DI transformer? OPT from st70 dynaco?

[shabtek]

HI, when I was in coledge studying sound engineering we did a shhot-out whith a bunch of passive DI boxes and an avalon mic pre/hi-z input...

The instructor had a homemade DI which he said was an OPT from a ST70 , a really big for a di tranny. is it gapped? Im not sure how the input/primary going to the bass was wired but the 8ohm secondary was feeding the mic pre input on the mixer.

I don't know if he was cheating the test or something but the consensus was it was as good or better than the avalon!

has anybody heard of this?

 

[gyraf]

Hi Shab,

Please resize your avatar to the recommended 60pixels width - as it is, it takes up a lot of screen real-estate in all the threads you post in..!

For your question - I don't really see how an output transformer should be useable - it has very different impedances that the ones we like for DI-boxes. But yes, you'll get a signal through it surely, just don't expect too much low-end.

Jakob E.

 

[Larrchild]

If the direct box is being fed directly from a guitar or bass pickup, the transformer is going to be too low impedance to have good lows, like Gyraf said. If it's an active instrument or keyboard with a low-z output, it might work ok. But it's probably 5-7k ohm instead of the 10k plus that is ideal for a real DI.

 

[pstamler]

Hmm...the Dyna tranny is something like 5k:8 ohms. That's a 625:1 ratio. Load it with 1.5k (typical mic pre input impedance) and you get an input Z of 937,500.

On the other hand, being loaded by 187x its proper termination, it'll probably ring like a bell.

Peace,
Paul

 

[gyraf]

Hmm...the Dyna tranny is something like 5k:8 ohms. That's a 625:1 ratio. Load it with 1.5k (typical mic pre input impedance) and you get an input Z of 937,500.

5K:8R is sqr625, that is, a 25:1 winding ratio.

But these transformed impedances tells us nothing about inductances - and it's the inductances that are responsible for how low-end will be transferred, not the turns ratio. It's probably a safe bet that the 5K-side of the transformer won't be over-spec'ed for inductance (because we want as few windings as possible to keep capacitance down, in order not to loose high-end), so it will probably not transfer full frequency range at impedances higher than the rated 5K.

Another thing is that these relatively-big standard-iron cores has quite a high hysteresis-distortion, making them far from perfect for low-level signals like DI. But then again, this might be exactly why it sounds good :razz:

Jakob E.