Potato Cakes

Re: Bo Hansen DI layout
« Reply #880 on: April 16, 2018, 04:56:23 PM »
After a couple of years of using this DI circuit with various transformer options, I now have a better opinion to share regarding the different possibilities.

Carnhill vtb9045- really shines on acoustic instruments that use a piezo pickup for live performances. There is drastic amount of clarity compared to the standard issue off-the-shelf options. Depending on the instrument and the environment, all that is required is a HPF and maybe some low mid cut.

Cinemag CM-9711 - I use this for anything that is a sample based instrument (keyboards, interfaces, drum machine, etc). For whatever reason, especially in a live environment, those type of sources are more easily heard in a thick live mix without the need to push them excessively loud. Typically for this application a passive Radial product (which are fine) is used, but when the dynamic of the music builds, things like keyboards tend to get lost, especially in a venue with subpar acoustics. This design plus the Cinemag seems to alleviate this problem. It even improves the clarity in IEM mixes and performers usually ask for less level due to being able to hear their sample based source better.

Sowter 9145 - If it is a bass, I only use this. It's the more expensive option and requires a larger case, but I have never had a bass guitar have as much clarity from the top to the bottom of its range with as little processing than with this DI configuration. It gets noticed with other bass players and engineers, which is really nice because it allows me to bypass all the Avalon and other gear that you typically see in a bass rig that cause problems more often then they should.

I have built versions with Haufe and OEP transformers, but compared to the ones mentioned above they do not seem to do anything special that would make them better than a passive Radial or some other similar product, but are more convenient as they have a footprint directly on the PCB. I built these with the components and construction methods exactly as prescribed in the assembly instructions, and all of them are noise free without the need for a ground lift. If there was noise it was because the source was generating it and not the DI. I have not found any need to tweak the circuit for different transformers or incoming impedance loads.

Two things that would be nice is for the PCB are to have a couple more options to directly mount different PCBs like JLM does with his Baby Animal preamp boards and a readily available compact enclosure option with engraved I/O labels. But as far as the circuit is concerned I think it is perfect. I definitely think it belongs among the ranks of other revered classic audio designs.

It would be interesting to see results of using different transistors and other components for the sake of science, but for those contemplating building some of these I would say don't bother with trying to figure out crazy modding schemes and to save that time and energy for other builds.

Brilliant work, Mr. Hansen. Thanks for sharing your brain with us DIY peasants.



Bo Hansen

Re: Bo Hansen DI layout
« Reply #881 on: April 16, 2018, 06:15:03 PM »

Thank you for sharing your test results for different transformers, very interesting.

The types you mention as Carnhill VTB9045, the old Neve/Marinair mike-transformer workhorse, use to be a definite winner with its nice character.

And also Cinemag CM9711, (similar to Jensen JT13K7) with its wide bandwidth, low distortion, nice phase  and level headroom, is almost a theoretically completed perfect 5: 1 transformer.

But I'm curious about what you think about Lundahl LL1538 or 1538XL.

I met a man with a dollar, we exchanged dollars, I still had a dollar.
I met a man with an idea, we exchanged ideas, now we each had two ideas.

Potato Cakes

Re: Bo Hansen DI layout
« Reply #882 on: April 16, 2018, 06:55:15 PM »
Mr. Hansen,

I haven't built one with the Lundahl option. I initially went with Haufe's as it was reported how close they both were sonically in this circuit. I have about 10 boards left that I am going to use for something, so I may try the Lundahl's then. But as of right now I'm very happy with the ones I currently use, so further testing on my end is a low priority. Maybe that can be something to put on the list for later this year.




Re: Bo Hansen DI layout
« Reply #883 on: April 22, 2018, 02:15:57 AM »
Sowter 9145 - If it is a bass, I only use this

Any bass players use Bo's DI with the LL1538 transformer?

Whats your experience, do you need to up the transformer to LL1538XL or even the Sowter 9045 for good bass?

Bo Hansen

Re: Bo Hansen DI layout
« Reply #884 on: April 22, 2018, 04:32:12 PM »
Today I am going to write something about DI-box transformers, regarding choice of type, character and quality.
I do it from my own point of view and experience.

My DI-box design is especially optimized for the Lundahl LL1538 or 1538XL.
It therefore performs very well when the balanced XLR output is loaded with a normal microphone input on a mixing console or preamp, that have a input impedance of about 1-2 kohm.
It is also capable of driving  very long cable lengths such as a multi-cable/stage box.

The other two options Haufe and OEP I have also tested, both practical and measurable, so I'm sure they can handle this too, but the LL1538 is still the best in all situations.

LL1538 is one of the best microphone input transformers in the world right now, along with some other types from Jensen and Cinemag.

Lundahl's dual coil structure and winding technology without bobin and their own core material production is unique, giving very high quality especially on small signal audio transformers.

Ok, now we talked about high quality with a great bandwidth, low distortion and high level capacity.

However,  there is another aspect of audio transformers, and sometimes it is a wish that they reproduce a special character for a particular purpose, such as for a bass guitar recording, and sometimes this does not have anything to do with "good quality", but more about music kind, production and taste, which is equally important.

Many times we think that older transformers made in the 1950s and 60s sound nice, such as UTC, Peerless, Stancor, Triad, Danish JS-Jørgen Schou and many others.

Unfortunately, microphone transformers from this time were mostly made for tube amps (low imp. to grid) and had high turns ratio around 1:15 to 1:30.

These types are not particularly suitable for active DI-box electronics, which are usually a unity gain impedance converter, and because these transformers can have a loss of -30 dB, the output becomes too weak to be an active DI-box.
A suitable useful loss from input to output for a active DI-box is about -15 dB.

So old high ratio microphone input transformer fit much better for passive DI-boxes, where you are used to weak output, and want it to be so.
For example, UTC type A10/LS10 or A12/LS12 has the reputation to sound great on some bass guitars.

Why these old transformers sound different and sometimes have a charming character, probably depends on the winding technology and core material from the past.
There are other guys in this forum who know this better than me, for example, "CJ" who has picked apart and analyzed more audio transformers than most of us.

Of course there are a lot of brands and types of microphone input transformers with ratio around 1:5 and 200 ohm to 5 kohm, that will work ok in my DI-box, and many of them are also great and also have more or less character,  but I have not tested all of them in same way as I did with Lundahl, Haufe and OEP.

 I have listed a number of possible types on my DIY page www.hansenaudio.se/techpage.htm and some I have tested, and some I have guessed are good for this application.

It's almost 15 years since my DI box design was published on this forum, when it was called ProdigyPro, and during this time, audio transformers have increased in their range in the market.

For a few years, a new caterogy of audio transformers has started to appear on the market.
These are microphone output transformers, where some types with lower turns ratio, are probably ideal for active solid state phantom powered DI-boxes.
But keep in mind, normally  these transformers are not encapsulated/shielded, so the DI-box case/box must be in metal, so the transformer does not pick up disturbing hum or buzz.

I have noted that Cinemag has some suitable types that are worth testing, according to the following types:

CM-2422-H or L
CM- 9711

Also noted that it sells some China-made microphone output transformer under different names, mostly copies of AKG and Neumann's well-known types.
How they are in quality, I have no idea, but I doubt that they can achieve the quality like Jensen, Cinemag, Lundahl, Sowter and  and some others with equal experience.

For the last 10 years I have served lots of China microphones, and those types that still have transformers, have not impressed me much.

« Last Edit: April 29, 2018, 01:03:26 PM by Bo Hansen »
I met a man with a dollar, we exchanged dollars, I still had a dollar.
I met a man with an idea, we exchanged ideas, now we each had two ideas.


Re: Bo Hansen DI layout
« Reply #885 on: June 11, 2018, 11:17:04 PM »
I cannot think of any advantage to making  a multichannel version of this DI.....but I made a couple anyway.
« Last Edit: June 11, 2018, 11:26:28 PM by godwin »


Re: Bo Hansen DI layout
« Reply #886 on: June 11, 2018, 11:18:18 PM »
another picture.


Re: Bo Hansen DI layout
« Reply #887 on: June 11, 2018, 11:39:39 PM »
The inside. I don't know why it's upside down. I tried three times. Sorry.

Bo Hansen

Re: Bo Hansen DI layout
« Reply #888 on: June 12, 2018, 02:30:24 AM »

Very nice done.

Stereo DI-box It is useful to have for various synthesizer with multiple outputs and similar.

I met a man with a dollar, we exchanged dollars, I still had a dollar.
I met a man with an idea, we exchanged ideas, now we each had two ideas.


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