DI Xfo on mic pre?

[hodad]

I'm planning a build of a preamp built around a Flickinger 535 card--I have the card, have a single Flickinger output xfo, a few different options for the input xfo, so it seems like an easy choice. As I consider possibilities for this design, the thought of a DI input using a separate transformer crossed my mind. The input trafo on a Flickinger is typically 150:600, occasionally 600:600, so I was thinking of wiring in something 15K:600 for the DI, & having a switch to go between inputs. I know that's a bit peculiar, but I'm the one who'll be using it, so I'm not that concerned about peculiarities. So: aside from the extravagance of having 2 input xfos on one pre, are there any pitfalls here I might be missing? Also, is there a more elegant way of switching between the inputs than a DPDT?

 

[Ike Zimbel]

I think you would be better off looking at an active DI circuit. 15k is really low for any passive instrument input and I think any "goodness" that you might get out of the transformer would be lost to the loading effect that such a low impedance would have on the pick-up. If, on the other hand, you're only planning to use active devices, like synths into the DI input, carry on with your plan.

 

[hodad]

15k is really low for any passive instrument input

What about setting the trafo up for 15K:150 (with the same circuit)? Would that have an impact on what impedance a guitar or bass would "see" when plugged in? The other option I found in my stuff is 50K:250, which might be more suited to a passive input.

 

[abbey road d enfer]

The nominal impedance of a xfmr is an abstraction.
The only thing it describes with a relative dose of certainty is that when the secondary is presented with the nominal load, the primary will measure as the nominal primary impedance, within the limits of rated frequency response.
In the particular case of your 150:15k xfmr, if you connect it in reverse, loading what is now the secondary with 150r will result in 15k at the primary.
Now, you would not connect it to a 150r load, as you would connect it to the input of a mic preamp.
According to usage, the actual impedance of a mic preamp rated for nominal 150-200r microphones is supposed to be about 10 times higher, for 1500/-000r.
The 15k:150 xfmr, presented to this 1.5-2k load, should reflect at the primary as 150-200k, if the xfmr was perfect.
Unfortunately it is not.
Particularly at low frequencies, where the inductance of windings become dominant, but also at high frequencies where capacitive losses intervene, putting an inferior limit to the actual impedance.
So the impedance seen by the instrument may well be about 150k at midrange, but much smaller at LF and HF. The result is a midrangey frequency response.
Reiterating the calculations with your 50k:250 xfmr shows that there is a predictable advantage. However, since the measurement conditions that produced data are probably not the same, it is impossible to predict the actual result.
Many passive DI boxes use a mic xfmr in reverse; results are variable, but the most significant variable is the user's expectations.
Personally, for years I've played a passive-piezo equipped guitar with a passive DI, which in theory should result in something unusable, however with the help of some EQ it gave a satisfying result.
Now all attempts I had with playing electric guitar through a passive DI resulted in nothing good, when active DI's produced a usable result.
However, I maintain that for electric guitar, there's nothing better than moving air via a piece of carboard, although some amp simulation softwares give pretty impressive results.
The latter is the justification for an active DI input, not passive.

 

[hodad]

Thanks for that explanation, abbey--I'm much more likely to DI bass or keys than guitar (I'm generally unhappy with the results I get with DI'ed guitar, so I tend to avoid it), so I'm a little less concerned about how a guitar would sound through it. Maybe the question I should be asking is whether I'd likely gain or lose anything by running a DI transformer straight to the amp card rather than having an external passive DI plugged into the preamp's mic input (with input trafo wired either 150:600 or 600:600). (My thought was that it might lighten the load on the DI trafo & open up the sound a little, but my understanding of impedance is pretty weak.)
If there's not likely to be any sonic edge to running the DI trafo straight to the amp card, it's probably not worth the trouble for me to have an onboard DI.

 

[abbey road d enfer]

Maybe the question I should be asking is whether I'd likely gain or lose anything by running a DI transformer straight to the amp card rather than having an external passive DI plugged into the preamp's mic input (with input trafo wired either 150:600 or 600:600).

It's definitely a better solution, going through one xfmr instead of two, but you may have a gain issue., or not.

 

[xazrules]

sorry for the intrusion, i've built 8 of similar mic pres, the poor man and easiest solution for me is to buffer the DI input with a fet, but remember to add a switch for mic/DI. to me is "the go to jail" of mic pre builds waste a trafo for a DI. or even easier add a DI input jack post input trafo without buffer but you have to buffer outside the box with a pedal.
you mention that you're going to use it more for bass, personally i like to load heavily bass pickups with for example a 1073, you're losing a lot of HF that more often than not is useful.

cheers