Questions about Bo Hansen's DI

[Julio]

Hello Everybody,

I've got questions about Bo Hansen's DI, which, by the way I find very good. I'd like to build a couple of them but I expect some problems.

I've tested two of them, made by a friend, with my electronic piano but when the volume was set a little too much, the sound was quite distorted. I know that the solution could be to feed the DIs with less level but as I want to use the DIs before my mixing desk which works with line levels, I don't want to loose too much level. Does a solution exist?

As my mixing desk couldn't feed phantom power through the line inputs, is it possible to modify the DI with a 9V battery?

May be my questions seems to be ridiculous for some of them but I'm a newbie in DIY. So thanks a lot for your help and your patience.

 

[Bo Hansen]

Julio,

The amp in this DI-box is a dual emitter follower and have no gain, so it will take a lot of input level before clipping, and even with a electric piano or synth with maximum output level, it is hard to overload this DI-box.

Maximum input capacity before clipping are +19dB (7,5 volt RMS), and gives +4 db (1,2 volt RMS) on the output, connected to a normal transformer or active balansed mike input. (around 1 kohm impedance/load)
And this is a lot of headroom to drive a mike input on a preamp or console.

But a electronic piano that have near line level output, connected thru the DI-box, can overload your console microphone pre-amp, evan that the DI-box have a approx. 15 dB step down transformer on the output.
(if you have a pad on your console, try if this work better)

Of course, first of all, you must check if the DI-boxes have a correct function in the amp circuit.

The demad for following test, is that the console Phantom powering can deliver +48 volt in 3,5 mA.
(the DI-box work ok with 12 or 24 volt "Phantom" power also, but with lower headroom)

Measure following points, (reference to ground) with a DVM/digital voltmeter,

1. Check that you have approx. +24 volt dc on the first transistor Q1 collector. (or + on the 100 uF capacitor)

2. If you have that, you shall also have approx. +12 volt on the Q1 emitter (crossing 100k and Q2 base) and also on the Q2 emitter (crossing 3,9k and 10uF).

And if you have this voltage indication, and also have a Lundahl LL1538 connected in 5:1 configuration, (as I have explain on my tech page) you have a ok DI-box with great hedroom.

BTW, you can easy do a -10 dB pad on the DI-box inbut, change the 10 kohm input resistor to a 2,2 mohm parallell with a 18 pF capasitor (to get flat high end responce)
Or if you want the pad switchable in/out, do same pad that you can see on my "API 312 DI-mod" http://web.telia.com/~u31617586/

Yes, it is ok to drive the DI-box from a 9 volts battery or a small dc-adapter (stomp box PSU/adapter, but you get around 10 dB less in hedroom.
I prefer two 9 volts battery connected in series to get 18 volt, or a 24 volts DC regulatet adapter to get better headroom.

1. Take away the two 6,8 kohm resistors
2. connect the battery + to the + on the 100 uF capacitor, and the battery - to - on the 100 uF capacitor, thats all.

Good luck

--Bo

 

[Yoannv]

Hi all,
I am the friend of Julio who have made the DI Box.

First, I have to say I really love the sound of these box !
For sure, it overclass my other samson DI Box, but it also clearly better than the DI input of my SPL Goldmike mk2.

But, I have also notice, like Julio, a distrorsion problem when using it with high level instruments.
I didn't take care of this, because I love the sound, and so simply turn down the level of he instrument.

But, now I read from Bo that I should get a great headroom, I'm asking myself if I haven't made a mistake somewhere (the Bo DI is one of my first DIY).

Distortion occurs with active accoustic or electric guitars, independently of gain level of the preamp after the box.
I've notice that distortion occurs more obviously, more "quickly" with my green gre (not totally finish yet) than with my SPL pre.

Transfos are Lundahl 1538.

We are going to check voltages, like Bo suggests, we'll reports results.

Thanks for your posts.
:guinness: :sam:
Yoann

 

[Bo Hansen]

Yoann, Julio,

Thanks for your appreciation of my old "1975 DI-box" design, I`m glad to hear that you build a couple of them.

When I read in your last posting that you have distrosion even when you use a regular electric or active accoustic guitar, and you also say it is independently of gain in following mike preamp.

I think I can guess what the problem is.
The two protecting zenerdiodes ZD1 and ZD2 on the DI-box input are probably wrong connected.
When this two zeners are connected in series to protect from very high input voltage, they must be connected with the same polarity together in the serial point, eiter cathode to cathode (ring mark) or anode to anode.

If the cathode are connected to the anode in serial, the input start to clipp with approx. 1 volt input level, (like a fuzzpedal but only in one edge of the signal) and this is near the peak level from a guitar with high output pickups when you play very hard, and use a preamp/Di-box with a high impedance input.

So take a look on the zener connection, and maybe you find the problem here.

BTW, I have done some add-on explanation in my first posting, that maybe can be interesting for you.

Please, tell us the result when you have check the zeners.

--Bo

 

[Yoannv]

Hi Bo,
Thank you very much for your answer end explanations.

I've check the two zeners diode, and I think that everything is right with them.

Here's the PCB I've used for the DI :

But, I have to say I have done a little mod, by adding a Led to show the presence of 48v.
The Led is after the two 6,8k resistor, maybe it will be clearer with a picture :

This mod could cause trouble on 48v, and so on the DI ? It a low current LED, and there is a 10k just before.

Anyway, we'll check voltage tonight with Julio.

Thanks again.
Yoann

 

[Bo Hansen]

Yoann,

Nice PC-card you have done, was it you that have make a similar card to fit in a passive DI-box case, for a year ago ??

Your LED modification with a 10 kohm serial resistor, steal near 2 mA current from the voltage feed, so the internal voltage drops appr. 6 volts.

If you want a LED, it is better to connect this in serial with the voltage feed, (with out any serial resistor)
In this case the voltage only drop around 1 volt, and this doesn`t matter.

Cut up the PC-trace after the two 6,8 kohm mix point, before the 100 uF capacitor, and connect the LED with the anode to 6,8 kohm resistors and the cathode to + side of the 100 uF capacitor, and you are ready.
Move "pin 2" 6,8 kohm nearer "pin 3" 6,8 kohm resistor, and you have more space for the trace cut up and the two new soldering points.

Use a small 3 mm red LED to get good brighten.

I have check your PC-layout, and it is look ok, so I can not find any more explanation for the distrotion you hear, except the four screw holes, that must be isolated from the fasten screws, so not the metal case/housing have connection with the PC-card ground.
The metal case shall only be grounded in XLR connector pin 1.
Also the two tele-jacks must be isolated from the metal case, use plastic jack types.

As I say in my first posting, the DI-box electronics will take +19 dB on the input, before clipping on the output, connected to a normal transformer or active balansed mike input, (around 1 kohm impedance/load) and this is a great headroom for a DI-box.
But even with your LED version, (with 6 volts drop) there are a enough headroom, so there must be a other mistake somewhare.

So, check all voltage points I talk about before, and if you have a oscilloscope and a tone oscillator, you can check that the output (between XLR pin 2 and 3) start to clipp symetrical wen you feed a 1 kHz sine tone,
with a level of around 6 to 7 volt RMS.
Of course, you must have the DI-box connected to a preamp microphone input with +48 volt phantom powering.

Please tell me the result, and I will carry on to help you with your distortion problem.

--Bo

 

[Yoannv]

Ok, I think I've find my mistake

After a better investigation in this problem tonight, we detect that only one of the two box had the distortion problem...

And, after a look on the PCB of the faulty box, I've found that... I miss to sold the 100k after the 2 Zener diode !!

And the sound is greatly better on the good box !

Really sorry for a such stupid mistake, and thank you very much for your patience.

Nice PC-card you have done, was it you that have make a similar card to fit in a passive DI-box case, for a year ago ??

In fact, that not really me who designed this PCB (and it's not me who make a similar card for a passive DI).
I've found a PCB version of your DI around the web, which include some modifications (phase inv. and other things). I take them out, redesign a little and go.

Thanks you very much for your advice for LED, ground, zener, etc...
I learn everyday in audio, and DIY, thanks to people like you who spend time and knowledge to everybody.

Really, thank again.
:guinness: :sam:

yoann

 

[Bo Hansen]

Nice to hear that everthing is ok now.

I think it is a good idea to change the LED connection, so you have the LED in serial with the voltage feed, this is a much better solotion, both for the DI-box total current consumption and the headroom.

And check also the grounding, I talk about in my last posting.

--Bo