Fox 460 Circuit Kit

The "Fox 460" kit is our 3rd-generation tube mic upgrade for the Alctron T11-A, Apex 460, Carvin CTM100, BadAax T11A, and clones. Based on a circuit design by Fox Audio Research, this kit replaces 100% of the audio signal path of the microphone: capsule, impedance conversion circuit, tube, and transformer.

The modded mic has excellent performance specs for sensitivity, noise, and distortion; its frequency response can be personalized at build time to mimic landmark vintage microphones. The Fox 460 mic will compete easily with commercial products costing 3-5 times more.

We revised and improved this circuit in January, 2022, to incorporate four upgraded capacitors, notably including the tube cathode capacitor. The board layout and build process were refined, along with the documentation.

Capsule Choices

It might be tempting to pick a cheap capsule from Reverb or Ebay. We advise caution, having diagnosed far too many bad-sounding customer mics whose cheap capsules ruined the project. The capsule determines the mic's frequency response, which is what you experience as the sound, "color," and tone of the mic. Cheap capsules often have low output, thin bass, and weird peaks in the mid- or high-frequency response. We've even seen terrible results from so-called boutique capsules. Consider the fact that Neumann tests their capsules extensively, because they know better than to assume that every capsule they make is going to sound good. We do the same thing. Suppliers of cheap capsules don't test their products, because you're not paying them to. Rather, you would be doing the audio QC on the cheap capsule, although you probably don't have the necessary gear to determine whether the capsule actually sounds the way it is supposed to.

We offer three distinctly voiced large-diaphragm capsules based on European designs. These allow you to choose from three sonic identities when building this kit:

• U47 → RK-47
  
• C12 / ElaM 251 → RK-12 capsule
• U67 → RK-87 capsule

The Fox 460 is not a clone of any of these vintage microphones, but can deliver a similar tone and frequency response... for a fraction of the price of a vintage European mic.

Upgrading the Apex 460

The core circuit in our Fox 460 mod was designed by Brian Fox of Fox Audio Research. Brian spent weeks reverse-engineering this microphone and testing numerous iterations of component changes. After countless electronic and acoustic tests, he settled on the recipe for our Fox 460 mod kit.

Then we took it a giant step forward by finding the best parts we could for every component on the board, and designing DIY-friendly custom circuit boards to accommodate those parts.

We took it another giant step forward by sourcing a custom-wound output transformer that gives us lower distortion and tighter bass response, and higher output (without raising the noise floor).

We took it yet another step forward by fully analysing the effect of the "tone" capacitor option, and providing clear documentation for the amount of rolloff created by a variety of cap values. This allows the circuit to be fine-tuned to suit every desired voicing.

Expert Review

Drummer, engineer, and microphone aficionado Don Gunn reviewed the Fox 460 circuit and RK-12 capsule combination for Tape Op magazine. Click the cover image at right for the full text. (The review refers to the "12-251" microphone kit, which is the Fox 460 circuit, RK-12 capsule, and our own version of the Apex 460 metalwork.)

Testing [a DIY microphone] build is always a bit nerve-wracking, as you hope for the best but always expect the worst, so I was overjoyed when the first thing I heard when I spoke into the mic with headphones on was my voice sounding big, resonant, and clear. It reminded me of what I like about my Soundelux E47 - a really pricey mic with a richness of tone that's just lush and "expensive" sounding...

I placed the 12-251, my Soundelux E47, and my Neumann U 89 capsules as close to each other as I could and recorded Lacey Brown playing her guitar... The consensus was unanimous that the 12-251 was the vastly superior mic on the acoustic guitar tracks - it provided the perfect balance of string attack/airiness and body from the guitar. I would be happy to have that acoustic guitar sound forever more.

Mod Kit Benefits

As compared to the stock microphone, this circuit offers:

• Increased headroom
• Superior power filtering
• Lower self-noise
• Reduced distortion
• Reduced transformer ringing
• Less congested sound
• Higher output

Before-and-after Apex 460 circuit mod comparison

Let's take a look at the behavior of the circuit. Here's a chart showing the circuit's performance via signal injection. There's no capsule in this test: just a swept sine wave injected directly into the input coupling capacitor.

The gain for both tests was fixed, so this chart shows that the modified circuit has nearly 2dB higher output. Further, it shows that the stock circuit inexplicably boosts high frequencies. Given that the capsule is already boosted in the highs, this combination results in a super-bright, unbalanced sound. More on that in the acoustic test, below.

Now let's look at the circuit's distortion, before and after the mod:

The chart shows 2nd and 3rd harmonic distortion for both the stock Apex 460 circuit and our modified circuit. The solid red trace shows 6% distortion in the first two octaves. This is a phenomenally poor result. The ~3% distortion figure from 1kHz–10kHz is not what we would consider better; all these distortion numbers are far too high.

The modified circuit behaves like a professional piece of tube gear: with low levels of distortion.

Acoustic sweep test, stock vs. mod

This graph shows an acoustic test of the stock vs. the modified microphone. What is not shown is the relative gain levels, as we've normalized them for display here. The modified microphone has nearly 5dB higher output than stock, with lower noise. This is one of the benefits of the modification.

The frequency response of your microphone would depend on both the capsule choice (RK-47, RK-12, RK-87) and on the value of the EQ or "tone" capacitor. In this microphone, we used an RK-12 capsule with a 330pF cap. The result has warmer, fuller lows than the stock microphone, and a much more neutral top end. The stock microphone is grossly boosted, +8dB @ 12kHz, while the modified microphone has a smooth response (±2dB) through 15kHz.

 

What's in the kit?

We provide everything you'll need to transform your Apex 460 (or compatible mic) into a studio powerhouse:

• Custom circuit boards. We designed these in house and manufacture them in the US. We use high quality two-layer boards with clear silkscreen markings and reasonably sized solder pads.
• Fantastic components. The kit includes 100% of components needed to build the mic circuit. All these parts are selected from among the best available, including low-noise Dale metal film resistors, a styrene input cap from Reliable, high-grade electrolytics from Panasonic and Nichicon, a NOS styrene tone capacitor, an excellent US-made output capacitor, selected European film capacitors, and a US-made output transformer that was custom wound for this specific circuit.
• Low-noise tube. The tube is a 6072a type, manufactured in Europe, then tested and selected in the US to conform to our specifications for noise and microphonics. We picked this 6072 over many other types because it has better distortion characteristics.
• Internal wiring. We've sourced pre-cut and tinned, color-coded, multistrand silver-plated OFC with Teflon insulation, made in the US specifically for this kit.
  
• Unparalleled documentation. Each kit comes with a full color, step-by-step instruction manual. Our installation guides are known throughout the DIY audio community, and have been widely praised as the best available. The Fox 460 manual was heavily revised and updated in early 2022.
  

Note: although the circuit can be used with (and can provide corrective EQ for) the stock Apex capsule, we strongly recommend that you order one of our excellent large-diaphragm capsules to accompany this circuit upgrade kit. The RK-12 is the best-selling choice for tube mic upgrades.

 

Compatibility Check

Our mod kit is designed as a drop-in replacement for the circuit in the Apex 460, Nady TCM1150, Carvin CTM100, Alctron T-11A (aka HST-11A), and BadAax T11-A.

If your tube mic has switches, such as the TnC ACM-1200, Avantone CV12, or A.I.R Liberator, see our V-251 kit, which is a better choice for those donor microphones.