DIY Factory - jlmsimpledi

DIY - JLM simple DI project
A project brought to you by Joe Malone of JLM Audio
Updated 27 March 2005

At Joe's site you will find all the basic drawings to get through this project.
However, we have also provide them here, just for ease.

I suggest you print the above documents and perhaps a couple of the pictures below to help with identification and orientation of the components.

Supplied in a plastic bag is a neat little pile of components and very small but quality PCB. Not the easiest things to solder are the JLM boards as they are quite small and compact but with a little care and without rushing, the boards can be soldered cleanly. This complete project was was assembled in around an hour, including drilling and fitting in the box.

I usually fit the components in order of increasing height, so the resistors come first. Then fit the IC socket followed by the small capacitors and then the 6.5 mm socket and finally the three electrolytic capacitors. Do use a Multimeter to check all components before fitting. This can practically eliminate the need for trouble shooting later.

Above is the blank PCB (bottom and top are show). Only one PCB is require for a single channel DI. The simple DI does not require all the components to be used and I have left out the screw terminals and the IDC connector to keep things even more simple and to have all connections soldered for reliability.

The Simple DI project requires only the output wiring and the addition of the battery connection, a power diode for reverse power protection and the 6.8k resistors for Phantom Powering.
Please note: Joe suggests an op-amp change to TL074, if you are to use the phantom power option.
The above picture (from Joe's site) has been edited to just show the output and the power connections. As I said before, I chose to leave the screw terminals and the IDC connector out for simplicity. Note that the output will instead of going to the Pre-amp, will go to the output of the DI box , a male XL3. A SPST switch will provide an earth lift. The use of the earth lift will mean the phantom can not power the unit and it will revert to battery power.

On the subject of battery power, a diode is used to provide some reverse voltage protection. I placed the diode nose first into the pad labeled 48v and soldered my RED lead to the back of that and the negative terminal of the battery is connected to the ring connection point on the PCB. Looking at the top side of the PCB (above) you can see a pad marked R . The use of the Ring connection on the 6.5 mm plug provides a convenient switch when a mono guitar jack is used. The Ring is connected to shield as the jack is inserted and so providing a path to earth to complete the power circuit and so turn the DI box ON .

Once the power is ready and before fitting the IC check that 9 volts appears at pin 8 and 0 volts on pin 1 of the IC socket. Fit the IC in the socket and you are now ready for a test.

You probably headed to this page because you are looking for a DI box so it might seem a strange time to ask why, as you have already decided you want one. However, it does give the chance to both explain what a DI box does and how it may help.

A Stratocaster guitar can not drive in to a low ohms input and generally even 100 k ohm may seem too low for some very weak output guitars. Most guitar amps are high input impedance and generally in the area of 1 to 2 meg ohms and the average Mic-pre is around 200 ohms. Clearly the Strat can't drive the average mic-pre directly and when the attempt is made, the level and sustain of the guitar is not at all normal. Yes the vibration in the strings is affected by the termination of the pickups and the energy is sucked out and the sustain is gone. So we need to get from a I meg ohm system to a 200 ohm system. Impedance Conversion is what some people call it. The object is to pass the same level but increase the current available so as to pass more energy. Sounds complicated but no more complicated than a power amplifier providing more current as you add speakers to your PA system. The voltage swing remains the same but the current demands go up as you add more speakers or reduce the impedance of the speakers and so the number of Watts increases.

Above you can see my trusty Passive VU meter (another cool DIY project) and an ordinary Digital Volt Meter. The right hand picture show the output of my signal generator which is unbalanced and does suffer into loads less than 1 k ohms. The output into the meter is set to 0dBV which is 0.774 volts RMS (0.776 is close enough). I should label the switched on the meters so it makes more sense. I'll talk you through it. The Passive VU meter has two switches. The left switch is three position and selects between 0dBv, +4dBv or +8dBv. Currently set at 0dBv. The second switch terminates the line with a 600ohm resistor. The picture on the left is UN-Terminated. The picture on the right shows the meter Terminated and you can see the output of my signal generator does fall by more than 3dB and the DVM shows 0.493v RMS. This sort of result may be similar to the average Synthesiser or domestic CD player. A Stratocaster guitar will suffer even more. 200 ohms is much easier than 200 ohm Mic-pre input.

The pictures above show a similar set up but with the DI box in circuit. You will note that there is a very slight increase in level from 0.776 to 0.838 volts RMS. This is not intended and could be because the VU meter is presenting a load to the signal generator and the DI box at 1.8 to 2.2 meg, isn't presenting a load to the signal generator. The op-amp on our DI is set-up to be unity gain. Although there is still a level drop when terminated, it is only by 0.03 volts.

As I didn't want to change to a TL074 and did want to keep the Burr Brown OPA2604 to a TL074, I chose to go without the phantom option.
In addition to this, after a little testing (see chart below) I chose to go with two 9 volts batteries for 18 volt power. During testing I had soldered and unsoldered a few pads and made a bit of a mess with flux and gunk so a clean up was required and a little reconfiguring of the way I have wired the battery leads.

Above is the completed DI PCB with the power leads with some wire through the unused mic input pad/holes. The yellow and black twisted pair is the loop cable for the extra 6.5 mmm connector that provides the loop through to the instrument amp. Remember that the PCB mounted 6.5 mmm connector provides the power ON/OFF switch.

Above you can see the complete set of parts that need to be fitted into an appropriate box. Finding a box can be more difficult and frustrating than any other part of the DIY process. Indecision is you worst enemy and finding a balance of convenience, availability and easy of build. Usually a trip to Jaycar on Sunday afternoon will give a result but even though I did purchase a couple of small boxes, I found nothing for this project. These boxes will come in handy for other future projects here at the factory. So I fumbled through some catalogs and then finally my surplus bin to find a likely candidate. An Eddystone120 x 95 x 34 mm. This gives some shielding a neat package to sit on top of your guitar amp.

I like to use masking tape to help mark out the metal work. It gives a clear surface that both pens and pencils can be easily seen. Markings like front back and connector type and perhaps even drill size, anything to help and eliminate mistakes.

Yes a very ugly yellow paint that was on special at my local Bunnings Hardware and I thought if I make it ugly enough I may just be able to hang on to a bit longer than I usually do. Even so a mate dropped in just before heading off to a rehearsal and spotted it ...
" Can I check this out for ya Kev ? "
" NO! the paint is still wet ... don't touch "

The chart below shows both 0dB and clip levels for three typical loads, at both 9 and 18 volts. The 18 volts is obtained by using two two 9 v batteries. I have not done any 48 volt operating tests. UN-terminated is an example of operation into a high impedance input like a Guitar Amp or some stomp boxes. The 600 ohm is an example of many Pro effects units and the 150 ohms is an example of many Mic-pres. This 150 ohms is likely to be the typical use of a unit such as this. Please note that 0dB is very high for the average Stratocaster style guitar but is quite possible from many Keyboards and Synthesises. Passive and Active Bass Guitars can fall anyway between the the Stratocaster level and more than 0dB. We can clearly see that the headroom even in to 150 ohms is very good.

Parts

1 IN4002 diode - ZR1034 - $0.84
2 x 6.8k resistors
1 x 6.5mm socket - Jaycar PS0192 - $1.95
1 x SPST switch - ST0335 - $2.20
1 x XL3 male - PP1023 - $4.95
1 x Metal Box - Jaycar - HB5064 - $14.35 - 119 x 93.5 x 56.5 mm (a little large and I didn't end up using this one)

For DIY'ers that want to take things further and go for a transformered output, JLM has a likely choice. A transformer can give even more isolation and the full floating balanced output. It is also possible to add a second transformer to solve some very tricky double destination situations that do come up occasionally.