Newbie question - DI box is going to be my first project

Okay - I have done my research
I am spending my hours in my white throne room readining about capacitors and reading Ethan Winners articles (very very good for us newbies)
So before spending a fortune I am going to make a DI box
I plan to build a modular desk based arounf the cassettes mentioned in another thread.
Value for money but good quality components required
I will try and make my PCB based on the eurocard
The aim is to take a unbalanced mono input from a guitar or synth and feed this though to tape
I am unsure on where to find a decent schematic for this
The main point though - is that I want to eventually build my own mixer - it will be a split line mixer (preamps and di boxes going to tape) - from tape the outputs will be going to compressors, limiters, eqs, pans and faders
So.... the Power Supply to the unit has to be compatable wiht all the other clones I plan to build

Advice, schematics, parts lists please...
(I was thinking £20-30 for the transformer)! - My instruments are worth thousands why skimp right at the beginning

Take a look at this thread:

http://www.groupdiy.com/index.php?topic=298

wherein Bo Hansén posts a link to his 1975 active DI box design, which looks to be simple to build and (according to the testimony of many in that thread) sounds excellent. You can use a variety of transformers from Lundahl, OEP or Sowter (all at a reasonable price in the UK).

I plan to build a few of these myself as soon as I get my new shed suitably equipped...

J

Incidentally here's a list of the parts from Bo's schematic to give you some idea of what's involved. Chassis not included.

Sockets:
Stereo 1/4" socket
Mono 1/4" socket
Male XLR socket

Resistors:
2 * 10k
1M
2 * 470k
100k
3.9k
2* 6.8k 1%
47 ohm

Electrolytic capacitors:
100uF 35v
10uF 35v
1uF 35v

Non-electrolytic capacitors:
0.1uF 100v
0.047uF 100v

Zener diodes:
2 * 12v/0.5w

Non-zener diodes:
2 * 1N4004

Transistors:
BC109C or BC550C
BC177B or BC560B

Transformer:
LL1538 (or equivalent from Sowter, OEP etc.)

Hope this is helpful,
James.

James,
Thanks for the nice help with the components list.

uk03878,
Good luck with your first DIY project, and if you choose to build my "1975 DI-box" please tell me if there are some problem.

--Bo

No problem. (Hope it's accurate - there are sufficiently few components that it should be!) Incidentally did you ever get chance to post a schematic of the DIY tube DI you mentioned in the previous DI thread?

cheers,
James.

James,

Yes, I have promise a FET and a tube DI-box, and it will be under the summer, the design is ready but only on lab-board so I must do a DIY style of the circuits.

--Bo

I've decided that a direct box is going to be my first "build from scratch" DIY project as well. I think I'll build Bo's circuit first and then try my hand at this Jensen:

http://www.jensen-transformers.com/as/as066.pdf

The parts for the Jensen, minus the transformer, come to about $30 US. Not bad.

Bo:

Do you recommend any particular type of non-polarized cap? (ie ceramic, polyester, polypropelene, aluminum)?

Just curious b/c I'm putting together a parts lists as I'm very interested in your DI and would love to build one (probably point to point).

Just two more questions..
Regarding the Bo circuit

I am having difficulty identifying what I should buy (even with the help above) - is it possible somebody could be very kind and post the part numbers from the RS catalog (I think I can get discount from RS - they are one of my clients and my collegues can go to the trade counter in Corby - in fact I got a job offer from RS yesterday but I turned them down!!)

Secondly - as this is a one off and my very very first - I don't really want to try and use a homemade PCB
Any suggestions for which is the best long term board for this (perfboard, breadboard, point to point) and any good web sites for instructions

Many thanks - and very very sorry for being a real newbie

I don't have any part numbers (or any time to look any up at the moment) but you should find all you need on the RS website, except for the transformer which you're probably best getting from Canford. The LL1538 is here:

http://www.canford.co.uk/commerce/productdetails.aspx?productid=23-103

They might also be a better source for the sockets.

The other components are pretty standard and the usual advice on resistors for audio applies (ie. prefer metal film to carbon). As for capacitors, RS sell the Elna Starget electrolytics which are well thought-of, so it might be worth having a look at this range for the 1uF/10uF/100uF caps in the DI. The other caps are small enough that you could get them in polypropylene. The schematic gives part numbers for the transistors and the diodes so you should find everything you need fairly quickly.

It sounds like most people are constructing these point-to-point, although I might try mine on Veroboard because I like that sort of thing! I'll gladly post a layout if I do end up doing this (won't be for a while though as I'm in the midst of moving house...).

Sorry to not be of more help,
James.

James
No problem - I think I will try and get everything from RS (I can get the OEP transformer there)
The good news is that I am 90% certain that I will be working in RS for a couple of months (starting next week) - so I will be visiting the trade counter to get discount!!!!!!!!

Greg,

It is not so important what type of 0,1 uF and 0,047 uF capacitors you use, that is ok with a normal polyester capacitor (Wima MKS02 or MKS2 series) or a PETP/polypropylene type Sprague, Xicon or BC-components "guitar amp parts capacitors"

For the 1 uF, 10 uF and 100 uF electrolytic capasitors you can use Panasonic NHG series, it is a good capacitor and easy to get, but other brands like Rubicon, Nichicon, Jamicon, ELNA, BC-component general purpose series are also ok for this project.

Regarding transformer,
The best choice quality/price are Lundahl LL-1538 (or LL-1538XL for higher level) in 5:1 configuration.
Try also older second hand microphone transformers as UTC, Triad, Sescom, Altec, OP-Amp Labs that have a ratio around 1:5 to 1:10, because they can have a nice sound for a base or synth DI application.

--Bo

Since this is your first project, and you don't want a PCB, point to point on a perf board is probably your best option. The perf board eliminates having to drill holes, since they're already there! PCB's are a little easier to troubleshoot (everything's labeled), but perf board is great for anything below radio freq. Vero board is just like a PCB, but predrilled, so it's not really point to point. Point to point is nice, because it eliminates having to search for microscopic solder bridges, between pins, which can cause some real headaches.
Something I never understood about Radio Shack, was their PCB kit; it has a drill bit, that is freakin' huge! There's no way you could get the right IC spacing with that bit! (OK, maybe with a microscope!) I solved my problem with the drill bits they use to clean MIG welders. I suppose jewelers might have them, as well.

Hello!

I'm also planning to build Bo's DI-box as my first project, but I have a question. Does this box need an external power supply? I'm a bit confused (yes I'm a newbie :wink: ) about the schematic. It says +24 V(3,5mA) at the top. Does this come from the phantom power of the micpre I'm connecting the box to, or do I need some kind of external power source?

/Thomas

Here's a PCB layout I did for Bo's DI Box, using a Lundahl LL1538xl. It's sized and arranged to be a retrofit for a Whirlwind EDB1 direct box.

3.2" x 2.9" (81.28mm x 73.66mm)
http://www.moosapotamus.com/boDIBOXpcb.gif
EDIT: corrected pin spacing for LL1538

The EDB1 sounds like shit. But, I picked up a couple of them for something like $7.50 USD each on eBay. So, I'm going to use the case, jacks and switches, but replace the PCB with Bo's DI circuit (thanks, Bo!).

All the jacks and switches are PCB mounted. The EDB1 has a couple of switches (pad and ground lift) that I'm going to use for phase (switching pins 2 and 3 on the XLR out jack) and a hi cut/norm/lo cut switch that will go in place of the 10uF cap that couples into the transformer.

My idea for the hi cut/norm/lo cut switch is to have two caps, like 10uF (C1) and maybe 0.1uF (C2), or something. The lo cut position would put the small value cap in the signal path. The norm position would put in the larger cap. The hi cut position would put in the larger cap followed by the smaller cap going to ground. Makes sense? I'd be real interested to know how folks think this might work out.

Also, added a resistor coming off of the 24V power rail (Rled, just below the 100uF cap) so a power status LED can be added. And, there's also a pad between the two 6K8 resistors for feeding 24V directly, instead of using 48V phantom power.

If anyone is interested in checking this PCB layout against Bo's scheme, please let me know if you find any mistakes.

Thanks!
~ Charlie

Hi Charlie,

Nice PC-board you have done, it is great here on the lab forum because all thing have a natural development and the DIY applications become more and more complete with improvments, PC-card, frontpanels and cases, I like this "relay race development"

But I must do some comments regarding LED, hi and low cut filter.

If you add a LED from the +24 volts rail to ground via a series resistor to get the normal LED current around 5 to 10 mA, the +24 voltage come to drop very low and this result to a very bad headroom.
The best way to add a LED are to connect a small LED (good light with low current) and cut up direct after the two 6,8k resistor and insert the LED between the 6,8k and the rest of the voltage rail include the 100 uF capacitor.

Low cut filter, yes you get bass roll of if you change the 10 uF farad transformer coupling capacitor to a smaller value, but the adaption/damp can be strange on some transformer types.
Maybe it can be better to play with the 0,1 uF input capacitor and change to a smaller value (maybe some nF)

The best way to add a hi-cut filter in this circuit are on the input side, after the 10 kohm resistor, a capacitor around some nF in parallel with the two Zener diodes, of course the slope and loss becomes on what type of instrument source you use.

The absolute best way to add a hi and low-cut filter are between the two emitter followers, but this is a re-design to get a new bias point for the Q2 and some other things.

You can do a lot of modification even in a simple circuit like this, but I recommend that you use it in orginal because this circuit is well balanced between headroom, current consume, symetrical clipp, transformer drive capacity and of course noise and frequency response, so all modification change the soul in this DI-box.

--Bo

Thanks for all the comments, Bo! Much appreciated! It's great of you to share this circuit with everyone, too.

I don't want to mess with this box's soul. So, maybe just a couple different values for that (10uF) coupling cap (I just can't leave well enough alone, sometimes :green: ). And, I think you even suggested trying that in another thread (47uF or even 100uF, maybe).

I also thought an indicator LED would just be cool to have. But, with only 3.5mA on the 24V rail (as shown in your scheme), I see what you mean. I'm not sure I understand how you are saying it could be done, tho. Pardon me for hacking your drawing (and, for being a bit thick, at the moment), but a picture says a thousand words... Is this what you mean?



Thanks, again!
~ Charlie

Charlie,

Yes, you get it,

Insert the LED as you have draw in the schematic, maybe you must try some types of LED`s to get one that light up ok with 3,5 mA current.

It is ok, to change the output capacitor value up to 22, 47 or 100uF, but I think I have choose 10 uF together with the old Sowter 3678 (5:1 configuration) that I use in my orginal version (same specs as Lundahl LL-1538) and I also have same transformer as a receiver load, and this give a flat response down to 20 Hz.

--Bo

Has anyone sourced a 1uF 35V electrolytic? 1uF 50V yes, but I can't seem to find a 35V. I checked Mouser and Digikey.

Why would you need a 35V-type?