WireWorld » Music » Music Blog » The Mic Preamp situation...

The Mic Preamp situation...

I've been interested in doing recording again. And the biggest gaping functionality hole is microphones and their accessory gear.

See, all I've got, really, is a cheap Radio Shack high-impedance microphone of uncertain vintage. And a few nasty-looking half-working oddball microphones. I used to like how my vocals sounded, back in the day, but I can't get anything that I really like these days. But, back in the good old days, I could borrow a Mackie mixer and SM58 microphones, so I suspect most of the stuff I like was done with a Shure Beta 58.

A poor artist blames their tools... but a smart artist also avoids getting frustrated with fingerpainting!

My friend Elea has a fancy USB microphone. It's one of the wide variety of modern condenser microphones, such that you just plug in the USB cable as a microphone cable and it works. Naturally, it works better on the mac because ASIO on the PC only lets you have one device active... forcing you to either use WDM or ASIO4ALL.... but that's another matter.

But, for a variety of reasons, the least of which is that I'm obsessive-compulsive... that doesn't appeal to me. I knew some stuff previously... and then I've been researching things lately, which just makes things worse.

In a high-end studio, the signal path usually starts with the instrument, then you have a fairly high-end microphone (which is never available in USB form), a dedicated mic preamp, optionally some amount of analog signal processing gear, and then a dedicated high-end analog-to-digital converter, often times separate from the audio interface. Each piece along the way is carefully picked out and characterized, such that a good studio engineer would know, from the vocalist or instrument he's recording, what gear to have patched together before the session starts.

As best I can tell, the individual pieces are not just chosen for audiophile-esque style points. There are differences between different mic preamps that can be revealed with blind A/B testing or analytical tests. The standard advice is that a good preamp ought to cost at least $300, preferably around $500, before it starts to sound good. Below that, you aren't buying anything that a mixer wouldn't give you.

The thing is, I can't take any sort of decent microphone.. even the standard SM57... and plug it into my computer's microphone port and expect it to work. The impedance is wrong, the microphone is a balanced device, and the gain isn't sufficient. At the very least, I need a microphone preamp or a matching transformer to adjust the levels before my computer's audio interface can have at it. As a result, any purchase other than crap or a USB microphone is going to require some extra hardware to be purchased.

Idiots on online forums try to point out that a USB plug has 5v power and phantom power is 48v and therefore, a USB microphone will always suck. Completely forgetting, of course, that 48v is because that's the maximum voltage a microphone would ever need and the presence of decent DC-DC converters that can easily step 5V up to 48v. And, like I said, USB microphones aren't total crap. Just not aimed at the serious recording marketplace.

Diving into various audio tests and reviews and even some schematic diagrams, my impression is as follows:

According to some comparisons I've seen, the "truest" signal is achieved by carefully balanced and adjusted components without using any transformers. Generally with discrete transistors. On the other hand, most folks doing recording of real music tend to prefer some color out of their microphone preamps. I've heard some allegations that the real thing people are looking for is a preamp that uses the right transformer, even more than something with tubes. After all, a bunch of the best-regarded preamps (the Neve ones being a famous example) are made with transistors, not tubes.

I've also looked up how much audio-grade transformers cost... and they quickly add up. I suspect that it's quite a specialized marketplace because you can't cut corners. Everybody else, outside of recording folks, who require that degree of accuracy is likely finding ways to not need a transformer. I've come to deal with the notion of audio transformers by thinking of them the same way I see the pickups on my electric guitar.

The standard advice is really to either get a mixer with built-in preamps or an audio interface with built-in preamps.. and if that's not good enough, get a real standalone preamp.

I did a quick survey of the preamps on the market. The cheapest preamps come from Behringer and ART. I found a schematic of the ART preamp and, given the commonality in the feature list to the Behringer, I don't think the design is very different. I would regard the design as being oriented towards being dramatically different from the sort of preamp that would be available in a mixer or audio interface. There's no transformers (remember... good audio-grade transformers cost!) there. Otherwise, it's mostly transistors and transistor networks. There's a single preamp tube in there, run in starved-cathode mode to give it tube color. It's almost like a little distortion box stuffed in there.

I did find a decent discrete transistor preamp schematic online and a mention of specialized IC preamp chips.

I drew up a possible design. Clearly the design must be dramatically more economical than a commercial design to be worth the trouble. As such, it doesn't have phantom power or the servo electronics necessary to be studio-grade. Nor does it have balanced outputs.

My opinion, and I'm far from a pro in this area, is that you could make a perfectly good preamp at the same price point as Behringer and ART but without the cloying fake-fuzz. And, realistically, you could probably simulate a perfectly adequate amount of fuzz and transformer behaviour inside of the computer to make it such that nobody would notice. But people won't buy that. The people who buy $30-50 mic preamps want something that's got a sound, even if it's not a good one.

I eventually got myself a Tascam US-1800 instead. It comes with 8 channels of allegedly decent preamplification. And, if I'm dissatisfied with it, I'll eventually get myself a standalone preamp of the appropriate price. But at least I'll be able to A/B into a working system...

Comments