DIY Sub Kick

The concept of a sub kick is very simple, take a large speaker and use it in reverse as a microphone specifically for picking up low frequency information, from a kick drum for instance. This can be used in the mix to reinforce the sound of the kick alongside another kick drum mic,  providing additional depth to the sound due to it’s focus on the sub 100Hz frequency range. The subkick won’t pick up much spill from other parts of the kit either, and a good trick is to use it to trigger a gate on the internal kick drum mic, to get a very tight and clean kick sound.

Various companies make them, the  Yamaha SKRM100 for example, which is essentially a 6.5″ woofer mounted in a small drum shell, with mesh either side and a built in stand. The price, £350, quite a bit for speaker on a stand! Over the years I’ve seen various DIY versions in studios around the way including an NS10 woofer clamped into an old worktop vice as a base, all these incarnations functioned well, and cost significantly less than £350 to make. Time to get the tools out I thought.

This is an easy DIY project, it involves mounting a woofer speaker into a suitable frame (in this case a tambourine shell), and then a little soldering to attach an attenuator and a male XLR connector.

20140904_113605

First start with a speaker, I went for an 8″ Wharfedale woofer of EBAY (approx £18), you can use anything from 6″ up, though some say it doesn’t work as well with larger speakers. I’m not sure about that, the 8″ seems to work pretty well. Most important is the speakers frequency response, it should go down to about 20Hz to get all those sub bass frequencies.

20140904_113623

I thought a lot about how to mount it, I wanted something that would isolate it and look good, but not cost too much. The cheapest and most elegant solution I could think of was to use elastic hairbands to suspend the speaker within the circular frame of a tambourine (without the jingles of course). So I ordered a 10″ tambourine (£7 Ebay) and removed the jingles by pulling out the nails holding them in place.

20140904_115038

Slip the elastic hairbands through the screw holes in the speaker, then feed each end through one of the slots in the tambourine rim and push the upper loop (the one that goes over the rim) round and under so the 2 loop ends overlap. Take the nail that used to hold the jingles in place, push it back through it’s location hole so that it pegs the 2 loop ends of the elastic in place. Push all the way in so it’s secure. That was probably the trickiest step in all of this, required a lot of patience to get the elastic and the nail to all go in the right place. Repeat this for the remaining screw holes.

20140904_121221

The next challenge was figuring out how to mount the the subkick using a normal mic stand. I happened to have a broken mic stand lying around so I took the pivot mount from it (the bit that screws onto the top of the vertical part of the stand and allows the boom to attach to it and swivel up and down). The advantage of using this was that it had a standard screw thread on the end which meant I could attach it to a regular mic stand (as shown in the picture above). The pivot mount itself attached to the tambourine rim with a nut, bolt and a couple of washers (through one of the existing slots in the tambourine).

Next step is connecting it up to a mic pre-amp. The output of the subkick is pretty hot, an attenuator (or pad) is recommended to bring the signal level down to the region expected by most pre-amps. You can buy an inline attenuator that connects between the subkick and the XLR cable to the pre-amp, or you can knock together a basic potential divider circuit to do the job for you. I opted for the latter as you probably guessed. Another point worth mentioning here is that I’m going to use an unbalanced connection, providing the environment isn’t to (electrically) noisy and cable runs are short, this shouldn’t be an issue. If you wanted to go for a balanced connection, you would need to wire in an audio transformer, and to get one that has good sub-sonic frequency response isn’t going to be cheap. You could also use a DI box, again, a good quality one with good bass response would be preferable.

Circuit 1

The schematic for an attenuator circuit is shown above, I used a simple potential divider, the values I chose for the resistors R1 and R2 are:

R1=680 Ohm , R2 =150 Ohm

There are many combinations you could use and they also depend on your speaker, Google “attenuator for Subkick” if you want to get lost in that world!

 

20140904_202512

With the resistor values chosen, I soldered them directly to the speaker terminals and covered them with heat shrink. Easiest way I found was to solder one end of R2 to the speaker – terminal, and one end of R1 to the speaker + terminal, slide a sheath of heat shrink over each resistor then twist the exposed ends together, before applying heat to shrink the insulation (see above).

20140904_203750

I then soldered a short length of guitar cable to the speaker – terminal and the exposed resistor ends, ensuring I had a sleeve of heat shrink in place to insulate the connection.

20140904_210922

Finally I soldered the male XLR connector onto the end of the cable, (Pin 1 to Speaker -, Pin 3 to the resistors, Pin 2 unconnected). I had a retaining clip lying around so I used that to secure the connector to the frame, so as not to put too much mechanical load on the soldered joints.

So there you have it, a fully functioning subkick that doesn’t cost an arm and a leg, doesn’t look to shabby, and is not to difficult.

20140904_214801

Here is a quick audio example of the subkick in action. The audio is as follows

Bars 1-2 : Internal kick mic only

Bars 3-4 : Subkick only

Bars 5-6 : Internal mic + Subkick

Bars 7-8 : Internal mic + Subkick (with gate on subkick)

Bars 9-10 : Full kit, no subkick

Bars 11 – end : Full kit with Subkick

https://wijproductions.files.wordpress.com/2014/10/example-subkick.mp3

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.

20140829_135735

– 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.

Screen Shot 2014-09-23 at 11.12.11

20140829_135927

20140829_141051

– 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.

20140829_140456

20140829_141122

 

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.

20140829_140318

– 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.

20140829_141807

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.

20140829_141905

 

20140829_142053

20140829_142527

 

– 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.

20140829_143308

– 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.

20140829_144913

– 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.

20140829_150204

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.

 Screen Shot 2014-09-23 at 11.48.22

– 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/)

20140829_145015

– 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

20140829_154006

– 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.

20140829_154236

– 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.

20140829_155242

– 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: