Post
by ashcat_lt » Sat May 31, 2008 9:54 pm
I've never tried either one, but I'll throw a little bit of "math" at you, FWIW.
It kind of depends on what DI you're talking about. Looking at the specs on their website, I'm seeing that their active DI has an input-Z of 220K. The passive says 100K, but I'm not happy with that answer. A transformer doesn't have its own impedance, but rather reflects the impedance of the mic pre into which it's plugged. Most won't get much higher than 200K even with extremely hi-Z mic pres.
The Fireface, on the other hand, presents 470K on input 3-4 when switched to "instrument".
Now, we can argue all day about what the ideal input impedance is for a guitar, but there's no denying that most modern effects and many amplifiers have "settled for" a nominal 1M standard input-Z. That is, if you were plugging your guitar into an amp, it would likely see something like 1M, about twice what the Fireface presents, and 5+ times what the Radials will give.
The self-capacitance of the pickup in conjunction with the inductance therein as well as the capacitance of the cable through which it is connected to it's load all add up to a pretty severe low-pass filter which cuts off at a surprisingly low frequency. Right around that cut-off frequency is what we hear in a guitar as sparkle, or sometimes spank, maybe even clarity. It's the part of the signal that we are missing when we complain about "tone suck".
Now, I put the word math in quotes because I'm not going to throw any formulas out. I don't understand them well enough to explain them, and imaginary numbers tend to freak people out. But I have seen a graph (can't link there right now because the GuitarNutz forum is down ) that showed the frequency response of a "typical" guitar circuit and how it changed as the input-Z to which it is connected changed. At 1M there was a very significant resonant peak right at the cutoff frequency. As the value decreases, this peak also decreases. At 500K (close enough to your FireFace) it's still pretty high up there, and you might not even be able to hear a difference. At 200K it is almost halfway down (from where it was at 1M) to flat.
That said, nobody really wants to hear that much of a resonance peak at that frequency range on a guitar. This tends to sound really strident and harsh, much like that horrid sound you get from a piezo-equipped electric/acoustic. The reason it "works" in an amplifier is that the amplifier and (especially) the speaker fail to reproduce those high frequencies. So you get something like a pre-emphasis/de-emphasis thing.
Based on all that, I'm thinking the Radials likely sound better when the guitar is to be heard without much in the way of further processing. The decreased high-frequency response* will act something like a speaker sim and mellow out the otherwise nasty sound of your guitar.
On the other hand - if I had to choose between these two boxes and nothing else - I think I'd choose the FireFace for anything where I intended to "re-amp". Whether ITB or O, I'd like for the amp/sim to receive a signal as much like the guitar connected directly to it as possible. Rather, perhaps, I?d like the guitar to act as much as possible as though it was connected to the amp. Or maybe I should say that the amp/sim is going to roll off that high end anyway, so I?d like to preserver it as long as possible.
Of course, the whole issue of impedance is rendered moot if there is a pedal between the guitar and the interface. In that case, the guitar "sees" the pedal and could give a fuck whether it hits a transformer or an opamp or whatever else on the other side. Most modern pedals have plenty low enough out-Z to interface fine with either one. A Boss or similar pedal will even help out in bypass mode.
Now, I haven't touched on any of the other aspects of the two options you've presented because they are something you'd have to hear, and I haven't. Which one is overall quieter? Which has more headroom? Which sounds better when it runs out of headroom?
* I think I saw that the Radial passive claims a 20-20K response. This means it will pass frequencies frequencies within this range. If it never actually sees these frequencies, though...
