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Retro Instruments 500PRE

So Many Tonal Choices from a Single-Slot Tube Mic Preamp

Retro Instruments expands its line of professional audio processors with the 500PRE, a single-channel hybrid/tube microphone preamp/line amp module that fits into a single slot of a 500 Series rack.

The 500PRE is Retro’s third 500 Series module, joining the original DoubleWide I and the updated DoubleWide II—both dualslot, single-channel tube compressors. The 500PRE incorporates the same basic double-balanced Class-A circuit topology as in the company’s Sta-Level limiter.


The 500PRE works both as microphone preamp and line-level amplifier; it has an input operating range of –72 dB to +12 dBu. The output clips at +18 dBu, with maximum output at +24 dBu. The 500PRE has two switchable gain ranges: high gain at 75 dB using all three of its TAD 12AT7 tubes, or low gain at 48 dB using just two of the tube stages.

All three tubes are mounted in sockets that hang upside down from a small sub-assembly board attached to the main circuit board. For longer tube life and less stress on the 500 rack’s power supply, and just like the DoubleWide compressors, the 500PRE uses a soft-start circuit that ramps up the filament voltage slowly when first powering up. This feature also extends tube life.

There are two subminiature relays for switching in/out the third tube gain stage. Cinemag CMMI-10BPC mic input and customized CMOQ-2S output transformers take up the remainder of the space inside this fully packed module.

The 500PRE’s front panel features full-size Carling toggle switches and control knobs for Input Gain, Low or High Gain, 48-volt phantom power on/off, Invert off/on toggle to reverse the polarity of the balanced mic input line, and an Output control for setting final line level.


The total slot current drain for the 500PRE is 160 mA. This exceeds the allowable current drain in the API Audio VPR Alliance specification of 130 mA for a single-slot module. A check in a tube manual shows that the 12AT7 requires 150 mA filament current each, and there are three.

Taking an idea from old ’50s tube television sets, the three tube filaments are wired in series and powered directly from the 500 rack’s ±16-volt rails, or 32 volts total—that’s nearly 11 volts DC for each tube. The tubes’ plate voltage is supplied by the 48-volt phantom power supply, and the current drain is just 3 mA.

If your Lunchbox or rack does not have phantom power, the 500PRE will still work, albeit with reduced headroom as the plate voltage will automatically switch over to the ±16-volt rails.

Microphone preamps are very low-power amplifiers; they are not intended to produce enough power to drive a speaker or low-impedance loads to a certain power level. However, to get sufficient power after the last tube gain stage, the 500PRE uses a solid-state buffer amplifier output stage to drive the output transformer for full line-level output.

I ran the 500PRE in an API Model 8P High Current 500 8-slot rack that provides up to 250 mA per slot, plus 10 mA per slot from the 48-volt phantom power supply. A word of caution: check your 500 rack’s specification to make sure it will support the current demands of the 500PRE. Just as with any audio processor, it will not perform as expected if it is starved for voltage and current.

In addition, you should avoid cheap and poorly designed power supplies. The 500PRE has extra filtering on the power rails, but if you have doubts, opt for a conventional well-regulated and filtered linear power supply of sufficient current capabilities.


I began my testing using the 500PRE as the preamp for vocals using my Bock/Soundelux U195 FET condenser microphone. I like to have a male lead singer close in on the microphone, and I set the mic in Norm mode without the Lo Cut filter or the –10 dB attenuator pad engaged. I had him about three inches away from my Pete’s Place Blast Pop Filter, which nearly touched the front of the mic.

At the same time I readied a Blue Microphones Blackout Spark on his acoustic guitar over the 12th fret but aimed back across the sound hole toward the bridge. I recorded that instrument on a separate pass using the 500PRE.

For the vocal on the U195, I used the 500PRE’s Low Gain position set to about “50,” while the Output control varied from about “50” up to “100” for quieter singing moments. The Output control can be used like a console channel fader for “riding” recording levels—it’s old school, and I have always loved this part of recording engineering. Using just enough Input gain and near maximum Output level achieves the cleanest sound. There is no input attenuator pad on the 500PRE.

I find that if I can anticipate big jumps in a singer’s volume by arriving at a lower Output control level just for those moments, it will cause less level going into the following EQ/compressor chain. This optimized gain structure will achieve a more open sound because the compressor squashes less when trying to contain momentarily big level peaks. Even momentarily lowering the output level by 2 dB helps a lot, and the Output control operates smoothly and noise-free for this method. I especially like that I can rely on this preamp to not distort, easily making the 500PRE a good choice for any microphone source.

For vocal compression, I patched the 500PRE’s output into another favorite of mine—Retro Instrument’s 176 Limiting Amplifier. This is a glorious combination that I can recommend to any engineer for an awesome and big sound on any microphone and singer. I instantly obtained natural dynamics using a 4:1 compression ratio with medium attack and release times. My singer said it was the best vocal sound on him he ever heard!

I tried that same chain for recording the acoustic guitar but switched in the low-frequency rolloff on the mic and added more Input gain for this much quieter instrument. Without equalizing at all, it was easy to get his 6-string Taylor sounding big, warm and clear, with good dynamics, “snap” and presence.


The 500PRE’s gain structure is extremely malleable by selecting either the Low or High gain range and adjusting the Input and Output controls to taste. In this way you’ll have many more variations of amplifier gain structure possible when recording.

If you drive the input hard (it’ll take up to a +10 dB line-level), the tubes will self-limit—so turning down the Output control achieves a proper final level without distorting the next processor (EQ or compressor) in your chain. The overload is smooth and consistent, without the low frequencies “blowing up” or distorting more than the other frequencies.

As I found with recording vocals, turning down the Input Gain and cranking up the Output level up at or near max takes the sound to totally legit and clean. I liked that you have this option at any time—even while recording once you get the hang of operating the controls on the 500PRE. You might want to notate the Input and Output settings that work so you can return to them instantly, or experiment by going back and forth between them quickly.


Fooling around with the 500PRE’s gain structure gave me many sonic options and ideas for a direct guitar recording using a Jensen Transformer Iso-Kit Direct box. My Fender Strat never sounded this good direct, with a crystalline sound yet full-bodied presence—none of the nasally and peaky sound I would usually have to try to EQ out.

For a clean, direct sound, I used Low gain and I made up some level through a UA 1176LN Limiting Amplifier compressing lightly. This is a sweet sound I now use all the time, and I find it great for tracking guitar or bass in a live room with other musicians.

But I had to try the High Gain mode for the same guitar and was surprised by getting the coolest overdriven sound immediately! I didn’t need any additional gain afterwards; I got loads of saturated level with a striking signature guitar tone and unique presence.


I used two 500PREs to record a full drum kit in my living room using just three microphones. For the kick, I used a single AKG D12 VR into a Sunset Sound S1P preamp and approximately 35 to 40 dB of gain. I tried two different overhead setups, starting with a pair of Royer R-10 ribbons.

The Low gain position was good for the most transparent sound, with Input control at 50 and Output at 100 (or 50/100). The space’s acoustics were well recorded using the R-10s figure-8 pattern, picking up the reflections off various diffusive surfaces and the 8-foot plaster ceiling. My drummer’s “tasty” and subtle stick work was easy to hear. Both the rack and floor toms sounded big in the overheads. Overall, this is an incredibly smooth sound without overwhelming cymbals.

In the same positions over the kit, I changed over to a pair of Gauge Precision ECM-84 small-diaphragm condensers with cardioid capsules and no attenuator pads or LF rolloffs used. The overheads sounded more like what I am used to, but required repositioning the mics to favor the tom-toms and reduce the level of the cymbals. The 500PRE also required resetting the Input level to only 20 for these much higher-output mics. But I like this sound, as well—bright overall and with plenty of attack.


You get a lot of sound choices in the single-slot Retro 500PRE. From the super-clean to colorful tube saturation, it is my first choice when I am recording and hunting for something special and different. Super highly recommended!


COMPANY: Retro Instruments
PRODUCT: 500PRE Tube Mic Preamp
PROS: A versatile mic preamp with many uses and colors
CONS: Requires a little more slot power from the 500 rack