[b0b]

Hello,
looks like I can't swing my boss to get a Radial J48 so I have to make do with my KT LB-100 DIs for a while longer. I've been trying to find an upgrade for them for a long time but I have had no luck finding schematics or mentions of any modifications. The problem is that the two LB-100s I have are quite noisy, mostly a high frequency hiss.

When it comes to possible upgrades I guess the transformer, the caps and the opamp would be the usual suspects? The transformer is a OEP J30A02/1 which I can't find any information on. The opamp is a Motorola mc33078p which to me look to perform quite well on paper... The electrolytic caps are EIC which is a make I haven't come across before but I'm quite the novice when it comers to electronics. If it would help I can post pictures at a later stage.

If anyone has any idea what would help to reduce the hiss I would be very grateful. Any suggestions are wellcome.

Cheers,
Andy

[Samuel Groner]

Without schematic there is almost no way to give informed advice--it really depends how the parts are used. First I'd check if the input connects directly to one of the opamp inputs or whether there is a discrete transistor (likely a JFET) in between.

Samuel

[b0b]

Ok, thanks. It's a simple enough circuit so maybe I'll be able to draw up a schematic of it. At least I should be able to identify most of the components used.

[b0b]

Hello,
while I'm working on the schematics I uploaded a few pictures if someone is interested in taking a look...

http://twin-x.com/groupdiy/thumbnails.php?album=309

As I mentioned earlier I'm a novice at this and I have no idea of how to identify some of the components. There's two identical components that I'd say are ceramic capacitors on each side of the opamp (yellow/orange with bluish/purple top) labeled C3 and C4 which have nothing printed on them as well as the blue and yellow component labeled LK1 beside the transformer.

Again any info and suggestions would help.

Cheers,
Andy

[abbey road d enfer]

C3 & C4 are indeed ceramic caps
LK1 is a link (0R resistor), a common necessity on single-sided PCB's.

[Samuel Groner]

There are no transistors in sight so this should be relatively easy. Is the transformer connected at the input (i.e. with the jacks) or the output?

Samuel

[b0b]

LK1 is a link (0R resistor), a common necessity on single-sided PCB's.

Thanks a lot for that! I've only ever seen just bare jumpers. Makes sense though. About the transformer, it's situated closer to the inputs but that doesn't really mean anything. Working on the schematic. Will be very busy the next few days but then I hope to get to work on it further.

[shabtek]

has 'MC____' opamp & led --active DI.
what is supplying power?

[b0b]

Phantom power... The DI is fed +48 volts DC from the mic pre trough the XLR lead.

[shabtek]

do you have different mic pres to check on (may be noise from phantom supply?) or make your own with 5 9v batteries in series and 2 6k8 resistors

[b0b]

Still determined to make my Klark Technik DIs perform as well as possible. I got a lend of a Radial J48 and tought I'd compare it to my KT. I also found a MTR active DI that I included in my test just for fun.

My findings are that there is definately something seriously wrong with the KT DI. There was no low end with the KT while the Radial and the MTR was similar enough. Also, I needed much much more gain (which explains the noise) with the KT compared to both the Radial and MTR which where again quite similar.

A problem with the opamp would explain the drop in gain would it not? Could that also be the source of the lack of low end?

Still working on that schematic...

[abbey road d enfer]

Still determined to make my Klark Technik DIs perform as well as possible. I got a lend of a Radial J48 and tought I'd compare it to my KT. I also found a MTR active DI that I included in my test just for fun.

My findings are that there is definately something seriously wrong with the KT DI. There was no low end with the KT while the Radial and the MTR was similar enough. Also, I needed much much more gain (which explains the noise) with the KT compared to both the Radial and MTR which where again quite similar.

A problem with the opamp would explain the drop in gain would it not? Could that also be the source of the lack of low end?

Still working on that schematic...

Regarding noise, the choice of an MC33078 is very strange; it's a bipolar opamp (dual), which is not very good in terms of noise if the impedance of the instrument is higher than ca. 10 kohms - that is the case with all passive electric guitars, and worse still with passive piezos. But the lack of low end seems to indicate some malfunction. Start checking voltages: pin 4 should be at or near 0v. Pin 8 should be anywhere between 9v and 36v. Then all other pins should be at half voltage respective to pin 8. Use a high Z meter on pins 2, 3, 5 and 6.

[b0b]

Thanks for that! Measured all the pins of the opamp and all seems to be ok. Pin 4 was pretty much 0V and pin 8 was 9V. The others where all 4,5V.

Would it not have to be a bipolar opamp to amplify a balanced signal?  Opamp pin 1 (output 1) is connected to XLR pin 2 and opamp pin 7 (output 2) is connected to XLR pin 3. Well there's a capacitor and a resistor in between but still...

[rodabod]

Would it not have to be a bipolar opamp to amplify a balanced signal? 

Possibly you are confusing bipolar and dual opamps. I think ARDE was suggesting a FET opamp might be a better choice since it would offer lower noise when connected to high-impedance sources.

So, am I reading this correctly that the OEP transformer is there for de-balancing balanced inputs? And the dual opamp is configured to drive the output?

[b0b]

Ok, I guess it's a dual opamp. From a previous post I thought that bipolar and dual are the same...

Here's my attempt at plotting out the input of the DI

The secondary of the transformer seemes to be connected to the input of the opamp. I'll try to figure out the schematic today.

[AudioJohn]

Klark Teknik have upgraded your design to the DN100 version - and boast of a 'ground up' re-design - with increased dynamic range and better low frequency response and lower noise. So perhaps the results you are getting with your box is normal?
The transformer now (DN100)  is on the output side - your initial schematic drawing is unusual, presumably giving all of the disadvantages of a passive transformer DI Box with few of the advantages of an active design!
It will be interesting to see the rest of the schematic - please put it up soon. 

[b0b]

I can't believe there could be such a drastic change between the new and old version. We have KT DI's in a few studios here. That the KT would be worse than the Radial, that I wouldn't be too surprised by but when the MTR DI was so close to the Radial then there must be something wrong with the KT because it didn't sound at all like the other two. The difference was really drastic. The bass sounded like a banjo when played trough the KT.

Here's an awful attempt at plotting out the schematic after the transformer. My software wouldn't let me have enough components to draw the whole schematic in one document.



I've left out the 15 dB swichable attenuator between the secondary of the transformer and the opamp inputs. Also the groundlift isn't included as it's between the input and output and I couldn't draw the whole schematic...

There might be some mistakes in this so if there's something that makes no sense, please let me know and I'll double check.

Thanks for all the help so far!

[EDIT] forgot to say that it's a 3:1 transformer. The data sheet can be found with the rest of the pictures and documentation here http://twin-x.com/groupdiy/thumbnails.php?album=309. And I don't know how the capacitor on the back of the transformer is connected. It's a ceramic capacitor that's connected to one of the primary pins. The other leg is also attached to the side of the transformer but I can't figure out where the connection goes...[EDIT]

[EDIT2]Updated schematic[EDIT2]

[EDIT3]Updated schematic to version 3. Fixed the 22K resistor to ve+[EDIT3]

[EDIT4]Updated schematic to include inputs and Earth link switch[EDIT4]

[abbey road d enfer]

There should be a 22k resistor going from V+ to the junction of R10/C8, in order to generate V+/2.
What a lousy piece of electronics!
No wonder they decided to redesign it from scratch.
Since it is used in a studio environment, galvanic isolation is probably not a serious concern.
I suggest you get rid of the goddamn INPUT xformer, connect the jack's tip directly to pin 3 of the opamp (and the sleeve to ground) - by the way, replace it with a good BiFet, a TL074 should work (provided your source of phantom power is in accordane to specs) .

[rodabod]

Might be nice to re-use the transformer on the output for balancing, and you can add an earth-lift switch too.

[abbey road d enfer]

Might be nice to re-use the transformer on the output for balancing, and you can add an earth-lift switch too.

The output is already balanced, and an earth-lift would not work since the box is phantom-powered. In addition, the xformer is designed (?) for high impedance; it probably has too much leakage inductance and stray capacitance.

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