The DIY preamp saga

There comes a point when many a recording engineer starts to consider the finer points of the recording chain, and inevitably finds themselves in the esoteric world of mic preamps. These days there are so many choices both off the shelf and DIY, so many opinions, and so many fuzzy descriptions of ‘warmth’, ‘open-ness’, ‘transparency’ etc…It’s hard to know where to begin.

I began by accident. I bought a job lot of electronic components off EBay, the seller claimed that all the components required to make a Neve preamp were included. Well, that was optimistic, I think there were about 4 capacitors and a handful of resistors that could be used, but anyway it piqued my interest. I found out the project board he was intending to use which led me to the fabulous www.groupdiy.com website, where Martin Adriaanse has created a complete guide to building the EZ1290 Neve preamp clone, including a bill of materials, circuit schematics, build guide and some basic troubleshooting points. You can purchase the PCBs directly from him (they are not available anywhere else).

My first decision was whether to go the complete DIY route (where you have to source all the parts yourself) or go for one of the many kits provided by the likes of Seventh Circle Audio or JLM Audio . Being the masochist that I am, I opted for buying the PCBs and sourcing components myself using mainly Mouser in the UK, how hard could it be? Of course, some of those nice polystyrene capacitors weren’t available on Mouser, nor were the BC184 transistors and a few other little things, so I ended up on EBay where I found pretty much everything else I needed.

The Neve sound is all about transformers, and not just any old ones; properly designed, carefully manufactured transformers made for the specific task of interfacing microphones and line level signals. You need 2 per channel, an input mic transformer and an output line transformer. These aren’t cheap, I got mine from Audio Maintenance Limited who also sell the Grayhill rotary switches used for the gain selector. All in, 2 channels worth of xformers and switches cost the best part of £200 once delivery was factored in.

Once the components were in, soldering the PCBs was a relatively straightforward task, pay attention to put things in the right place and use a heatsink on those capacitors. Use a fine tip soldering iron (especially for that rotary switch, the pins are tiny and close together) and audio grade solder.

Soldering the components onto the ez1290 pcb
Soldering the components onto the ez1290 pcb

I started with resistors first, then smaller caps, then transistors and larger caps.

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Almost complete ez1290 pcb

Note the overkill polystyrene capacitor on the right, couldn’t find the right voltage spec one for that capacitance so had to go for a 400V spec, hence the size. Still fits and works!

Building the boards was the easy bit to be honest. Once they were done the fun of trying to fit everything into a case and route the wiring began. I got a 1U rack case off EBay again, it was from a network router so needed a bit of tidying up, and I also ordered a plain aluminium front panel which I was going to attach on top of the existing front plate. I’ll admit that the rack case was probably my biggest cock-up. I thought it was full depth but when it arrived it was a shallow one, so stuffing all the PCBs, transformers and connectors in was going to be a challenge.

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I put the transformers for each channel down the sides and the PCBs in the middle. This just about worked, I had to leave space for input, output and power supply sockets too.

I wanted to make use of the input transformers multi-taps, so I could make a switchable impedance for the mic inputs. Although not strictly necessary these days as most mics are designed to work with the typical high impedance of most mic preamps, it can help when interfacing with older mics and it also offers another tonal option for a given mic. The taps on the primary side of the transformer allow the two halves of the primary to be wired in series (1200Ω) or parallel (300Ω), I wired in a switch to change the configuration via the front panel.

Nearing completion, fitting the front panel
Nearing completion, fitting the front panel

The only other switch I fitted was a +48V phantom power switch. Phantom power is introduced to the hot and cold connectors on the input XLR via 6.81K resistors. I left out phase and pad switches for now. I also opted not to fit the output trimmer. You ideally need to use a high quality pot, I think I may add this for the next build as it allows you to drive the input stages harder for more grit if you want it. Some people say that the trimmer degrades the sound quality slightly, that’s a better reason than not being arsed to fit it.

The other important piece of the puzzle is the power supply. These boards need a +24V supply as well as +48V  for phantom. I had no choice but to locate all of this in a separate enclosure, though with careful planning you could fit all this into a larger rack unit (watch out for induced mains hum from the power supply transformer). JLM do a suitable power supply, I already had a toroidal transformer so found a PSU kit from a Russian supplier on EBay. These do the job, but I blew the first one with an accidental short circuit.

Once everything is connected, you need to bias the output transistor, for this you ideally need an oscilloscope and a signal generator. I used a 1kHz test tone from my sound card and a borrowed scope from the local university, very useful diagnostic tool and I might have to invest in one if I’m going to do this more often. Biasing is explained in the build guide and is a pretty straightforward procedure, just adjust the trimpot till you get symmetrical clipping and your done. Pushing it too far in the wrong direction can make things get very hot.

My first board wouldn’t bias properly, I ended up swapping all the transistors in the output driver stage and it all worked ok after that, took a bit of head scratching though, the build guide gives a few tips such as the typical DC voltages at each of the transistor pins but it wasn’t obvious which was the culprit.

I had some intermittent hum issues, I was using the case as a ground so I implemented a star grounding scheme, seems to have worked.

I used a powder coated metal enclosure for the power supply, this is a pain as the individual panels don’t necessarily conduct with each other so the safety ground had to be wired to each panel to ensure proper earthing.

I blew the 1W resistors on the + side of the board supply rails by letting the underside of the board accidentally touch the case (which is earthed) when testing, won’t be doing that again.

The finished product (Roof off)
The finished product (Roof off)

So here is the finished version. Sure, it’s a little chaotic and Heath Robinson in there, I used whatever material I had to hand to make things like cable guides and mountings for the transformers. But this is my first build.

I’m glad I chose to do it this way, you learn so much more by going through the stages yourself, figuring out details like how to package and connect up the system and then troubleshooting it, making mistakes, blowing components, ordering the wrong thing etc….I know what to do next time. Speaking of which, I’m considering this project as a prototype; I’m planning to get another 2 boards and build a 4 channel unit into a 2U (full depth) case, and add pad and phase switching for each channel, and maybe a switched mode power supply instead of that bulky and noisy toroidal transformer.

All in all it has been a lot of work (and expense) to get these mic-pres up and running, however, they work beautifully and I’ve been getting some great results on guitars, vocals and drums. I’m even starting to use words like ‘warm’, ‘open-sounding’ and ‘big’ when I describe the recordings. In the next post I will put up some examples to illustrate why I think they are worth the effort.

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.

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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.

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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.

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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.

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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!

 

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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).

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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.

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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.

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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.

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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.

Screen Shot 2014-09-23 at 11.12.11

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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: