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. (Order the capsule separately.) 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.
Choose Your Color
Choose from three core sonic palettes by ordering one of our capsules to complement this kit:
- U47 → RK-47 or RK7 capsule
- C12 / ElaM 251 → RK-12 capsule
- U67 → RK-87 capsule
The Fox 460 is not a clone of any of these vintage microphones, but we can certainly achieve a similar tone and frequency response... for a tiny fraction of the price. Then fine-tune the mic's high-frequency response using the instructions in the kit to establish exactly the voicing you desire.
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.
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 hotter 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, RK7, etc) and on the value of the EQ or "tone" capacitor. In this microphone, we used an RK-12 capsule with a 330pF tone 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 Elna, a NOS styrene tone capacitor, an excellent US-made output capacitor, Wima 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 Russia, 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. One exception to this: the main input wire that carries the high-impedance audio signal from the capsule to the tube is solid-core, 100% silver Kimber Kable.
- 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.
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.
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.
It should also work in any donor body that will physically accommodate its circuit boards and a 6072A tube and transformer. Practically speaking this means it will fit the 46mm diameter tube mics with switches such as the TnC ACM-1200, Avantone CV12, A.I.R Liberator, although you would have to remove the switches. We choose to use a higher-quality output cap rather than leave room for the switch PCB in these models.