data sheet
data sheet

buy OEP transformers

Transformer comparison

I have done some measurements on the Lundahl LL1538, Haufe ST8456 and Walters OEP A262A3E, in my DI-box circuit.

Here is a brief comparison of the three transformers that fits the circuit board layout.

Lundahl and Haufe have near same behavior from 100 Hz and up, but Haufe are a bit better in low end small signal distortion, and Lundahl have lower distortion in low end near saturation point.
Both have a very good square wave response, and have 14 dB loss with 1 kohm total load.

OEP have approx. 10 times higher distortion from 100 Hz and down, and have also higher distortion in the high end, but have ok square wave response, slightly rounded, with out ringing or overshoot.
Because the 6,45:1 ratio, it have 16 dB loss.

All three sounds nice in my DI-box, but Lundahl LL1538 are still the best with a steady and clean low bottom and a fast and clear high end.

Lundahl LL1538XL have very near same specification as LL1538, but because it have approx. 45% bigger core, it can handle 3 dB more level.

Other transformers

There are lots of other good mic transformers with ratios around 1:5 which can be used backwards in 5:1 configuration for this DI-box design.
These do not fit directly on the pc-card so they may be connected with loose cables.
The transformer housing must be isolated from the DI metal box if it is mounted outside the pc-card, but it must be connected to the pc-card electronic ground system.

here are some good brands and types that can have a nice character and personality.
Amplimo TM2P
Cinemag  CMMI-5C, CMO-5/200CT
Jensen JT-13K6
Lundahl LL1528, LL1578
Sowter 3678, 8230, 9045, 9145
Steven & Billington 829

The old famous Neve/MarinAir/St.Ives 10468/VT22670/T1454 mic input transformer, configurated in ratio 4,8k to 300 ohm can also fit very nice.
It is manufactured today by:
Carnhill VTB-9045
Sowter 9145

boxes/enclosures and metal work

Metal boxes of die cast aluminium is very practical and easy to use.
An example is my prototype pictured here, where I have assembled everything in the lid.
This box is manufactured by Deltron, type number 461-0040, LxWxH 114x65x55 mm, painted in light gray.
The disadvantage of this box is the small inside depth, only Lundahl LL1538 and Haufe ST8456 will fit,
LL1538XL and OEP A262A3E are too high.

If you are not so mechanically handy, it is better with a larger box where the pc-card is mounted on the bottom and contacts around the sides.
A suitable box for this is Hammond type number 1590-TBK, LxWxH 121x80x55 mm, painted in satin black.

Install the pc-card with 4 pcs. 6 mm spacers with 3 mm hole (or three nuts), to get a distance between the pc-card and box.

Both boxes are available from ELFA, Farnell, Mouser, Newark, RS-components.

Of course, you can find nicer small metal instrument cases of various brands on the market, but these are usually much more expensive.


Is it not necessary to use shielded cables between pc-card and connectors. (reason is the metal box)
The usual "hook up wire" type AWG22 for internal wiring will do just fine, which can be twisted in pair nicely.

final testing

To get a first indication that the DI-box electronics are working properly, only two measurments needed to be done.

Connect the DI-box XLR connector to a mixer or preamp mic input with true 48 volt phantom powering.
If the LED is used and it light up, this is a first sign of health.

Note, if the console/preamp phantom powering not have the right specifictions, will not following test measurments be accurate.

Use a digital voltmeter in dc mode and connect the negative test cord to the input jack ground and the positive test cord to the junction av the two 6,8k resistors, or the long leg of the LED if this is used, or the link if the LED not used, and here you should measure +24 volts, +/- 1 volt, if everything works ok.

The next measurment is to check symetrical clipping and headroom.
Move the positive test cord to the junction of 3,9k resistor, 10uF capacitor positive end and emitter on BC560 transistor, and here you should measure around +12 volts, +/- 0,5 volt.

technical specifications

Measured with Philips BC550C/BC560C and Lundahl LL1538

Input impedance: 1 Mohm.
Max input level before clipping: +19 dBu/7,5 volt.
XLR output impedance: 200 ohm transformer balanced.
Max output level from balanced XLR before clipping: +4 dBu/1,2 volt over 1 kohm load.
Loss from input to balanced XLR output:  -15 dBu over 1 Kohm load
Loss from input to amp output: 0 dBu/unity gain over 100 Kohm load, -6 dBu over 10 Kohm
Frequency response: flat from 20 Hz to 100 kHz, 10 Hz to 200 kHz +/- 1 dB
Distortion: 0.05% 40 Hz - 20 kHz (0,01% 20 Hz) un-weighted, 100 mV out over 1 kohm load. 
Noise figure, shortet input: -106 dBu weighted CCIR 468-2/QP (add only 4 dB noise to the mic preamp) 
Max current consumption: 3,5 mA at 48 volts phantom powering.

Active DI box 1975 , Bo HansÚn DI box , Bo Hansen DI box , DIY DI box , build a DI box , bygg en DI box ,  bygg en line box , direct box ,  line box , tappbox , direct injection box , aktiv DI box , active DI box , passive DI box , transformer DI box , line driver, impedance converter,


one of the first
original versions
around 1975
old box inside.JPG (177657 byte)


DIY versions 2010

mounted in the lid 
Deltron 461-0040 box
small box outside.jpg (359499 byte)
front and card.jpg (85250 byte)

mounted in the box
Hammond 1590-TBK
big box inside.JPG (155912 byte)

thanks to Lukas Meier
who etched the first
black prototype pcb

old orginal
Active DI box 1975.jpg (241074 byte)

new schematic
the pcb-layout
thanks to
Ray Thomson
Active DI-box 1975, new schematic.png (150175 byte)

the first pcb-layout
january 2010 rev.1
layout by
Luny Tune

no longer for sale,
only used as 
model for

silk V9.jpg (270222 byte)


the new pcb
may 2015
same as rev.1
except some small
changes in
placement and text

BoDI-rev2PCB-01.gif (18191 byte)
BoDI-rev2PCB-03.gif (23466 byte)

this new pcb layout
is a collaboration
between my self,
Luny Tune and
Volker Mayer

ordering of PCB
can be done here
















Lundahl LL1538Lundahl LL1538.jpg (9487 byte)Lundahl LL1538 5-1 config..jpg (8957 byte)



Haufe ST8456
Haufe ST-8456 pin out.jpg (24137 byte)


OEP A262A3E.jpg (229553 byte)

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API 312 mic amp with DI input modification

click on images 
to enlarge


I discover the API 312 mic amps sound quality in the middle of 1970's, when I use them on some recording productions in the orginal Bohus studio from 1975.

My old friend and then partner Ake Eldsater purchase 8 units of these direct from API old factory in 
New York, Long Island when he where on visit around 1975.
This old pre's is still used in recording productions here in Sweden, and have same 2520 op-amps. 

The 2520 discrete op-amp in combination with API 2622 (or Jensen JT-110) mic input transformer and the API 2503 "quad fillar" output transformer, give the 312 mic preamp a unique sound with a own personality, ok some time it have a hard and a bit dirty character, in this time one of the better sounding pre┤s on drums and electric bass.

Also we modify one of the 312 amp cards, so we can use it as a DI-box with hi-Z input. and use it close to the base player with short guitar cable, and then from line output direct in to the control room via a Urei LA-3 leveling amp/compressor and then stright in to a Ampex MM-1100 24 channel machine.

We take away the mic-transformer and then add a 1 megohm resistor direct to the +non invertet input to ground, and a 0,1 uF polypropylene capacitor in series with one meter guitar cable. (of course, the 312 card was included the orginal 22 kohm reverse logaritmic gainpot)

This modification I have done here, is a further development from 1970┤s orginal version, and I have add -10 dB pad, input protection, switching for mic and DI mode, ground lift, variable monitor amp output, line and mic level output.

This is a graet DI-box, try this on bass guitar, and you have a very nice bass sound.


orginal schematic
API 312 mic amp.jpg (232802 byte)

modified schematic
API 312 DI mod..jpg (200915 byte)

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API 325 line amp with DI input modification

click on images 
to enlarge

Here can you do same hi-Z modifications as the 312 amp card, 
but with out the microphone input mode.

orginal schematic
API 325 line amp.jpg (214962 byte)

modified schematic
API 325 DI mod..jpg (158999 byte)

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power supply for API 312 or 325 amp card

click on images 
to enlarge

Here are a power supply usable for one 312 or 325 amp card.

API psu.jpg (192768 byte)

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simple FET impedance converter

click on images 
to enlarge

A very simple construction with a minimum of components.
It was designed to get a a hi-z input on a pc-soundcard, to use as a measure input.

Because the 10 k preset for the gate bias, you can use near all n-channel JFET transistor on the market.
Adjust the preset for approx. the half of the battery voltage, measured between drain and ground.
(or set the preset for best symetrical output clipping, with the real load connected)

Of course, you can use lower or higher "battery voltage", but check the maximum Vds limit for the
FET-transitor you want to use.

FET impedance converter.jpg (34236 byte)

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48 volt phantom power supply

click on images 
to enlarge

Three well tested and short prof circuits.

Easy to build, with a minimum of componets.

Phantom PSU.jpg (301777 byte)

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updated 2021.11.14