DIY Factory

Country Hick Di Box
A project brought to you by Bill Yacey
Updated 31 July 2004

Born from a simplified version of an obvious commercial unit but with a couple of omissions for simplicity.
This one can not be powered via phantom volts like the original but may end up being powered at a higher voltage which may offer a little more headroom. This may also cause some problems along the way.

Either way this should be a cool learning experience.

If you want the phantom powered option , then do go buy the original as it is a fine product and worthy of your tool kit.

The above circuit was the fist scratchings of one of the DIY Factory Team members and see below that it was transferred to CM2000 for a little simulating.
Note: some of the resistor changes, R8 and R7 and R9 for ease of supply.
R3 at 10 Meg may be a little difficult to obtain for some people so a multiple resistors method may be included on the PCB

Simulated fine.
This could be helpful in predicting voltages around the circuit in the event that some trouble shooting may be required.

A prototype PCB was knocked up for testing.
This one has no onboard output trafo but it is expected that this will be an option.

PDF Files

Generic_DI.pdf
same as the above circuit

2N4401.pdf
transistor data

2N5459Fairchild.pdf
FET data

Some thoughts

This DI box is well suited for recording as well as stage use. It is the closest thing that I have found to the ideal "What goes in is what comes out".

The circuit is quite simple. R1 and R2 create a voltage divider that is switchable enabling speaker level signals to be padded to a usable level for the FET input. R4 and R5 create a voltage divider to bias the FET at ½ of the B+ voltage. C2 creates a virtual AC ground at the junction of R4 and R5. R3 determines the input impedance of the DI box at 10M ohm. D1 and D2 dump any over voltage condition to ground or the B+ rail to protect the FET gate. The FET in turn drives the base of Q3, a class A buffer. Q2 works as a constant current source for Q3. The AC component of the signal is coupled by C3 to the output transformer, which balances the output of the direct box.

The transformer chosen will alter this slightly, and I have tried various transformers with consistently good results. Any reasonable quality audio transformer that has a turns ratio of anywhere from 2:1 to 8:1 should work nicely provided it is wound for lower impedances in the 600 to 10,000 ohm range. Due to the higher operating signal level (-20 to 0 dB) compared to microphone level (-30 to -60) the transformers tend to exhibit more linear frequency response. The transformers I have tried were just from my junk bin, and they provided very good performance, so it isn't necessary to spend a lot of money for really high end transformers if this doesn't fit your budget.

The other parts in the high impedance buffer circuit may be had for very little money as well. For about $30 U.S. you should be able to buy all the parts as well as an enclosure to house it in. There really isn't anything esoteric about this circuit, and similar circuits can be found on the net with almost identical configurations. This particular circuit is set up as a buffer, that is to say with a gain of 1. Additional loss is introduced with the transformer to prevent mic preamp overload.

Sound wise, this direct box excels with high impedance magnetic guitar and bass pickups, as well as piezoelectric transducers. The 10 megohm input impedance does not load the transducer, thus maintaining the natural frequency response of the transducer. Instruments such as keyboards and other electronic derived output signal sources work happily into the high impedance as well. With the additional input pad shown on the schematic, speaker level signal may be fed into the direct box input enabling the user to capture the sound of the guitar amplifier electronics (albeit without the sound of the speaker).

Just a note of caution: If you choose to feed the output of an amplifier into the direct box, ensure that the speaker load is still connected to the amplifier directly or via the loop through jack on the DI box, otherwise your amplifier is likely to suffer damage - especially tube output amplifiers. Ignoring this warning may create a new task, DIY output stage rebuild.

Bill Yacey

Parts List

Parts list for:
Country Hick DI Box

qty Label-Value PACKAGE DESCRIPTION Designation (JAYCAR Part# - to come)

Capacitors

1 0.02uF RAD0.2 C1
1 1uF RAD0.2 C2 - Tantalum to minimize leakage
1 10uF RAD0.2 C3

Resistors

1 10k 1W AXIAL0.4 R1

2 150 AXIAL0.4 R2
2 1000k AXIAL0.4 R3
2 2500k AXIAL0.4 R4 R5
2 2.7k AXIAL0.4 R6 R8
2 100k AXIAL0.4 R7 R8
2 270 AXIAL0.4 R10
2 10 AXIAL0.4 R11

Silicon

1 2N5457 TO-92B Q1
9 2N4401 TO-92B Q2 Q3
4 1N4148 DIODE0.4 D1 D2 D3 D4

Miscellaneous

4 screw terminals 2 way ST1 ST2 ST3 ST4
2 Battery housing 9V BH1
1 Eddystone Box ? mm X ? mm BH2
2 switches SPST toggle S1 S2

brought to you by the team at the Factory.

Farnell Parts numbers
682366 TRANSISTOR JFET N TO-92 2N5457 $2.22

different brands of 2N4401
358976 TRANSISTOR NPN TO-92 2N4401 $0.51
358988 TRANSISTOR NPN TO-92 2N4401 $0.51
4399705 TRANSISTOR NPN TO-92 2N4401 $0.37

More suggested parts and future enhancements

An enclosure option that looks nice and is somewhat robust is the Hammond 1455K1201. This is an extruded aluminum box that is a nice size for use as a DI box. This box has slots for holding PC boards, and has enough room with some careful planning to contain the two 9V batteries we have planned for the future.

We are planning to do some tests on the DI using some of the Dukane surplus transformers the DIY Factory has in stock. Although they are 600 ohm
to 30K and a little higher ratio than we would have liked , but as they are cheap and in need of a project we though we would try them.

Poor Man DI

Charlie knocked up a variant and had his fiend Kyle, do some custom pin striping. This is just the CountryHick circuit with an A-22 o/p trafo (because he had one !), two 9V batts for the headroom ( Kev agrees ). Made a little foam box for the batts and there is some foam attached to the lid to keep them in place. Input jack switches the power and second 1/4 inch is just straight through. Ground lift switch is there for the hummmm ....

Box is Hammond 1590C (actually 1590CBK for the black textured powder-coated
part number, that paint is pretty tough stuff!)
Input jack is Switchcraft 14B, TRS, with shorting switches on each T and R.
UTC A-22 transformer is wired 600:150. Speaker level switch was omitted.
MPF102 was used for 2N5457 with no other changes.

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this page is still under development
more will be added

brought to you by Bill Yacey and the DIY Team at :
www.diyfactory.com