Two mics into one preamp

[punkomatic78]

The producer/engineer Brendan O'Brien is known for micing toms with two mics (one on top, one under) plugged into a Y cable, which feeds the combined signal into one mic pre. I'm a big fan of his production, and clearly this works for him. I just wanted to ask a newb question about what's happening when you plug in two mics this way.

As I understand it, the two mics are in parallel, so showing less impedance to the preamp(?). What effect does this have, both subjectively in terms of tone, but also does the preamp circuit suffer in any way as a result?

Also, does combining the signal before the pre affect the performance of the mics, and if so, if the mics' output impedance is unmatched, could you predict which one (the one with the lower or higher Z) would be affected more? Would two dynamic mics together behave differently than a dynamic and a condenser?

This is something I intend to try on sessions where I could do with a few more spare channels and where appropriate (e.g on double miked guitar cabs) but I want to understand what's happening and what the limitations or problems might be.

Thanks!

[Mike Stranks]

I find this somewhat puzzling, but maybe I'm missing something...

Others will doubtless address the two paralleled mics and their impedances, although I think that in general terms it's nothing to be overly concerned about as long as the mic input has a reasonably high (for a mic input) impedance.

I'm more puzzled about the Y cable. Are you sure it's a straight Y with nothing else on one of the legs? I would have thought that miking above and below will result in two signals that are out of phase with each other. I know that it's customary when miking a snare above and below to put a phase invert on one of the mic channels to keep everything tight.

But as I say... I could be missing something...

[punkomatic78]

Thanks for your response, that's helpful.

In the case of the two drum mics, one of the female ends is wired with the hot and cold the opposite way round in respect to the other female end to invert the polarity of the bottom mic. I obvs wouldn't do that for two mics on a guitar cab, but good to bear in mind.

[James Perrett]

Technically this isn't the best way to do things as each mic will be trying to drive the other mic. The mics will be seeing a low impedance load which will change their response if they are dynamic mics. Now this may be part of the sound but you can achieve the same thing with a preamp with variable input impedance (like certain Audient models) or just by sticking a 200 ohm resistor across the output of the mic. You also have no control over the mix between the mics - and I usually find that I want mainly the top mic in a mix with just a little of the bottom mic.

I guess that he has hit on a particular combination of mics, snare drum, drum heads and tuning that works for him but I'd prefer to experiment with things in a more conventional way to give more flexibility.

[Hugh Robjohns]

Given the affordability of multichannel mic preamps this feels like an unnecessarily compromised bodge.

As already acknowledged, the top/bottom tom mic idea requires a polarity inversion in the bottom mic feed of the Y cord.

It would also work best with two identical dynamic mics, ideally with output transformers.

I wouldn't recommend combining a dynamic and capacitor mic. Not only will they have very different sensitivities, the capacitor mic is likely to get upset with the other mic's signal being forced up its output!

[Funkyflash5]

My understanding is that he uses a 421 on top and a 57 on the bottom. The complex frequency dependent interaction between the 2 is why it's supposedly different than mixing the 2 signals after amplification/recording. If you have a spare couple cables and the ability to solder, it's easy enough to give it a try, but it's very much a particular sound and more like turning 2 mics into a single third option than actually using 2 mics. Also, it's only useful with dynamic mics!

[Hugh Robjohns]

The complex frequency dependent interaction between the 2 is why it's supposedly different than mixing the 2 signals after amplification/recording.

It would certainly have complex frequency dependent interactions!

[punkomatic78]

Thanks folks, that’s great food for thought. I’ll go and experiment, knowing I’m unlikely to fry anything in the process.

[Martin Walker]

Thanks folks, that’s great food for thought. I’ll go and experiment, knowing I’m unlikely to fry anything in the process.

Do post again with your findings, as I suspect folk here would be fascinated to see how successful this technique is in practice.

Martin