The COVID pandemic has changed so much for so many lives, mine included. Deciding to use the gift of discretionary time to full advantage I set out to do something I’ve always wanted to do – experiment with building tube microphones! (Let me be clear that I am hardly an expert on tube circuit design so please be gentle if there are any inaccuracies below.)
For a number of reasons, and fueled by our success in building four extremely musical and often used Neumann U87 clones in 2015 (read that article here), I chose this time to build a pair of U47 type mics. I am fortunate enough to own many great microphones, including 2 Telefunken U47’s (more on those in a minute) but was lacking a set of mics with what I fondly think of as “47-ness,” the warmth, open bottom end; silky smooth extended highs, and a notable proximity effect boosting lows without an inherent compression signature.
Unlike the 87 build, there is an element of an original Neumann / Telefunken U47 microphone that is extremely difficult to attain – namely the VF14 vacuum tube which is a big part of why the microphone sounds and acts like it does. Given that every original U47 still in use sounds a bit different from any other, it is a challenge for mic builders to find a tube substitute that definitively “replaces” the VF14. Virtually every microphone worthy tube choice (requiring extremely high input impedance, extremely low noise, extremely minimal self-microphonics) has been tried and claimed by someone or other as the new champion. I just wanted something that would sound great, be reliable and replaceable and so I settled on a NOS Telefunken EF800 tube. I found a pair of them in the UK and was super happy with the price, shipping and quality of the tubes.
A little bit about my pair of Telefunken U47’s. These were bought in the late 90’s as original mics “reconditioned” by BLUE, at the time a new company in Latvia which partnered with Jazz artist Skipper Wise to re-vamp old mics. I was told at the the time that these had been nuvistor tube models (Neumann still made U47’s after the superior VF14 tube was out of manufacture and used a small nuvistor tube instead) reconfigured to use an EF86 tube. The EF86 tube is used in the amazing U67 microphone and tends to have a bit of self compression, quite a bit of compression if the circuit has any feedback like the U67 does. The only thing about these mics that is original are the bodies, so they are not nearly as valuable as unmolested U47’s. That said, while these mics really don’t sound like any other U47 they do sound fantastic!
OK, back on my quest. Aside from the tube, two other major elements to the sound and quality of a tube mic are the capsule and the output transformer. There are MANY opinions on what is “best” or most like the original component, but I try to go with what I know personally and so chose the AMI BV-8 classic replica transformer (uses the same alloy and similar wrapping technique to the Neumann original) because we used their transformers in our U87’s and I loved them. I also bought a pair of PSU’s from AMI. Similarly, I chose the Thiersch M7 capsules because Sigmund Thiersch is one of the last great Neumann capsule experts and did a great job on re-skinning original capsules for one of my M49’s and my M269.
Microphone genius Oliver Archut experimented quite a bit with replacements for the VF14 tube, so I decided to start with the circuit he drew up around an EF800. I bought a handful of resistors and capacitors and was ready to go!! See Oliver’s schematic here.
My first mistake
“Hey, why don’t I experiment in one of my existing 47’s, leaving the BLUE capsule in place so that I could compare it to the other BLUE 47?” Sounds reasonable, right? So, I disassembled BLUE 47 #2 (all of our mics and psu’s are numbered so that any adjustments to supply voltages stay consistent) and whipped up a prototype EF800 circuit.
After I burned in the EF800 for a few hours I was anxious to hear it.
It was horrible. 🙁
Though it had tons of low end, the mids were dominant and the highs were splashy. And it got worse at distance. Hmmm. So I looked first at the circuit and decided that Oliver’s use of extremely high impedance at the input to the tube was putting it in an uncomfortable working range for me so I swapped that resistor as well as one at the capsule from 1G Ohm to 400M Ohm (values closer to Neumann’s 100M Ohm) and had a listen.
Better. Still bright though.
While I was pondering next steps, I was examining the circuit I took out of the BLUE 47… I’d never really looked at it before and had no schematics for these mics. As I followed it through, I noticed some voicing capacitors and realized that it was very like a U67 circuit after all! I began to wonder if maybe the BLUE capsule, which I had assumed was a K47 type, had some pre-emphasis (intentional brightening that is later removed by a bit of EQ in the mic circuit) like a K67 capsule (used in a U67 mic) does. So I replaced it with the M7 capsule and – Ah Ha! It sounded great. Damned BLUE mic is a U67 clone in disguise!!! Now when I compared the revised EF800 mic to BLUE 47 #1, it was much more evenly balanced and quite 47-ish. Unfortunately, through all of this critical listening I’d reinforced how much I actually do like the BLUE 47’s – I’m no purist and though the BLUE’s are no U47, a good mic is a good mic. So now I had to set about getting some mic bodies for my final EF800-47 builds.
My second mistake
I should have just bought the DIY47 kit from AMI. It comes with everything including PSU, circuit boards and a Chinese made body and is very reasonably priced. But I decided to buy some beautiful brass 47 style bodies from BeesNeez in Australia as I was disappointed in the Chinese made bodies we got for the 87’s. Sadly, one head basket arrived damaged. I spoke to Bruce at BeesNeez who is incredibly nice, but oddly never sent the promised replacement and dropped out of contact. I hope that he and his family are okay during this difficult time. With the cost of these (arguably superior) bodies, I had now spent way more than the AMI kit would have cost.
I built point to point turret boards (like a guitar amp would have) and a transformer deck using supplies from Tube Depot and I think they came out great!
After wiring and PSU voltage tweaks, it was time for final assembly and burn in.
COVID has all but shut the studio down until it’s safe to resume recording ensembles, but I did have an actor come in to record some vocals for a cast album and used the opportunity to shoot out the new EF800 47 against a U67, BLUE 47 and M269 (It’s a pop musical so I chose the U67 for it’s mid-thrust and compression.) It held up against this competition beautifully. Clear, open, great headroom, silky top. Certainly I will be using these for all kinds of things once it’s safe to resume orchestra recording!
