Implementing DI (Hi z inputs) on preamps

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Implementing DI (Hi z inputs) on preamps

Postby harry » Wed Nov 01, 2006 5:09 pm

hi all,

i have been collecting a heap of parts transformers, meters, doa's etc.

it is my plan to build a pair each of about 6 diff kinds of pres (1272/1290 style, api 312 style, ssl 9k, jlm twin servo etc, and rack up an audix 35102 and 35302 channel strips calrec pq1253 strips, and yamaha pm1000 channels.

I will be mainly recording hi z instruments (acoustic, and electric guitars - passive pickups, and through old guitar pedals (usually have a high output impendance) and low line level signals (analog synths and keyboards (fender rhodes, hohner pianet, yamaha cp30 and effects)

I have convinced myself that the magic 'funk' im after is gonna be imparted by those lovely transformers ive been collecting (marinair, sowter, carnhill, lhundahl etc).

i have noticed that the usual way to implement a di input is to bypass the input transformer and connect to the first gain stage (transistor or op amp).

synths and samplers can often sound a bit too squeaky clean and 'static' when the outputs are going straight into the soundcard. I want to hopefully impart that 'iron' on to the sounds.

My question are: (sorry if i am a bit unclear)

1. Will implementing a (HI-z) di input to run through the transformer give me the 'funk' or the 'iron' sound that comes from transformers?

2. Is it desirable or even possible to do this??

3. Would i be better off racking like these 12 pres without D.i inputs and then make simple (are there simple circuits around) di boxes that i could connect between the hi z sources and mic pres when needed.

i know on a technical level it would seem stupid to go unbalanced to balanced and then back again just to get to use an input tranny but maybe it would flavour the sound the way i would like.

p.s - im into 70's disco and funk so those huge tape sounds. i want my hi hat's to go 'ssssh' instead of 'tuk'. I also love those 'shit' cardboard drum sounds that many people laugh at. Good old rock records also have the type of sound im after.

thanks guys
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Postby chris p » Wed Nov 01, 2006 8:35 pm

Hi Harry

LOTS of issues in that post, and I'm not sure I can answer them all.

Re putting stuff through the input trannies, remember they are generally wound to give some, or even a lot (like between 12 and 18db) of gain just through the tranny, before it even hits the gain stage. If your signal starts out hot, adding this much gain is just going to massively overload your gain stages and drive them into distortion of the not nice kind.

So to answer (2), its not that you CANNOT wire your DI input into the input tranny, its rather than most DI signals (including pedals, low line signals, etc) are going to be way too hot for that to be a good thing .... unless you have dropped the signal level down to something like a mic level beforehand.

To return to (1), "iron" is somewhat mythical - yes, there are measurable things happening to the audio when the core of a tranny gets saturated blah blah blah, BUT the smooth velvet you are looking for is NOT obtained by just throwing in as much iron as you can between signal in and signal out. The sound you are looking for is actually better called "great design" and "great components matching to the job they have to do". Trannies play a part of this to be sure, but its the tranny in relation to the gain stage in relation to the buffering in relation to the circuit design in relation to the quality of the knobs. You can't just pick out the tranny and say eureka. Tranny's were not added to circuits for their sound, remember, but for electrical isolation and impedance matching.

Re (3), what you suggest is they way I do it, but as noted above, you need your external DI to return the signal level to something like a mic output if you want to pass it through a preamp (the alternative is to have your DI bump the signal up to +4 line for direct plugin to your recorder / DAW interface). I have external DI's (like the DI Joe project, as well as some nice heavy passive DI's I bought dirt cheap off ebay), and all can attenuate the signal to achieve this. But this one's a matter of taste, choice, and how you work best. I'm a nut for flexibility, so a wired in DI would only work for me if ALL my preamps had DI ins, and that just not necessary.

Finally, you've got an ambitious DIY list there - are you sure you would not be better just getting a PM1000, or even a PM700, mixer instead?
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