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

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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.

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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.

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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.

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

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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.

Modifying the T.Bone SC1100 mic

Modifying the T.Bone SC1100 Large Diaphragm Condensor Mic

T.BONE SC1100

I bought one of these mics a while ago on the recommendation of a vocalist friend. Considering it cost about £100 from Thomann it was a good deal; 3 patterns, transformer coupled and smooth-sounding with none of the top end harshness of other mics in this price bracket. It also came with a nice metal case and decent shockmount. However, it is still flawed, there is a distinct scooped sound to it which can be flattering but also leads to lack of clarity, and the low frequency response is muddy or ‘woofy’, which is probably a lot to do with transient response.

I did some research to see if anyone had performed a mod on it, but very little came up, until I came across a this thread on the Advanced Audio Europe forum:

“…The SC1100 has a discrete class “A” transformer coupled circuit based on the original AKG 414 from the early 70’s. This circuit has 14db more headroom than a U87.

The capacitors in the SC1100 are already high quality tantalum and polypropylene. R10 can be changed to a 2.2K which will increase the output level and headroom by 3db.

The SC1100 has a dc to dc converter board similar to the U87AI in order to polarize the rear diaphragm via the pattern switch with 110 v dc for FIG 8.

The SC1100 has a low tech single winding transformer that works remarkably well when driven from the much lower output impedance of the 414 circuit.

We can supply a 2.25:1 transformer with dual bobbin windings and bi-metal laminations for $59. Our 2.25:1 transformer will take 6db more level than the stock transformer and recover much faster from percussive transients.

The SC1100 has no pre-emphasis and there is lots of room in the head grill for either our AK47 or AK67 which will both work well with that circuit.

The AK47 will give it a more U47fet tone but with 3 patterns and the AK67 will give it a more U87 tone but with more headroom….”

Looks like someone else had the same idea, I contacted Advanced Audio and they supplied me with a new capsule (AK67), transformer and resistor to perform a relatively simple upgrade to this mic.

The result? A clearer, more precise mic with nice tight bass response. You can opt for a different capsule of course, and this will change the character of the mic, they recommend their AK47 or AK67 capsules. I found the AK67 added back the mids that seemed scooped in the original, giving a lot more presence to recorded sources, I’ve included some quick audio examples at the bottom of this post.

The basic cost of this mod was 145 Euros plus p&p for the AK67 capsule BV2.25 transformer, and they very kindly threw in the 2.2k resistor as well.

If you’re interested in performing this mod yourself, then I will outline the procedure I followed. I would say this is an easy to medium mod, you will need to be confident with soldering and desoldering components (see here for some good tutorials), and not be squeamish about completely dismantling the microphone!

Disclaimer: Be aware that you could damage your mic if you get this wrong! I am in no way responsible for any damage that may occur as a result of following my guide. Use common sense and take care and you should be fine, if in doubt or lacking confidence, find someone who can help!

You will need:

  • Suitable replacement capsule , transformer and resistor (many suppliers can provide these components, I found Advanced Audio Europe to be very helpful in recommending the right parts, get in touch with them here)
  • A temperature controlled soldering iron with a fine tip
  • Desoldering pump
  • Audio quality solder (4% silver)
  • 1mm dia. heat shrink
  • wire cutters / strippers
  • Set of small Phillips screwdrivers
  • 99.9% Isopropyl Alcohol, a toothbrush and / or cotton buds (for cleaning the solder joints afterwards)
  • (optional) A crocodile clip to use as a heat sink when soldering near heat sensitive components.
  • (optional) A camera

Time required is about 1-3 hours depending on how skilled you are (took me about 3 and I’m not that skilled!)

1) Prepare your workspace

Not essential but this is how I like to work: have a clear, clean workspace with good lighting ready, warm up the soldering iron and lay your tools out. I grounded myself for ESD protection, not sure if it’s strictly necessary but I did it anyway, I have a wire connected to the radiator pipe which I wrap round my finger.

I took photos every step of the way so I could refer back to see where wires were connected or how things fitted together, very useful and no hassle these days with camera phones.

2) Dismantle the mic

– Unscrew the base of the microphone, slide off the body sleeve and put both parts to one side.

unscrew the microphone body

-With an appropriate screwdriver, undo the 2 screws either side of the head basket and then remove the basket and he plastic locating ring that mounts the headbasket to the inner metal runners.

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– I’m not sure if it’s strictly necessary, but I found it easier to completely dismantle the mic to work on it, so I removed the 4 screws on each PCB that hold them to the frame, and the 6 screws holding the transformer case and XLR connector to the frame too.

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– desolder from the PCBs the 3 wires coming from the capsule. I found it better to do this than cut them as the blue wire on the replacement capsule was not quite long enough so I had to exchange it for the one from the original capsule. (Use a heat sink on the leg of the capacitor to prevent damage when desoldering). If necessary, ensure you take a picture(s) of where the wires connect so there is no confusion when reconnecting.

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3) Remove the existing transformer

– As you’ve seen, the transformer is located in the metal can at the bottom of the mic, open this and pull the transformer out, it is usually stuck to the lid with an adhesive pad.

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– cut the wires to the transformer fairly close to the transformer itself, the replacement has short wires and you will need to splice them to the existing wires in order to reach the PCB.

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4) Install the new transformer

-The replacement BV2.25 transformer is dimensionally quite different, you will need to install it lying on its side in order for it to fit in the can. I wrapped the metal core of mine in electrical tape, not sure if it’s necessary but the original was wrapped up too so I figured it might be useful to do. Route the wires through the 2 entry holes on either side of the transformer can.

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– Prepare the ends of each sets of wires for soldering (strip and tin where necessary), place a sheath of heat shrink on the long wires leading to the PCB and push down out of the way.

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– match up the colours of the wires from transformer to PCB and solder them together, you may need some clamps to hold the wires in place as you do this.

– slide the heat shrink sheaths over the joins and apply heat till they contract, ensure no bare wire is exposed.

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– relocate the wires in the metal runners and assemble the transformer can and XLR connector back into the frame. Replace the 6 screws taking care not to pinch any wiring in the metal runners.

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5) Swap the resistor

– Locate the resistor labelled R10 on one of the PCBs, flip over and locate the solder joints corresponding to this component.

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– using the desoldering pump, desolder and remove the resistor (I will assume you know how to do this, but if not sure then here is a good guide: http://www.tangentsoft.net/elec/movies/)

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– fit the replacement resistor, solder and trim the terminals

6) Capsule swap

– If your not intending to re-use the existing capsule, then you don’t have to be so careful about handling it. With the new capsule, only hold if from the sides and don’t pull on the wires connecting to the front and rear faces of the capsule.

– remove the existing capsule from the plastic saddle by removing the 2 holding screws either sideScreen Shot 2014-09-23 at 11.59.53

-Take the new capsule from it’s case and locate it on the saddle so that mounting holes are aligned with those on the saddle. Using the screws provided with the capsule, screw the new capsule in place and tighten, ensuring there is no play.

– feed the capsule wiring through the holes in the top plate of the microphone leading to the PCBs. If the blue wire appears to be too short, then you can remove the blue wire from the old capsule by undoing the retaining screw and swap it with the wire on the new capsule.

– to avoid accidental damage to the exposed capsule, I slipped the metal sheath from the mic body over the capsule, this is particularly usefull when it comes to soldering the wires onto the PCB as any spurts of solder could potentially damage the diaphragm

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– resolder the capsule connections, the blue wire goes to the shared terminal with the capacitor (use a heat sink to protect the capacitor), and the red wires go to the terminals on top of the other PCB. To avoid confusion, the PCB with the 3 way pattern select switch is mounted on  the front side of the mic, the wire from the front side of the capsule goes to the right hand side connection when looking at the back of the PCB.

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– clean the back of the PCB and all new solder connections with the isopropyl alcohol20140829_145823

 

7) Re-assemble and Test

– re-attach the PCBs using the screws, be aware of the capsule and avoid damaging it, replace the plastic ring and headbasket, now you can relax a bit as the capsule is protected! slide on the body sheath and tighten the base, the black backing plates to the pattern select and filter switches are likely to have come off and will need inserting before you put the body back on.

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– plug it to your preamp, apply phantom power and test, speak into it first and make sure you are getting signal, then try it on a range of instruments or vocals and see how you like it.

Good luck, I hope you enjoyed and / or have found this useful, I’ve made 2 of these mics now and am very happy with the recordings I’m getting from them. Here are some quick audio examples to illustrate the change in character of the modified mic compared to the standard version:

Zen and the Art Of Microphone Modification

 

 

 

After much standing on the side procrastinating, reading various websites, blogs and how-to guides, I decided to get my hands dirty and take on a microphone modification. I had recently bought an Apex 460 valve mic from the states, and an upgrade kit from the very helpful people at Microphone-parts.com. The kit comprises everything you need to convert the stock Apex 460 into a very nice sounding mic. The stock mic sound is nothing special, but it’s only when I did a comparison with the modified version did I realise how big a difference the mod really makes.

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Basically, the modification involves changing the tube to a better quality, lower noise one, removing the RF filter circuit, swapping critical capacitors, diodes and resistors with higher quality components, and changing the capsule for a better quality version.

I spent the best part of a day doing this, but I was going slowly because it was

a) My first time, particularly doing desoldering work

b) I was ultra paranoid about wrecking the mic

Here’s what I got, where I got it and how much it cost:

– APEX 460 Tube mic from Ebay (MegaToneMusic) Total cost including shipping to UK and customs = £205

– Apex 460 mod kit (The Fox SG version from Microphone-Parts.com), approx £140

– Thermostatically controlled soldering iron and desoldering pump – Ebay – £15.99

– Desoldering Braid – Ebay – £3.49

– Helping Hand crocodile clip and magnifying glass (for holding PCBs whilst soldering) – Ebay – £2.88

– 4% silver solder (1m) for audio connections – Ebay – £1.99

– 99.9% Isopropyl alcohol – Ebay – £3.95

These items I already had lying around:

– A clean toothbrush and some cotton buds

– regular solder for de-soldering

– wire cutters, screwdrivers

– clean containers for storage

So you can see that it’s actually quite cheap (compared to spending several hundreds of pounds on a valve mic). I’m not a particularly experienced solderer, I know how to do it and have done the odd repair job, but nothing major, and I’ve never desoldered and replaced components on a board before. I found the overall process of moderate difficulty, once I got the hang of desoldering it was actually quite quick. The kit recommends using the vacuum pump to desolder, but I also ended up using the braid in some places because it was easier and cleaner.

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The first task is to swap out a pair of diodes in the power supply, this is fairly straightforward and a good way of easing yourself into the job, you have to make sure the power supply has been disconnected from the mains for at least 1 hour so all the caps have fully discharged and you avoid the risk of electric shock. Once this swap is done, fire up the mic and check all is ok, then it’s time to work on the mic itself.

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Opening up the mic, firstly you remove the old tube, which is held in place by a spring loaded pad at the bottom, once done you can unscrew the 2 PCBs from the frame, desolder the capsule wires and open up the 2 PCBs to access the joints on the underside. The very detailed and comprehensive instructions tell you exactly which components to desolder. A useful tip here is to always bring the soldering iron in from the edge of the board, to avoid burning any wires or components on it by accident (as I did!). The “helping Hand” tool was also quite useful here, allowing me to hold the PCBs at the best angles to facilitate access to the joints.

Once the necessary components had been removed, it’s time to repopulate the board with the new components (taking care to observe correct polarity for certain capacitors). The FOX SG kit comes with a bit of 4% silver solder (audiophile grade apparently), however I bought a meter of the stuff off Ebay, probably my soldering but I ran out of the stuff they provided before the last component was done. Once the components are in, time to clean the boards with the alcohol to remove flux and other contaminants (taking care not to get any on the polystyrene capacitors).

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At this stage, they recommend testing the mic, probably wise, so put the new tube in, reconnect the capsule wires and plug in. Providing this goes successfully it is then time to remove the headbasket and swap the capsule. This was the bit I found most fiddly, probably because I was so paranoid of touching or damaging the new diaphragm that I ended up being over cautious. Suffice to say, it was actually quite straightforward in the end, and the new capsule was on in no time at all. The wires onto the PCB need to be trimmed as short as possible to reduce capacitance, but be careful here, I cut mine a little too short and had real trouble accessing the joints to reattach them.After a final clean and inspection it was time to reassemble and test.

Initial results were very encouraging, on vocals the mic sounded big, warm and deep, listening to the pre-mod version it sounded thin and harsh, this new mic is very smooth indeed, looking forward to putting it too work in the coming days. I did a quick comparison with a few other mics to get an idea of how it sounds:

I had been lent a Sontronics Aria for a few days to play with, which was perfect timing as it gave me a high quality reference to compare with. Although the Aria ultimately performs better (slightly quieter and smoother), it is over twice the price and the 460 holds it’s own very well.

As a final mod, I decided to remove 2 of the 3 layers in the headbasket, this took a bit of patience and fiddling with a pair of pliers to get them both out, I found using a sharp pointed object to prise up some of the mesh before getting stuck in with the pliers worked best.

So there you have it, if you are thinking about doing this and are reasonably handy with a soldering iron then I would thoroughly recommend it, the mic is quite easy to work with and the upgrade kit is great, well documented and the choice of components really makes a massive difference. I’ll be doing this again I think!