# Calling all electronic wiz's! (iaresee, mhammer, wild bill et al...)



## kw_guitarguy (Apr 29, 2008)

Hi everyone,

I have assembled (with LOTS of help from Greco) one effects unit, I repaired a cable the other day, and am now thinking I would like to build a passive DI. I need a new DI box and thought it would make a good project.

A few questions: 

1) Does this look like a suitable schematic?










2) If not, does anyone have another one?

3) How affordable would it be to build? I mean, I can buy a basic passive DI for $40.

4) Would it make sense to combine the DI in the same enclosure as my A/B pedal? (would need an enclosure big enough I guess...)

I think that's all my questions for now.

Thanks!

~Andrew


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## kw_guitarguy (Apr 29, 2008)

After some more reading, looks like the transformer is the key to a quality and they can get expensive?? If so, I may just buy a DI instead of build one...


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## greco (Jul 15, 2007)

Hi Andrew:

I can't answer your questions (although I feel confident enough to make some "somewhat educated" guesses).

If you decide to go ahead with this, I have some of the components in the schematic and could get you started with an enclosure and the board.

If you need components, I would start by going to Orion Electronics on Lancaster St. (near Victoria St.)

Cheers

Dave


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## greco (Jul 15, 2007)

kw_guitarguy said:


> After some more reading, looks like the transformer is the key to a quality and they can get expensive?? If so, I may just buy a DI instead of build one...


You and I were typing at the same time. 

What transformer do you need?
An 18VDC power supply?

Cheers

Dave


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## mhammer (Nov 30, 2007)

Consider the Craig Anderton-designed Direct Interface, the documentations and schem for which can be found here: http://www.paia.com/talk/viewtopic.php?f=6&t=91

It's not necessarily "better" but it provides more information with respect to construction and such. You may find that useful. Check out the entire site for other very useful manuals, too.


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## kw_guitarguy (Apr 29, 2008)

Thanks for all the info Dave!

The schematic I posted may be incorrect, as I would be looking for a passive DI, not an active DI. Would I need a transformer for a passive DI? I know I wouldn't need power.

These transformers are apparently the best:

http://www.jensen-transformers.com/db.html

The Radial JDI DI box uses them...

~Andrew


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## kw_guitarguy (Apr 29, 2008)

Thanks mhammer!

I have found this one as well:










~Andrew


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## greco (Jul 15, 2007)

Andrew..let me know what you decide and I'll help you as best as I can.

mhammer's link (to the .pdf) is very informative and thorough.

Cheers

Dave


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## Guest (Nov 13, 2009)

kw_guitarguy said:


> The schematic I posted may be incorrect, as I would be looking for a passive DI, not an active DI. Would I need a transformer for a passive DI? I know I wouldn't need power.


You spotted it, eh? I was going to point out that that DI uses opamps and is active.

A transformer decouples two electrical circuits -- they are no longer physically connected. It also helps you step up or step down the voltage. The induced voltage in the secondary coil is based on the voltage change in the primary coil multiplied by the ratio of the coil windings. Make sense? 

In audio applications we use transformers to help us do impedance matching. Taking high impedance instrument output and matching it to a low impedance mixer input.

Edit: here's a gap in my knowledge. I always thought the transformer in a DI was low windings on the primary, high windings on the secondary -- so the signal out was bigger than the signal in. I though the guitar signal was too weak to play nicely at a low impedance source. But that schematic above has the opposite setup -- the windings on the mixer side are less than the windings on the instrument side so it's a step-down in voltage, not up. Maybe I should read CA's PDF? :smile:

The two signal lines plus the ground are just another means to reject noise: one signal is in phase, one signal is out of phase. Any noise introduced on the line is introduced equally to both the in phase and out of phase signals. So if you phase shift the out-of-phase signal and combine it with the in-phase signal they add, but the noise is now out-of-phase, so it subtracts. Noise rejection is the end result.



> These transformers are apparently the best:
> 
> http://www.jensen-transformers.com/db.html
> 
> The Radial JDI DI box uses them...


They are really nice. So are the Radial boxes that use them. I know a lot of people go on about CountryMan DIs but I love the Radial DIs and dig that they're Canadian owned and made.


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## greco (Jul 15, 2007)

iaresee said:


> The two signal lines plus the ground are just another means to reject noise: one signal is in phase, one signal is out of phase. Any noise introduced on the line is introduced equally to both the in phase and out of phase signals. So if you phase shift the out-of-phase signal and combine it with the in-phase signal they add, but the noise is now out-of-phase, so it subtracts. Noise rejection is the end result.


Read this and took 2 Tylenol extra strength....LOL

Cheers

Dave


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## kw_guitarguy (Apr 29, 2008)

WOW!.....Ummm.....since I didn't understand the majority of iaresee's post (thank you though for posting it!!!) I think I may be in over my head.

So iaresee, mhammer, in your opinions, is it worth it to build one? The Jensen transformers look to be about $90 CDN.

~Andrew


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## Guest (Nov 13, 2009)

greco said:


> Read this and took 2 Tylenol extra strength....LOL


Gah. I hate when I can't explain things well. Maybe Wikipedia says it better? See: http://en.wikipedia.org/wiki/Balanced_line


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## Guest (Nov 13, 2009)

kw_guitarguy said:


> WOW!.....Ummm.....since I didn't understand the majority of iaresee's post (thank you though for posting it!!!) I think I may be in over my head.


You're not! I just didn't explain it very well. Don't let my ramblings scare you out of this.



> So iaresee, mhammer, in your opinions, is it worth it to build one? The Jensen transformers look to be about $90 CDN.


Shop around. Maybe you can find some used for less? A used Radial DI can be had for about $100. I know that's what I sold one of my pair for. So $90 for just the transformer seems a bit steep. You could also start your build with something less than a Jensen and swap one in later.

The advantage to doing an active DI is you can use opamps which are many, many times cheaper than a carefully crafted audio transformer like the Jensen. Does it absolutely have to be passive?


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## kw_guitarguy (Apr 29, 2008)

It doesn't have to be passive...I just hate having more wall warts to lug around!

However, If I can do it cheaper as an active, that works for me 

~Andrew


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## greco (Jul 15, 2007)

iaresee said:


> Gah. I hate when I can't explain things well. Maybe Wikipedia says it better? See: http://en.wikipedia.org/wiki/Balanced_line


Ian...you explain things very well. Sorry that you possibly misunderstood me (and my often very warped sense of humour).

Many of us have complicated things to try and get our heads around in our professions (neurophysiology damn near did me in)...but electronics theory goes beyond what I think any normal brain should be capable of understanding. My hat is off to you, Sir...:bow: This is likely beacuase I have no formal training and have tried to teach myself in a very casual/unstructured manner. 

I have learned a lot in this forum, from yourself and others, and I owe many thanks to all.

Cheers

Dave


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## Stickman (Oct 27, 2009)

Cost-wise, I think I can shed some light. Look at it this way for the active one:

Hammond box: $10
Jacks: $1.25 x 3 (including DC input)
XLR Jack: $3 (???)
Switches: $1 x 2
Components: $3 
Perfboard: $2

Which is something in the range of $25. At least it doesn't have any pots as they run the cost up about $2.50 each counting the knob. Or a footswitch, which will run you $5 or so. So you'll save $15 off the $40 price of buying one off the shelf. If you can't source all the parts out locally, you'll have to add a few more dollars for shipping. You can get all that stuff in Toronto/Mississauga, but I'm not sure about KW.

I think the general rule of thumb is that any stompbox is going to start off running about $30 without even thinking about what's inside it. If it has an outrageous number of pots or footswitches, or bizarre components like germanium transistors or NOS RC4558 chips made in a particular factory in the 70s, the cost will go up from there. 

For the passive version it looked to me like those transformers were running around $70. The Radial DI looks like it costs around $100.

Just a personal opinion here, but I don't think I'd find a DI box a great deal of fun to build. There's not really any room to tweak it or play around with mods and tone changes, which I think is half the fun of DIY stompboxes. It would be way more interesting to build something like a RAT clone and fart around with different kinds of clipping diodes, tone stacks and stuff like that.


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## greco (Jul 15, 2007)

kw_guitarguy said:


> It doesn't have to be passive...I just hate having more wall warts to lug around!
> 
> However, If I can do it cheaper as an active, that works for me
> 
> ~Andrew


If others feel that it won't be a problem, we can build the power supply into the enclosure. My main concern is that I would need to know if it needed special shielding or something. 

I have some power supply options that *I can give you for free*.

Cheers

Dave


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## kw_guitarguy (Apr 29, 2008)

I agree Dave,

The wealth of knowledge shared on this board is phenomenal!

I think I will still take this project on, hopefully I get a little bit more feedback before starting!

~Andrew


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## kw_guitarguy (Apr 29, 2008)

Wow! 

Lots of replies at the same time!

@Stickman - Thanks for the breakdown. I hear you about the lack of tweaking available...this project is more from a soldering practice, schematic reading practice and lack of funds prospective 

@Greco - You will certainly be asked to participate!  Once I/us get a better handle on what's required we can go from there.

~Andrew


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## greco (Jul 15, 2007)

Stickman said:


> Hammond box: $10
> Jacks: $1.25 x 3 (including DC input)
> XLR Jack: $3 (???)
> Switches: $1 x 2
> ...


Andrew...I have most of the stuff listed above.

Now you just have to decide if you want to give this build a try. 

Can you get these as a kit? (i.e., without an enclosure or power supply)

Cheers

Dave


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## greco (Jul 15, 2007)

kw_guitarguy said:


> @Greco - You will certainly be asked to participate!
> ~Andrew


The LCBO will have some "components" that we will need.

There is one conveniently located about 3 blocks away.

Cheers

Dave


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## kw_guitarguy (Apr 29, 2008)

Hi Dave,

I took a look, and there are some kits, but the prices appear to be quite high...I took a quick look at Mouser Electronic's website, and the parts don't seem too expensive to buy individually.

I am sure the LCBO will have what we are looking for 

~Andrew


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## greco (Jul 15, 2007)

Another place that is worth going to is Sayal Electronics (across from the Toyota factory).

The problem with ordering is the shipping costs.

It is up to you, obviously.

I'll have a look around at the LCBO...just to be sure.

Cheers

Dave 



kw_guitarguy said:


> Hi Dave,
> 
> I took a look, and there are some kits, but the prices appear to be quite high...I took a quick look at Mouser Electronic's website, and the parts don't seem too expensive to buy individually.
> 
> ...


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## Guest (Nov 13, 2009)

kw_guitarguy said:


> It doesn't have to be passive...I just hate having more wall warts to lug around!


That first schematic you posted to used 18VDC (so two 9Vs in series) or phantom power from the board. No wall warts to lug around if you build that.


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## kw_guitarguy (Apr 29, 2008)

Hi Ian,

Does it look like a good schematic overall?

Thanks!

~Andrew


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## Guest (Nov 13, 2009)

kw_guitarguy said:


> Hi Ian,
> 
> Does it look like a good schematic overall?
> 
> ...


Argh. I had a nice answer written and then Firefox crashed. So here's the crib notes rewritten. Sorry. 

Does it look alright? Yes. Will it work _well_? I can't say. I'm not very good and judging operational characteristics from schematics. TL072s, as long as you keep them in their linear operating range, should be nice and transparent: what goes in, goes out. Buy a dual IC package so you get a matched pair. Otherwise you'll have a nightmare of a time trying to align the two signals. They need to be exactly 90 degrees apart in phase for this to work and a matched pair makes that easy to do. Also means one set of power and ground pins to deal with.

I highly recommend breadboarding that circuit before committing it to perf board. You can simplify it a bit when breadboarding: cut out the line/speaker selection at the front end of the unit. It's not likely you're going to want to plug this inline with a speaker right? That's a setup I'd venture down cautiously. You can also lose the filter on the ground pin (pin 3) of the XLR plug if you like. Handy to have, but for testing purposes not strictly necessary.

If you have a scope and a signal generator you can test the circuit performance pretty readily: the output signal should look like the input signal, just lower peak-to-peak spread. (Again, I'm revising how I think of DIs based on that second schem -- someone please fill in my gap here, see my first post in this thread)


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## greco (Jul 15, 2007)

iaresee said:


> Argh. I had a nice answer written and then Firefox crashed. So here's the crib notes rewritten. Sorry.
> 
> Does it look alright? Yes. Will it work _well_? I can't say. I'm not very good and judging operational characteristics from schematics. TL072s, as long as you keep them in their linear operating range, should be nice and transparent: what goes in, goes out. Buy a dual IC package so you get a matched pair. Otherwise you'll have a nightmare of a time trying to align the two signals. They need to be exactly 90 degrees apart in phase for this to work and a matched pair makes that easy to do. Also means one set of power and ground pins to deal with.
> 
> ...


Andrew..for amatuers like us, this bite seems like it is getting bigger and bigger to chew.

Dave


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## Guest (Nov 13, 2009)

greco said:


> Andrew..for amatuers like us, this bite seems like it is getting bigger and bigger to chew.
> 
> Dave


You guys will do just fine. Do you have a breadboard? It's a great $20 spent. Lets you wire up a circuit without burning your fingers. Part-wise there's only a handful of physical parts in that circuit to place. It'll breadboard easily.


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## greco (Jul 15, 2007)

iaresee said:


> You guys will do just fine. Do you have a breadboard? It's a great $20 spent. Lets you wire up a circuit without burning your fingers. Part-wise there's only a handful of physical parts in that circuit to place. It'll breadboard easily.


I have a breadboard, function generator (same as signal generator that you mentioned in a previous post? ... I assume/hope) and a scope.

Thanks for your support and vote of confidence that we will "do just fine"...seriously...we WILL need it...LOL

We'll let you you know how it turned out.....in 2012 or so...LOL

Cheers

Dave


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## Guest (Nov 13, 2009)

greco said:


> I have a breadboard, function generator (same as signal generator that you mentioned in a previous post? ... I assume/hope) and a scope.


I thought you had all this stuff. Yea, you're all set -- you can build and test without risking any gear.


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## Stickman (Oct 27, 2009)

iaresee said:


> Argh. I had a nice answer written and then Firefox crashed. So here's the crib notes rewritten. Sorry.
> 
> Does it look alright? Yes. Will it work _well_? I can't say. I'm not very good and judging operational characteristics from schematics. TL072s, as long as you keep them in their linear operating range, should be nice and transparent: what goes in, goes out. Buy a dual IC package so you get a matched pair. Otherwise you'll have a nightmare of a time trying to align the two signals. They need to be exactly 90 degrees apart in phase for this to work and a matched pair makes that easy to do. Also means one set of power and ground pins to deal with.


A TL072, by definition is a dual op amp. I'd suggest socketting up in case you want to audition some other op amps in the circuits. I think that the OP52s (????) are maybe a little bit more hifi than TL072, something that might matter in a DI box.

If figured this was probably more of a "get some practice soldering" project than anything else. It should be pretty good for that. Have fun!


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## Guest (Nov 13, 2009)

Stickman said:


> A TL072, by definition is a dual op amp.


You're absolutely right. The TL071 is the single amp version.


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## Stickman (Oct 27, 2009)

And the TL074 is the quad version. I've got a couple of those in my box of bits. They're handy for building EQ circuits.


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## greco (Jul 15, 2007)

Stickman..if you don't mind me asking...are you an EE?

Thanks

Dave


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## Stickman (Oct 27, 2009)

greco said:


> Stickman..if you don't mind me asking...are you an EE?
> 
> Thanks
> 
> Dave


Oh god no. I'm a computer programmer that fiddles with this stuff. I'm really no where near an expert on this, and certainly nowhere near the experience that Mr. Hammer has. You guys are lucky to have him hanging around here.


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## greco (Jul 15, 2007)

Andrew...any decision on building this?

Cheers

Dave


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## kw_guitarguy (Apr 29, 2008)

Hi guys!

Thanks for all the info.

Hi Dave, if the total parts cost is reasonable, then I think I will do it. I have to be in Ottawa for a couple of weeks, so it's looking like December at the earliest to build it.

~Andrew


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