Bo Hansen DI layout

[godwin]

Another picture.

 

[Bo Hansen]

Godwin,

I suppose you have already compared the three working PC cards with the PC card that is not working, that all components are in the right place, are correctly turned and have the correct value.
Also check all voltage test points, as I describe on my DI box DIY page www.hansenaudio.se/techpage.htm

If all this is ok then the next step will be to test if the unbalanced telejack output sounds right.

If it does, then there is something wrong with the Lundahl LL1538 output transformer, or its connection.

Try an additional time to solder all connection pins on the transformer.

NOTE, never cut the pins on a Lundhal transformer, after soldering, these pins are hollow and contain the transformer's thin leads from coils.

--Bo

 

[godwin]

Thank you for your reply. The unbalanced output DOES sound correct. I have another transformer on order. I will reply here after I swap it out.

 

[musika]

I really like your DI box made from thick aluminum rectangular tubing.  Can you tell me what the thickness is that you ordered?  What are the final dimensions and how much length of tubing is necessary to create one DI box?

 

[godwin]

All the tubing is 1/8" thick. The outer case is 2' x 4" x 6". The inner case is 2" x 5" x 3 & 3/4" The supplier cut the pieces to length .
6" long for the outer case and 3 & 3/4" for the inner case. I cut the top of the inner case  off with a hack saw at 1 & 3/4" then filed as needed until it slid inside the outer case. 5 #4-40 countersink 1/2" bolts mount the PC board and ground wire to the inner case.
2 #4-40 countersink 1/2" bolts secure the inner case from the bottom of the outer case. The inner case is tapped at #4-40 to receive those bolts. 

The aluminum is rough when received. I filed the edges square and smooth and polished the outside with fine sandpaper and steel wool.

I purchased the aluminum at Metals Depot online. The approximate case cost per DI Was $22.00.

 

[godwin]

Another picture.

 

[godwin]

Last one.

 

[musika]

Thanks for the info and additional pics...  you did some nice work with those and they look great! 

 

[godwin]

I replaced the LL1538 transformer with a new one and the unit is working correctly. Thank you for your guidance.

 

[godwin]

Added some labeling.

 

[godwin]

other side.

 

[Bo Hansen]

Godwin,

Nice to hear that you solved the problem and that you like my DI box design.

--Bo

 

[Di_Sousa]

Hello Bo,
I would like to use your DI box circuit with Piezo pickups that need a much higher input impedance than electric guitars.

Could I change the input impedance of your DI box to 10 Megas?
What components should I change?

Thank you so much

 

[Bo Hansen]

Whoops,

Since the input starts with a BJT transistor, which draws some current on the base, you can not go as high as possible in impedance.

However, it is possible that it runs with a 10 Mohm bias resistor, as there is quite little current through this transistor, but I want to try and do some measurment on this for you and come back later.

It is also possible to replace the first NPN transistor and replace it with an N-channel JFET transistor, such as 2SK30, 170, 246, 2N3819 or 2N4391, 2N5457  series, and then only replace the 1 Mohms bias resistor to 10 Mohm or higher if desired, without having to change something else.

But even this I want to test for you, so everything works as it should.

--Bo

 

[Di_Sousa]

But even this I want to test for you, so everything works as it should.

--Bo

Hi Bo , thank you so much for your help and for caring.

actually having the option of 10Mega input impedance would even make this DI more flexible in terms of uses.

Thanks you

 

[analogguru]

This small modification should deliver you approx. the desired input impedance:

 

[Bo Hansen]

Whoops,

Sure, lots of choices and features are always fun, but jacks with "make and brake" functions, extra switches and potentiometers, cause poor contact and error over time.

So that's why I chose to present this DI-box design in its simple version, to always work and sound good.

Why did I choose an input impedance of only 1 Mohm, when almost all other active DI boxes have 10 Mohm.

Well, there are three reasons for this, one is that my original design from 1975 has this, the other is that BJT transistors are easier to get with good quality for all of you DIY people, and the third is that 90% of all guitar and bass amps have 1 Mohms input impedance, and then you'll recognize the sound characters and the behavior of your knobs on the guitar.

But wait a while, and I'll come back with some good suggestions for you.

Although I can guess and propose a modification, I'm careful about testing and measuring it, so I do not leave something by chance.
If I suggest something, I want it to work fully.

--Bo

 

[Di_Sousa]

Although I can guess and propose a modification, I'm careful about testing and measuring it, so I do not leave something by chance.
If I suggest something, I want it to work fully.

That's totally understandable and I appreciate that

 

[Bo Hansen]

Analogguru,

Yes, bootstrap from emitter to base bias ref. resitors, rise the input impedanse a lot, theoretically about 10 times, what I remember.

But it is also important to see how electronics behave after this modification in reality.

As an example, will the transistors' work point be correct and stable, so the output still clipping nicely with max headroom ?

--Bo

 

[analogguru]

.....
Yes, bootstrap from emitter to base bias ref. resistors, rise the input impedanse a lot, theoretically about 10 times, what I remember.
.....

Exactly - bootstrapping is an old (at least since the 50's) and well established technology.
Here is some easy-to-understand article about it:
http://www.hawestv.com/amp_projects/amp_hi_z_transistor/bootstrap-preamp.htm

.... will the transistors' work point be correct and stable,.....

The DC-"work point" of the transistor will not change with bootstrapping...

....  so the output still clipping nicely with max headroom ?

I doubt that anything clipping will be "nicely" (and I doubt that a piezo pickup will be able to drive this unity-gain-circut into clipping) - but why don't you just try it out if it fits your desires ?  It is only moving one leg of a capacitor from ground to the emitter (the already installed 1µF will be at least sufficient for lo-mid-frequencies and up)

BTW, I also doubt that it is really necessary to have an input impedance of 10M for a piezo pickup, especially when the pickup should have its on impedance converter [think about 5m cable from pickup to the DI-Box with 100pF/m {= 320kOhm @ 1kHz, 32kOhm @ 10kHz}], but....