Microphone Collection

Microphone Collection

Microphone Collection.

We’ve been making some changes at Agent Audio HQ recently, amongst them, updating our Microphone Collection.

The most recent additions to the collection are the Northern Electric 633A “Salt Shaker”, often badged under Altec or Western Electric, and the STC 4021c “Ball and Biscuit” or “Apple and Biscuit” Omni-direction pressure gradient.

STC 4021c

I’ve been after one of these for a while.  The famous  STC 4021c “Ball and Biscuit” or “Apple and Biscuit” (depending on which BBC engineer you ask).  The STC 4021c is an Omni-direction pressure gradient microphone which resembles a small cannonball.  It sounds best as a drum mic with a genrous amount of FET based compression.

Altec 633A

The Altec 633A, “Salt Shaker” microphne was sold under different brands, such as Western Electric.  This one is branded Northern Electric.  A quick look inside to rewire it for XLR operation revealed that this particular design is from 1941.  It’s interesting to note the tube which equalises the pressure at the rear of the capsule making it uni-directional.

Sound Engineers EDC

Sound Engineers EDC

EDC is a term used by Survivalists and Preppers, it means Every Day Carry.  EDC can just be the thing you carry on you each day – sunglasses, wallet, keys, etc.  However, it can also mean a prepared kit of useful tools and objects.  With this in mind, I have developed a Sound Engineers EDC over the past couple of years, bear in mind that this is always a work in progress and gets updated regularly.

Equipment List:
Maxpedition Fatty Organiser
Mini Sharpie Pen
Fischer Space Pen
Gaffa/Duct Tape
Insulation Tape
Gas Soldering Iron
Desoldering Braid
Heat Shrink
Test Leads
Mini Multimeter
Drum Keys Square and Hex
Moon Gel
Mini Clip-on Tuner (+ Decent Tuning Phone App)
Selection of Guitar Strings
Plectrums (different thicknesses)
PP3 and AA batteries
Various adaptors
FlatKeys P!NG
FlatKeys S!NG
MEDC Compact Medical Kit
MagneticPickup Tool with Plumbers Mirror
Rolson Mini Precision Screwdriver
Stanley Mini Screwdriver
Leatherman Multitool (Note: It is currently not legal to carry a lock-knife in the UK without having a specific purpose, it’s classed as a fixed blade, so probably best to buy one without a lock on the blade.)
Beyerdynamic M260 N Repair

Beyerdynamic M260 N Repair

We have just acquired a classic ribbon microphone in the form of the Beyrdynamic M260 N circa 1965.  The seller said it was rattling and might not work so, personally, I wasn’t holding out much hope.  This is an interesting microphone in that it’s polar pick-up pattern is hypercardioid as opposed to the usual figure-8 associated with most ribbons, this is to do with the way the ribbon is mounted in the microphone.  It’d great on guitar cabs, hi-hats and sounds of a similar nature and boasts a flat frequency response from 40hz to 18khz.

Beyerdynamic M260N

Top removed to reveal the ribbon element

After bit of research I discovered that the windscreen comes apart in the centre, revealing the ribbon capsule. Upon opening it was fairly obvious what was causing the rattling sound, the plastic peice which protects the ribbon had come loose.  I managed to carefully unpick the entire piece to reveal the ribbon sat in the centre of the magnet.  The ribbon looked in very good condition, no marks, dents or anything.

Beyerdynamic M260N

Cleaning up the magnet.  Ribbon visible

Next I very carefully cleaned the magnet each side of the ribbon to remove some of the rust.  I use isopropyl alcohol and a swab, being very careful not to get any particles on the ribbon.

Beyerdynamic M260N

Re-assembly of the capsule.

Next some superglue was supplied to the outside of the magnet, again being careful to get it nowhere near the ribbon.  Only using the minimal amount.  The plastic cover was then re-fitted and the small pieces of tape re-attached.

Beyerdynamic M260N

Beyerdynamic M260N Repaired

The repaired microphone was then tested.  The microphone does not have the standard XLR fitting of a modern microphone instead having a Klein Tuchel type connector, fortunately I already have and old AKG which used the same connector so was able to borrow the lead for testing the microphone.

More information on this microphone model can be found here, at the excellent Martin Mitchell’s Microphones.

Pedal Building

Pedal Building

Pedal Building.

A post which examines the electronic components, skills and knowledge required to build simple guitar effects pedals.

I often get approached by people asking me how to get started with audio electronics.  The best way is probably to build one of the excellent kits available from Fuzz Dog’s Pedal Parts or smilar websites.

However, in the age of the Maker, there’s nothing quite like building your own circuits from scratch.  Getting the basics down provides a good platform to try more complex builds later including etching your own circuit boards and designing custom enclosures.

Please note this post covers basic through-hole audio electronics for the hobbyists,  I’m sure there’s more professional level advise out there but, when it comes to electronics, I’m a hobbyist and happy to remain so.

The most important thing you can do, as a beginner, is actually build circuits. Sure you’ll fry a few compents but it’s all part of the  fun! Sure, you can (and should) support your experiments by reading up on the theory that accompanies the components and circuits, but nothing beats actually building circuits … especially when you get them to work.

In order to make the most out of your prototyping and development, you’ll need to buy some basic gear to build and test your circuitry.  None of these things are expensive but can help save you time and frustration in analysing and diagnosing any faults.


A multimeter for testing circuits


For connecting components

Battery Clip

For powering the breadboard


Crocodile/Aligator leads


For prototyping  circuits


For testing your circuits

Safety: It’s important, when first starting out only to power your circuits with a battery, including the amplifier.  Thus, if you do make a mistake, like we all do occasionally, the worst you can do is pop a component or melt something! It’s a rite of passage in learning electronics.

Once you have collected all your prototyping gear togther, you will then need to purchase a handful of components to experiment with.  Sometimes it’s hard to ascertain which components are suitable for audio circuits, especailly when starting out.  Below are some common values of the most common through-hole components found in audio circuitry.

You’ll need to have a basic understanding of the following components to begin building circuits: Batteries (Cells), Resistors, Potentiometers (Pots), Capacitors (Caps), Operation Amplifiers (Op Amps), Transistors and Diodes.

With a good, sound knowledge of how these components operate, you can build most buffers, boosters, distortion, fuzz, overdrive pedals plus a few more, such as tremolos and phasers.

Remember audio is represented as A.C. (Alternating Current) in a circuit, so you need to know how these compenents work with both D.C. (Direct Current) and A.C.

Below are some of the most common values of components found in various audio circuits.  I compiled this info from building a variety of audio circuits.  However, having it in one place would have been invaluable when I first started my foray into audio electronics so, to save you some time, here they are…



Transistors come in all shapes and sizes from the simple BJT (Bipolar Junction Transistor) to the J-FET (Junction – Field Effect Transistor) and the MOS-FET (Metal Oxide Semiconductor – Feild Effect Transistor).  They also have polarity like diodes this is denoted as NPN (negative, Positive, Negative) or PNP (Positive, Negative, Positive).

Common Silicon BJT types:
NPN: 2N2904, 2N2222A, 2N5088, 2N3903, 2N2369A, 2N5089, 2N4401, MPSA18, MPSA13.
PNP: 2N3906, 2N5087, 2N2907.

Common J-FET types:
N-Channel: 2N5457, MPF102 and J201 (Note the pin out with the J201; it’s different!).

Common MOS-FET types:
BS-170 (These are susceptible to static so be careful handling them).



Theses can be divided into two groups, Polarized and Non-Polarized, of which, there are various types.  Arguably film are better for audio.

Non-Polarized Kit
Polarized Kit



Again there are various types made out of different materials such as Germanium.  However, Silicon are the most common: 1N4148, 1N914  it doesn’t matter which one, they’re both effectively the same.



Another type of diode which is extremely common is the king that emits light, a.k.a. Light Emitting Diode.  These are, mainly, useful for two things; 1) telling if a pedal is engaged and, 2) using in place of a regular silicon diode for some interesting sounds.



The easiest way to tackle these is to buy a “kit” of values:

E12 Resistor Kit



Very common as a volume control: 100K-A (audio/Logarithmic taper).

Other common values: 470 or 500 ohms, 1K, 5K, 25K, 47K or 50K, 100K, 250K, 1M.


Op Amps

Perhaps the most common of all is the 741 but a TL071 can be less noisy.  You can also get these in multiple OpAmp packages, e.g. the TL072 contains two OpAmps and the TL074 contains 4 OpAmps. Some less common values include the JRC4885D (Ibanez Tube Screamer) and the LM308 (ProCo RAT) although both are tricky to source these days.

I can’t stress this enough, you need to build circuits, using these components, to really appreciate how they work.  Providing you run from battery, the very worst you can do is melt or pop some components (we’ve all done it, it’s part of the learning curve).  Here are four basic links containing fairly simple circuits to build…

LM386 Amplifier

A Simple Power Amp is a great place to start and will save you money on buying a battery powered amplifier.  Note the LM386 is NOT an OpAmp, it’s a power amp in a chip

Fuzz Face

Once you have mastered that, the next step is to try a Fuzz Face circuit, which is, probably the most famous fuzz circuit.  It is based around two transistors but pay close attention to the polarity of the voltage on these; some are -9v (minus 9 volts) with a positive ground.

Transistor Buffers

One of the most basic circuits you can build is a Transistor Based Buffer, linked below is a great resource. The common values of transistors and OpAmp detailed above will work with all these buffers.  Pay close attention to the part about Vr, as most transistor (and OpAmp) circuits need to be “biased”

Op Amp Booster

Next you may decide to build an Op Amp Booster. Linked here is an example with the appropriate maths to work out the gain.

Electronics is a large field and, therefore, it’s not possble to cover everything in this initial post.  However, there should be enough information here to get you started building circuits and researching into how the components work within the field of small signal audio electronics.

Get building and share your thoughts and creations with us.

About the Author

Stuart Welsh

Stuart Welsh

Resident Audio Guru

Stu has been teaching Music Technology since 2002 at dBs Music where he is currently a degree Course Leader. He is an Avid Certified Expert Instructor in Pro Tools and holds a Masters Degree in Computer Music.

Systems Integration (Part 3)

Systems Integration (Part 3)

The second episode in Systems Integration: XLR Patching.

The studio has four wall boxes, only two of which are in the same room.  I wanted to create an effective way of getting these wall boxes  to the sixteen mic pre’s in the console, so I opted for an XLR patchbay, because of slight danger of using a jack style patch chord with phantom power (the power briefly connects to the wrong side first).  As the studio will be used by a variety of engineers and I also have some vintage ribbon mics, I wanted to make sure the patching was as logical as possible.

A great bulk of thee effort, involved with building this, was in bolting the XLR sockets into the patch panel, each socket requiring two tiny bolts and self-locking nuts.  This process, in itself, took a good couple of hours to work through all 96 fixings.

Once that was complete, I had to de-solder all the tails that previously occupied the end of the cables before I could move onto soldering them to the new sockets.

Once this was complete I cable tested every single socket on the panel and wall boxes for continuity and polarity.  Finding just two minor errors which were very easily corrected.  It pays to be methodical with these things.

And, here it is, the finished panel, all labelled up.  The bottom panel is the 16 mic pres in the Audient, and the top is the wall boxes.  Of course, this also means I have effectively created a “wall box” in the Control Room too.

Should be incredibly easy to route signals from now on.

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