FMR Audio RNP8389 Really Nice Mic Pre  From FMR

WHAT'S COOL...

Lots o' headroom
These days, the trend is for microphones to increase their output signal levels while reducing their output noise levels. This means either employing a front-end pad and/or increasing the headroom of a mic pre. We decided to do the latter: at unity gain, the RNP 8380 has a clip point of +27.5dBu (almost 25Volts peak-to-peak)! Even with a hefty microphone output, the RNP8380 should take what your mics have to give.

Fidelity

When designing a pre-amp, there are many ways to balance the sometimes conflicting operating constraints. Unfortunately, with the proliferation of integrated-circuit (IC) based mic pres in recent years, one of the typical casualties has been fidelity. At this point, I won't bore you with the geeky details, but I want to assure you that fidelity was not sacrificed in the RNP8380. Even though there are many mic pres that are quieter, there are very few that have the spurious-free (i.e., distortion-free) spectrum of the RNP8380. Now, more than ever, what you put into a mic pre is what you'll get out.

Ugly box

If you look beyond the RNP8380's utilitarian ugliness, you'll see that we opted to throw the money we saved in cosmetics into the quality of the audio electronics. I like having nice-looking gear as much as the next guy, but given the choice between something that sounds nice versus looks nice, the sound wins out everytime.

Really Nice Compressor (RNC) inserts

Each channel on the RNP8380 has an unbalanced send/receive jack that allow you to connect an RNC1773 using a single Tip-Ring-Sleeve (TRS) cable (per channel). With proper cabling, you could also use this feature to balance any other unbalanced piece of gear. In addition, with the right external configuration, the RNP can provide a separate +22dBu unbalanced output and a +28dBu balanced output simultaneously to allow nifty things like separate M/S decoding while tracking the M/S signal without decoding.

Balanced/unbalanced operation

Both the mic inputs and mic pre outputs are electronically balanced, differential signals. Want to run to/from unbalanced sources? No problem, the RNP automatically adjusts...no special cabling required.

Balance an RNC

Using the above features, you can allow/provide a balanced signal to/from an RNC/RNP set.

Hi-Z DI inputs

These inputs are designed to take a source, such as a passive bass, and not "suck the tone" away from the instrument. This input features a high-impedance unbalanced 1/4" jack.

Slowly-ramping 48V supply

To reduce the chance of destroying mics connected to the RNP that can't tolerate +48V applied to them, we ramp up the +48Volts slowly just in case you accidently turn on the phantom supply with a phantom-intolerant microphone attached to the RNP. In addition, this ramp helps reduce "settling time" of the RNP's servos (see Geek Stuff below) and reduces the size of the output pops/clicks.

EMI-resistant

The pre-amp's front end includes a third-order EMI filter. It is specifically designed to attenuate RF while giving a smooth audio performance up to 200kHz. This reduces the chances that you'll have trouble with RF interfering with the RNP's operation even at high-gain levels. When used with a nominal 150 Ohm balanced source, this filter yields exceptional audio performance.

Output mutes

During phantom engagement/disengagement and DI/mic switching, we momentarily mute the output signals to reduce output pops & clicks that may otherwise annoy you and others. This mute actually occurs before the insert send signal, so even devices connected to the insert are spared large pops & clicks.

Stepped gain control

To allow precise gain-setting between multiple channels, we're using a Grayhill 12 position switch to set the RNP's gain.

Phase inversion control

This is fairly standard stuff to allow you to accommodate any phase problems due to things like system cabling, etc.

Precision metering

Three LEDs are provided for metering each channel (signal presence, +18dBu and CLIP). In addition, the clip LED trips at 1dB below actual clip and stays on for 3 seconds to make signal-chain troubleshooting easier. The metering is digitally-calibrated and driven by the internal microprocessor.

Flexible power supply requirements

The RNP will run off of either AC or DC as long as the voltage range is within 9-12V (and has a current capacity of at least 1.5Amps). Polarity of the wallwart connection is unimportant so that you don't have to worry about that detail either. In addition, if you happen to connect a wallwart that doesn't have enough oomph to power the RNP (like the RNC's wallwart), the RNP will tell you by periodically flashing it's front panel LEDs.

Geek stuff

An internal microprocessor provides control and monitoring of many RNP internal activities: power supply operation, source (mic/DI) selection, precision metering, push button control and phantom voltage control. The preamp itself is a unique, full Class A self-biasing-fully differential-DC servoed-transimpedance-100MHz GBP-instrumentation amp with an input impedance of 5k. The RNP's maximum level is +28dBu (differential mode) with an EIN of -120dB. High common-mode impedances help reduce the effect of interfering noise sources. Both phantom coupling caps and gain port voltages are independently servoed.

WHAT SUCKS...

Now I will violate a very important marketing rule by telling you what I think sucks about the RNP. Why? Because nothing is perfect and compromises always have to be made. By giving you some perspective on the RNP's shortcomings, at least you'll have some insights into the whys about my choices. Let your ears and application requirements be the ultimate guides.

The RNP uses a wallwart

As I explain on our website for the RNC, the RNP also uses a wallwart to: (a) reduce internal noise induction, (b) to make the national/international regulatory compliance less costly, and, (c) to permit easy adaptation of the RNP to countries other than the U.S. The upside is that we've designed the RNP to use a range of wallwarts (see what's cool, above) instead of the pain-in-the-ass one used on the RNC.

The RNP is relatively noisy when evaluated by lab measurement

Many mic pres these days (including the really cheap ones) have very low noise floors (EINs of -127dB or better). The RNP's EIN of -120dB is obviously not as "good" as these others.
I decided that the sonic character (or lack thereof) and a decent price point were more important than the absolute noise floor. Why? First, many of the sought-after vintage mic pre noise levels are much worse than the RNPs. So in actual use, I concluded, many folks (particularly those "in-the-know") prefer good tone, even if it's slightly noisier. Second, even though we have internal versions of the RNP with a lower EIN, we'd have to charge at least $100 more for the privilege of meeting lab measurements that few actual applications would challenge. Third, the trend in microphone development has been to raise the output level of microphones, thereby reducing overall gain requirements of external mic pres. Are there some applications using the RNP that may be problematic? Yes. Will most of us encounter them? No.

The RNP has coarse gain steps of 6dB/step

Under ideal circumstances, when gain staging your signal path, you want to only use as much gain as necessary to do the job. Too much gain and you possibly run out of headroom. Too much or too little gain and you possibly get more noise than you'd like for a given application. So, why not use a gain pot or a switch with more positions to allow finer gain steps?

First, pots are notoriously inconsistent and imprecise for gain setting without using a two stage pot...one stage for low gains and the other for high gains. That's OK, but I personally don't like to have a "gain range" switch that can cause the gain to jump 30dB or more. This is primarily 'cause I've been known to inadvertently push the button at an inopportune time (don't tell anyone)! Second, 16 and more positions make for a very expensive switch. In an ideal world, I'd have a switch with an infinite number of steps to allow us to smoothly and precisely maximize the mic pre's dynamic range...allowing us to exactly dial-in the gain we need without too much noise or too low of a clip point.

Given these trade-offs, we've made the RNP with a twelve step switch. The lowest gain setting is 0 dB with a clip point of almost +28dBu. Each step then adds 6 dB of signal gain. Even though this requires that we (the users) are more careful in setting the maximum gain before clipping, its gain setting is a little less critical given the high pre-amp clip point. So, a user is likely to be more concerned with the signal being too hot further down the signal chain, at which point the signal can usually be attenuated to the appropriate level.

Of course, you can always get finer amplitude control by using an RNC in conjunction to your RNP.