The circuit is rather clever, using a dual triode tube with the first triode taking the very high impedance input from the capsule backplate and amplifying it. The second triode is directly coupled to it plate to grid, and operates as a cathode follower whith no voltage gain, but a lot of current gain for driving the transformer. The output transformer will distort less and have a wider frequency response driven from the lower impedance at the cathode pin 8. Voltage gain stage - current gain stage - transformer.

Now to the details. The Monster uses a 12AX7B instead of a 12AT7. The 12AX7 has higher gain, higher harmonic distortion, and higher output impedance than a 12AT7, all of which will increase the “character” of the mic.

C9 and C10 are 0.1uF, which is unusually large for RF filter capacitors. The capacitors resonate with the transformer secondary at about 25KHz, just above the audio band, and cause a rise in response above 8 KHz. Added to the capsule’s rise at high frequency, the mic sounds quite bright and sibilant. I changed C9 & C10 to 100pF NP0 ceramic caps, 1000 times smaller. Ceramic, for low inductance and good RF performance at WiFi and cell phone frequency.

C8 is a 1uF/450V electrolytic. 1uF is reasonable, but I prefer a film type for audio coupling, so i replaced it with a 1uF polypropylene 250V cap. The film cap is larger and has to lie flat against the circuit board. It can connect to the board on the tube end. It was easiest to unsolder the transformer lead from the PCB and solder it directly to the cap. Cover the joint with a piece of shrink tube.

C9 & C10 are small and lie flat against the board under C8.




New C6    C8    C9



Circuit board before modification

Stock C4 is polypropylene 1000pF 630V. Not bad, but I replaced it with a very low leakage polystyrene cap, 1500Pf 150V.           C4

Following the Fox Research guidance, I picked a 12AY7 / 6072 tube to replace the 12AX7B. The tube is a JJ gold pin 12AY7, about $35 from Tube Depot or Eurotubes. The most expensive part of the electronic upgrade, and the most important for reducing distortion. The 12AY7 needs a slightly higher bias, so a 5.6K 1/4W resistor is tacked across R7 on the solder side of the circuit board.

That completes the circuit modifications except for a possible added capacitor to flatten the high end response of the stock capsule. I’ll get into capsule choices and voicing in the next installment.

Onwards ... to measurements !

The original circuit’s frequency response is fairly flat, down about 1/3dB at 20Hz and rising about 3/4dB at 20KHz. After modification, gain is 1dB less and the high frequency rise is gone, rolling off gently above 40KHz.

With the replacement tube, gain is down about 2.5dB from stock, and the top end is flat as a ruler well past 50KHz. The 1/2dB drop at 20Hz is inaudible. Some people replace the transformer with a Jensen, Lundahl, or Cinemag. The stock transformer measures well. I kept it.









But the lower distortion was mostly due to lower signal output in the transformer at low frequencies. I’m testing at a pretty loud level. And of course the preamp has to be turned up 5dB to compensate. I decided the 12AY7 hit the sweet spot, and that’s what’s in the mic for capsule tests.

The mic electronics are now clean enough for professional recording, in my opinion. Third harmonic is commendably low in any configuration. So far, parts cost is under $50, most of that spent on the tube. You can try less expensive tubes. I picked the JJ gold pin 12AY7 based on web reviews, which gave it top marks for low noise and microphonics. It may not be hand made in Germany by virgins, but it does come from Over There In Yurp. The Slovak Republic, to be precise. A NATO country, not Russia, so there’s no embargo on importing them. They are pricey but worth it IMHO.

Next step: picking a capsule, and choosing the sort of “sound” the mic has.

Replace C6 with a 47uF / 25V non-polarized cap.

Solid lines = 2nd harmonic

Dashed lines = 3rd harmonic

One last tweak is adding a resistor from the junction of C8 and transformer to ground, to damp the transformer’s primary. It should be in the range of 200K to 270K, depending on the individual transformer. I used 220K because I had one on hand, and didn’t have 250K which is the calculated value. That cut distortion at 20Hz in half. I can live with that.

If a medium gain 12AY7 is better than a 12AX7, would a 12AU7 which is lower gain yet do better? Sort of. A JJ gold grid 12AU7 delivered 5dB less output and 5dB less noise than a 12AY7. Distortion was less over most of the spectrum too.