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Curt's Mic Preamp Page

One of the first things that ZHP clients who come to the studio may notice is that my outboard preamps are built either from kits or from a set of plans. This may seem unsettling to some. They usually expect to see "name brand" preamps that you'd expect to find in a pro studio. There are valid reasons for this. Name brand preamps are just that because everybody's heard them, or heard of them, and they deliver expected results. On the other hand, something custom built, or built from a kit, doesn't necessarily have to sound crappy and/or unpredictable. A well designed, home-built preamp can sound every bit as good as one from the local gear shop.

Some advantages to hand made gear: 1) Building it yourself gives you more intimate knowledge of the design. Not only do you know what it can do, you know what it can be made to do. 2) There's no hesitation in modifying the gear. If you see something you want to change, change it. There's no warranty to void. 3) Home made gear can, in many cases, be less expensive than store bought gear for the same features. Since you build it yourself, all you're paying for is the parts.

Over the last several years, I have become a student of mic preamp design. After all, the preamp can pretty much make or break your recording. All the cool digital editing tools in the world are no substitute for a good signal to tape or hard disk. So here are a few designs I have come across that have worked well for me.

PAiA Electronics 9407 Tube Mic Preamp

This gorgeous design from John Simonton and Jules Ryckebusch has been my personal favorite for years. It's basically a "Starved Plate" tube design. Pretty common in most inexpensive tube mic preamps. Basically, the tube is fed with a low plate voltage, usually around 45-50 volts. This causes the tube to saturate much quicker, giving an exagerated tube sound. What the guys at PAiA have done is add a little twist to the design, called the "Blend" control. This allows you to dial between a straight solid state sound and a full-out distortion fest, and anywhere in between. The skinny on "Starved Plate" design can be found at PAiA's website. It's a cool sounding preamp right out of the box, but I've made a number of modifications to really make it cook! Special thanks go to Scott Lee at PAiA. The man is a genius with the patience of Job. Also, I need to thank Paul Stamler for his series of articles on equipment modifications that appeared in Recording magazine from March, April, and May of 1996. His insights led to most of the major alterations in this unit.

Modifications to the PAiA Tube Mic Preamp

1. All resistors were replaced with 1/2 watt Metal Film. This was done to elimenate thermal noise from the stock 1/4 watt carbon comp resistors.

2. All input op-amp IC's were replaced with Burr-Brown OPA-2228's. All output op-amp IC's were replaced with Burr Brown OPA-2604's. These have better performance than the stock 5532 op amps.

3. Input coupling caps were replaced with 100uF 50V Bipolar Electrolytics with .01mF polystyrene bypasses. This solved two problems. First, the stock tantalum coupling caps sound awful for audio. The high value of the replacements reduces low frequency phase shift. (Special thanks to Wally Duguid for this one.)

4. Sovtek tubes were replaced with matched Yugoslavian Ei tubes. Why? They sound better.

5. Feedback cap on the final gain stage was replaced with a 3pF Poly cap. This improved high frequency phase shift characteristics.

6. Increased the value of the phantom power filtering cap for improved phantom filtering.

7. Increased the gain of the first gain stage. Just a preference of mine.

8. Powered the tube filaments with filtered DC instead of AC.

9. Replaced electrolytic stage coupling caps with 1uF polypropylene caps.

10. Replaced output coupling caps with bi-polar 50V caps for better coupling and circuit protection.

So how does it sound? It rocks!! It's surprisingly quiet, and has great frequency response. Dialing in a little tube sounds really fat!

How much did it cost? About $170 for the kits (one for each channel) and rack case, plus about $80 for modifications. I'd put it up against anything in the under $1000 price range.

Paul Stamler Project R Solid State Mic Preamp

This design appeared in Recording magazine (Oct-Dec 1998). It was designed by Paul Stamler, whose written work has also appeared in Recording. This was a big challenge for me. I had to build it completely from scratch. I had to print and etch the circuit boards, track down all the parts, and punch and drill the case. Not for the faint of heart!!

The Project R could easily be characterized as being overdesigned. Mr. Stamler has taken into account just about everything that makes a good mic preamp. It's inputs are transformer coupled, using high quality Jensen transformers. All capacitors that go anywhere near the signal path are high-end polypropylene. All resistors are 1/2 watt metal film. The power supply alone takes up half the hardware. The raw supply lives in a separate case to keep the power transformers away from the audio circuit. The power is filtered out the wazoo before it even gets into the main box. Then it gets filtered and pre-regulated, and regulated again at each channel. By the way, while most audio circuits run at +/- 15 volts, this unit runs at +/- 21.5 volts. That translates to increased headroom. This power is CLEAN! And you cannot have clean audio without clean power!

The audio circuits themselves are a sight to behold. Every op amp has a current amplifier circuit attached to its output to make sure the thing runs in Class A. It's got plenty of gain for just about any pro mic out there, plus a three band EQ and high pass filter to boot.

P.S. A BIG thank you to Mr. Paul Stamler for all of his help with answering my questions on this, plus the several months of work he put into chasing down problems . I couldn't have done it without you.