rolls MP13 Mini Single Ch. Mic Preamp

My use-case is voice and for that this has a very high noise floor. Sound quality and gain are fine otherwise though.Output impedance is not listed in the datasheet but it's about 100 Ω and is AC coupled---except for the "clipping" detector. That last part is important if you are plugging this into a laptop or phone, as I do, since the voltage from those devices (used for electret microphones) may create a false clipping indication. In this case just ignore it.The instructions claim that it will drive stereo headphones but be aware that only the left channel will have sound. I was thinking that something was wrong but, in fact, the jack itself is a true mono jack with only two contacts; no ring connection at all. This particular model of jack does incorporate an internal switch but it is not used nor connected in this circuit.The pot, a little unbranded 10k job, is a bit scratchy in spots. I may try DeoxIT sometime.This unit comes with a wall wort but if you are going to buy or build a power supply mind the polarity: center is negative. To avoid ground loops (hum) try to use an isolated supply. Batteries would be fine.Finally, the Noise:This is a two stage amp with the first being an instrumentation amplifier comprised of two op-amps. The op-amps used are inside a 4560's. Those have fair voltage noise of 8 mV/√Hz but current noise is unspecified. For a dynamic preamp this can usually be ignored since these mics are only around 200 Ω and large current noise amounts to near nothing. In this design, however, there are two large 10 kΩ resistors in series with the mic (R4 & R6). I'm not sure why they are there but they add thermal noise and make the current noise significant. I found that shunting them drops the noise by 6 dB! With the shunt, op-amps like LM833 or LM4562 would likely do better still but with the 10 k's in place even an expensive FET op-amp wouldn't help too much.

This works well for PC/Laptop microphone volume control.HOWEVER... You do need a lot of connectors because there is a dark little secret about most computer microphones most people do not know.If you ever looked at a computer microphone connector, you've probably realized that it has a 1/8th inch TRS connector. On headphones or speakers these 3 contacts are: Right-Channel / Left-Channel / Common-Ground.These 3 connectors are different on PC microphones and the microphones on headsets(headphones with a microphone attached). For these types of microphones the contacts are as follows: Microphone Signal / +5 volts / Common-Ground. You see, computer microphones need to be energized with around +5v to work properly. Otherwise the sound signal they produce is very VERY bad and weak.I bring this up because the Rolls MP13 amplifier here is designed for 1 channel microphones that DO NOT NEED TO BE ENERGIZED! This means if you just plug a computer micropone(or headset microphone) into the Rolls MP13 input and then connect the output directly to your sound card's Microphone Input: You will get a horrible array of loud random noises sent into your microphone line and it will seem like the Roll MP13 is broken.If you wish to use the Rolls MP13 as a computer microphone amplifier you need to split the Microphone signal and the +5 volt line.1) The +5 volt line must go directly from your soundcard to the +5 volt line on your microphone.2) The Microphone Signal line from your soundcard must be connected to the OUTPUT of the Rolls MP13 amplifier. The Microphone Signal connection from the MICROPHONE must be connected to the INPUT of the Rolls MP13 amplifier.If this is confusing to you, you should NOT try to use the Rolls MP13 on a computer microphone.

I bought this unit to use with the "house sound" at an outdoor public pool. It's between a dynamic mic and a 70volt PA amplifier's balanced line input. It replaces the second of two  DBX 286s Microphone Preamp Processor  channel strips that each worked brilliantly until being killed by winter temperature fluctuation in the unheated facility. After the second DBX failed (each one lasted two years), I knew I had to find a different preamp. I chose the Rolls hoping that a simple circuit would be more rugged to better handle the harsh weather. If the weather kills this too, at least it's a lot cheaper than a DBX.The sound from the Rolls is a little raw and edgy compared to what we were used to. This Rolls doesn't have all the signal processing that the channel strips had, but that little extra bite might make it easier to understand the announcements from a distance. If the Rolls is noisy, I sure haven't noticed it. This seems to be a good unit for the application. Not sure I'd buy this for recording music or voice over; probably not for live sound for a band either, but for a pool? It works just fine. DBX 286s Microphone Preamp Processor

Works well with no distortion or hum. Level control is smooth and quiet. Recommended.

This is a relatively high quality product, that would be great for musical application, but clashes with my use for a clean mic signal. The gain control is self advertised as being slightly overdriven, and that can give a rich sound, but adds notable distortion to an otherwise clean signal. If that feature could be disabled by the user, this would be a 5 star product. Still a quality item for musical application.