Modifying the Rode K2 mic Part 1: Tube Change

I originally intended to replace both tube and capsule on this mic, but after experimenting and comparing I ended retaining the original capsule but with a replacement tube. In this post I’ll describe the simple tube change and give some audio examples. In the next post I’ll go over the capsule change. To be clear, the capsule upgrade made the mic sound different, rather than better, and based on what I already had in my mic locker I chose to return to the original capsule.This highlighted the importance of following your ears rather than what you read on the internet!

Okay, so here’s what you need:

A Rode K2 valve microphone ( I got mine second hand off EBay for £300 which is a fair price). These are decent mics and are very useable straight out the box. They are multi-pattern, very well made with a good quality power supply, cable and shockmount (unlike the Chinese valve mics often used for mods such as the Apex 460). The case is a bit tacky but you can’t have it all.

A NOS (New Old Stock) Telefunken E88CC (6922) valve or equivalent


I learned a lot about the vintage tube market whilst researching these. Firstly, they are not cheap! You have to be careful as there are a lot of rip-offs out there, people trying to pass off newer tubes as vintage ones but fortunately there is a wealth of info out there to help as well as several reputable dealers that get recommended by many different people. I paid approx £125 for one from NOS Tube Store , their service was great and the tube was as described. This particular one is a sought-after Telefunken made in West Germany in the 1950’s. There are many other fine vintage tube options but I didn’t have the time and money to experiment, and this tube was highly recommended by a lot of people. If you want to know more and have time to burn, just check out the forums. Different tubes will impart a different sonic character to the mic so the ultimate test is to use your ears!

How to do it

This is very easy and I would recommend you try this first and see if you like the way it sounds. I recommend you record some different sources with the stock tube before you proceed, so you can compare afterwards and understand for yourself how the mics character has changed.



Firstly, undo the metal retainer ring at the base of the mic and remove it. Then undo the metal sleeve that forms the main body of the mic (hold the mic by the grill end and unscrew). It should then slide off to reveal the tube and circuitry of the mic as shown in the picture.


The tube is secured in position by a plastic clip which sits on top of the tube (pointy end) and the clip is held in place by 2 screws. Using a Phillips screwdriver, loosen both these screws and fully undo 1 of them to flip the plastic clip up and out of the way.


The tube can now be pulled carefully from its socket, a slight amount of wiggling (of the tube, not you) may help!


Carefully insert your new tube into the socket, it only fits in one orientation, make sure it is fully engaged. Replace the plastic retaining clip over the top of the tube and tighten the screws.

To my ears the NOS Telefunken tube makes the mic more mid-focused, the low frequencies are still clear, warm and big, the highs are a little less bright and sharp but the mid presence makes the mic cut through the mix more. I also find it more pleasant to listen to than with the stock tube, but it may not suit every application. I’ve presented some comparisons here to give you an idea of the difference. All clips were recorded via the preamps on my UA Apollo interface, I’ve endeavoured to maintain the same conditions and distances between mic and source for each comparison, but there may be small variations.

Firstly, male voice (don’t worry, I’m not singing)

Stock Tube

NOS Telefunken E88CC

Female voice

Stock Tube

NOS Telefunken E88CC

Double Bass

Stock Tube

NOS Telefunken E88CC

Judge for yourself and decide if this mod is worth the money, for me it’s a yes, but I will qualify it by saying I have other flavours of mic tonality in my locker, this adds something different, and it does it rather well.

In the next post I’ll describe changing the capsule and post some examples of the difference that makes.





AKG C414B Hum problems

I thought I’d write a quick post on this in case some of you experience a similar problem, it may save you hassle and expense.

I have a pair of AKG C414B mics, they are real workhorses and as the cliche goes, the studio Swiss army knife. On top of that they are solidly built and very reliable. But sometimes things go wrong. I recently did a bit of drum recording for myself and was listening back when I noticed a faint but distinct mains frequency hum on the overheads (my 414s). I had them running through my home made Neve preamps and then into a pair of line-level inputs on my UA Apollo interface. I automatically assumed the hum was coming from my home-made preamps rather than the solid, Austrian-engineered AKG mics. I duly set out to test, swapping cables, microphones and trying various configs of phantom power on / off on each channel, until it was clear that the problem lie with one of the 414s.

Faced with an expensive repair, I thought, well, let’s open it up and see if anything obvious was up with it.

Hmmm, lots of surface mount components and no obvious signs of trouble. A quick Google search led me to this post. Not exactly the same problem but similar, so I read on. Turns out that the grill / mesh of the mic connected to ground via pin 1 of the XLR forms a Faraday Cage, which helps shield the high-impedance capsule from EMI noise in the room. The mesh makes contact mechanically and if this connection is a little dodgy, it won’t work and there will be noise. I tested the resistance between pin 1 on the XLR and the mesh, it was a variable which immediately suggests a problem. I went about gently squeezing the base of the mesh in an attempt to improve the mechanical connection. Checking again, the resistance was now consistently minimal, so time to test. I put the body back on and plugged it in, powered it up and hey presto, no noise. Phew!…. So if you’re having this kind of issue, try this first, the mic is easy enough to open , just remove the 2 screws on the base (star driver or a flat head screw driver will do) and the smaller cross head screw in the XLR connector base and then slide the body off.

Flying mics and ominous cymbals

I recently read an article on the flying mic technique – not sure where, may have been TapeOp – but being a fan of the more esoteric side of recording, I thought I have to try that out.

The idea is simple, you suspend a mic from a pulley via a length of string which allows you to smoothly raise and lower the microphone. You then record a source and vary the mic position as the source sound decays, allowing you to pick up more of the detail in the decay of the sound.

This technique is particularly interesting with cymbals. If you have the mic too close, the initial attack will likely overload it so you won’t capture the detail in the moments immediately after the transient, but with the mic far away you can’t readily pick up the fine details of the decaying sound. And it’s in that decaying sound that a lot of the interest lies; cymbals produce lots of non-harmonic tones, i.e. frequencies not related to each other nor a fundamental pitch by integer multiples as we find with strings or pipes.

I set up a mic stand over my ride cymbal and improvised a makeshift axle from the clamp for a reflection filter, this would function as a pulley and allow a length of string to travel freely over it and up and down over the cymbal. For the microphone I chose my trusty Oktava MK-012  (modified by Micheal Joly) with an omni-pattern capsule so that I could get really close into the cymbal without the proximity effect. I tied the mic to one end of the string and made sure there was enough free XLR cable to allow the mic to travel freely up and down.  With the other end of the string in my left hand and a drum stick in my right, I practiced striking the cymbal whilst simultaneously lowering the mic. It took a few goes to get the timing right but eventually I could ride the mic in just after the initial hit and get it very close to the cymbal surface for the decay.

I also experimented with moving the mic around the cymbal surface to capture different harmonics. Monitoring on headphones also allowed me to get the movement right in terms of riding the volume.

So here are some results:

I think there’s some great scope for sound effects and soundscapes using these. With that in mind, here are a pair of hits reversed, hard panned and with an added splash of reverb to create an eerie, ‘something bad is about to happen’ kind of sound:

Happy experimenting…..


“Boutique” DI box on a budget

I’ve been meaning to build a passive DI box for sometime, I have a couple of active ones and it’s nice to have an alternative. Also, I recently bought a Korg Minilogue and figured a good quality passive DI would be a good match.

Passive DI’s are very simple, they are essentially a step down transformer used to bridge the high impedance output of an instrument to the lower impedance input of a mic preamp, as well as to convert from an unbalanced to balanced signal. As they are based on a transformer, the performance of the DI box is pretty much uniquely governed by the transformer, and good quality transformers aren’t cheap. There are a number of high quality DI boxes out there, such as the Radial JDI, but being a DIY fiend and a slight cheapskate, I thought I could have a go at making one, how difficult (and expensive) could it be?

There are a few great posts giving instructions on building a DI box from scratch, but what I noticed was that as you get all the parts together, the bill of materials creeps up and the cost benefit of doing it yourself  becomes less attractive. Now, cost is not the only reason why I build things, in fact, those EZ1290 preamps ended up costing me way more than I expected (they were a labour of love), but frankly, if I can build 2 good quality DI boxes for the price of buying one, then that’s worth the effort for me.

Whilst doing my research I noticed there were a number of cheap passive DI boxes out there, and I’m guessing the reason they were so cheap was because they used a poor quality transformer, so rather than build a box from scratch, why not just upgrade a cheap one? I found this Cobra DI box on EBay, it only cost £9.95 (+£2.95 p&p) but comes in a metal case with pad and ground lift circuitry and a thru jack. Perfect as a donor, all I needed was a transformer.


I went for this Cinemag CM DBX (EBay £64.99)


Good alternatives include:

Jensen JT-DB-E/EPC (as used in the Radial JDI)

Lundahl LL-1530, LL-1576/77/78

Sowter 4243, 8044

The Cinemag website includes schematics for a DI circuit using their transformer, comparing this to the COBRA DI box there were a few components missing but I thought I’d wire it up as a direct replacement for the existing transformer first and then see if there were any issues.

Firstly, open up the COBRA DI box by unscrewing the 4 retaining screws on the base.


With the cover removed you can see the simple and robust construction, the original transformer is through-hole mounted on the PCB.


Flipping the box over and working with a solder sucker, remove the original transformer.


You can see the obvious differences between the original and upgrade transformer, the mu-metal can for shielding, but the Cinemag also has individual shielding for each winding and will use better quality wires and laminations.


I soldered the Cinemag wires into the existing PCB holes following this wiring scheme:

  • Yellow – Input Tip
  • Orange, Black, White – Input Sleeve (PCB ground)
  • Gray – XLR 1
  • Red – XLR 2
  • Brown – XLR 3

I tested this configuration and it worked fine, but I noticed that in the Cinemag schematics they wire only the white cable to case ground (not the black and orange too), so I modified the wiring slightly such that black went to the PCB ground and white connected to the case via the existing green cable.

I will admit that I was feeling lazy and didn’t fancy drilling the case or the PCB to mount the transformer, so instead I used cut up neoprene pipe insulation to snuggly hold the transformer in place between the case and PCB. Once the case is on the transformer isn’t going to move anywhere and there is no strain on the connectors.


And that’s it, job done, a very simple and quick route to getting a high quality passive DI box on a budget.





EQ Ear Training using Max For Live

I love Max for Live, it’s DIY audio in the software domain and it makes trying out ideas and building new audio tools extremely easy.

I built this device to help improve your ability to judge and apply EQ, you will of course need Ableton Live and Max For Live to be able to use it, you can download the device here:

The concept is simple; you listen to some source material and the plugin applies random EQ boosts (or cuts) and you have to guess what the frequency band is. If you do this regularly enough your ear will become attuned to the different EQ frequency bands and their effect on the source material. Just as with musical interval training, the more you do the quicker and easier it becomes to identify and apply EQ effectively, ultimately leading to better mixes / masters.

The idea came from Bob Katz book Mastering Audio: The Art and the Science , which I thoroughly recommend if you want some in-depth knowledge on some of the concepts and practices in mastering. In an early chapter he outlines various exercises to improve your ear in the context of mastering, and this is based on one of those.

I’ve put together a video to explain how to use it, it’s pretty straightforward:


Main features are:

  • Audition mode to hear the effect of each EQ band
  • training mode to test your ability to identify the frequency band in use
  • Apply EQ cut or boost, 3 or 6 dB and with Q factor of 0.5 (broad) to 1 (narrow)
  • Choose between your own source audio material or pink noise

Building it was relatively straightforward, there are a few quirks with max that I’m still getting used to, for instance, I only just figured out the sequence of execution of objects triggered off the same bang depends on the position of the objects in the patch editor (bottom to top, right to left).

Hope you find it useful, let me know what you think.



Piano Re-Amping

I recently watched a brilliant online recording masterclass with Sylvia Massey courtesy of Creative Live, very inspiring and it reminded me of something I’d lost sight of, the spirit of experimentation. During the class she records a band live and demonstrates some of the techniques she uses as well as her creative process. What struck me was how playful it was. Among the many things she did (including vocal feedback delay using 2 mobile phones a mic and a monitor speaker,  and passing a guitar signal through a power tool), she re-amped a snare by passing the recorded track back through a speaker with a snare strapped to it. The recorded sound of the re-amp was blended with the original snare to get fatter drum sound. This got me thinking, what else could you apply that too?

I’ve been doing a few tracks recently where we’ve used midi piano, I’ve been using the sampled grand piano in Ableton which isn’t bad at all (to my ears at least), however, it’s still a sampled piano and lacks a little dimension. I had the idea of re-amping the piano by passing the sampled piano track back through a speaker playing into the body of the upright piano I have at home, then micing the piano to get a new piano track. The idea being that this new track contains the string and body resonances of the real piano. The final piano track would then be a blend of the 2.

I set about doing this, it was relatively simple. A spaced pair of AKG C414s aiming inside the top of the piano, close in on the strings, and then a speaker placed at the bottom of the piano aiming into the body (I used my Avantone Mix Cube for the job).I removed the lower panel of the piano so the speaker could play into the body (see photo)

Using a real piano to enhance the sound of a sampled piano
Using a real piano to enhance the sound of a sampled piano

Once all this was set up, it’s just a case of pressing play and record. In the mix, just blend the original with the re-amp track according to your taste.

Here’s the original sampled piano track:


Here’s the re-amped track:


Here’s a blend of the 2:


You can hear how it adds a depth and dimension to the sampled piano without recourse to loads of plugins.

I’ve also tried passing other signals through it, including vocals and guitar tracks,it lends an interesting colour.

I’m liking the natural colouration you get with re-amping, it’s something I’m definitely going to explore more.



Comparing Convertors

I regularly review music gear for Sonic State, it’s an interesting job as I have to look in detail at a particular piece of hardware or software. Often when you are just using a piece of kit, you don’t get time to delve into all the subtleties of it and learn all its’ full capabilities. I recently had a look at the new range of Apollo audio interfaces from Universal Audio, the full video review will be coming out in due course over on the Sonic State website, but in the meantime I thought it would be interesting to post up some audio illustrating the performance differences compared to the previous generation of interfaces.

The new Apollo thunderbolt-only interfaces boast different cosmetics and some enhanced functionality, but what I was most interested in was the sonic differences due to the revised A/D and D/A signal paths. On paper the specs show improved SNR, THD and Dynamic Range. Given that the previous generation of Apollo interfaces were already very well specified in this department, I did wonder how much of an audible difference these improvements actually made.

In order to test the differences I set up a simple drum recording session: 4 mics on a 1950’s Premier drum kit (EV RE20 outside kick, AKG C414 on snare, Oktavamod Oktava MK012’s for overheads) and my good friend and ace drummer Mark Whitlam laying down some funky yet consistently repeatable grooves. We tracked first through the 4 preamps on the older generation Apollo, then immediately switched the mic cables into the first 4 preamp channels on the Apollo 8P. Mark layed down the same groove again (clicked for ease of comparison) and that was it. Admittedly, this is more a real-world test rather than a carefully controlled scientific experiment, there will always be slight differences in the playing between each take no matter how consistent the drummer, but it gives a good indication of the differences.

Here are the audio files, listen for your self and decide if you can tell the difference:

First, the old generation Apollo Quad:

And next, the new generation Apollo 8P:

Can you hear a difference?

You can download the 24bit 44.1kHz audio files yourself from this link, load them into a DAW and flip between solo’d tracks to get a more direct comparison.

To my ears, I hear a different emphasis in the mid to high range, I can hear it in the hi hats and ride cymbal. I also perceive a bigger stereo image somehow, the the whole thing sounds a little smoother, more open and less boxy compared to the old generation. These are very subtle differences, but good monitoring or headphones do show them up.