cheap DI box?

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cheater

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Joined 2004
#1
Hi guys, I searched high and low, but just can't find: a cheap DI box that I could make for 10 dollars. Starving musician here, can't even afford a transformer.

I'd be most grateful if someone helped me find a transistor/op amp/etc based one
 
#3
Dave, thanks for the link.

The circuit doesn't seem to do any impedance matching though? What do you know about that? It sees it's just 1:1 impedance, while you want a high-impedance input... no?

I never figured that out: what's the usual impedance of the output of a bass? And a normal (consumer) line input?
 
#4
The input to this circuit is about 100k ohms and the output is 100 ohms. the output is pretty standard (usually about a 10kohm input impedance for line level inputs) however the input IMHO could be greater for a bass guitar ( impedance of guitars can range from 50kohms to average 100-200k ive seem some as high as 500kohms) as a general rule of thumb i was taught your input impedance should be 10 times (or greater) that of you output impedance. Most fx pedals i make have a 1M ohm input impedance. You could increase R4 = R2 = R5 to a higher value ( 500K ohm - 1M ohm ++) if it seems you are not getting enough signal.

here is a great link for audio related schematics / information

http://www.epanorama.net/links/audiocircuits.html

if you are not familiar with opamps try reading these articles, they should be some help.

http://sound.westhost.com/dwopa.htm
http://sound.westhost.com/dwopa2.htm
http://sound.westhost.com/articles/dwopa3.htm


Dave
 
#5
Just to reapeat what has allready been said, You can make the Input impedance pretty much anything you want by changeing the Input resistor......

I generally use a 1M Ohm resistor for the input for Guitar and Bass inputs, You can even use a Rotary switch with different Resistors so you have selectable input impedance depending on the Audio source.....

Cheers
 
#6
Hey guys! I've built the box. Or jar, rather, because it's in a glass jar for now as a prototype 😉
I've built it with an ST TL072. I haven't been able to check with the LM358 (suggested as an alternative in the article) but right now the output seems too small. Also I'll try swapping the resistors. Minion, I guess the input resistor would be R2?

Dave, you suggested making R2 (input to U1B), R4 (feed from U1A to U1B) and R5 (feedback in U1A) higher - what about R3? Doesn't that need to be made higher?

Right now the whole thing is battery-operated only (I don't own a phantom supply). Do you think that might be the reason the output is so low? Will I be able to get more output with a regulated supply - and if so, what can you suggest? Building your own? Buying? What power should that supply have?

Thanks for the great tips guys!! You are all a great bunch 🙂

Cheers,
Damian
 
#7
In that circuit there was no gain at all( gain of 1), all it did was allow for impedance matching . after looking at it in more detail i noticed a couple things i didnt like (grounding) so i quickly drew a new one up here at work. Read those articles on Basic opamp operation and you will understand what is going on, all this circuit is, is a high input impedance (R2 and R4), low output impedance(R6 and R7), and now a 2x gain. I included the formulas so you can mess around with resistor values. for the 200kohm resistor just put two 100 k ohm's in series (back to back). R4 and R2 create the input impedance to each of there respective opamps. If you increase one, match the other to the same value and then calculate the gains accordingly. if you are going to be doing lots of electronics projects i would suggest building a simple power supply to test the stuff. I included a simple supply design also, look up "Regulated dual bench supply" or check into the 7815 and 7915 regulators, also there is lm317 and lm337 for variable regulators. PM me if you got any questions and i would more then happy to try to help you in any way i can. Ooops, i better get back to work

http://i195.photobucket.com/albums/z105/zdavesf/diboxmodified.jpg


Dave

P.S. let us know what your signal chain is and what your using this for?

electric guitar --> di box --> mixer --> amp?
 
#8
Hey Dave!

Thanks a lot for the input. Sorry for the long delay between my answers; I don't have easy access to the Internet right now.

Thanks a lot for the new schematics! I have looked at them briefly and will study them, along with the op amp pages, taking them home.

I have fit the original schematics on a single 16-pin project board (3 holes x16 plus three "buses"). Very tight fit but both the DI and the voltage supply circuit fit on the real estate of a stamp! Took me a bit of time to figure out.. But this also taught me a bit about the topological structure of that circuit (where voltages come from etc - the box has two voltages in it, one is ~ 20V and the other ~ 10V - did I guess right?)

I will read the ESP articles of course.
Are you the maker of ESP? That seems a good page. It somehow reminds me of Rene Schmitz's page.

How I'm using the DI:
Fretless Bass -> DI -> line input in my trusty SB Live Value
Then that goes through anything I have running on my (again trusty) P3, and then through the (unavoidable) graphical EQ in the kX project DSP. Then out to my (..trusty) 80s radio/amp (repaired it myself!) then to my DIY 4" speakers. Done.

I think I'm gonna build some analog boxes afterwards? 🙂

But for now let me build the DI box and have it working allright 🙂

BTW, how do I know the input/output are clipping?
Any idea what kind of clipping it will be? Hard, soft?
I'm just gonna have to try.. 🙂

Thanks again, Dave
 
#9
The supply voltages of the opamps are +9 Vdc on pin 8 (Vcc) and
-9 Vdc pin 4 (Vee). I am not sure if that is the 20 v and 10 v voltages that you are talking about? Good luck with that new circuit and remember to double, no triple NO QUADRUPLE check your wiring to make sure everything is correct, i dont know how many times i have cursed at a circuit i made because i forgot one wire. All the best to you
and don't worry about the long time in between posts, we are all busy and cant simply devote our lives to this stuff (yet I try desperately, this is my carrer and my hobby 😉 wow i am a loser!)

keep us all informed on your progress, if you get it built up and start using it in your actual rig, get your hands on a digital camera and show us some pictures

Cheers

Dave
 
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